GAS PROCESSING SIMPLYFIED

There is a demand for effective ways to treat both associated petroleum gas, common at oil production sites as well as gas condensate produced alongside natural gas at gas condensate fields. This demand is boosted by several reasons: growth in oil and gas production, tightening of the ecological norms and emission standards, gradual decline in dry Cenoman gas deposits and a consequent shift to development of new gas condensate fields.

Most of the problems faced by gas processing professionals are typical, really. However, it is not uncommon to see owners starting to ‘invent a bicycle’ anew instead, leading to a complicated project that takes longer to build. All there is to gas processing are 5 distinctive steps.

IT IS ENOUGH TO COMPREHEND 5 STEPS

Step 1. Compression

To compress associated petroleum gas screw or reciprocating type compressors are usually employed. They could be driven by either a stroke piston or an electric motor.

When designing a compressor, the following has to be taken into consideration:

  • A gas engine requires some sort of fuel gas treatment
  • Condensate has to be returned to feed gas for treatment
  • Compressor station is not a standalone project but rather a part of a larger gas treating/processing establishment and it will have to cope with all of the tasks and requirements of this larger facility.

Step 2. Sour gas removal and utilization

Further stages of gas processing and receipt of spec gas products require that mechanical impurities, sour components and inert gases are removed from the feed gas.

Mechanical particles and contaminants are removed via separators, micro filters and water scrubbers.

Sulfur, carbon dioxide and inert gases are removed via either absorption or adsorption or a membrane processes.

Sour gases are then utilized by one of the following methods:

  • Reinjection
  • Licensed sulfur removal processes (Lo-Cat, Crystall etc.)
  • Use of non-regenerative chemical reagents
  • Incineration

Step 3. Gas dehydration

Based on the sequent processes of gas treatment on of the following gas dehydration methods is used:

  • Required water dew point -10°С or higher – hydrate inhibitor injection;
  • Required water dew point -30°С – glycol contactor drying of the gas;
  • Required water dew point -30°С… – 100°С – adsorption dehydration.

Step 4. Hydrocarbon extraction

The purpose of the plant shall define the method of extraction (i.e. is dry gas to spec the priority product or is it NGL?). Feed gas composition and pressure also play a part in choosing the process.

Generally, cryogenic processes of separation are employed:

  • Expander or a JT valve as a source of refrigeration
  • External refrigeration (i.e. propane chillers)
  • Some special cases may be feasible with membranes

Absorption (oil absorption) or adsorption methods both are outdated while still find some niche use.

Step 5. Fractionating

To get to the stabilized products upon completing the previous step, NGLs are further fractioned. Ethane, LPG (С3/С4 mix) and stable gas condensate are among the most typical end products at this stage.

Where large volumes of gas are concerned, it may be appropriate to review an option of further fractioning of the gas certain NGLs to produce separately ethane, propane, butane isomers, pentane isomers etc.

Process wise fractionation units are little more than a set of separation columns. When a high spec of the end product is necessary those are complemented by a purification skid: methanole extraction out of LPG, CO2 removal from ethane, etc.