Gas separation membranes for separating components from a gas stream are gaining increasing popularity. They are often used to separate CO2 from hydrocarbon gases, hydrogen and ammonia.

Membranes are simple, modular in nature, have no moving parts; and, can package a high membrane area in a small volume. Most membranes are packaged commercially in either a spiral-wound or hollow-fiber configuration. All membranes act as very fine filters and trap particles and other contaminants.

Once they are plugged, even partially, their separation efficiency (directly dependent on the available porous volume) is decreased. In addition, membranes themselves are expensive and care must be taken to protect them from contaminants which have a serious and irreversible effect on membrane life.

Membrane System Solutions for Gas Separation Applications
Using semi-permeable membrane systems for gas separation, especially CO2 removal from hydrocarbon gas streams, is definitely state of the art technology. The process is environmentally attractive and offers capital cost advantages, as well as operational advantages. The technology is commercially proven and advantageous in a multitude of applications – most notably in offshore environments where space and weight savings are primary drivers, and in applications where bulk CO2 recovery is necessary.

Membrane types
The membranes commercially available today for CO2 separation are glassy polymers such as cellulose acetate and polyimide, and Rubbery Polymers such as Silicone Rubber. Figures 1 and 2 represent the two types of membrane fabrication configurations used in commercial applications, which are Hollow Fiber and Spiral wound. The membrane materials mostly used for CO2 applications are manufactured from Cellulose Acetate polymers

Applications

• Existing process applications for CO2 removal range from a few MSCFD to over 1 BSCFD, while CO2 Inlet compositions range from 3% CO2 to over 90% CO2 with product requirements down to less than 3% CO2. There are many small membrane systems in service that are of similar design.
• However the larger systems should be designed specifically for the given application. Since membrane performance can be significantly affected by the operating conditions it is important that the supplier have a good understanding of the inlet conditions, outlet specifications, and other process design and equipment requirements of the project. Some existing membrane system designs and applications did not perform well initially due to poor specifications, lack of proper integration with the overall processing facility, or process design issues which required pre-treatment modifications for successful operation. Past experience is a good reference for future designs and their expected performance, and experienced membrane suppliers offer the best opportunity to achieve future project success.
• Just as you would choose only an experienced pulmonologist (lung doctor) to take care of your personal lung membrane, the facility engineer should request only experienced membrane system suppliers with references to projects similar to the application being designed. How similar is the new facility to those in service, and is there data from actual operations or from test run on field gas that is close to the requirements of the new facility? Having a membrane supplier with good reference projects, like having a good doctor, will certainly build confidence in the ability to meet the desired facility requirements.