Hydrocarbon Compression

Archive for the ‘Gas Sweetening Processes’ Category

Gas Permeation

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Gas permeation is based on the mass transfer principles of gas diffusion through a permeable membrane. In its most basic form, a membrane separation system consists of a vessel divided by a single flat membrane into a high- and a low-pressure section. Feed entering the high-pressure side selectively loses the fast-permeating components to the low-pressure side. Flat plate designs are not used commercially, as they do not have enough surface area. In the hollow-fiber design, the separation modules contain anywhere from 10,000 to 100,000 capillaries, each less than 1 mm diameter, bound to a tube sheet surrounded by a metal shell. Feed gas is introduced into either the shell or the tube side. Where gas permeability rates of the components are close, or where high product purity is required, the membrane modules can be arranged in series or streams recycled.

The driving force for the separation is differential pressure. CO2 tends to diffuse quickly through membranes and thus can be removed from the bulk gas stream. The low pressure side of the membrane that is rich in CO2 is normally operated at 10 to 20% of the feed pressure.

It is difficult to remove H2S to pipeline quality with a membrane system. Membrane systems have effectively been used as a first step to remove the CO2 and most of the H2S. An iron sponge or other H2S treating process is then used to remove the remainder of the H2S.

Membranes will also remove some of the water vapor. Depending upon the stream properties, a membrane designed to treat CO2 to pipeline specifications may also reduce water vapor to less than 7 Ib/MMscf. Often, however, it is necessary to dehydrate the gas downstream of the membrane to attain final pipeline water vapor requirements.

Membranes are a relatively new technology. They are an attractive economic alternative for treating CO2 from small streams (up to 10 MMscfd). With time they may become common on even larger streams.

Written by Jack

September 19th, 2009 at 8:05 am

Distillation Process

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The Ryan-Holmes distillation process uses cryogenic distillation to remove acid gases from a gas stream. This process is applied to remove CO2 for LPG separation or where it is desired to produce CO2 at high pressure for reservoir injection.

Written by Jack

September 19th, 2009 at 8:03 am

Sulfide Scavengers

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Sour gas sweetening may also be carried out continuously in the flowline by continuous injection of H2S scavengers, such as amine-aldehyde condensates. Contact time between the scavenger and the sour gas is the most critical factor in the design of the scavenger treatment process. Contact times shorter than 30 sec can be accommodated with faster reacting and higher volatility formulations. The arnine-aldehyde condensates process is best suited for wet gas streams of 0.5-15 MMscfd containing less than 100 ppm H2S.

The advantages of amine-aldehyde condensates are water (or oil) soluble reaction products, lower operating temperatures, low corrosiveness to steel, and no reactivity with hydrocarbons.

Written by Jack

September 19th, 2009 at 8:03 am