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REACTION ENGINEERING OF COAL LIQUEFACTION
Ken K. Robinson Mega-Carbon Company St. Charles, IL
Table of Contents
INTRODUCTION
A Brief History of Coal Liquefaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Process Overview: What Goes In and What Goes Out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Need to Motivate Remaining Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COALS AND CONVERSION CHEMISTRY
Coals: Classification, Nomenclature and Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Coal Reaction Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Coal Expts ---> Solubility Lumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Model Compound Expts ---> Elementary Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
THE ROLE OF CATALYSTS Catalysis
Coal Liquefaction Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Coal Liquids Upgrading via Hydroprocessing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Mass Transport Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Catalyst Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REACTION MODELING
Global Models (Lumped Models) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Molecular Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Mechanistic Models
REACTOR TYPES/MODELS AND PROCESS DESIGN
General: What are Reactors? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Laboratory Reactors
Coal Liquefaction Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Model Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pilot Plant ProcessesConjecture About Commercial Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
INTRODUCTION
A Brief History of Coal Liquefaction
Interest in coal liquefaction has had several high and low periods over the last 60 years. It starts just
prior to World War II with the Bergius Process used by Germany to fuel their war effort, continues
through the 50s and 60s with the research by the U.S. Bureau of Mines, and then hit a extensive pace
following the Arab oil embargo of 1973. Since the early 1980s, interest has waned due to the decrease
in crude oil prices, but the recent invasion of Kuwait by Iraq has caused the world to reevaluate their
position and think about a national energy policy which will provide incentives to develop U.S. natural
resources. The largest source of liquid hydrocarbons is crude oil and the United States has found and
used up the cheap domestic crude that it once had. When demand exceeds supply, or an errant ruler
decides to invade a neighboring country, the price of crude oil will increase. This makes a wide variety
of options available to produce liquid hydrocarbons from alternative energy resources. Coal liquefaction
would be an attractive option for the United States due to the vast coal reserves.
In a 1988 article, Lumpkin (Lumpkin 1988) said "the economics derived early in the 1980s established
the price of transportation fuels from coal liquefaction at $80 per barrel or higher. However there have
been dramatic improvements in the technology since 1983 that have not been widely appreciated.
Recent designs and cost estimates show that a 60% decrease in the cost of liquid fuels from coal relates
to about $35/barrel for crude oil".
Direct liquefaction involves the addition of hydrogen to coal in a solvent slurry at elevated temperature
and pressure. The solvent provides a convenient transportation medium for the coal and enhances heat
and mass transfer during chemical reaction. In many processes, the solvent shuttles hydrogen from the
gas phase to the coal and is called a donor solvent. The elevated temperature cracks the coal molecules
by thermally rupturing carbon-carbon linkages and increases the rate of reaction. High pressure keeps
the solvent and products in the liquid phase, prevents coke build-up on the reactor walls and catalyst
surface, and promotes hydrogenation by maintaining a high partial pressure of hydrogen. Catalysts are
normally used to increase the rates of the desirable reactions which include cracking, hydrogenation ,
and removal of oxygen, nitrogen, and sulfur.
Coal consists of complex macromolecules without repeating monomer units that are built primarily of
carbon and hydrogen but also consist of significant amounts of oxygen, nitrogen, and sulfur. The
constituent units tend to be mostly substituted aromatics or hydroaromatics and the degree of
condensation increases as the coal matures. Coal has a hydrogen-to-carbon ratio which is significantly
less that petroleum ; converting coal into liquids requires either the addition of substantial amounts of
hydrogen or the removal of excess carbon.
The coal time line for the 20th century (Schlesinger 1980) helps to trace the major events that have
occurred in our recent history. Referring to Figure kkr-1, direct coal liquefaction technology developed
from work on hydrogenating coal tar by Bergius in Germany in the 1920s (Fischer 1925). A major
improvement was the development of sulfur-resistant hydrogenation catalyst by Pier in BASF's
laboratories in Ludwigshaven, Germany in 1924. The first German brown coal was hydrogenated in
1929. In the 1930s, several plants were built in Germany, and one in Billingham, England. The original
Bergius plants consisted of liquid-phase hydrogenation of coal/recycle slurry followed by vapor-phase
hydrogenation of middle oils (180 - 325°C). Both hydrogenation reactors operated with catalyst and
pressures of 5000-10,000 psig.
German capacity increased to the point that during World War II, eight additional plants were built in
Germany. In 1943, the installed capacity was over 100,000 BSD in 15 plants processing about 50,000
tons of dry coal per day. The US Bureau of Mines tested the German technology after World War II in a
200 barrel per day pilot plant. All of these efforts were technically successful but could not compete
economically with inexpensive petroleum from the Middle East in the early 1950s.
Second generation technology came along in the late 1970s and early 1980s. The pilot plants to develop
these processes included:
1. SRC-II(solvent refined coal) in Tacoma, Washington
2. EDS (Exxon Donor Solvent) in Baytown, Texas and
3. H-Coal in Catlettsburg, Kentucky.
Both the SRC-II and EDS process depended on a donor solvent for hydrogen during the liquefaction and
used the minerals in the coal for a catalyst. EDS did use a catalytic stage to hydrotreat part of the recycle
solvent in a second reactor but no catalyst was used in the coal liquefaction unit. The H-Coal process
was developed by Hydrocarbon Research Inc.(HRI) and was derived from their H-Oil process for
petroleum resid upgrading. The basis of the process was a novel catalytic reactor in which the catalyst
was ebullated in the liquid phase, similar to the more familiar gas-phase fluidized bed processes used in
the petroleum industry. The advantage of this type reactor is that the reactor contents are well mixed
helping to alleviate the excessive heat release associated with coal liquefaction plus the ability to add
and withdraw catalyst from the reactor while it is operating so that the catalyst activity is maintained at a
relatively high and constant level. The U.S. Department of Energy (DOE) and a consortium of industrial
sponsors built a 250 ton per day pilot plant at Catlettsburg, Kentucky which ran from June 1980 to
January 1983. The plant was a technical success and confirmed yields by smaller scale equipment and a
wide variety of mechanical equipment was tested including ebbulation pumps, pressure letdown and
block valves, and hydroclones to concentrate the solids-containing product. Although the process was
not economic, two recent developments promised further cost reductions. The first was a new catalyst
developed by Amoco researchers with a unique bimodal pore size distribution resulting in less catalyst
deactivation, and most importantly higher liquid yields. The second improvement came from Chevron
Research, a two stage liquefaction scheme which lowered hydrogen consumption by tailoring reaction
conditions in each stage to favor the reactions of coal dissolution, hydrogenation of the donor solvent.
thermal/catalytic conversion of the high molecular weight species (residuum) and removal of sulfur,
nitrogen, and oxygen.
Process Overview of Coal Liquefaction
Direct coal liquefaction refers to the direct reaction of coal with hydrogen to form liquids. The hydrogen
reacts with oxygen, sulfur, and nitrogen in coal to remove them as water, hydrogen sulfide, and
ammonia. The hydrogen is also required to substantially increase the H/C ratio before it becomes a
liquid. One of the key differences between coal and petroleum is the much lower H/C atomic ratio of
coal (-.7 vs 1.2 for petroleum). Consequently the conversion of coal to petroleum-like products requires
direct hydrogen addition and this adds considerable expense to the product.
The ideal technology for direct coal liquefaction for gasoline production would:
- produce only C5-400°F products which contain no nitrogen, oxygen, or sulfur
- produce no light hydrocarbon gases, since these gases consume large amounts of
hydrogen
- produce aromatic liquid for octane requirements to reduce hydrogen consumption
- remove oxygen from coal by carbon rejection to carbon oxides rather than
hydrogenation
None of the current coal liquefaction processes accomplish this. Second-generation liquefaction
processes now under development are descendants of the classic Bergius technology. Most involve,
more or less, catalyzed interactions between molecular hydrogen and coal-oil slurries of elevated
pressure and temperature. The new processes (2500 psi, 800-850°F) are different; 1. They operate at
less severe conditions, 2. They place a great deal of emphasis on H transfer from the slurry oil to the
coal/ 3. Careful attention is paid to producing process-derived solvent to recycle to the front end of the
process for slurrying the coal.
The following table lists these "second-generation" processes and how they compare:
Table 1Coal Liquefaction Process Summary
Name Reactor Type Demonstration Unit Capacity, TPD
H-Coal ebulated bed w/recycle Catlettsburg, KY 200
Exxon Donor Solvent plug-flow dissolver +catalytic solventhydrotreater
Baytown, TX 250
SRC I & II plug-flowpreheater/dissolver
Tacoma, WA andWilsonville, AL
506
Two stageliquefaction
plug-flowpreheater/dissolver &ebullated bedhydrotreater*
Wilsonville, AL 6
* Interstage deashing (moderate coupling) or Post-2nd stage deashing (tight coupling)
The Need for Reaction Engineering in Coal Liquefaction
Commercial coal liquefaction plants represent huge investments in capital. For example, the
Breckenridge H-Coal plant was sized to produce 80,000 BSD of liquid products and cost estimates were
in excess of 2.5 billion dollars in the mid 80's. Small errors in equipment sizing or yield translate to
millions of dollars in unnecessary expense. Thus it is extremely important to do the best job possible on
sizing, modeling, and specifying reaction conditions for the coal liquefaction reactor(s).
We need to have a clear understanding of reactors at three stages of development. At the earliest stage,
the laboratory reactors explore new reaction conditions, catalyst formulations, or coal types. The
reaction time should be well defined (avoid long heat-up and cool-down periods) and sufficient mixing to
minimize mass transfer limitations. Reaction kinetics may also be studied at this stage so the rate
models should not have some artifact of the experimental test embedded in one of the parameters.
The second stage of development is typically aimed at mimicking commercial operations by employing
recycle streams to achieve realistic simulations of the integrated process. Isothermal conditions are
usually maintained in the reactor. This is one of the more bothersome aspects of coal liquefaction, since
the coal liquids produced must be fractionated and then a solvent oil sent back to the slurry tank so that
the "process runs on its own juice". The challenge is significant. For example the H-Coal process
demonstration unit (PDU) at Trenton, NJ was a vertical 4" tube with a single bubble cap at the bottom to
introduce liquid feed and hydrogen gas. This was scaled to a 5 ft. diameter H-Coal reactor at
Catlettsburg with over 250 bubble caps and now the added complexity of localized flow variations in the
ebullated catalyst bed.
Finally we move to commercial scale and adiabatic operation. High hydrogen consumption translates to
substantial heat liberated in the liquefaction reaction. Reactor control and stability are extremely
important. Higher-than-desired temperatures lead to cracking reactions and subsequent lowering of
liquid yields. Furthermore coke will accumulate on the inner reactor walls and eventually alter the flow
regime and plug the reactor. We need to design the commercial reactor so that it is sized properly, can
be started-up and shut-down safely, and operated stably at steady-state conditions. It is a formidable
problem for the reaction engineer but if he/she is careful and rigorous, the end product will be success.
COALS AND CONVERSION CHEMISTRY
Coals: Classification, Nomenclature and Structure
The following is an excerpt from Peter Given's (Given 1986) course on coal science at Pennsylvania
State University. His excellent and vibrant description of coal cannot really be improved upon by the
author so it has been left relatively untouched.
"Coal can reasonably be described as an organic rock. As such, it is, like other rocks, a heterogeneous
assembly of more specifically definable components, and the plant debris from which it originated has
undergone a progressive series of alterations as a result of geological or geochemical processes.
Whenever materials are used, it is desirable to be able to relate properties as determined by simple
measurements in the laboratory to behavior in an industrial process; if such a relationship can be
established, one has a basis for selecting the best material for a given process, or for modifying
operating conditions to utilize most effectively the material available. Therefore, as with any other
material, it is important to be able to classify coals and coal components in some manner helpful to
utilization, on the basis of standard laboratory tests.
There are at least seven characteristics of coals that are or may be relevant to their behavior in
conversion processes: rank, geological history, mineral content, trace element distribution, petrographic
composition, chemical structural parameters and pore structure. The coal "series" allows coals to be
classified by the carbon content as:
peat ---6 lignite ---6 sub-bituminous ---6 bituminous ---6anthracite
When we study coals, what we see in nature is an apparently continuous series of alterations from living
plant material through dead material, peat, lignite, sub-bituminous and bituminous coals and anthracite to
graphite. The process of alteration from the peat stage onward is called metamorphism, and the term
"rank" is used in the scientific literature to signify the degree of alteration or metamorphism. Rank is of
such importance in defining the properties of coals that we must discuss it at some length.
Table 2
ASTM System for Classifying Coals By Rank
Class Group Fixed
Carbon,maf
Volatile
Matter,maf
Heating
Value,BTU/lb
Anthracite metaanthracite
anthracite
semianthracite
>98
92-98
86-92
<2
2-8
8-14
Bitumenous low-volatile
med volatile
high volatile A
high volatile B
high volatile C
78-86
69-78
<69
14-22
22-31
>31 >14,000
Subbitumenous subbitumenous A
subbitumenous B
subbitumenous C
10,500-11,500
9,500-10,500
Lignitic lignite A
lignite B
6,300-8,300
<6300
For simplicity, Table 2 makes it appear that coals are distributed along a single development line. In fact
any plot of a coal property against a measure of the degree of metamorphism is a band, rather than a
line. That is, there have evidently been a number of related pathways by which plant debris has been
progressively altered and passed along the coal series. Nevertheless, the degree of alteration is of
overriding importance in determining what set of properties a coal will have. If you are going to use a
coal, its rank is always the first thing, and sometimes even the only thing, that you want to know about it.
Table 3.Approximate Values of Some Coal Properties in Different Rank Ranges
High Vol. Bit. Bituminous Lignite Subbit. C B A Med Vol Low Vol Anthracite
% C (ash free) 65-72 72-76 76-78 78-80 80-87 89 90 93
% H 4.5 5 5.5 5.5 5.5 4.5 3.5 2.5
% O 30 18 13 10 10-4 3-4 3 2
% O as COOH 13-10 5-2 0 0 0 0 0 0
% O as OH 15-10 12-10 9 ? 7-3 1-2 0-1 0
Aromatic C atoms% of total C
50 65 ? ? 75 80-85 85-90 90-95
Av. no. benz.rings/layer
1-2 ? 2-3 2-3 2-3 2-3 5? >25?
Vol. Matter, % 40-50 35-50 35-45 ? 31-40 31-20 20-10 <10
Reflectance, %,Vitrinite
.2-.3 .3-.4 0.5 0.6 0.6-1.0 1.4 1.8 4
Density
Total surface area
Plasticity andcoke formation
Calorific value,maf free, BTU/lb.
7000 10,000 12,000 13,500 14,500 15,000 15,800 15,200
"Rank" is as a qualitative concept. Obviously we need some sort of quantitative parameter to characterize the
stage of alteration reached by a coal in its progression from plant material to anthracite. Since metamorphism of
coal involves systematic changes in so many of its properties and structural characteristics, we should expect that
several parameters would be needed simultaneously to specify adequately the degree of metamorphism. But
there are obvious practical difficulties in following this course...
The A.S.T.M. system used for commercial purposes in the U.S., and the International Systems used in Europe,
depend upon volatile matter to classify the higher rank coals, and calorific values for the lower, which is fair
enough as far as it goes. These properties are used to classify coals into about ten categories, as shown in Figure
KKR-2. People sometimes speak of a coal as being, for example, of high volatile A bituminous rank, as if the
statement uniquely defined the rank. It would be more accurate and less liable to lead to misconceptions if one
spoke of a coal as being in a certain rank range. Each rank category in fact covers a portion of the total rank
range, and the high volatile A category in particular contains coals of considerably different degrees of
metamorphism (e.g. the carbon content may be between about 80 and 87%, and the oxygen content between 11
and 4%).
A further characteristic of coals that may be of practical importance is their geological history. Coals are known in
almost every state of this nation, though in some states the deposits are too small to be of much commercial
significance. There were two principal coal-forming periods, the coals of the central and western states being
much younger than those of the eastern and southeastern states. This means that coals on different sides of the
continent were derived from plants of quite different type, since a great deal of evolution occurred in the 150
million years between the Carboniferous and the Cretaceous. Moreover, for any one geological age, coals were
deposited in a number of distinct basins in different parts of the country, and experienced distinctly different
geological histories.
For these various reasons, coals from the different major deposits may have properties that differ in ways of
considerable importance to the coal user. Therefore, it is of practical significance to use a geological
classification for the nation's coal deposits, as shown in Table 4 below. In some of the Provinces, notably
Provinces 2 and 4, there are several distinct basins of somewhat different histories. With this reservation, one
might expect to find some degree of homogeneity among coals of any one province, but to find differences
between Provinces.
Ranks of Coal
Schobert's (Schobert 1987) description of the various coal ranks follows:
BROWN COAL
Compared to lignite, Brown coal has a very high moisture content, sometimes more than 60% as it is mined, and
it is a younger coal geologically. Deposits of brown coal show distinct layering; the layers are distinguishable by
color. When brown coal dries on exposure to air, large pieces tend to disintegrate, a process called slacking.
When stored in stockpiles or bins, brown coal occasionally catches fire by spontaneous combustion. Brown coal
is easy to ignite but has a low heating value, about 3000 Btu/lb as mined. An enormous deposit of brown coal
occurs in the state of Victoria, Australia. Brown coal also occurs in central and eastern Europe.
LIGNITE
Among the coals of the United States, lignite is the lowest in rank, containing less than 75% carbon on a moisture-
and ash-free basis. The moisture content of lignite is high, but not as high as that of brown coals. The highest
moisture content of lignite mined in the United States is 40-42% (Gascoyne mine in Bowman County, North
Dakota). Lignite is relatively soft and ranges from brown to black. Because lignite has not progressed very far in
maturation, many lignite deposits contain easily recognizable plant remains up to the size of branches or stumps.
The reproductive parts of plants - spores and pollen - are well-preserved in lignite.
Two major deposits of lignite occur in the United States. The Fort Union region spreads over North Dakota,
Montana, Wyoming, and parts of Saskatchewan and Manitoba. This lignite deposit is the largest in the world.
The Gulf Coast lignites stretch from southern Texas, across Louisiana and Arkansas, and into Mississippi and
Alabama.
SUBBITUMINOUS COAL
Subbituminous coal is intermediate in rank between lignite and bituminous coal. Subbituminous coals have
matured to a point at which the woody texture often seen in brown coals or lignites is no longer apparent.
Subbituminous coals are black, having none of the brownish color seen in some lignites. Years ago
subbituminous coal was sometimes called black lignite. Subbituminous coals have the same tendency toward
slacking and spontaneous combustion as lignites and brown coals have. Large deposits of subbituminous coals
occur in the United States in the Rocky Mountain states.
BITUMINOUS COAL
Bituminous coal supplies most of the energy that comes from coal. Until the 1970s the low-rank coals provided
only about 2% of the national coal consumption. Bituminous coal and anthracite made up the rest; bituminous
coal had about a 6:1 margin over anthracite.
Bituminous coal is black but frequently appears to be banded with alternating layers of glossy black and dull black.
It breaks into prismatic blocks. The moisture and volatile matter contents are lower and the heating value is
higher than those of subbituminous coal. Bituminous coal shows little tendency to decrepitate on weathering or
experience spontaneous combustion.
When some varieties of bituminous coal are heated in the absence of air, they soften. Heating also removes
volatile matter as gases, which bubble through the softened mass of coal. The product resolidifies as a shiny,
porous, hard, black solid known as coke. Coke is the fuel used in blast furnaces to make iron.
Bituminous coal occurs in the Appalachian Mountain chain, running from Alabama up into Pennsylvania. A
second major deposit lies in the center of the United States, particularly in Illinois. Bituminous coal is widespread
throughout the world. The principal countries having bituminous coal include Great Britain, Germany, the Soviet
Union, China, and Australia.
ANTHRACITE
Anthracite ranks highest among coals, a position merited for several reasons. Having a very low volatile matter
content, anthracite burns with a hot, clean flame with no smoke or soot. This feature makes it an ideal domestic
fuel. Compared with bituminous coal, anthracite burns more slowly and gives off heat more uniformly. Anthracite
is very low in moisture, about 3%, and is low in sulfur. It is stable in storage and, unlike other coals, can be
handled without dust forming. Its heating value comes close to that of the best bituminous coals.
Anthracite is jet black and usually has a high luster. It is the hardest and most dense coal. In the days when coal
meant only bituminous coal and anthracite, the terms soft coal and hard coal were often used to distinguish the
two. Anthracite was occasionally called stone coal. It was also known as black diamond because of its hardness,
luster, and commercial value.
The many good qualities of anthracite make it a premium fuel. Unfortunately, its geographic distribution limits its
use. In the United States, significant deposits of anthracite occur only in a small region of northeastern
Pennsylvania. Therefore, it is not as readily available on the market as are other coals and thus commands a
premium price.
Coal Analysis
PROXIMATE ANALYSIS
The first approach to examining coal is based on the behavior of coal when it is heated. Heating is one of the
most easily available ways of bringing about changes in substances. All processes that make use of coal involve
heating it. For these reasons, an examination of the effects of heat on coal is a likely starting point for
characterizing the various kinds of coal.
If coal is heated gently at 105°C, it loses weight. The material that is removed at this temperature is water. This
test is the basis for determining the amount of water in coal. Usually, water is referred to as moisture in coal
analysis.
After the moisture has been removed, the temperature can be increased. Because coal burns if heated in air, this
test is done in an inert atmosphere. Heating coal at several hundred degrees Celsius causes another weight loss.
Some of the material that escapes in this experiment is a mixture of gases including carbon dioxide and methane.
The remainder condenses as an oily organic liquid and a tar. Ordinarily, the gases, oil, and tar are not analyzed
further to determine the amounts of pure compounds present in each. Instead, the total amount of gas, oil, and
tar evolution is lumped together and called volatile matter.
The solid material remaining after the volatile matter has been removed is a black char resembling charcoal. If
air is admitted to the apparatus, the black char burns and leaves behind an inorganic solid called ash. The
amount of coal material in the char is called fixed carbon and is calculated by subtracting the percentages of
moisture, volatile matter, and ash from 100.
The set of procedures for determining moisture, volatile matter, and ash and for calculating fixed carbon is known
as the proximate analysis. Use of this term is misleading because proximate is often thought to be short for
approximate. In fact, nothing is approximate about proximate analysis. The procedures for performing a
proximate analysis are very carefully defined and rigorously followed to ensure repeatability from one laboratory
to another. The specific details are prescribed by standards-setting organizations in various countries. In the
United States, the proximate and other analyses are governed by standards established by the American Society
for Testing and Materials (ASTM). The term proximate refers to the practice of lumping together a group of
constituents and reporting those constituents as one generic item, for example, volatile matter.
ULTIMATE ANALYSIS
Although the proximate analysis is helpful in practical applications, it doesn't provide information about the actual
composition of coal. The elemental composition is determined by a second set of analytical procedures, which
collectively are known as the ultimate analysis.
The elements that are normally determined in the ultimate analysis are carbon, hydrogen, sulfur, and nitrogen.
Carbon and hydrogen are measured by burning the coal and collecting the resulting carbon dioxide and water in
chemicals that will absorb them. By weighing the absorbents before and after the analysis, the amounts of carbon
dioxide and water can be determined. The carbon in carbon dioxide and hydrogen in water are known with great
accuracy.
In one of the common methods used to determine sulfur, the sulfur can be converted to sodium sulfate, which in
turn can be converted to barium sulfate. Barium sulfate is highly insoluble in water and therefore can be
collected, dried, and weighed.
Nitrogen is usually determined by converting it to ammonia and absorbing the ammonia in a known amount of
acid. The amount of acid remaining after the ammonia has been absorbed is subtracted from the amount used
initially. The difference represents the acid neutralized by the ammonia and thus tells the amount of ammonia
made in the experiment. Then, the amount of nitrogen present in the coal sample can be calculated.
Relatively simple analytical methods to determine oxygen in coal are not available. The most commonly used
method today involves bombarding the coal with neutrons to convert the oxygen to a very unstable isotop 22 of
nitrogen (nitrogen-16). The nitrogen-16 decays rapidly by emitting beta (ß) and gamma (() rays, which can be
detected and counted. A comparison of the results from a coal sample with those from a pure compound in which
the oxygen content is well-known provides a measure of the oxygen in the coal. Very few laboratories have
access to the appropriate neutron source, that is, a nuclear reactor. Many others do not want the expense, time
delay, or bother involved in sending samples to a reactor facility. Consequently, the amount of oxygen is
determined by adding together the percentages of the other four elements and subtracting this amount from 100.
The disadvantage of this procedure is that all of the experimental errors in the carbon, hydrogen, nitrogen, and
sulfur measurements are propagated into the calculated value for oxygen.
SULFUR IN COAL
When coal is analyzed carefully for both major and trace constituents, almost every element will be found. About
the only exceptions are the highly radioactive elements that are made artificially and some of the noble gases. Of
the roughly 80 elements found in coal, the one besides carbon having the most significant effect on coal use is
sulfur. When coal or coal liquids are are burned, the sulfur oxides formed cause air pollution if allowed to escape
into the environment. The sulfur in coal has caused the spending of millions of dollars on equipment and
processes to capture the sulfur oxides or to reduce the sulfur content of coal before it is burned. Sulfur occurs in
coal in three forms:
1.organic sulfur, which is a part of the molecular composition of the coal itself;
2.pyritic sulfur, which occurs in the mineral pyrite and some related minerals; and
3.sulfate sulfur, which is mostly iron sulfates.
In most coals, sulfate sulfur is a small fraction of the total sulfur content. In low-rank coals, organic sulfur may
contribute half or more of the total sulfur, but in most bituminous coals, the majority of the sulfur is pyritic sulfur.
Inorganic Constituents in Coal
Coal contains a rich variety of inorganic constituents. Mineral grains in coal can sometimes be seen with the
unaided eye and are easily seen through a microscope. When coal is burned it leaves behind the inconsumable
inorganic residue known as ash. The ash is the product of reactions or transformations of the inorganic
components of coal caused by the high temperatures of the combustion process. Several questions concern
these inorganic species: Where did they come from? What are their components? How do they behave?
SOURCES
Several sources contribute inorganic material to coal. The first is the coal-forming plants themselves. Most
plants contain at least small amounts of inorganic substances; that is why burning logs in a fireplace always leave
some ash. However, some plants concentrate significant quantities of inorganic material. For example, the
scouring rushes concentrate enough silica in their cell walls to give them a gritty texture that pioneers found useful
for cleaning cooking utensils or scouring floors. The scouring rushes are examples of the last surviving genus
(the horsetails) of a class of plants that were prevalent in the coal swamps of the Carboniferous period. Some of
the inorganic components of coal were in the original plant material.
During the diagenetic stage of coal formation, as the organic sediments were accumulating in the swamps, ample
opportunities occurred for inorganic debris to be deposited by either water currents or wind. This debris would
settle into the organic sediments and become part of the coal that eventually formed. Clay particles are deposited
into coal in this way. Minerals transported into the coal-forming environment by wind or water are called detrital
minerals.
Although all types of inorganic material in coal are important, either because of their deleterious effects in coal
processing or because of their adverse environmental impact and potential health hazards [76; see also Section
15.1], the relative abundance of different inorganic constituents makes it appropriate to distinguish between major
and minor constituents. The major constituents are
1.clay minerals (aluminosilicates), which occur mostly as illite, kaolinite, montmorillonite, and mixed illite-
montmorillonite and commonly make up as much as 50% of the total mineral matter contents;
2. carbonate minerals, principally calcite (CaCO 3), siderite (FeCO 3), dolomite (FeCO 3@MgCO3), and
ankerite (2CaCO 3@MgCO 3@FeCO 3), but frequently also present as variously composed mixed
carbonates of Ca, Mg, Mn, and Fe;
3. sulfides, which are mainly present as pyrite and marcasite (i.e., dimorphs of FeS x, which only differ in
crystal habit), but which have also been reported in the form of galena (PbS) and sphalerite (ZnS); and
4. silica, which is ubiquitous as quartz and usually accounts for up to 20% of all mineral matter.
Coal Petrology
The following is an excerpt from W. Spachman's (Spachman 1986) course taught at Pennsylvania State
University.
Coal petrology is the science concerned with the nature, origin, and evolution and significance of coal as a rock
material, and coal petrography is the subscience concerned with the description of coal materials and the practical
use of compositional descriptions. Widespread acceptance of "coal" as a class of rock materials has come about
only recently, hence, most "rock" petrologists are not well informed with respect to the composition of coal, and
coal petrology, accordingly, is a young science.
Coal may be usefully characterized at several different organizational levels, including:
1. the elemental level,
2. the molecular or sub-molecular level,
3. the phyterallevel,
4. the maceral-mineral level,
5. the lithotype level, and
6. the lithobody level.
From the standpoint of the coal petrologist, coal consists of two basic classes of materials, i.e. inorganic,
crystalline, minerals and organic, phytogenic, non-crystalline macerals. The macerals form the carbonaceous,
combustible fraction of the coal and by definition they must comprise more than half of the rock mass. Both
macerals and minerals occur in the coal as grains, particles, or fragments, commonly ranging in size from 2 cubic
microns to several cubic centimeters or larger.
The minerals commonly encountered in coal seams are:
Table KKR-6
I. Silicates
2 2 3 2Montmorillonite 4SiO @Al O (1+x)H O4 y 8-y 4 4 4 6 20Illite (OH) K (Si @Al @Fe @Mg @Mg )O
2 2 3 2Kaolinite 2H O@Al O @2SiO2 2 6 4 2 4Muscovite [K ] [Al Si ] [Al ] O (OH)
XII IV VI
8 5 2 3 8Chlorite H (Mg, Fe) Al Si O
II. Oxides
2Quartz SiO2 3Hematite Fe O
III. Sulfides
2Pyrite FeS2Marcasite FeS
IV. Sulfates
4 2Gypsum CaSO @2H O2 6 12 4 4Jarosite K Fe OOH) (SO )
V. Carbonates
3 3 3Ankerite 2CaCO @MgCO @FeCO3Calcite CaCO3Siderite FeCO
THE ROLE OF CATALYSTS
Coal Liquefaction Catalysis
Catalysis plays an important role in coal liquefaction and has led to important technical advances and improved
economics for producing liquid synfuels. A catalyst is defined as a substance which accelerates or changes the
course of a chemical reaction without being consumed itself. The influence of the catalyst on liquefaction rate is
actually not very noticeable since many of the liquefaction reactions take place in the absence of the catalyst. It is
generally believed that some of the minerals (i.e. pyrite) present in coal provide a catalytic function, particularly
for bituminous coals. However, once the coal has been initially liquefied, the catalyst will help to hydrocrack the
coal liquid constituents, remove sulfur, nitrogen, and oxygen as hydrogen sulfide, ammonia, and water and
rehydrogenate the hydrogen donor solvent.
Possible improvements to coal liquefaction catalysts can be summarized below:
Table 8Catalyst Improvements for Coal Liquefaction
Improvement Benefit
Higher activity Smaller reactor
More selectiveLess hydrogen usage
More effective heteroatom removal
More stable Lower catalyst usage
The way in which one makes catalyst improvements is not straight-forward. A heterogeneous catalyst is generally
preferred in coal liquefaction since separation of products from the catalyst is simplified. However, mixing of a
soluble catalyst with the coal slurry provides intimate contacting at the molecular scale. The diagram below
illustrates the catalyst parameters that have to be considered in an effective application:
(INSERT FIGURE KKR-3)
Table 9
Active Portion Nonactive Portion
Type of active components Type of support
Level of active components Support geometry
Stabilizer - Promoters Surface area
Bifunctionality Pore size distribution
Impurities
Reaction Steps in Catalytic Liquefaction
In catalytic coal liquefaction, with a heterogeneous catalyst, we will first list the various steps that occur in the
reaction path. In a slurry reactor, hydrogen-rich gas is bubbled through coal liquid containing suspended catalyst
particles. For simplicity, lets consider the reaction, hydrogenation of the donor solvent to replenish atomic
hydrogen lost (donated to reactive coal fragments to stabilize them). The catalytic hydrogenation reaction is:
According to film theory, the various steps in the overall process are the following:
1.Gas-liquid interface - hydrogen gas dissolves into liquid phase according to Henry's law
2.
Liquid film - dissolved hydrogen is transported to bulk liquid phase through the liquid film next to bubble.
3.
Catalyst film - dissolved hydrogen in bulk phase diffuses from the main body of the liquid to the catalyst surface.
4.
Catalytic reaction with pore diffusion - dissolved hydrogen diffuses into pores of the catalyst and reacts.
4'.Product transport out of pore - hydroaromatic diffuses out of catalyst particle to outside surface.
3'.Catalyst film - hydroaromatic transfers from the catalyst surface to bulk fluid through catalyst film.
2'.
Liquid film - bulk phase hydroaromatic is transported to gas phase through liquid film to gas-liquid interface of
bubble.
1'.
Gas liquid interface - a portion of the hydroaromatic vaporizes into the gas phase according to Henry's law.
Most of the above resistances occur in series but a few are in parallel, such as pore diffusion. The electrical
analog of a catalytic reaction helps to illustrate this resistance concept as shown below:
If the resistances were all in series, we could combine them by straight-forward algebra. However, pore diffusion,
a parallel resistance, complicates the mathematics and prevents a simple combining of terms. If the slowest step
in the process (i.e. rate controlling) is a series step, we can derive a simple rate expression for use in the reactor
performance equation. We will develop these in more detail later.
Nomenclature
Aip - partial pressure of A at gas-liquid interface
AiC - liquid concentration of A at gas-liquid interface
AH - Henry's law distribution constant for A
S - interfacial bubble area
AdN - molar flux of A, moles A/time dt
Alk - liquid film transfer coefficient
AlC - concentration of A in bulk liquid phase
exS - catalyst external area
Afk - catalyst film transfer coefficient for A
AsC - concentration of A at catalyst surface
cdN - molar flux of C, moles C/time df
cfk - catalyst film coefficient for C
csC - concentration of C at catalyst surface
clC - concentration of C in bulk liquid phase
ciC - concentration of C at gas-liquid interface
clk - liquid film transfer coefficient for C
cH - Henry's law distribution constant for C
cip - partial pressure of C at gas-liquid interface
The Porous Structure of Catalysts
Coal liquefaction catalysts are highly porous materials, and typically show some aspects of pore diffusion control.
Molecular transport in pores depends on the size of the pore. We can categorize pores as follows:
1. Bulk flow - Large pores
2. Ordinary diffusion - Large pores
3. Knudsen diffusion - Small pores
The relative balance of the surface reaction rate to the diffusion rate establishes the concentration profile in the
catalyst particle. If diffusion is rapid or surface reactions slow, then the concentration of reactant will not drop
very sharply as we travel down the pore. On the other hand, a slow diffusion rate or fast surface reaction will
produce a sharp concentration gradient in the catalyst particle; then the reaction on the external surface is much
higher than in the particle interior.
Ideal Catalyst
For the moment, consider a single cylindrical pore in a catalyst. The
differential mass balance for a first order reaction down the pore (Levenspiel 1972) is:
(INSERT FIGURE KKR-6)
Solution of the above differential equation leads to effectiveness factor plots like the following
sThe Thiele modulus, N, contains the intrinsic kinetic constant, k , that is frequently unknown. To get around this
problem, a new dimensionless modulus, M, is sometimes defined as:
M = R observed rate2
eff s D C volume
This produces the following modified effectiveness factor plot:
Real Catalysts
The mathematical development for the effectiveness factor, 0, as a function of the Thiele modulus is based on a
single size catalyst pore. We need to remind ourselves that real catalysts have a distribution of pores of many
different sizes. When catalyst extrudates are formed, the powder is compressed into a single large pellet and can
be visualized as:
The table below lists the properties of some catalysts developed by Amoco and W. R. Grace under an EPRI
contract.(Kim 1979) The "bimodal" pore size distribution is characterized by the inclusion of macropores greater
than 1000 Å in diameter.
This inclusion of these larger transport pores is important for activity maintenance and a higher catalyst
effectiveness factor. The mean diameter of the smaller pores (APD) should be around 120 Å to accommodate
the large coal-derived molecules. When the larger pores are "squeezed" out of the catalyst particle during
extrusion, or simply not built in, then the pore size distribution is called unimodal with a single hump in the pore
size frequency plot.
Table 10Catalyst Inspection Data
Composition, Wt% APD SA PV PV of 10 -10 Å ABD3 5
CoO MoOCatalyst
3
Å m /g cc/g % of total g/cc2
Bimodal Catalysts
HDS-1442A 3.1 13.2 58 323 .64 26.1 .567
CoMo-G120B
3.2 16.2 122 154 .60 11.1 .665
Mo-G120B --- 14.9 123 167 .65 9.6 .656
Unimodal Catalysts
CoMo-G120U
3.0 15.5 115 162 .62 4.7 .733
0.5 CoMo-G120U
0.5 16.0 121 160 .64 3.2 .723
Two of the bimodal catalysts are plotted in the frequency plot below along with one unimodal Amocat . AlthoughTM
the bimodal American Cyanamid HDS-1442A has a significant amount of macropore volume, the APD of the
smaller mesopores is too small for hydroliquefaction of coal. When exposed to a coal liquefaction environment,
the smaller pores rapidly fill with coke causing the pore mouth to neck down and eventually plug.
Not only does the catalyst effectiveness factor drop but ultimately the interior of the catalyst particle is isolated
from the exterior by the coke-filled pores. This will be discussed more in a later section on catalyst deactivation.
Which Rate Equations to Use
The rate equations for catalytic hydroliquefaction can take many forms depending which step or steps are slowest
and rate controlling. Referring back to the mass transport steps (Figure 2) the rate equations for these various
situations are listed below:
External Mass Transport
- Hydrogen Transfer Controls at Bubble Interface
When the transport of hydrogen gas from bubbles to the bulk liquid is the slowest step the rate
equation is:
iThe interfacial area of the gas bubbles, a , is a critical parameter in this case. If the gas bubbles are large, we can
increase reaction rate by better dispersion of the gas into the liquid phase, to produce smaller bubbles (increasing
ia ). Obviously there is a practical limit, because an unstable foam will eventually develop as bubble size is
decreased.
- Film Resistance Effects at Catalyst Surface
When the catalyst is functioning well and the catalytic rate fast, then external mass transfer may be the rate
limiting step. As soon as the reactants reach the external catalyst surface, they react. This causes the film
around the catalyst particle to limit transport of fresh reactant to the surface and, in turn, to control the overall
reaction rate.
Catalytic Reaction Controls
Pore Diffusion Limitations
We defined the effectiveness factor earlier as the ratio of the diffusion affected rate to the non-diffusion
rate or
Thus to account for pore diffusion limitations, the effectiveness factor is included in the rate equation as a
multiplier of the catalytic rate constant. If we begin to include other transport effects, the effectiveness factor will
still be associated with the catalytic rate constant. Referring back to the case where film resistance effects were
present at the catalyst surface we would use, the effectiveness factor as follows:
Estimation of the effectiveness factor was described earlier in the section on porous structure of catalysts. We
restate the key aspects.
Research Data for Reactor Design
Fulton (Fulton 1986) has pointed out that reactor design for a heterogeneous catalyst involves two major phases.
First, the researcher collects experimental reaction-rate data that are free of heat- and mass- transfer effects (i.e.,
intrinsic kinetic data). Second, the design engineer combines the intrinsic rate with the specific-heat and mass-
transfer effects appropriate to the plant-sized reactor to be designed.
In the second phase, the design of a commercial liquefaction reactor, the design engineer must take into account
the variety of heat-transfer, mass-transfer and reaction phenomena that occur.
In the first phase, the generation of intrinsic reaction-rate data, the researcher must:
1. Build and operate the reactor to minimize axial and radial transport (intrareactor) effects; e.g., isothermal
operation is highly desirable. We should define time "zero" (no long heat-up) in batch reactor test by
using a charging bomb, to inject the reactants into the reactor after heat-up.
2. Operate the reactor at a mixing rate large enough to remove heat- and mass-transfer gradients between
the catalyst particle and the bulk fluid (particle-fluid or interphase).
3. Use catalyst particles small enough to eliminate the influence of intraparticle heat and mass gradients.
With these precautions taken, the observed reaction rate will most likely be the intrinsic rate -- i.e., the true
catalytic reaction rate. As a result, the researcher needs methods for determining whether transport effects are
influencing the laboratory data. The "limiting criteria" allow the researcher to calculate or experimentally
determine whether these transport effects are important. They, in turn, guide the researcher to reactor operating
conditions that will provide data free from transport effects.
@ Mixing Intensity to Eliminate Mass Transfer Effects
@ Reduce catalyst particle size to reduce pore diffusion effects
Key rate parameters are modified by the influence of different transport regimes. For example, if diffusion is
strongly influencing the overall reaction rate, the energy of activation determined experimentally will be one-half
the intrinsic reaction energy of activation. As a result, the Arrhenius correlation for reaction rate would be grossly
in error.
The kinetic parameters (order and activation energy) are altered as the data-collection basis moves from the
chemical-reaction, and through the intrapellet and interphase, controlling domains. The influence of pellet size
and fluid velocity on rate becomes greater as bulk mass transfer becomes more important. Displayed below is
the characteristic behavior of kinetic parameters appropriate to each controlling regime.
Table 11
A A A-r = kC = A exp[-)E/RT}Cn n
DomainActivationEnergy Order
Pellet Size
FluidVelocity
ChemicalReaction )E n Independent
Independent
Diffusionreaction )E/2 (n+1)/2 1/L
Independent
Bulk masstransfer 5 kcal/mole 1st (1/L)3/2
v1/2
Catalyst Deactivation
In general, catalyst deactivation occurs by three mechanisms, 1.) sintering of the supported metals on the surface,
2.) poisoning by compounds which strongly and irreversibly adsorb on to the active sites and 3.) fouling, where
coke forms on the surface of the catalyst which limits access to the active metals and also reduces the diameter
of the pores, eventually leading to pore mouth plugging.
Coking, sintering and ash deposition all contribute to catalyst deactivation in coal liquefaction, the degree
depending upon the coal type and processing conditions. Used catalyst characterizations are described below for
work in pilot plant studies at Amoco. (Kim 1979)
Effect of Coking
Table 12 presents the loss on ignition (LOI) for various used catalysts. Irrespective of catalyst type and on-stream
hours, the loss on ignition (LOI) is almost always 20 wt% based on spent catalyst weight. Elemental analyses
show that a majority of this deposition is undoubtedly "coke" with the atomic H/C ratio ranging between 0.78 and
1.0. The second largest constituent is sulfur which amounts to about 5 wt%. The H-Coal catalyst in the table was
obtained from the bench scale H-Coal bench unit of HRI after 600 hours on stream operating in the syncrude
mode followed by testing in the Amoco continuous pilot plant for 50 additional hours; a somewhat higher LOI
(26%) is observed. The two unpromoted molybdenum catalysts (at the bottom) are no different from the rest of
the catalysts as far as LOI.
Table 12IGNITION LOSSES OF USED CATALYSTS
Amoco Pilot Plant Tests (Kim 1979)
Type Run No. CatalystHours on Slurry LOI, Wt% S, Wt%
5140 CoMo-G200B 136 21 4.5
Cobalt-molybdena 5149 CoMo-G120B 166 20 -
5138 CoMo-CT100 200 21 -
5135 HDS-1442A 190 21 -
5147 H-Coal (H-CoalUnit-Syncrude
650 26 -
5142 NiW-G120B 48 21 5.5
Nickel-molybdena orNickel-tungsten
5141 NiMo-G120B 137 24 5.1
5146 NiMo-G120B/Si 147 20 4.1
5145 NiMoRe-G120B 160 22 5.1
Miscellaneous 5144 SnMo-G120B 112 20 4.9
5143 Mo-CT100 95 22 4.5
Unpromoted molybdena 5148 Mo-G120B 140 20 3.7
The electron microprobe analysis of carbon profile across the catalyst pellets is shown in Figure KKR-13.
Both HDS-1442A and the Amoco CoMo catalyst show very high carbon concentrations at the outer
perimeter with some lower level of carbon across the entire cross-section, indicating pore mouth
plugging. An aged catalyst from the pilot plant was ground and retested in a batch unit; performance was
almost equivalent to the fresh catalyst. Other observations from the microprobe analyses are that HDS-
1442A shows stronger and more sparse carbon intensities in the catalyst interior, compared to the more
uniform distribution for CoMo on Grace 120 D catalyst.
Contaminants on Used and Regenerated Catalysts
The used as well as regenerated (calcination in air) catalysts were analyzed for metals contaminants by
emission spectroscopy. The results on calcium, iron and titanium are presented in Table KKR-13 on a
coke free basis. Titanium is clearly the major contaminant followed by, to a less extent, iron, silicon,
sodium, magnesium and calcium. The trace metals deposited on the catalyst are primarily from the coal.
The slurry oil typically contained less than one ppm titanium, 24 ppm Fe, 9 ppm Na, 5 ppm Mg, 2 ppm
Si, and 1 ppm Ca.
It appears that the titanium on spent catalyst is present in the carbonaceous deposit as well as the
catalyst support. When compared to the regenerated catalyst, about 60-70% titanium measured on the
spent catalyst is retained on the regenerated catalyst. It is not clear how the titanium leaves during the
regeneration. Calcium on the H-Coal syncrude catalyst is unusually high, perhaps due to the difference
in H-Coal slurry oil and a longer time on stream.
Table KKR-13TRACE METALS ON USED AND REGENERATED CATALYSTS
Amoco Pilot Plant Tests (Kim 1979)
Catalyst Hours
on StreamWt%, Coke-free BasisCalcium Iron Titanium
H-Coal (Syncrude)UsedRegen
650-
0.940.29
0.80.7
2.61.8
CoMo-G 120BUsedRegen
130-
0.140.12
0.60.4
2.31.4
NiMo-G120B/Si
Used 147 0.49 1.0 3.7
Regen - 0.24 0.9 2.4
Mo-G 120B
Used 140 0.13 0.8 3.0
Regen - 0.15 0.4 2.3
Mo-CT 100
Used 95 0.26 0.6 3.8
Regen - 0.15 0.4 2.3
Pore Size Distribution of Used Catalyst
Pore size distributions of fresh, used and regenerated catalysts were compared to examine patterns of
pore plugging. Figure KKR-14 shows pore size distributions obtained by mercury porosimetry for fresh
HDS-1442A and Amocat-1A, CoMo on 120 D alumina, tested by HRI. Pore volume is plotted versus
pore diameter and both catalysts clearly show bimodal pore distributions. HDS-1442A, which has
smaller pores than the Amoco CoMo catalyst, has a macropore (>1000 D diameter) volume about 27%
of the total, whereas the macropore volume of the Amoco catalyst is somewhat lower (18% of the total
pore volume). Based on performance data, the Amocat-1A catalyst is believed to have a more favorable
pore distribution, due to the larger mesopores around 125 D diameter.
The pore size distribution of the used HDS-1442A catalyst which had 190 hours is compared with the
fresh catalyst in Figure 3-17. The pore volume of the used catalyst is shown on a coke free basis in
order to make a one-to-one comparison with the fresh catalyst. The macropores on the right are virtually
unaltered after use. The macropores probably serve as feeder pores which transport materials into the
catalyst interior. On the other hand, the small pores in the range of 40-100 Å diameter, which contains
most of the surface area, have been greatly reduced. The reduction in pore volume and correspondingly
surface area is caused by coking and/or sintering. When regenerated, these pores are only moderately
restored. Small pores are usually more susceptible to sintering.
The pore size distribution of fresh and used Amocat-1A (CoMo on Grace 120 alumina) are shown in
Figure 3-18. This particular sample has been aged for 170 hours in the continuous pilot plant. Like the
HDS-1442A catalyst, the macropores of Amocat-1A are unaltered. The pore size distribution for the
used catalyst implies a whole new range of small pores had been created. However, this is due to partial
blockage of the pores by the coke causing a downward shift in the size distribution. Regeneration
restored most of these active pores in the 40-200 D diameter range. The net effect was that only about
20% of the fresh pores are destroyed during the course of liquefaction. The used catalyst retains more
than one half of its micropore volume, based on the regenerated pore volume.
Deactivation Models
In order to translate the differences in benzene soluble conversion to catalyst activity, a simple kinetic
scheme was proposed, which consists of two first order parallel reactions; one path represents coal going
to asphaltols, defined as THF soluble-benzene insoluble material, and then proceeds to benzene soluble
product; the other path represents coal going directly to benzene soluble material.
The above reaction scheme is not intended as a process model but serves as an estimation of catalyst
activity consistent with observations. Two assumptions were made; the first assumption deals with which
one of the two reaction paths is likely to be affected the most by a catalyst. Step 3 is assumed to be the
catalyzed reaction step. Since THF soluble conversions are the same for both thermal and catalytic
2liquefaction of Illinois No. 6 coal, Step 2 should be primarily thermal allowing k to be estimated from
thermal data. Thermal reaction also results in some benzene soluble product directly from coal as
shown in Step 1. We assume that a major function of catalyst is to convert asphaltols to benzene
soluble material. Of course, the mineral matter of coal has some small catalytic action on preasphaltene
conversion, but benzene solubles thus produced may be regarded as if derived directly from the thermal
reaction of coal. The second assumption is that the stirred autoclave reactor approximates perfect
mixing. Then, the following simple algebraic expressions for the rate constants can be derived.
Using the data obtained with HDS-1442A catalyst at 427°C and 48 minutes residence time, the values of
1 2 3 3k , k and k are calculated as 10.4, 6.3 and 7.0 hour , respectively. The rate constant k is a measure of-1
catalyst activity for liquefaction conversion. Since the space velocity remained constant in the aging
3test, the values of k /LHSV are plotted in Figure 3-11 for the unpromoted molybdenum and two other
CoMo catalysts.
The plot suggests first order deactivation since activity declines with time in a logarithmic fashion. The
unpromoted molybdenum catalyst is no more active than CoMo version initially, but it clearly maintains
activity better. For instance, at 100 hours on stream, the molybdenum catalyst shows a liquefaction
activity 40% higher than CoMo on the same alumina support and 160% higher than HDS-1442A.
Studies on spent catalysts from the Wilsonville pilot plant (Stohl 1985) give even more insight on catalyst
deactivation. It should be noted that the catalyst was in the second of two stages and not directly
exposed to the initial coal liquefaction environment, in the first stage dissolver.
The catalyst used in all three runs was Shell 324M with 12.4 wt % Mo, 2.8 wt % Ni and 2.7 wt % P on an
alumina support. Catalyst samples were withdrawn periodically throughout each run with only minor
(5%) catalyst additions to the hydrotreater. All samples were Soxhlet extracted with tetrahydrofuran and
dried under vacuum at 100°C prior to characterization and activity testing. A portion of each extracted
sample was regenerated by slowly heating to 500°C in a muffle furnace in air. Regeneration removed
the carbonaceous deposits but did not affect the contaminant metals. Studies of both aged and
regenerated catalysts enable separation of the effects of both carbonaceous deposits and contaminant
metals. The types of processing options practiced at Wilsonville include:
Run 242 ITSL integrated two stage liquefaction (interstage deashing)
Run 246 DITSL doubly integrated two stage liquefaction (two recycle streams)
Run 247 RITSL reconfigured integrated two stage liquefaction (post 2nd stage deashing)
Characterization of the catalysts included studies of the buildup of carbonaceous deposits and
contaminant metals as well as changes in physical properties. Quantitative carbon analyses show that
the carbon buildup is very rapid in samples from all three runs. Catalysts from Wilsonville run 246 attain
a carbon content of 7.3 wt % during DITSL processing which then increases to 9.0 wt % during ITSL
processing. Catalysts from Willsonville run 242 (ITSL processing) attain a carbon content of 9.3 wt %
after about 133 lb resid/lb catalyst which then remains constant throughout the remainder of he run.
Samples from Wilsonville run 247 with RITSL processing attain the highest carbon content of 10.8 wt %,
which decreases twice during the run to values of -10 wt % after 100 lb resid/lb catalyst and to 8.9 wt %
after 300 lb resid/lb catalyst. These two decreases in carbon are correlated with temperature increases
in the reactor from 680 to 700°F and from 700 to 710°F, respectively. The carbon contents in catalyst
samples from these three runs are correlated with the quantity of heavy resid and unconverted coal in
the hydrotreater feed. DITSL processing has the lightest feed whereas RITSL processing has the
heaviest feed because deashing, (which normally removes some unconverted coal and heavy resid) was
performed after the 2nd stage catalytic hydrotreater.
Many contaminant metals are deposited on the catalyst during coal liquefaction processing. However,
Fe and Ti are the most abundant so only data obtained on these two metals is shown. Iron contents,
determined by atomic absorption, as a function of catalyst age are given in Figure 2. It is interesting that
the catalyst exposed to high-ash feed has the lowest Fe concentration. This suggests that the high Fe
content in run 242 catalyst is not derived from the Illinois No. 6 coal, which was also used in run 247, but
from upstream equipment perhaps related to the deasher.
The lowest Ti content was observed on the run 246 catalysts which processed subbituminous coal. This
result is in agreement with previous studies on catalysts from single stage direct coal liquefaction
processes which showed that Wyodak coal resulted in lower Ti buildup than Illinois No. 6 coal. Since the
Wyodak coal and Illinois No. 6 coal have similar Ti contents, the difference must be due to the presence
of Ti phases in the subbituminous coal that are less reactive with the catalyst than those of the
bituminous coal. Run 247 catalysts with an age of 546 lb resid/lb catalyst show about a 30% higher Ti
content than run 242 catalysts with a similar age. This suggests that deashing is only partially effective
in removing the Ti phase which reacts with the catalyst.
Electron microprobe analyses of samples from each run with ages between 500 and 600 lb resid/lb
catalyst were used to determine the distributions of the contaminant metals. These distributions vary
widely in regard to maximum contaminant metal content but show less than 25% variation in penetration
depth. Figures 4 and 5 show typical Fe and Ti distributions in cross sections of the catalyst extrudates
that were about halfway down the length. The run 242 sample with the highest Fe content shows a
penetration depth of -50% of the pellet radius. The lowest Fe penetration is observed on the run 247
sample and the lowest Ti penetration is on the run 246 sample. Both run 242 and run 247 samples show
Ti penetration depths on the order of 50% of the pellet radius.
An equation has been derived which relates extrudate activity to the physical properties of the catalysts(2)
and the fraction of intrinsic activity lost due to deactivation by carbonaceous deposits and contaminant
metals.
At the start of coal processing, the largest activity lost is due to carbonaceous deposits. However, losses
due to contaminant metals also become significant and cause a continuous decline in activity after the
effects of the carbonaceous deposits have levelled off. The most significant difference between runs
242 and 246 is due to the lower deactivation caused by contaminant metals in run 246. The overall
deactivation rates, however, are similar in these two runs. Run 247 has a deactivation rate due to
contaminant metals between those observed for runs 246 and 242. However, the loss of extrudate
activity early in run 247 which is due to carbonaceous deposits is significantly greater than observed in
either run 242 or 246. This high deactivation is related to the very low effective diffusivity of this sample.
Catalyst Design for Coal Liquefaction
The design of a coal liquefaction catalyst is an extremely challenging problem. Before embarking into
the discussion, it is helpful to reexamine the triangular diagram for key catalyst properties, in Figure 3,
that must be balanced.
Due to the large heat release associated with coal liquefaction, the reactor contents should be mixed
sufficiently so that temperature gradients are maintained at some acceptable level. This usually means,
we use a reactor (typified by the ebullated H-Coal type) with a recycle pump to circulate the exit steam
back to the reactor inlet.
This sets the physical constraint on the catalyst, its geometry, density, and abrasion resistance. The
catalyst particles are suspended in the upwardly flowing coal liquid. The particles have two key forces
acting on them, the buoyant force of the coal liquid they displace and the fractional drag from the flowing
liquid. Typically, extrudates about 1/8" in diameter are used in the H-Coal reactor. In contrast, the H-Oil
reactor (for petroleum resid upgrading) can use the smaller 1/16" particles with little trouble since the
liquid is less dense and provides less buoyant force for a given volumetric displacement. The minimum
fluidization velocity (Kuni 1969) can be estimated as:
The second aspect is activity of the catalyst. This implies a highly porous support which provide plenty
of active surface. We select the particular combination of catalytic metals based on which of the
reactions we need to promote.
Levy and Cusumano (Levy 1977) have noted, "Before a material is tested for catalytic coal liquefaction,
2its chances of survival in the liquefaction environment should be examined. The presence of H S poses
the most severe problem. A large number of compounds that may ordinarily be considered promising
candidates sulfide in this environment. It is therefore fruitless to spend considerable effort in the testing
of these materials. Compounds that are expected to resist sulfidation include a number of oxides,
nitrides, borides and silicides."
For the tried and true hydrotreating catalysts, we include the following table for reference.
Table 14Examples of Typical Sulphide Catalysts Practically Used in the Industry.
(Weisser et al. 1973
Catalyst type Catalystnotation
Catalystcomposition
in the fresh(nonworking) state
Origin ofthe catalyst
Application Reference
For destructive hydrogenationof the heaviest fuels andpetroleum residues
4 2 310927 FeSO + Na CO ondust from Winklergegenerator (5%Fe)
I.G.Farbenindusstrie
Destructive hydrogenation ofthe heaviest fuels (I.G. F.,Varga-process
[317] [1107] [1172]
3- CoO + MoO onaluminosilicate(?)
Gulf Research Dev.Co.
Hydrocracking of residualpetroleum fractions in theGulf-HDS process
[155] [2137]
3 2 3- CoO + MoO on Al O HydrocarbonResearch
Hydrocracking of residualpetroleum fractions in the H-Oil process
[1618] [1621] [2137] [2148]
For hydrocrack- ing tar andpe-troleum distil-lates
26434 10%WS on clayactivated with HF
I.G.F. Hydrocracking of tardistillates (vapor phase)
[1101]
- NiS onaluminosilicate
CaliforniaResearch Corp.
Hydrocracking of mediumpetroleum distillates by theIsocracking process
[2148]
Refining and destructive-refining catalysts
33510 53.5% MoO + 30%ZnO + 16.5% MgO
BASF Hydrotreating and hydrocacking [793] [1101]
25058 WS I.G.F. Hydrotreating and hydrocrackingof medium oils
[793] [1101]
39062 7.5% MoO + 7.5%3 2 3WO + 35% Al O +
250% SiO
VEB "Leuna Werke" Hydrotreating and hydrocrackingof medium oils
[1429]
23076 NiS:WS = 2:1(unsupported)
I.G.F. Hydrogenation catalyst of highefficiency (hydrogenation ofolefins)
[317]
2- NiS-WS (40% W and25% Ni,nonsupported)
Shell Oil Co. Unsupported desulphurationcatalyst
[1743]
28376 27% WS + 3% NiS2 3on (-Al O
I.G.F. Hydrotreating of tar-oils invapor-phase, MTH-process
[1101]
38376 ox. 26% WO + 4.5% NiO2 3+ 69.5% Al O
VEB "Leuna Werke" Hydotreating of tars andpetroleum distillates, MTH-process etc.
[1101] [1429]
8376 ANTIAS Type 8376 withincreased Nicontent
Chemical WorksZáluží (�SSR)
Hydrotreating of arsenic-containing tar
[2015]
37846 10% MoO + 3% NiO2 3on Al O
I.G.F. resp. BASF Hydrotreating of medium oils [1429]
Refining and destructive-refining catalysts
38197 17% MoO + 4.5%2 3NiO + 78.5% Al O
VEB "Leuna Werke" TTH-process {1429}
3 3U 63 12% WO + 8% MoO+ 4.5% NiO + 75.5%
2 3Al O
VEB "Leuna Werke" Hydrotreating of tar andpetroleum distillates
[1429]
BASF 0852Nalco 471Nalco 810Aero HDS-2OroniteGildler G-35BHoudry 200-AComoxP.SpenceCyanamid HDS1Co-Mo (USSR)7362 (�SSR)
For composition,see Table 78
Different Americanand EuropeanCompanies
Hydrorefining anddesulphuration catalysts fordifferent purposes
[1743]
BR 86 2.3% CoO + 0.8%3NiO + 17% MoO +
2 380% Al O
VEB "Leuna Werke" Improved refining Co-Mocatalyst
[1429]
- 1.0% Co + 0.5% Ni2 3+ 8.3% Mo on Al O
Davison Chem. Co. Improved hydrorefining Co-Mocatalyst
[1743]
2Dehydrogenation catalysts IG 5615 NiS:WS -1:2(nonsupported)
I.G.F. Dehydrogenation of cyclanes toaromates
[1347]
2- NiS:WS -2:1 Shell Dev. Co. Dehydrogenation of cyclohexaneand methlycyclohexane toaromatic substances
[1049]
Finally we must consider catalyst stability or how well it survives the coal liquefaction environment. Pore
structure, specifically a bimodal pore size distribution of macro (> 1000 Å diam) and meso (20 Å < d <
600 Å) pore is the key to acceptable activity maintenance. Coking of the catalyst causes a rapid initial
decline in the activity. Then a more gradual deactivation occurs due to sintering and additional coking.
The inclusion of macropore volume is accomplished in the catalyst forming step such as extrusion. The
drawback of the macropores is that the catalyst is "softer" and can sometimes grind up when it is in the
reactor being ebullated. Thus, there is a practical upper limit (25% of total) of macropore volume that
one aims to stay under.
In summary, design of a coal liquefaction catalyst represents a balance of properties. The type of
reactor, coal type, reaction severity, and desired product quality will all enter into the design of the
catalyst. As a base point, the properties of the Amocat™ family of catalysts is given below for use in
catalytic hydroliquefaction of coal. They have enjoyed considerable success in Wilsonville, AL and the
H-Coal pilot plant at Cattettsburg, KY.
Table 15
Catalyst Name Amocat™ 1A Amocat™ 1B Amocat™ 1C
@ Composition, wt%
3MoO 16±0.9 16±0.9 16±0.9
CoO 3±0.4
NiO 3±0.4
CaO #0.1 #0.1 #0.1
4SO #0.5 #0.5 #0.5
2 3Al O balance balance balance
@ Surface Properties
BET Surface Area, M /g 170 min ))))))))))))))))>2
Pore Volume, cc/gm (<1200 Å diam) 0.65 min ))))))))))))))))>
Total pore volume, cc/gm 70.82 ))))))))))))))))>
Pore size distribution bimodal ))))))))))))))))>
Total pore volume in macropores (>1200 Ådiam), %
18±5 ))))))))))))))))>
Pore Volume in micropores <40 Å diam) <2 ))))))))))))))))>
@ Physical Properties
Shape extrudate ))))))))))))))))>
Size, in. 1/16-1/10 ))))))))))))))))>
Dry Attrition, 24 hr, 30 mesh, wt% <3 ))))))))))))))))>
Bulk density, compacted, lb/ft 37±3 ))))))))))))))))>2
Fines content, 16 mesh, wt% <1 ))))))))))))))))>
@ Suggested Use
Liquefaction X X X
Desulfurization X
Denitrogenation X
Aromatic saturation X X
LIQUEFACTION REACTOR TYPES/MODELS
Ideal Reactors - A Brief Review
Many types of reactors enter into the field of coal liquefaction. The researcher may be using a simple
tubing bomb reactor which is operated batch wise. In another situation, coal liquids may be pumped over
a fixed bed of catalyst in a plug flow trickle bed reactor. The continuous pilot plant or commercial
liquefaction reactor will frequently be well mixed, by either mechanical agitation or high recycle; this
reactor type is the continuous stired tank reactor (CSTR). The reactor performance equations for these
three ideal reactors are given below:
Reactor Type Batch Plug Flow CSTR
Shematic
Rate Equation
Reactor Equation
Coal liquefaction is a complex reaction process comprised of both homogenous and catalyzed
heterogeneous reactions. Thus, we must use a reactor performance equation which combine accounts
for both thermal and catalytic reactions for the depletion or formation of a reaction species.
There are four primary tasks that involve an understanding of liquefaction reactors and include:
1. Reaction model development
2. Evaluation of catalyst performance
3. Evaluation of coal liquefaction behavior
4. Scale-up from one level to the next size up (i.e. commercial)
We will discuss each at these situations in turn and give some examples.
Reaction Model Development
The development of a reaction model to describe coal liquefaction is a common situation that a research
engineer encounters. The data are generated from small scale tests with a batch or continuous reactor,
operated isothermally. It is obviously important to measure the intrinsic kinetics and not to have the data
disguised by mass transfer limitations.
A micro-batch reactor (-5 ml) such as the tubing bomb reactor is a common, inexpensive device to
develop the data. The coal, donor solvent, and optionally, catalyst are changed into the small reactor,
sealed and then pressured with hydrogen gas to about 500 psi. To start the reaction, the tubing bomb is
immersed in a heated fluidized sand bath for a specified length of time with agitation. Shortly after
immersion in the heated sand bath, the reactor pressure is increased to a final level, close to commercial
conditions. To stop the reaction, the micro-reactor is pulled out of the heated bath and rapidly quenched
in a cooling fluid.
A more expensive, but superior, batch reactor is a stirred high pressure autoclave (-300 cc). The
internal mixing by the spinning turbine blade is far better than the gentle up-and-down agitation of the
micro-batch unit. The dispersion of small gas bubbles into the liquid phase enhances mass transfer and
eliminates stagnant zones, where coking might occur. The problem with the large size reactor is in
defining time zero, when the reaction starts. The reactants cannot all be changed into the cold reactor
and then heated up without some of the reaction taking place during heat-up. A solution to this is to put
only slurry oil (2/3 of total) in the stirred autoclave and slowly heat to slightly over the desired reaction
temperature. When the desired temperature is reached, the coal and remaining slurry is rapidly charged
into the top of the reactor with a pressured bomb fitted with ball valves on the top and bottom. The
liquefaction reaction is the allowed to proceed and then thermally quenched at the desired time by
circulating cooling fluid (water or oil) through internal cooling coils.
For either of these batch reactors, the rate equation and reactor performance equations are:
The data are in the integral form for a batch reactor and do not lend themselves well to model
discrimination or parameter estimation. A technique developed by Churchill (Churchill 1974) translates
integral data to differential rate data where it can be manipulated conveniently.
The difference table above illustrates how the integral data on the left graph are translated to rate data
on the right graph. A smooth curve drawn through the rectangular tops (so that area in triangles above
and below are equal) gives the instantaneous rate at time, t.
The continuous stirred tank reactor (CSTR) is a frequently a better tool to obtain kinetic data if you can
afford the time and expense of setting one up. The reaction rate can be derived directly from
experimental data as pointed out by Mahoney, Robinson, et al (Mahoney 1978). The reactor has no
concentration or temperature gradients and conversion is controlled by changes in slurry feed rate or
reactor temperature. An overflow tube controls the level and an annular catalyst basket is fitted near the
wall. The reactor schematic for the multiphase CSTR is given below with more details in Autoclave
Engineers Bulletin 1200.
The reactor performance equation for the CSTR is:
A AThus the reaction rate, -r or -r ', can be directly calculated from conversion data and process conditions
and then used to develop a kinetic rate expression
Two familiar test reactors are the Robinson-Mahoney (stationary catalyst basket) and Mahoney-Robinson
(spinning catalyst basket). In Figure 28, these two reactors are shown.
Evaluation of Catalysts or Coals
The next two tasks that involve liquefaction reactors are catalyst performance and coal liquefaction
behavior. Compared to kinetic modeling, the experimental constraints are not as demanding. It is not as
critical to define time zero, if the reactor can be heated up in the same way each time. Usually coals are
run at the same severity and a relative comparison made on the conversion level, and product quality.
Catalyst evaluation/ranking, however, is best done when compared at constant conversion level. For
continuous reactions, the relative catalyst activity is defined as the ratio of space velocities as shown
below. If it is not possible to keep conversion constant, then the constant reaction severity mode will
certainly suffice.
Coal reactivity is frequently compared with a micro-batch reactor. This reactor was described earlier and
the important aspect is to keep temperature- time history constant. Researchers at HRI (lb) developed a
scheme to translate coal conversion data at various reactions times and temperature a common basis.
The reaction severity parameter they developed is called to Standard Temperature-Time Unit or STTU.
It is defined as:
Liquefaction Catalyst performance should almost always be in a continuous flow reactor. Catalyst life is
a critical factor in coal liquefaction and fresh catalyst activity is only important to establish where the
catalyst starts out. Coke deposits rapidly form on the catalyst the first few days that it is on-stream and
then a more gradual increase occurs. Catalyst is usually evaluated for at least 10 days and preferably
around 30. A CSTR high pressure autoclave is preferred and these are described by Autoclave
Engineers (Autoclave Bulletin 1200). A typical plot at constant reaction severity is shown below:
Commercial/Pilot Plant Reactor Design and Scale-Up
The design of larger liquefaction reactors provides a significant challenge since heat release is
substantial and the flow regime deviates from the ideals of plug flow and perfect mixing. We will
examine the commercial reactors in increasing order of complexity.
Exxon Donor Solvent (EDS) Reactor
The liquefaction reactor used in the EDS process is a network of tubes with no catalyst inside. Coal
slurry and hydrogen gas are introduced in the inlet with a nominal residence time of 30-45 minutes. The
key issue is establishing the coal slurry residence time since gas bubbles occupy some fraction of the
g lreacter volume (gas holdup, , + liquid holdup, , = 1).. Although hydrogen is consumed along the length
of the reactor, light gases are also formed, so it is not easy to predict the gas and liquid holdup volumes.
The "axial dispersion" model, a one-parameter model, is a good approximation for the EDS reactor. Its
flow regime has a small deviation from plug flow conditions. The reactor coils are immersed in a heat
transfer fluid so that the temperature rise along the length of the reactor can be controlled by transferring
heat out. The material and energy balances are the following:
The above equation are solved simultaneously with a numerical differential equation solver. Estimation
of the Dispersion coefficient can be accomplished by the following equation by Shah (Shah 1981).
Liquid and gas holdup are also estimated from formulas in Shah's book.
l l lThis sets the superficial velocity (u = Q/,A)and, in turn the residence time (J = L/u ) of the liquid in the
reactor.
H-Coal Reactor
The H-Coal reactor is rather unique and called an ebullated bed catalytic reactor. A recycle pump,
located either internally or externally, circulates the reactor fluids down through a central downcomer and
then upward through a distributor plate and into the ebullated catalyst bed. The reactor is usually
insulated well and operated adiabatically. Frequently the reactor mixing pattern is defined as backmixed,
but this is not strictly true. A better description of the flow pattern is dispersed plug flow with recycle.
Thus the equations for the axial dispersion model are modified appropriately to account for recycle
conditions.
The schematic below shows the key elements of recycling a portion of the exit stream with the feed
stream and how that affects the feed concentration(s). The recycle increases the superficial velocity, u,
and changes the feed concentration due to dilution with the product stream.
Although the H-Coal reactor is loaded with catalyst, not all of the reactions are catalyzed; some are
thermal reactions, like thermal cracking, which depend on liquid holdup and not on how much catalyst is
present. Thus the material balance equations need to be divided into two categories, one set for the
non-catalytic thermal reactions and another set for the catalytic reactions. A convenient parameter to
use is the thermal/catalytic ratio, T/C, which is the ratio of liquid holdup to catalyst volume. In a
commercial ebullated bed, this ratio is close to 1.0 under ebullation conditions. Consequently the
material balance equations for the catalytic reactions with no recycle is:
The final set of equations for the H-Coal reactor can now be written to account for the recycle situation,
and thermal reactions in concert with catalytic reactions.
The model for the H-Coal reactor introduces two complications beyond the axial dispersion model. First
the boundary conditions are modified to account for the recycle and second the catalyst in the reactor
means that both thermal and catalytic reactions are occurring simultaneously. The equations are solved
numerically with a differential equation solver. This allows the reactor size to be determined by
Aintegrating along dimensionless distance z until concentration, C , equals the desired final
concentration .
Two Stage Liquefaction
A two-stage liquefaction reactor system represents one additional level of complexity beyond the single-
stage H-Coal reactor. Now the exit stream from the first reactor becomes the feed stream for the second
reactor. The reactor system may be visualized as below.
The approach to solving the above reactor system is to first develop the exit values for Reactor 1 and
then to use to these outlet variables as input for Reactor 2. The reactors will usually be operated at
different temperature levels with stage 2 cooler than stage 1 so that the equilibrium for aromatic solutions
reactions is favorably shifted. The interstage cooler is shown in the diagram for this reason.
Furthermore the second stage catalyst may be different with more activity for denitrogenation and
aromatic saturation. The basic algorithm to solve the 2-stage reactor system is listed below:
1. Define the operating conditions for stages 1 & 2
1 2i.e. temperature, T & Tcatalyst, A & B
1 2recycle reaction, R & R
feed concentration, ....
2. Target outlet concentration of stage 2 for reaction species that you are monitoring.
i.e. ....
3. Select outlet concentrations leaving stage 1 which are midway between inlet and final values.
i.e.
4. Calculate diluted feed concentrations to stage 1 by the recycle equation.
5. Solve the set of differential equations for stage 1, stopping when the outlet concentrations satisfy
your criteria. This defines reactor volume and/or catalyst charge for stage 1.
6. Calculated dilated feed concentrations to stage 2 by the recycle equation.
7. Solve the set of differential equations for stage 2, stopping when the outlet concentrations satisfy
your criteria, set in step 2.
8. Compare relative size of stage 1 and stage 2. If imbalanced too much, modify interstage
concentration or temperature level, and then resolve system of differential equations, starting at
step 2 of algorithm.
Table 16Liquefaction Reactor Guide
Reactor Type Positive Negative Best Application
@ Research and Development Scale
Micro-batch- Inexpensive - moderate mixing - coal screening
- small feed charge - difficult product work-up - catalyst screening
Stirred Batch autoclave- Good mixing/contacting - more expensive
- Product work-up no problem - hard to set time zero
Mahoney-Robinson continuous stirredautoclave (spinning basket)
- excellent mixing, bubble dispersion - smaller catalyst charge - catalyst aging
- expensive - coal comparison
- heavy feeds may not work well - kinetic studies
Robinson-Mahoney continuous stirredautoclave (stationary basket)
- Handles heavy viscous feeds - expensive - kinetic studies
- Holds most catalyst - catalyst life studies
Trickle Bed- simple to operate - isothermal operation difficult - catalyst aging
- simple to design - flow maldistribution common - coal liquid upgrading
@ Commercial Scale
H-Coal- Extensive operating history. - Tricky to operate - commercial operations
- Handles exothermic reactions well
Exxon EDS- Simpler operation - Heat release hard to control - commercial operation
- hydrogen starvation possible
Two-stage
- Better contol of reactions - More complex to operate - commercial operations
- Improved yield of distillates
- Builds on H-Coal technology
Pilot Plant Processes
The following section contains "short-form" process descriptions for some of the leading technologies in
direct coal liquefaction. There are no direct coal liquefaction processes currently in commercial use. Not
only are the production costs expensive, the disposal of waste streams is a big issue. For example, the
residue cannot be burned in a conventional boiler due to its high ash content (-230 %) and sulfur level.
Options have included gasification to generate hydrogen but current thinking leans toward fluid bed
combustion for process heat. Natural gas and process-derived fuel gas appear to be the better raw
materials for generating hydrogen for use in the process.
The four processes described in this section include:
1. H-Coal Process
2. Exxon Donor Solvent (EDS)
3. Solvent Refined Coal (SRC I & II)
4. Two Stage Liquefaction
Process: H-Coal
Description: Direct catalytic hydroliquefaction process developed by HRI, Inc using a novel ebullated
bed reactor (Merdinger et al. 1982?). Typical reactor conditions are 3000 psi, 454°C, and 75 lb coal/hr-ft3
for fuel oil mode or 30 lb coal/hr-ft for syncrude mode.3
Background:
@ Patented process (US 3,321,393) developed by HRI, Inc.
@ Extension of H-Oil process for residuum hydrotreatment
@ Bench scale and process development unit (PDU) operated at the Trenton, NJ laboratories
@ H-Coal pilot plant (200 TPD syncrude or 600 TPD fuel oil mode) built and operated at
Catlettsburg, KY
- Operator: Ashland Synthetic Fuels, Inc.
- Cost: $320 MM
- Participants:
US Department of Energy(DOE)
Kentucky Department of Energy
Ashland Oil
Electric Power Research Institute (EPRI)
Mobil Oil
Standard Oil (Ind)
Conoco Coal Development
Ruhrkohle AG
Process Schematic:
Reactor Conditions/Schematic:
Total Pressure, psi 3000
Temperature,°F 845
Slurry Concentration,% 38
Space Velocity,lb coal/hr-ft 323
Table 17
Process Yields (lb/lb mf coal)
Illinois
Bituminous
Wyoming
Subbituminous
Product Syncrude Fuel Oil Syncrude
2H (5.3) (3.4) (6.2)
2 2H O, CO, CO 7.1 6.5 20.0
2 3H S, NH 3.6 2.2 1.6
1 3C -C 11.2 6.8 12.3
Naphtha (C4-204°C) 18.7 13.4 25.8
Fuel Oil (204-524°C) 29.1 20.8 18.6
Bottoms (524°C+) 35.6 53.7 27.9
TOTAL 100 100 100
Design Considerations:
-Reactor Performance Equations
- Liquefaction Catalysts
Table 18
Name Amocat 1A Amocat 1B HDS 1442ATM TM
Composition 3 3 33%CoO/10% MoO 10% MoO 3.3%CoO/14.7%MoO
Surface Area, m /g2 154 167 323
Avg Pore Diam, Å 123 122 58
Pore Distribution bimodal bimodal bimodal
Size, in diam 1/16 1/16 1/16
- Start-Up/Shut-Down
Start-up of reactor consists of slow heat-up of "cold wall" insulated vessel with
internal refractory. Alternate "hot wall" design requires a high alloy steel liner and is
more susceptible to temperature excursions.
- Reactor Process Control
Catalyst bed level measured by gamma ray detectors which will either raise or lower
ebullation pump rate by speed control on motor. Temperature controlled by temperature
of fresh slurry entering reactor through feed preheater.
- Safety Aspects
Temperature runaway is controlled by quench oil and replacement of hydrogen gas in
reactor with methane to stop the exothermic hydrogenation reactions.
Process: Exxon Donor Solvent (EDS)
Description: The Exxon donor solvent (EDS) process was developed by Exxon Research and
Engineering (ER&E) and is a thermal liquefaction process where the donor solvent is catalytically
hydrotreated in a fixed bed reactor external to the liquefaction zone (Epperly et al. 1981; Hsia et al. 1981;
Ansell et al. 1980). Typical liquefaction reactor conditions are 840°F and 2000-3000 psia with 45
minutes residence time.
Background:
@ Process developed by Exxon Research and Engineering
@ Two pilot plants (75 lb/day) and a third (1 ton/day) at Baytown Research
and Development Division in Houston, TX set the design scale-up basis
@ Larger EDOS process (250 ton/day) built and operated at Baytown, TX
- Operator Exxon
- Cost $341 MM
- Participants
US Department of Energy (DOE)
Exxon
Electric Power Research Institute (EPRI)
Japan Coal Liquefaction Development Company
Phillips Coal Company
Anaconda Minerals Company
Auhrkohle AG
ENI (Italian National Oil Company)
Process Schematic:
Reactor Conditions/Schematic:
Process Yields:
Table 19Product Yields of EDS Process
(2500 psig, 840°F)
Illinois No. 6bituminous(Monterey)
Wyomingsubbituminous(Wyodak)
Texaslitnite(Big Brown)
Residence time, min 40 60 25-40
Yields, wt% maf coal;
2H -4.3 -4.6 -3.9
2 xH O+CO 12.2 22.3 21.7
2 3H S+NH 4.2 0.9 1.7
1 3C -C gas 7.3 9.3 9.1
4C -1000 oF liquid 38.8 33.3 33.3
1000 oF + bottoms 41.8 38.8 38.1
Design Considerations:
- Donor Solvent Hydrotesting Catalyst
- Start-Up/Shut-Down
- Reactor Process Control
- Safety Aspects
Process: SRC - I & II
Description: The Solvent Refined Coal liquefaction process referred to as SRC-II is a thermal
liquefaction process; it is an outgrowth of an earlier Solvent Refined Coal process tested by Gulf Oil in
the 1960s (Rao et al. 1983; Moniz et al. 1984; Gir and Rhodes 1983; Nalitham et al. 1985; Moniz and
Nalitham 1985). The earlier process, known as SRC-I, was aimed at boiler fuel production of an ashless
low-sulfur solid fuel. In contrast, the SRC-II is geared to the production of synthetic liquid fuels. Typical
reactor conditions are 870°F, 1500-2500 psig, and 15-45 minutes with no catalyst.
Background:
@ Two major pilot plants
- 50 TPD at Fort Lewis, Washington under Gulf Oil and DOE
- 6 TPD at Wilsonville, Alabama originally built by
Catalytic, Inc. under sponsorship of Southern Company
Services, Inc. eventually set up as:
Sponsors Participants
US Department of Energy Catalytic, Inc.
Electric Power Research Institute Kerr-McGee Corporation
Amoco Corporation Hydrocarbon Research, Inc. Southern Company
Services, Inc.
Integration of H-Oil hydrotreater into process employed at Wilsonville so that some of recycle slurry oil
came from catalytic hydrotreater; it led to the process being called two-stage liquefaction with the
standard integrated mode (ITSL) and double integrated mode (DITSL).
Control Solvent Deashing (CSD) by Kerr-McGee eliminated filters and allowed recycle of heavy residue
as major component of slurry oil
Process Schematic:
Reactor Conditions/Schematic:
Table 20SRC II With Interstage Deashing
ITSL DITSL
run no. 246F 246G 246A 246B 246C 246DE
THERMAL STAGE*
average reactor temperature (°F) 826 813 812 812 804 825
inlet hydrogen partial pressure (psi) 2,050 2,040 2,040 2,130 2,190 2,160
coal space velocity [b/hr-ft (>700°F)] 24 17 23 27 30 243
solvent-to-coal ratio 1.8 1.8 1.8 1.8 1.8 1.8
solvent resid content (wt %) 34 34 0.7 1.0 1.0 0.9
CATALYTIC STAGE
reactor temperature ( F) 623 623 624 643 606 624o
space velocity (lb feed/hr-lb cat) 1.1 1.0 0.7 1.0 1.0 0.9
feed resid content (wt %) 34 34 22 14 24 22
catalyst age (lb resid/lb cat) 360 496 69 128 204 252-279
2 3*additon of Fe O at 2.0% MF coal and DMDS at 1.1 x stoichiometric requirement for conversion of2 3Fe O to FeS.
Table 21Process Yields
ITSL DITSL
run no. 246F 246G 246A 246B 246C 246DE
yield* (% MAF coal)
1 3C -C gas (total gas) 8(19) 9(19) 7(17) 6(14) 5(13) 9(17)
water 12 11 12 11 12 12
4C +distillate 51 53 51 50 49 55
resid 3 1 -2 -1 0 -1
hydrogen consumption -5.1 -5.4 -5.0 -4.6 -4.1 -5.4
hydrogen efficiency4 2(lb C +dist/lb H consumed
10.0 9.8 10.3 11.0 12.0 10.2
distillate selectivity1 3 4(lb C -C /lb C +dist)
0.17 0.18 0.14 0.12 0.10 0.15
energy content of feed coalrejected to ash conc.(%)
22-24 22-24 26-29 29-32 30-35 21-24
3*elementally balanced yield structures, SO -free ash
Process: Two Stage Coal Liquefaction
Description: Compared to SRC-I & SRC II, two-stage liquefaction tightly couples the dissolver and
catalytic hydrotreater so that the time between the two stages is minimized. Ash removal occurs after
the second catalytic stage rather than interstage deashing. This This accomplishes higher liquid yields
with less hydrogen consumption with two stages operating at different temperature. Second stage
always has catalyst in it but the first stage can be either catalytic or thermal. Typical reactor conditions
are 2500 psig, 2/1 Slurry Oil/Coal, 45-70 lbs/hr-ft cat (each stage) and 650-775°F first stage/810-825°F
second stage (Comolli, MacArthur, and McCleary 1985).
Background:
@ Original concept introduced by Chevron Research in late 70s (US)
@ Pilot plant tests by HRI, Inc. in mid 80's with exceptional success noted
by high distillate and low residuum yields
@ Wilsonville pilot plant operating as SRC was modified to tightly couple
the 1st stage dissolver with 2nd stage H-Oil hydrotreater in 1986-87 and
called RITSL (Reconfigured Integrated Two-Stage Liquefaction)
operating mock.
@ Two stage process under some debate with high/low and low/high
temperature staging not showing yields too different. HRI favors
low/high staging while Amoco prefers high/low to favor aromatic
saturation in 2nd stage.
Process Schematic:
Reactor Conditions:
Table 23CATALYTIC TWO-STAGE LIQUEFACTION PROCESS (HRI tests)
WYODAK COAL YIELDS AND PROCESS PERFORMANCERun 227-22
Process Yields (lb/lb mf coal)
RANGE(1)
REPRESENTATIVEYIELDCONDITION 7
YIELD, W % M.A.F. COAL
1 3C -C 5 - 9 8.1
4C -390 F 17 - 27 25.0o
390-650 F 30 - 47 36.5o
650-975 F 0 - 11 4.2o
975 F Resid 0 - 5 1.9o +
Unconverted Coal 7 - 21 10.7
2 2H O, CO, CO 17 - 23 20.0
2 3H S, NH 1.4 - 1.8 1.6
Hydrogen Consumption 6.3 - 8.1 8.1
4C -975 F Liquid 54 - 68 65.7o
PROCESS PERFORMANCE
Coal Conversion 83 - 92 89.3
975 F Conversion 74 - 90 87.4o +
Hydrogen Efficiency 8 - 9 8.2
Table 24RITSL yield structures (thermal/catalytic) (Wilsonville pilot plant)
run no. 247C-II 247D 247E**
yield*(%MAF coal)
1 3C -C gas (total gas) 7(12) 6(12) 5(11)
water 10 9 11
4C +distillate 60 62 61
resid 5 3 3
hydrogen consumption -6.1 -6.1 -6.3
hydrogen efficiency4 2(lb C +dist/lb H consumed)
9.8 10.2 9.7
distillate selectivity1 3 4(lb C -C /lb C +dist)
0.11 0.10 0.09
energy content of feed coalrejected to ash conc. (%)
22 22 22
*elementally balanced yield structures**catalytic stage feed sensitivity studies
Design Considerations:
- Liquefaction Catalyst
HDS - 1442A, Amocot 1A(Co-Mo), and Amocot IC(Ni-Mo) have been
evaluated by HRI, Inc. at Trenton, NJ laboratory.
- Start-up / Shut-down
- Reactor Process Control
- Safety Aspects
References
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