Gas Plant processes gas streams collected from oil fields and gas zones to produce commercial standard associated/natural gas, for master gas system, and natural gas liquid (NGL)/condensate. Gas and liquid products are shipped and distributed through pipelines.
Process operations consist essentially of three steps: (1) separate the acid gas (H2S and CO2) from the gas stream in order to reach commercial specification, (2) recover NGL from gas stream, and (3) process the acid gas in order to recover sulfur. NGL and sulfur are thus two major products of process operation. NGL has a high value, while sulfur has a low value. Products, therefore, are NGL, sulfur, and sales gas.
Typical Gas Processing Plant Overview
Usually, gas plant processes get a mix of gas: Associated Gas; a product of Gas/Oil separation and natural gas; which is produced from deep gas reservoirs. Such gas is produced to make-up for sales gas demand unmet by associated gas alone.
In addition to gas processing, the gas facilities normally contain what is known as shipment center which receives NGL from other gas processing facilities and ships it to NGL fractionation plants. Such activity increases the electricity consumption mainly due to pumping.
The function of the gas plant is to process raw gas into two primary products-Sweet HP gas for export to the grid, and natural gas liquids (NGL) for use as a petrochemical feedstock. For instance, the raw gas comes from two sources: Associated gas from GOSPs (220 psig) and natural gas from gas fields (950 psig). The process consists of 4 main steps:
– Gas/Liquid feed receiving and conditioning
– Gas sweetening (H2S and CO2 removal)
– Gas dehydration (water removal) and gas compression
– Liquid recovery
Associated gas from GOSPs enters the plant battery limits at a pressure of about 220 psig. Each gas stream is passed through dedicated slug catcher to knockout heavy HC condensates and sludge of liquid. High pressure natural gas at – 950 psig is brought in to supplement the associated gas. It too goes through a KO drum to dropout HC condensates. The separated condensate is then sent to crude mixing/stabilization plants. The HC condensate comes from the GOSPs through the HC condensate stripper to remove H2S and light hydrocarbons (C2-). Bottom from the HC stripper is a relatively clean NGL stream, and goes straight to NGL product sales.
The first step in the process is to remove H2S and CO2 from the incoming feed gas. There are gas treating units for LP associated gas and for HP natural Gas. All employ the conventional absorption stripping process, using DGA as the absorbent.
Sweet gas from the contactors is first compressed from 150 psig to 465 psig in the feed gas compressors, which are driven by steam turbines and/or motors. The compressed gas is cooled down below 80°F and passed through a separator to drop out the condensed moisture. It is then passed through molecular sieves, which take the moisture content down to 0.1 ppm. Normally, there are several packed bed gas dehydrators in each of the production lines. For instance, if three are used two of them are generally on- cycle and the third is generally on the regeneration cycle.
Regeneration is accomplished by passing heated “residue gas” (recycled from downstream in the process) to desorb moisture from the resin. The exhaust gas is cooled below 80°F against air and propane, and the condensed moisture is removed in a knock-out drum. Similar dehydration system is also used for the liquid hydrocarbon stream that is separated out in the first separator.
Liquid Recovery Unit
First Stage Feed Gas is introduced to the Liquid Recovery Unit after going through the sweetening process in Amine Treating Units followed by the Compression and Dehydration Unit. The temperature of the feed was determined to be 143°F on average at a pressure of 465 psig. The unit produces mainly three products; High Pressure Residue Gas (HP Gas) from the last chill-down separator, Low Pressure Residue Gas (LP Gas) and NGL from the De-methanizer Column. After a series of heat exchangers, the LP Gas is directed to the Fuel Gas Compressor. The discharge of the Fuel Gas Compressor combines with the heated HP Gas combines where they are directed to the suction of the Sales Gas Compressor.
The refrigeration system may consists of one ethane level at about -80°F and three propane levels at about 60°F, 10°F and -40°F. The ethane system consists of a single stage compressor with around 17 psig suction and 139 psig discharge pressures. The discharge vapor at about 85°F is condensed against propane and then sub-cooled to -80°F against LP residue gas. The propane refrigeration system has 3 suction pressures at around 17 psia, 31 psig and 84 psig and a discharge pressure of about 260 psig. The primary condenser is air-cooled, and operates at about 125°F.
Normally, there are 2 propane compressors, both of them are in operation at any given time. Each compressor motor/steam turbine is rated at about 20000 HP, and represents a significant portion of the plant electrical power load/steam load. Propane is sub-cooled down to 120°F using NGL stream at 42°F, and LP residue gas at -42°F. Small amount of refrigerant is usually lost from the system due to leaks, and must be made up. The plant extracts ethane and propane from NGL product by fractional distillation either on site or at another NGL fractionation facility.