Dehydration of natural gas using membranes. Part II: Sweep/countercurrent design and field test

Membrane technology is an attractive alternative to conventional glycol dehydrators for natural gas dehydration. While membranes with high water/methane selectivity are available, the separation performance of membrane systems is restricted by the moderate feed-to-permeate pressure ratio encountered in this application, resulting in high membrane area requirement and high methane loss. This paper (Part II of a two-part study) addresses that challenge by evaluating suitable process designs to overcome the limitations of feed-to-permeate pressure ratio, using a process simulation package. A design with a dry gas on the permeate side as a countercurrent sweep shows the best combination of low methane loss and minimum membrane area requirement. The design was experimentally evaluated by field testing a spiral-wound module that incorporates countercurrent operation, at a natural gas processing plant. The use of a dry sweep gas on the permeate side significantly increased water vapor flux across the membrane. For example, as sweep/feed flow rate ratio increased from 0% to 5.9% at a permeate pressure of 4.8 bar, water flux increased by 2.5 times, from 0.13 slpm/m2 (standard liter per minute per m2 membrane area) to 0.47 slpm/m2. The measured water flux reached about 80% of the value calculated with the process simulation package for a perfect sweep/countercurrent module, confirming the suitability of the spiral-wound modules with countercurrent configuration for natural gas dehydration.