reactor kinetics of urea formation

7/25/2019 Reactor Kinetics of Urea Formation

1/21

See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/284593787

REACTOR KINETICS OF UREA FORMATION

RESEARCH NOVEMBER 2015

DOI: 10.13140/RG.2.1.3574.4086

READS

26

1 AUTHOR:

Prem Baboo

National Fertilizers Ltd.,India

16PUBLICATIONS 0CITATIONS

SEE PROFILE

Available from: Prem Baboo

Retrieved on: 17 December 2015
http://www.researchgate.net/profile/Prem_Baboo?enrichId=rgreq-e98c58c0-2b0f-4456-a78f-bd6f9e84e4fb&enrichSource=Y292ZXJQYWdlOzI4NDU5Mzc4NztBUzoyOTk1NjY5MDI1OTU1ODRAMTQ0ODQzMzcyMzgxMg%3D%3D&el=1_x_7http://www.researchgate.net/?enrichId=rgreq-e98c58c0-2b0f-4456-a78f-bd6f9e84e4fb&enrichSource=Y292ZXJQYWdlOzI4NDU5Mzc4NztBUzoyOTk1NjY5MDI1OTU1ODRAMTQ0ODQzMzcyMzgxMg%3D%3D&el=1_x_1http://www.researchgate.net/profile/Prem_Baboo?enrichId=rgreq-e98c58c0-2b0f-4456-a78f-bd6f9e84e4fb&enrichSource=Y292ZXJQYWdlOzI4NDU5Mzc4NztBUzoyOTk1NjY5MDI1OTU1ODRAMTQ0ODQzMzcyMzgxMg%3D%3D&el=1_x_7http://www.researchgate.net/profile/Prem_Baboo?enrichId=rgreq-e98c58c0-2b0f-4456-a78f-bd6f9e84e4fb&enrichSource=Y292ZXJQYWdlOzI4NDU5Mzc4NztBUzoyOTk1NjY5MDI1OTU1ODRAMTQ0ODQzMzcyMzgxMg%3D%3D&el=1_x_5http://www.researchgate.net/profile/Prem_Baboo?enrichId=rgreq-e98c58c0-2b0f-4456-a78f-bd6f9e84e4fb&enrichSource=Y292ZXJQYWdlOzI4NDU5Mzc4NztBUzoyOTk1NjY5MDI1OTU1ODRAMTQ0ODQzMzcyMzgxMg%3D%3D&el=1_x_4http://www.researchgate.net/?enrichId=rgreq-e98c58c0-2b0f-4456-a78f-bd6f9e84e4fb&enrichSource=Y292ZXJQYWdlOzI4NDU5Mzc4NztBUzoyOTk1NjY5MDI1OTU1ODRAMTQ0ODQzMzcyMzgxMg%3D%3D&el=1_x_1http://www.researchgate.net/publication/284593787_REACTOR_KINETICS_OF_UREA_FORMATION?enrichId=rgreq-e98c58c0-2b0f-4456-a78f-bd6f9e84e4fb&enrichSource=Y292ZXJQYWdlOzI4NDU5Mzc4NztBUzoyOTk1NjY5MDI1OTU1ODRAMTQ0ODQzMzcyMzgxMg%3D%3D&el=1_x_3http://www.researchgate.net/publication/284593787_REACTOR_KINETICS_OF_UREA_FORMATION?enrichId=rgreq-e98c58c0-2b0f-4456-a78f-bd6f9e84e4fb&enrichSource=Y292ZXJQYWdlOzI4NDU5Mzc4NztBUzoyOTk1NjY5MDI1OTU1ODRAMTQ0ODQzMzcyMzgxMg%3D%3D&el=1_x_2


2/21

REACTOR KINETICS OF UREA FORMATION

Author

Prem BabooSr. Manager (Prod)

National fertilizers Ltd, India

Mob. +919425735974

[email protected],[email protected]

An Expert forwww.ureaknowhow.com

Fellow of Institution of Engineers (India)
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]://www.ureaknowhow.com/http://www.ureaknowhow.com/http://www.ureaknowhow.com/http://www.ureaknowhow.com/mailto:[email protected]:[email protected]


3/21

Abstract

The paper intended to the kinetics model of urea synthesis and process from ammonia

and carbon di oxide compatible with thermodynamically justified stoichiometric modelof a complex process. The obtained kinetics equation of the process, in a complete and

in a simplified form. Kinetic models for ammonia and urea reactors are presented and

simulation was performed. The performance of ammonia and urea synthesis reactors

was carried out by varying temperature and pressure and satisfactory results wereobtained.How to improve urea reaction rate with installing of high efficiency trays in urea

reactors and petern of trays for turbulence. Ammonia is an important raw material for the

production of urea in an agricultural sector. The current ammonia and urea productions

are done separately. Ammonia is produced from nitrogen and hydrogen feedstocks inreactor at the conversion of only around 27%. Urea is further produced by reacting

ammonia with carbon dioxide in a downstream urea reactor .

Kinetic Model

The Urea reaction favoured at High Pressure140-200 Bar and high temp 160-2000C.Following

parameters are involve in Urea reaction.

1. Carbon di oxide.

2. Ammonia,

3. Ammonium Carbamate

4. Urea &

5. Water

And the fraction of components formed in the result of urea decomposition process asBiuret& Triuret. By means of analytical method known so far, however, it is possible only to

determining in the liquid phase, the urea concentration, the overall concentration of carbon

di oxide and ammonia not bounded in the form of urea and the concentration of water.

Basing of the analytical determined composition the following fundamental parameters of

the process are found the molar ratio NH3/CO2and H2O/CO2 in the liquid phase and total

conversion of Carbon di Oxide to urea. The Conversion of Urea in the terms of Carbon di

oxide because the carbon di oxide is the limiting reactant in this process.

Urea is formed according to following reactions

2NH3+ CO2 = NH2COONH4 + 157.5 KJ/mole .(1)

(Ammonium Carbamate)

NH2COONH4 = NH2CONH2 + H2O - 26.44 KJ/mole ...(2)

(Urea)

2 NH2COONH4 = NH2CONHCONH2 + NH3 .................................................(3)

(Biuret)

NH2CONHCONH2 + NH2COONH4 = NH2CO-N-CONH2 + NH3 . (4)INH2CO (5)

(Triuret)


4/21

The development of urea synthesis model is based on the reaction rate equation of the

formulation of Urea, Ammonium Carbamate and Carbon Dioxide along the reactor

length. The conversion of reaction (1) and (2) will be denoted as k1 and k2 while the

overall conversion as k These will give,

K1 =(Fc +Fu)/(FCo+ FUo+ FDo) (6)

K2 =Fu/(Fc+Fu) (7)

And hence ,k=k1*k2 = Fu/(FCo+FUo+FDo) (8)

The total for initial flow rate, FTowill be the sum of the initial flow rate of the individual

components. For the flow rate of each component at any point, the following equations

will be used,

FU = k(FCo+FUo+FDo) .....(9)

FC= ( k1-k)( FCo+FUo+FDo) .(10)

FD =(1-k1) )( FCo+FUo+FDo) ..(11)

FA =(a-2k1) ( FCo+FUo+FDo) (12)

FW = (b+k)( FCo+FUo+FDo) ...(13)

Where flowrate of urea is denoted as F U , carbamate as FC , carbon dioxide as FAammonia as FAand water as FW. The total flowrate as FTof the components will be the

sum of all individual flow rate.

The rate of disappearance of carbon dioxide, rDis as given by,

rD = -k1f(CA2 CD -CC/k1) .(14)

Where k1f is kinetic for the forward reaction in equation (1) and k1 is equilibrium

constant. The rate of formation of Carbamate,rCand urea,rUare,

rC = =k1f( CA2 CD -CC/k1) - k2f (CC -CUCW/k2) and, ..(15)

rU= k2f (CC- CUCW/k2) .(16)

Where k2f represent the forward reaction in equation (2) and k2 is the equilibrium

constant. The mole balance equations for the species, urea, carbamate and carbon

dioxide in an ideal plug flow reactor are,

dFU/dz =ArU ..(17)

dFC/dz = ArC (18)


5/21

dFD/dz = ArD .(19)

Where A is the area of the reactor.

Reaction No (1) given above is exothermic in nature and 157.5 KJ/mole heat is produced in the

formation of ammonium Carbamate. Reaction No (20 is endothermic and 26.44 KJ/mol is

consumed in the dehydration of ammonium Carbamate. If Ammonia is to be supplied in the

liquid form and water that is formed in the reaction is to be separated by evaporation the net

heat evolved will be the 47.69 KJ/mol. Therefore it can be said that the urea process is heat

generating Process. But in the actual practices, the conversion being restricted by equilibrium

and cost optimization. It means high level energy is the form of steam and electric power and

low level heat to be removed by cooling water, air etc. Both the reaction are reversible and

depends upon temperature, Pressure & residence time in the reactor.

Formation of Ammonium Carbamate

This exothermic reaction is slow at ambient Conditions but almost instantaneous at 100 bar and

1500C. At a particular pressure the rate of reaction in increase with temperature. Reaches a

maximum and then rapidly decrease to zero value at a temperature corresponding to

dissociation pressure equalizing the working pressure.

The dissociation pressure of Carbamate increases rapidly with temperature it 100 bar & 200 0C

The dissociation pressure is considerably increased when an excess CO2 is used. This

dissociation pressure is much less when an excess of ammonia is used.

Formation of Ammonium Biuret & Triuret

Following are the favourable conditions of Biuret and Triuret formations.

1. At High temperature.

2. At high concentration of urea.

3. At high residence time of urea solution in holder & piping.

4. Less contents of ammonia.

Adverse effect of Biuret & Triuret on plant growth. The fate of Biuret & Triuret in

soils and its phytotoxicity is reviewed. Biuret & Triuret are mineralized by many soil

microorganisms, but the process is much slower than for urea. Excessively high

Biuret & Triuret concentrations can damage seedlings and, like urea, should not be

placed in close proximity to germinating seeds. Crop tolerance to Biuret & Triuret

varies according to the plant species, soil conditions, fertilizer placement, and

method of application. Biuret applied to soil or to plant foliage interferes with N

metabolism and protein synthesis. The current standards in the fertilizer industry

supply adequate protection against Biuret & Triuret induced damage to crops.


6/21

CONVERSION OF UREA CAN BE IMPROVED WITH INSTALLING HIGH EFFICIENCY TRAYS

Urea reactor performance improvement by adopting the high efficiency trays which are

the most efficient trays available in the market and are also an essential element to

make Split Flow Loop as efficient as possible.

PURPOSE

1. The main purpose HET to improve the redistribution of unreacted carbon

dioxide inside the liquid phase rich in free ammonia.

2. To reduce the back mixing phenomenon due to density increase of carbamate

and urea solution from bottom to reactor top.

3. To reduce also channelling which has a negative effect on the solution residence

time.

ADVANTAGES

The activity contributes to environmental and social aspects and eventually to

sustainable development by: Reduction of consumption of non - renewable fuel like NG,

which is a step towards conserving natural resources. Reducing steam consumption

which results in reduction in energy consumption.

Steam is used in the strippers and varies proportionately with the urea production. Dueto the improved conversion efficiency of the process (again due to improved tray design

and increased number of trays), the steam utilisation in the overall manufacturing

process has reduced. Hence the parameter of the specific consumption of steam to urea

gives a clear indication of the energy saved. As elaborated above, the specificconsumption of the steam to urea forms the critical parameter and hence the urea

production and accordingly the steam consumptions are monitored.


7/21

Fig.-1

A typical reactor therefore contains a gaseous phase and a liquid phase flowing in co-current

flows inside a pressurized reaction chamber. Conversion of ammonia and carbon dioxide to

ammonium carbonate and ultimately urea is enhanced as fig-1, i.e. to increase urea output,

using tray reactors. Urea tray reactors substantially comprise a normally cylindrical shell, which

extends substantially along a normally vertical axis, and is fitted inside with elements, i.e. trays,

defined by respective metal sections shaped and/or perforated to divide the reaction chamber

into compartments and form specific paths for the substances inside the reactor. The trays are

normally perpendicular to the vertical axis of the reactor, and equally spaced along the axis to

the full height of the reactor. The trays are very often perforated, i.e. have holes variously

arranged and possibly of different shapes and/or sizes. Fluid dynamics and its influence on

heat and mass transport rates in gasliquid reactors is, in general, an important starting

point for development of a process design. Improvements in the understanding of these

aspects can be particularly fruitful in the case of urea reactors where the fluid-dynamic

patterns are complicated by the co-current flow of two phases and the bubbling mode of

the vapours. The analysis of such systems highlighted the non-optimal design of existing

reactors and led to the conception of new reactor internals. Several industrial

applications demonstrate the ability of the new configuration to improve reactorefficiency. Both energy-saving and production increases were obtained. This is a further

demonstration that even mature technologies can be improved, leading not only to

economic advantages, but also to a reduction in their environmental impact. With

CO2conversion in the reactor ranging from 56 to 70%, depending on the particular

technology adopted, efforts to obtain improvements have mostly been addressed to the

recycle system. The efficiency of the synthesis reactor itself has been particularly under

evaluated, probably following the common conviction that the optimum performance

had already been achieved. Urea reactors consist of cylindrical vessels (generally 20 40

m high), having diameters from 1 to 3 m, containing, in most cases, several traysgiving rise to a stage-wise structure. The aim is to reduce axial back mixing and


8/21

redistribute the vapour phase. Some-times, reactors operating at high NH3/CO2ratios

and pressures, and having relatively small diameters, are used without trays. In the

most widespread process configurations, the unconverted reactants are recycled to the

reactor through a series of decreasing pressure stages using heat provided by steam.

The higher the CO2 conversion, the smaller the amount of heat and the size of theequipment needed to reach a certain capacity. During the formation of urea, vapour

and liquid are present all along the reactor, flowing co-

Currently and exchanging mass and enthalpy fluxes through their interfaces. As the

process is also characterized by reversible reactions, the overall behaviour is

controlled by both physical and chemical equilibrium, coupled with physical andchemical kinetics.

The trays are preferably designed for insertion through the manhole reactors are

normally provided with, so they can also be fitted to existing reactors and/or removed

and replaced. For which reason, the trays are normally made in a number of parts thatfit together.

The trays have various functions, and in particular:

1. Maximize the hold time of the light (faster) phase; distribute the reactants as

evenly as possible along the reactor section, to prevent back-mixing;

2. Enhance mixing of the gaseous- and liquid phases; and

3. Reduce bubble size' to improve diffusion of the ammonia in the carbon dioxide.

Numerous urea reactor tray designs and configurations are known.

The Principle of high efficiency trays:-

1. Mass transfer factor

2. Contact pattern of phase

3. Fluid dynamics factors

4. Interfacial surface area

5. Geometry of reactor vessel

6. Chemical kinetics factors

7. Temperature & pressure

Generally speaking, known solutions fail to provide for thorough mixing of the light

and heavy phases (both consisting of supercritical fluids) , which, because of the

difference in density, tend to flow along separate preferential paths defined by the

design and arrangement of the trays, and in particular by the shape, location, and


9/21

size of the holes in the trays. This drawback also impairs final conversion of the

reactants, thus reducing urea output.

1. The geometry of the reactor tray according to the present invention provides for

thoroughly mixing the gaseous and liquid phases in a urea reactor and ureaproduction process, and so greatly increasing urea output.

2. The reactor tray according to the present invention and the reactor as a whole

are also extremely easy to produce and install.

3. Urea producers can reduce consumption and/or increase production of their

plants by introducing the various revamping technologies developed

The installation high efficiency trays are giving an increased production of urea and

reduced steam consumption; the financial benefits are determined by the urea sales and

energy prices. It has been demonstrated that the installation of high efficiency reactor

trays in existing urea plant is very profitable.

THE USE OF REACTION KINETICS TO IMPROVE THE CONVERSION IN

VERTICAL UREA REACTORS

The conversion of Carbamate into urea is a relatively slow reaction and requires heat.

Non converted ammonia and carbon dioxide, passing the high-pressure carbamate

condenser, supply the heat needed for this reaction. Because of the equilibrium

reaction, the reaction is preferably done in a plug flow type of reactor. Installing a

number of continuous stirred tank reactors in series can approach plug flow. Thus the

urea reactor is divided into a number of compartments, mostly separated with sieve

trays, and each compartment acts as a continuous stirred tank reactor. As a result plug

flow is approached in such a cascade type reactor.

Fig.-2(a) Fig.-2(b)

To obtain a continuous stirred tank reactor, stirrers should be applied. However urea

reactors are not equipped with mechanical stirrers. The driving force for mixing the

liquid in the compartments of the reactor is the gas phase. The urea reactor is a so-

called high-pressure bubble column. By adding the gas phase through the center of a

compartment via carefully designed holes, a Torus circulation exists and thus the


10/21

required mixing of the liquid in such a compartment is obtained, as fig. 2(a) the

principle of such a Torus circulation exists and thus the required mixing of the liquid in

such a way Compartments obtained. The principle of such a torus circulation is shown

in the figure. Because the urea reaction is a relatively slow equilibrium reaction a

relative large retention time in the reactor is needed to approach the maximumequilibrium level. Fig.-3, However an infinite large reactor volume is required to reach

this equilibrium. For economic reasons the installed reactor volume in the designing of

urea plants is such that the fraction approach to equilibrium (FAE) is 95 percent. The

fraction approach to equilibrium is defined as:

FAE = 100*C02actual/C02equilibrium

Fig.-3

The relation between the fraction approach to equilibrium and the retention is shown in Fig.

In large-scale urea plants (> 1500 MTPD), equipped with reactors with large diameters and

conventional type reactor trays, it is observed that the expected fraction approach to

equilibrium is not reached resulting in a relative low reactor conversion. The consequence is

that at a specified plant capacity the steam consumption on the high-pressure stripper is

larger than expected. The reason for the observed relative low reactor conversion was a

non-optimal mixing rate in the urea reactor compartments and, thus, these compartments

did not act as an optimal continuous stirred tank reactor. The non-optimal mixing behavior

in such reactors can be caused by:

1.

Back mixing2. Channeling (fig-5,a)3.

Stagnant zones

Back mixing occurs when the liquid phase passes the sieve trays through the gas holes. This

occurs when the height of the gas cushion below the sieve tray is small. In reactors with large

diameters, when the reactor tray is not perfectly horizontal then the

. gas holes are in contact with the liquid phase. This is illustrated in

Retention

F.A.E

co2


11/21

Fig.-4

Reactors with a relative large diameter are sensitive for stagnant zones. Stagnant zones

are caused by poor mixing in the compartments and have a negative impact on the

reactor conversion since the compartments will not optimally act as the required

continuous stirred tank reactor. To avoid the negative effects of back mixing and

channeling, Stamicarbon developed in the beginning of the 90s the high efficiency trays

as illustrated in the fig.

Channeling occurs when the liquid phase is partly bypassing a compartment. In urea

reactors, equipped with conventional sieve trays, the liquid is transported from the one

compartment to the other compartment via the annular spacing between the tray and

the reactor wall. In urea reactors with large diameters it appears that the mixing rate bythe Torus circulation may not be large enough to avoid these channeling effects. The

channeling effect is shown in Fig.

Fig.-5(a) Fig-5(b)

HIGH EFFICIENCY TRAYS, HET

These high efficiency reactor trays are equipped with liquid risers where the liquid

enters the following compartment. By staggering the liquid risers, the liquid is forcedinto the Torus circulation and channeling is eliminated. To avoid back mixing, the gas


12/21

cushions were increased and this makes the trays less sensitive to horizontal variations

of the tray. Because the conversion of carbamate into urea is an equilibrium reaction,

the reaction is preferably done in a plug flow type of reactor; that is to say, one in which

the flow of reaction medium is uniform and non-turbulent over the entire cross-section

of the reactor interior. It is difficult to prevent turbulence and back mixing in a largeunconfined body of fluid; however, an approximation to overall plug flow can be

attained in a number of continuous stirred tank reactors arranged in series. Therefore

the urea reactor is divided into a number of compartments, separated from one another

by sieve trays, and each compartment emulates a continuously-stirred tank reactor. The

driving force for mixing the liquid in the compartments of the reacto r is the gas phase.

By forcing the gas phase to pass through the centre of a compartment via carefully

designed holes, a Torus-shaped circulation prevails and thus the required mixing of the

liquid in such a compartment is obtained. However, in larger reactors non-optimal

mixing behaviour has been identified and investigated. Identified causes were back

mixing, channelling and stagnant zones. To address that problem, Stamicarbon has

developed a new generation of high efficiency trays known as Siphon Jet Pump trays.

The compartments, separated by sieve trays, are equipped with a draft tube. Inside the

draft tube there is a two-phase flow of gas and liquid. The effective density of this two-

phase flow is considerably lower than the liquid density on the outside of the draft tube,

and the density difference further enhances liquid circulation, promoting mixing. The

deflector plates in the pool reactor and pool condenser, which work on the same

principle as the draft tube, have amply proved this effect. Using Siphon Jet Pump trays

provides the closest approach to a continuous stirred tank reactor without necessitating

any mechanical agitation. The mixing rate is increased significantly and the negative

effects of back-mixing and channelling are avoided. The first Siphon Jet Pump trays

were installed at SKW Piesteritz, and the result was so satisfactory that Siphon Jet

Pumps have been installed in all three plants and are currently in operation. They have

had the effect not only of making operations very smooth and raising the capacity of the

existing plants, but also of reducing the HP steam requirement of the HP stripper.

Amongst others, Fauji Pakistan, ABF Malaysia and Qafco Qatar have also installed iphon

Jet Pumps in their

Although the high efficiency trays 'improved the reactor efficiency significantly, they didnot improve the mixing rate. The mixing is still reliant upon the Torus circulation. In

practice it appeared to be difficult to keep the strict tolerances for the gap between the

reactor tray and the reactor wall because of the no roundness of the reactor. To improve

the mixing rate in the reactor compartment sand to avoid strict Mechanical tolerances,

Stamicarbon recently developed a new generation of H.E.T. Known as siphon jet pumps


13/21

Fig.-6

The compartments, separated by sieve trays, are equipped with a draft tube. Inside

the draft tube there is a two-phase flow with the density of this two-phase flow

being considerably less than the liquid density at the outside of the draft tube. By

this density difference liquid circulation is enhanced further stimulating the

mixing. The deflector plates in the pool reactor and pool condenser, in which the

deflector plates have a similar function as the proposed draft tube, have provedthese phenomena

Because of this heavy circulation effect and thus improved mixing rate it is no

longer necessary to equip the reactor trays with liquid risers. The liquid can

enter the following Compartment via the annular spacing between the tray and

the reactor wall in a similar fashion as the conventional Reactor trays. The strict

tolerance regarding the gap between The tray and the reactor wall for the new

generation HET is No longer required

The first Siphon Jet Pumps were installed in one of the plants of SKW Piesteritz.Because the trays were operating very satisfactory, two other reactors of SKW

Piesteritz are now also operating with Siphon Jet Pumps. In the following table

the current references for Siphon Jet Pumps are presented.


14/21

Table-1

Client

Capacity Year in Number of New/Modified

(MTPD) operation trays trays

SKW Piesteritz 3 1050 2001 11 New



Fauji Fert.Pakistan 1670 Completed 10 Modified

ABF Malaysia 2250 Completed 11 New

Qafco II 1400 Completed 11 New

Daqing 2300 Completed 11 New

Qafco III 3000 Completed 11 New

The gas holes in the tray are more centered than in the conventional tray design to

improve the driving force and the tray is equipped with a ring that acts as a Venturi to

improve the mixing rate,as fi 8 & 9.By installing these siphon jet pumps all aspects to

approach the continuous stirred tank reactor are included. The mixing rate is increased

significantly and the negative effects of back mixing and channeling are avoided.

Fig.-7

F.A.E.

A- convention trays

B- high eff. Trays

C -Siphon jet pump


15/21

CASALE TRAYS

Fig.-8

1. Inverted U type , better mixing due to generation of smaller bubbles

increasing interfacial surface area and improving the contact pattern causinghigher CO2conversion

Fig.-9


16/21

2. Small perforation at top and sloping area for vapour space and large perforation

for liquid at bottom area

3. But CASALE trays suffers from corrosion due to sharp configuration

Fig.-10

In combination with other Casale technologies such as the High Efficiency Trays, the

Split Flow Loop/ Full Condenser configuration is applied for increasing the capacity of

CO2stripping plant with very low investment.

High Efficiency reactor trays, the HP loop is drastically debottlenecked even for a large

capacity increase (Up to 50% over its original design. Casale, therefore, foresaw to

install the Casale Dente High Efficiency Trays in order to debottleneck the HP synthesis

section.

TABLE.-2 - Plant performance after Casale trays installation

Plants Country Year Process No. of

Trays

CO2conversion

Increase(%points)

MP Steam

Consumption

reduction(kg/MT)

Capacity

increase

(%)

Togliatti

Azot

Russia 1993 NH3

Stripping

14 6.4 300 17

Togliatti

Azot

Russia 1993 NH3

Stripping

14 4 200 17

Arcadian Trinidad 1994 NH3

Stripping

14 2.8 183 9

Yuman

Chem(*)

China 1994 CO2

Stripping

10 3.5 148 3

Agrium

Can

Canada 1994 CO2Stripping

10 5 65 -

Chemco Bulgaria 1995 NH3

Stripping

14 NA 170 6

CFI(**) USA 1995 CO2Stripping

10 3.5 70 10

Agrium

USA

USA 1995 NH3

Stripping

10 5.3 251 9

Amonil Romania 1996 CO2Stripping

11 5 178 8

NFCL India 1996 NH3

Stripping

14 4.5 95 3

Shriram India 1996 Total 14 6 >100 -


17/21

Recycle

NFL,Nangal India 2001 Montedition 14 6 110 6

Note.-(*) only 5 HET installed

(**) Data after trays installation based on Casale Survey

Casale has been, in the last decades, very active in revamping existing plant and hasextensive experience in the design and implementation of complete plant revamping

projects, including major modifications to key equipment. Casales plant revamp

strategy has always been to develop and apply new, advanced technologies to obtain the

best possible improvement in plant performance at the minimum cost; with the aim of

reducing the energy consumption and/or increasing the capacity

The Casale-Dente High Efficiency Trays (HET) are the most efficient trays available on

the market and are also an essential elemen t in making the Split-Flow-Loop as efficient

as it is. The improved geometry of these trays has a profoundly beneficial effect on the

mass transfer efficiency of NH3and CO2from the vapours into the liquid phase where

urea is formed.

The new trays are designed in such a way that: Vapours and liquid follow separate, but

adjacent cocurrent paths through the space between the trays. This guarantees stable

flow of the two phases and a better approach to an even uniform flow of the two phases

throughout the whole reactor. These separated paths through the tray are chosen so

that very efficient mixing takes place between vapour and liquid. Consequently there is

a very high degree of both mass and heat transfer within the liquid phase is realised. It

is possible to generate vapour bubbles with a far smaller diameter than with anyprevious design. As a consequence, the interfacial surface, for mass and heat transfer, is

increased. There is also a much larger interfacial surface for exchange between the

vapour bubble emulsion and clean liquid. The relative short path length of the

recirculation streams into the emulsion phase significantly decreases transfer

resistances. The trays are plates corrugated into a series of parallel linear ridges and

troughs. The ridges are flattened at the top and the troughs are similarly flattened at the

bottom. Large perforations are provided in the trough bottoms for liquid to pass

through and there are small perforations in the tops of the ridges for gases

accumulating beneath them to pass This unique design produces extremely smallbubbles and, as a consequence, a very high specific surface area for mass and heat

transfer enhancing the highly efficient mixing between vapours and liquid mentioned

above.

SNAMPROGETTI (SAIPEM) SUPERCUPS TRAYS

1. The innovative M/S. Saipem Super Cups design for Urea reactor trays has been

conceived and developed by Saipem with the support of Engin Soft by means of


18/21

CFD(Computation Fluid dynamics) simulation. Latest super cup trays the third

generation of high efficiency trays recently invented and patented by Saipem2. Computational Fluid Dynamics (CFD) provides a qualitative (and sometimes even

quantitative) prediction of fluid flows by means of mathematical modelling (partial

differential equations)

3. The computer code (software) which embodies this knowledge and provides detailedinstructions (algorithms) for the computer hardware which performs the actualcalculations. CFD is a highly interdisciplinary research area which lies at the interface offluid dynamics.

4. The Reactor trays that prevent back-flow of the heavier solution from the upperpart downwards and favour the gas absorption in the liquid phase.

5. The support of a systematic plan of fluid-dynamic simulations gave a significant

contribution to the development of the innovative design.

6. The proprietary M/S Saipem Super Cups (New Design) greatly increases themixing of the liquid and gaseous phases, respectively ammonia and carbamate, and

carbon dioxide, thus optimizing the product conversion rate in the reactor. The

immediate benefit is the lower specific steam consumption requirement to

decompose carbamate to CO2and NH3in downstream sections.7. This represents a further step ahead to get closest to the theoretical equilibrium

conversion in the reactor. In fact, the increase in the reaction conversion is strictly

dependent on the mixing conditions of ammonia, carbamate and carbon dioxide

through the reactor so that the main purpose of these innovative trays is to furtherimprove the contacting conditions among the reagents.

8. The peculiar behaviour of the Super Cups is characterized by a triple fluid-dynamic

effect Gas Equalizer, Mixer Reactor and Gas Distributor.

9. The first effect of Super Cups is to uniformly distribute the concentration of the

gaseous phase reagent on the entire section of the tray. In this way, the gas bubblesmoving upward lose the memory of the non -uniformity of the previous reaction

stage and the non-reacted CO2 can be evenly fed to each cup of the tray. Figure

shows the formation of the gas-cushion (blue area) just below the tray externally

to the cups. The cups behave as multiple confined reaction volumes in which thereagents - gaseous CO2 and liquid ammonia & carbamate heavily swirl inside,

thus reaching a high mixing degree. Each cup performs as a static mixer where the

phases are strongly contacted.

10. In this way the Super Cups Trays do not simply behave as gas distributors as inother commercial designs. But perform as additional active reaction stages which

can be modelled as a Continuous-Stirred-Tank Reactor (CSTR),as fig 12. The

CSTR behaviour (ideal perfect mixing) of each single tray can be clearly observedby the comparison of RTD curves for the new and standard designs.

11. The mean residence time increases by about 70% with respect to the standard

design, thus strongly improving the urea formation yield.

12. The CO2 gaseous phase forming the gas-cushion below the tray can be partially

streamed inside the cups to create a mixer reactor and partially distributed on theupper stage. This split range is one of the most critical design parameter since it

allows the customization of the RTD curve of each reactor stage and the increase or

decrease of the CSTR (perfect mixing) or PFR (plug flow) behaviour according to

the composition of each stage.


19/21

13. The Super Cups Trays permit an increase in the urea reactor efficiency with

consequent beneficial effects in terms of higher return on investment, lower

energy consumptions and reduced environmental impact.14. The CFD study of the traditional perforated plate vs. the innovative tray facilitated

the ability to compare the fluid dynamic behaviour of several designs in terms of

mixing performance of the reactants, flow patterns, pressure drops a nd residencetime.

Fig.-11


20/21

Fig.-12


21/21

CONCLUSIONS

With the combination of skilful modelling and original design, the possibility was

proven of increasing the efficiency of urea reactors, which were considered for a longtime to be operating close to their optimum. This new tray design represents a

significant upgrade of the urea reactors and, by consequence, of the whole plant. Thenet improvement of the CO2 conversion in an existing plant has, in fact,

The following advantages:1. The reduction of the energy consumption and of recycle.

2. The possibility of a sensible increase of the production with the same reactor.

The development and successful design of the High Efficiency trays in the reactor waspossible through a very accurate fluid dynamic simulation of the system combined with

the modelling of the chemical-physical equilibriums and of the heat transfer

phenomena. The most important of these consists of a sharp reduction in specific

steam consumption. This feature was con-firmed by a number of test run results

carried out in the field. Reductions of specific steam consumption up to 250300 kg perton of urea have been obtained and capacity increases up to 10 20 % .

References

1. Kinetic Model for Ammonia and Urea Production Processes

( Z. Umair,, P. Balasubramanian, and M. Shuhaimi)

2.

Kinetics Equation of Urea synthesis process by Maria Zolotakin,Jozef Szrawaraand Jerzy Piotrowski.

3. Biuret in urea fertilizer by R. L Mikkelsen

****************************************************************************************

reactor kinetics of urea formation

Documents