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Stage Separation of Gas-Condensate


Stage separation of gas-condensate is a process in which hydrocarbon mixtures are separated into vapor and liquid phases by flashing to low pressure in several steps. Main purposes of stage separation are:

  1. To obtain a more stable stock-tank liquid
  2. To increase liquid recovery

Figure below shows typical stage separation process.

Stage Separation Process

I want to share to you some terminology that I also just knew about stage separation. A two-stage separation requires one separator and one storage tank, and a three-stage separation requires two separators and one storage tank. I never thought before that the storage tank is always counted as final stage of vapor-liquid separation.

How Does Stage Separation Work?

Stage separation reduces the fluid pressure a little at a time, in stages or steps, resulting in a more stable stock-tank liquid. Liquid in the stock-tank contain a large percentage of light molecules, especially methane and ethane, which will flash to gas in the tank. Condensate stabilization is the process of increasing the amount of intermediate component (C3-C5) and heavy components (C6+) in the condensate.

Usually a stable stock-tank liquid can be obtained by a stage separation of not more than four stages (three separators plus one storage tank).

Stage Separation to Increase the Recovery of Stock-Tank Liquid

In high pressure separation system of gas-condensate, liquid condensate stage-wise pressure reduction can significantly increase the recovery of stock-tank liquids. Three or four stages of separation theoretically increase liquid recovery compared to two-stage separation. However, further economic evaluation must be performed to check if incremental liquid recovery worth cost of additional separator. It has been generally recognized that two separators plus one stock-tank are practically optimum.

Operating Condition of Stage Condition

Operating pressure of first-stage separator is generally determined by flowline pressure and operating characteristics of well. The pressure is usually ranging from 600-1200 psig. If the flowline pressure is greater than 600 psig, it is practical to let the first-stage separator ride the line or operate at the flowline pressure.

When This Stage Separation is Applied and when it is not?

Additional separator will add capital investment of the project. Unless the gas well produces at pressure (less than 500 psig) and the gas contains very little condensate (less than 100 BPD), the additional investment for separator is almost always economical when balanced against increased liquid production.

Bibliography

Handbook of Natural Gas Transmission and Processing Principles and Practices Third Edition [Book] / auth. Mokhatab Saeid, Poe William A and Mak John Y. – Oxford : Gulf Professional Publishing, 2015.

Petroleum Production Engineering A Computer-Assisted Approach [Book] / auth. Guo Boyun, Lyons William C and Ghalambor Ali. – [s.l.] : Elsevier Science & Technology Books, 2007.

Surface Production Operations Volume 2 [Book] / auth. Arnold Ken and Stewart Maurice. – Houston : Gulf Publishing Company, 1999.

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