1. NATURAL GAS LIQUIDS RECOVERY
INTRODUCTION
Most natural gas is processed to remove the heavier hydrocarbon liquids from the natural gas stream.
These heavier hydrocarbon liquids, commonly referred to as natural gas liquids (NGLs), include ethane, propane, butanes, and natural gasoline (condensate).
Recovery of NGLcomponents in gas for dew point control & yields a source of revenue.
Lighter NGL fractions, such as ethane, propane,and butanes, can be sold as fuel or feedstock to refineries and petrochemical plants, while the heavier portion can be used as gasoline-blending stock.

2. Gas Condensate Reservoirs
Gas condensate reservoirs have been defined as those hydrocarbon reservoirs that yield gas condensate liquid in the surface separator(s).
A retrograde gas condensate reservoir is one whose temperature is below the cricondentherm (the maximum temperature at which liquid and vapor phases can coexist in equilibrium for a constant-composition multicomponent system).
As pressure decreases below the dew point due to production, a liquid phase develops within the reservoir, which process is called retro grade condensation.

3. Figure 1: shows a pressure-temperature phase diagram.

4. Options of Phase Change
To recover and separate NGL from a bulk of gas stream, a change in phase has to take place. In other words, a new phase has to be developed for separation to occur. Two distinctive options are in practice depending on the use of ESA or MSA.
Energy Separating Agent
(Refrigeration)
(Distillation)
Mass Separating Agent
To separate NGL, a new phase is developed by using either a solid material in contact with the gas stream (adsorption) or a liquid in contact with the gas (absorption).

5. Parameters Controlling NGL Separation
Operating pressure, P
Operating temperature, T
System composition or concentration, x and y
To obtain the right quantities of specific NGL constituents, a control of the relevant parameters has to be carried out:
1. For separation using ESA, pressure is maintained by direct control.
Temperature, on the other hand, is reduced by refrigeration using
one of the following techniques:
Compression refrigeration
(b) Cryogenic separation; expansion across a turbine
(c) Cryogenic separation; expansion across a valve

6. 2. For separation using MSA, a control in the composition or the concentration of the hydrocarbons to be recovered (NGL); y and x is obtained by using adsorption or absorption methods.
In Summary
The efficiency of condensation, hence NGL recovery, is a function of
P, T, gas and oil flow rates, and contact time. Again, absorption could be coupled with refrigeration to enhance condensation.
A proper design of a system implies the use of the optimum levels of all operating factors plus the availability of sufficient area of contact for mass and heat transfer between phases.

7. Figure 2:Thermodynamic pathways of different NGL recovery technologies.

8. Mechanical Refrigeration
Mechanical refrigeration is the simplest and most direct process for NGL recovery. of condensate are expected. This process may also lead to the recovery of liquified petroleum gas, where for LPG recovery up to 90%, a simple propane refrigeration system provides refrigeration at temperatures to −40o F.
Flow sheet of a mechanical refrigeration process

9. Salient Features
propane as the refrigerant
gas-to-gas heat exchanger recovers additional refrigeration
The temperature of the cold gas stream leaving this exchanger “approaches” that of the warm inlet gas
The chiller in is typically a shell and tube, kettle-type unit
The refrigerant (often propane) boils off and leaves the chiller vapor space essentially as a saturated vapor.
The thermodynamic path followed by the gas in an external refrigeration process is shown as line ABC in Figure 2. From A to B indicates gas-to-gas heat exchange; from B to C, chilling.
Hydrate formation is prevented either by dehydration of the gas or by injection of a hydrate inhibitor.

10. Choice of Refrigerant
Any material could be used as a refrigerant. The ideal refrigerant is nontoxic, non-corrosive, has Pressure-Volume-Temperature (PVT) and physical properties compatible with the system needs, and has a high latent heat of vaporization.
The practical choice reduces to one, which has desirable physical properties and will vaporize and condense at reasonable pressures at the temperature levels desired.
Cascade Refrigeration
Cascade refrigeration refers to two refrigeration circuits thermally connected by a cascade condenser, which is the condenser of the low temperature circuit and the evaporator of the high-temperature circuit.
A cascade system utilizes one refrigerant to condense the other primary refrigerant, which is operating at the desired evaporator temperature. This approach is usually used for temperature levels below −90◦F, when light hydrocarbon gases or other low boiling gases and vapors are being cooled.

11. Mixed Refrigerants
An alternative to cascade refrigeration is to use a mixed refrigerant. Mixed refrigerants are a mixture of two or more components. The light components lower the evaporation temperature, and the heavier components allow condensation at ambient temperature.
The evaporation process takes place over a temperature range rather than at a constant temperature as with pure component refrigerants. The mixed refrigerant is blended so that its evaporation curve matches the cooling curve for the process fluid.
Heat transfer occurs in a countercurrent exchanger, probably an aluminum plate fin, rather than a kettle-type chiller. Mixed refrigerants have the advantage of better thermal efficiency because refrigeration is always being provided at the warmest possible temperature.

12. Self-Refrigeration
In this process, the nonideal behavior of the inlet gas causes the gas temperature to fall with the pressure reduction, as shown by line ABC’ in Figure2. The temperature change depends primarily on the pressure drop.
Flow sheet of a self-refrigeration system

13. If the objective is to recover ethane or more propane than obtainable by mechanical refrigeration, a good process can be self-refrigeration, which is particularly applicable for smaller gas volumes of 5 to 10 MMCFD.
The self-refrigeration process is attractive if the inlet gas pressure is very high. It is important that the reservoir pressure remain high for the intended life of the plant.
Low-pressure inlet gas favors a cryogenic refrigeration plant or straight refrigeration process

14. Cryogenic Refrigeration
Cryogenic refrigeration processes traditionally have been used
for NGL recovery.
These plants have a higher capital cost but a lower operational cost.
In the cryogenic or turboexpander plant, the chiller or Joule–Thomson (JT) valve used in two previous processes is replaced by an expansion turbine.
The expansion process is indicated as line ABC” in Figure 2.
The turbine can be connected to a compressor, which recompresses the gas with only a small loss in overall pressure.

15. Typical flow sheet of a cryogenic refrigeration plant

16. Schematic of Ortloff gas subcooled process

17. Schematic of Ortloff residue split-vapor process

18. Simplified flow diagram of an oil absorption plant

19. Schematic of a solid bed adsorption plant