GAS ENGINEERING NOTES S.M. Taha Rehman Siddiqui

Institute of Chemical Engineering and Technology

University of the Punjab

S.M. Taha Rehman Siddiqui E11-PG08

Uses of Natural Gas:

Natural Gas is primarily used as a fuel and raw material in

manufacturing Industries.

As a Domestic Fuel:

Home Furnaces Water Heater Cooking Stoves

Gas Ovens Boilers

As an Industrial Fuel:

i) It is use in Kiln for baking Brick, Cement and Ceramic tiles.

ii) In Glass Industry

iii) Generating Steam in Boilers.

iv) Clean Heat Source for sterilizing Instruments and Processing Foods.

As a Raw Material in Petrochemical Industries:

i) Natural Gas is use to produce Hydrogen, Sulfur, Ammonia and Carbon

Black.

ii) Ammonia is used in range of Fertilizer and as secondary feed stock for

manufacturing other chemicals including HNO3 and Urea.

iii) Ethylene an important chemical is also produce from Natural Gas.

Miscellaneous:

Gas is used as a fuel for vehicles (CNG & LNG) and Gas Generators.

Question:

Why Natural Gas is Preferred over Liquid and Solid fuels?

Answer:

Natural Gas is Environmental Friendly Clean Fuel that offers environmental

benefits as compare to other fossil fuels. Environmental qualities over crude oil

and Coal are that of less emission of CO2, SO2 Nitrous Oxide emission.

It also helps to reduce problems such as acid rain, ozone layer detrition and

greenhouse gas.

It is safe source of energy when transported, stored and used.

Lecture 1

October 31, 2014

S.M. Taha Rehman Siddiqui E11-PG08

Constituents of Natural Gas:

Principle Constituents of Natural Gas is methane other constituents are paraffinic

hydrocarbons such as ethane, propane and butane. Many natural gas sources also

contains Nitrogen, H2S and CO2.

Trace quantities of H2, Argon and Helium are also found in some sources.

Natural Gas Composition before Refinement:

C5 Plus hydrocarbons in small proportions and when separated this fraction is light

gasoline. Some aromatics such as benzene, xylene and toluene can also be present.

Acid contaminants such as Mercaptans, Carbonyl Sulfides and carbon Disulfides

maybe present in small quantities.

Typical Composition of Natural gas form Source.

Component Formulae Percentage Composition

Methane CH4 >85 %

Ethane C2H6 3-8 %

Propane C3H8 1-2 %

Butane C4H10

S.M. Taha Rehman Siddiqui E11-PG08

Bacterial:

The gas formed due to the action of bacteria of the organic debris accumulating in

the sediments is called bacterial gas or Biochemical gas.

Methane is the only hydrocarbon forms by this procedure, the temperature

ranges between 65-80C corresponding to the depths of 2000-2500 m.

Thermal Gas:

Formation of thermal gas from organic matter present in the *

Organic matter is incorporated in the sediments at the time of deposition. Debris

from organic matter which accumulates at the water sediment interphase

degraded by living organism.

In an aerobic environment (in the presence of free oxygen dissolved in water) this

degradation is rapid and nearly complete and carbon initially present in organic

debris is nearly converted to CO2 which returns to atmosphere.

In an anaerobic environment (in the absence of air) by contrast degradation is slow

and incomplete.

The residues accumulates in the sediments in the form of complex structures and

debris which have resisted biodegradation. The overall mass is insoluble in an

organic solvents and is called KEROGEN.

The sediments are progressively buried and raise to increasing temperature and

pressure. Temperature generally below 200C.

However the periods during which this process occur are so long that the

KEROGEN is eventually thermally degraded. This thermal degradation generates

hydrocarbons as well as non-hydrocarbons. Such as CO2, H2O, Hydrogen, H2S,

Nitrogen and a residue very rich in carbon similar to coke produces in a refinery by

thermal cracking of oil called PYROBUTEMEN.

S.M. Taha Rehman Siddiqui E11-PG08

KEROGEN Degradation Mechanism:

S.M. Taha Rehman Siddiqui E11-PG08

Basic Concepts of Natural Gas Processing:

Background:

Raw natural gas after transmission through the field gathering network must be

processed before it can be moved into long distance pipelines system for use.

Objective:

To separate natural gas, heavy hydrocarbons, condensate, non-condensate, acid

gases and water from a gas processing wells and conditioned these fluids for sale

or disposal.

Simplified figure for Basic Natural gas processing:

Different Natural Gas Fractions and its various forms:

Natural gas and its different fractions can be transported in various forms:

i) Compressed Natural Gas

a) In Gas Pipelines

b) CNG ( Further Compressed after receiving from distribution System)

ii) Liquefied Natural Gas (LNG)

iii) Liquefied Petroleum Gas (LPG)

iv) Chemicals (Methanol, Urea, Ammonia)

Lecture 2

November 14, 2014

S.M. Taha Rehman Siddiqui E11-PG08

Reason for Gas processing:

A number of Natural gas Components must be extracted either for reason imposed

by the subsequent production or transport steps or to comply with Commercial or

Regulatory Specification.

Components to be removed during Natural Gas processing:

a) Hydrogen Sulfide (H2S)

It is Toxic and Corrosive

b) Carbon Dioxide (CO2)

Corrosive, has no heating value and can crystallize in Cryogenic processes

c) Mercury (Hg)

toxic and Corrosive with aluminum based alloy.

d) Water (H2O)

It can lead to the formation of hydrates and can cause corrosion.

e) Heavy Hydrocarbon

It can condense in the transport system

f) Nitrogen (N2)

No heating Value

g) Helium (He)

No heating Value and valuable in other processes.

Pipeline Transport of the Commercial use:

The specification to be met for processed gas are related to transport condition

of use (Commercial gas).

Pipeline Transport Objective:

i- Aimed at preventing the formation of a liquid phase.

ii- The clogging of line by hydrates and excessive corrosion.

Commercial:

For a commercial use the specification are more severe and also include a range

impose for heating value.

Typical specification of commercial gas:

Gross Heating Value (GHV) = 39100-39500 KJ/Nm3

Hydrocarbon dew point: Less than -6C

S.M. Taha Rehman Siddiqui E11-PG08

Water Content: Less than 50ppm

C5+ Content: Less than 0.50 mole%

Process Module:

The

first module is the physical separation of the distinct phase, which are typically

gas-liquid hydrocarbons, gas-water or gas solid separation. Transmission lines

supplying gas operate with 2-3 phases and consequently liquid slugging is common.

Question: When Slugs are formed ?

Answer:

From Elevational Changes in the inlet supply line

Changes in gas supply flow rate

Changes in temparature and pressure

Problems associated with Slugs:

The arrival of slugs at production or processing equipment impacts the operation

negatively carrying both mechanical problems (due to high velocity and momentum)

and process problems (increase in liquid level causes surging and trips).

In some Cases operator can minimize the liquid accumulation in a way to create a

suitable fluid flow regimes (mist flow regimes) in which the gas velocity is high

enough to keep liquid dispersed continuously.

In addition suitable process equipment are also installed in a pipeline to reduce the

effect of slugging. For example slug catchers are designed to separate

hydrocarbons, condensate, inlet water and gas.

S.M. Taha Rehman Siddiqui E11-PG08

Flow of Multiphase Mixture:

Multiphase flow is important in many areas of chemical and process engineering and

the behavior of the material will depend upon the process, the flow rate and

geometry of system.

Some of the major systems to be considered are:

Mixture of liquid with gas or vapor.

Liquids mixed with solid particles (Hydraulic transport)

Gas carrying solid particle wholly or partially in Suspension

(Pneumatic Transport)

Multiphase system containing solids, liquid and gases.

Mixed material may be transported horizontally, vertically or at the inclination to

the horizontal in pipes,

Liquids may have densities up to three order of magnitude than gases but they do

not exhibit any significant compressibility.

Liquids themselves can range from simple Newtonian fluids such as H2O to non-

Newtonian fluids with very high apparent Viscosity.

These very large variations in densities and viscosity are responsible for large

difference in behavior of solid-gas and solid-liquid mixture must in practice be

considered separated.

For all multiphase flow system it is important to

understand the nature of interactions between phases and how these influence the

flow pattern (The ways in which the phases are distributed over the cross section

of pipe or duct).

In design it is necessary to be able to predict pressure drop which, usually,

depends not only on the flow pattern, but also on the relative velocity of the

phases, this relative velocity is known as Slip Velocity.

This slip velocity affects the holdup (The fraction of the pipe volume which is

occupied by a particular phase). In the flow of two component mixture, the holdup

(in-situ concentration) of a component will differ from that in mixture discharged

Lecture 3

November 21, 2014

S.M. Taha Rehman Siddiqui E11-PG08

at the end of pipe because, as a result of slip of the phases relative to one

another, their residence times in the pipeline will not be the same.

In order to study the flow of this complex mixture, the following point are

required to be considered.

(1) The flow patterns.

(2) The hold-up of the individual phases and their relative velocities.

(3) The relationship between pressure gradient in a pipe and the flow rates

and physical properties of the phases.

Vertical flow:

The difference in density between the phases is important in determining flow

pattern. In gas-solid and gas-liquid mixtures, the gas will always be the lighter

phase, and in liquid-solid systems it will be usual for the liquid to be less dense than

the solid. In vertical upward flow, therefore, there will be a tendency for the

lighter phase to rise more quickly than the denser phase giving rise to a slip

velocity.

For a liquid-solid or gas-solid system this slip velocity will be close

to the terminal falling velocity of the particles. In a liquid-gas

system, the slip velocity will depend on the flow pattern in a complex

way. In all cases, there will be a net upwards force resulting in a

transference of energy from the faster to the slower moving phase,

and a vertically downwards gravitational force will be balanced by a

vertically upwards drag force. There will be axial symmetry of flow.

Horizontal Flow:

In horizontal flow, the flow

pattern will inevitably be more

complex because the gravitational

force will act perpendicular to

the pipe axis, the direction of

flow, and will cause the denser component to flow preferentially nearer the bottom

of the pipe. Energy transfer between the phases will again occur as a result of the

difference in velocity, but the net force will be horizontal and the suspension

S.M. Taha Rehman Siddiqui E11-PG08

mechanism of the particles, or the dispersion of the fluid will be a more complex

process. In this case, the flow will not be symmetrical about the pipe axis.

Two-phase Gas (vapour)-Liquid Flow:

Two phase gas (vapour)-liquid flow is important in various chemical engineering

application this ranges from the flow of mixture of oil and gas from well heads to

flow of vapour-liquid mixture in boiler and evaporator.

In most cases the gas phase which may be flowing with a much greater velocity

than the liquid, continuously accelerates the liquid thus involving the transfer of

energy.

It is important to study the different flow regimes which exist in two phase (gas-

liquid) mixture the regimes are important to calculate and predict the holdup of

phase during flow and to calculate pressure gradient for gas-liquid flow in pipes.

In addition, when gas-liquid mixtures flow at high velocities serious erosion

problems can arise and it is necessary for the designer to restrict flow velocities

to avoid serious damage to equipment.

Flow Regimes and Flow Pattern:

Horizontal flow:

The flow pattern is complex and is influenced by the diameter of pipes and the

physical properties of fluid and flow rate. In general as the velocity are increase

and gas liquid ratio increases changes will take place from bubble to mist flow as

shown in Diagram:

S.M. Taha Rehman Siddiqui E11-PG08

Flow Regimes in Horizontal two phase flow

Regimes Description

Typical Velocity

(m/s)

Liquid Vapor

Bubble Flow

Bubbles of gas

dispersed through the

liquid

1.5-5 0.3-3

Plug Flow Plugs of gas in liquid

phase 0.6 < 1.0

Stratified Flow Layer of liquid with a

layer of gas above < 0.15 0.6-3

Wavy Flow

As stratified but with

a wavy interface due

to high velocity of gas

< 0.15 NA

Slug Flow Slug of gas in liquid

phase < 0.3 >5

Annular Flow

Liquid film on inside

walls with gas in

center

6

Mist Flow Liquid droplets

dispersed in gas cannot measured >60

At high liquid-gas ratios, the liquid forms the continuous phase and at low values it

forms the disperse phase. In the intervening region, there is generally some

instability; and sometimes several flow regimes are lumped together. In plug flow

and slug flow, the gas is flowing faster than the liquid and liquid from a slug tends

to become detached, to move as a relatively slow moving film along the surface of

the pipe and then to be reaccelerated when the next liquid slug catches it up. This

process can account for a significant proportion of the total energy losses.