PEH:Gas Treating and Processing: Difference between revisions

<br/>When natural gas contains a high concentration of CO₂ , the options for reducing the CO₂ concentration to acceptable levels are either to use a regenerative solvent, such as an amine, potassium carbonate or Selexol, or to install a membrane separation process.<br/><br/>Separation by means of membrane technology makes use of thin layers of polymeric material.<ref name="r44">Koros, W.J. 1995. Membranes: Learning a Lesson from Nature. Chemical Engineering Progress (October): 68.</ref> This polymeric material can be manufactured in two main types—porous and nonporous. The porous membrane makes use of differences in diffusion rates and acts like a sieve in separating molecules based on relative size. The main application of this type of material is the separation of small molecules such as hydrogen or helium from gas mixtures. The nonporous membrane promotes the separation by dissolving some compounds in the polymeric material and allowing these compounds to diffuse through the material more rapidly than hydrocarbon compounds, which do not dissolve but diffuse through the material. In natural gas separation, the nonporous material is used. The compounds specifically found in natural gas, which have the ability to dissolve in the polymeric material, are the polar compounds— CO₂ , H₂S, and H₂O. Hydrocarbons also diffuse through such membranes, but at a much lower rate.<br/><br/>The membrane material is manufactured as a very thin film, which has no strength. It is supported in applications by a porous layer, through which the gas molecules that have passed through the membrane (permeate) flow to the low pressure side. In one type of application, the membrane and the porous permeate spacer are spirally wound around a perforated tube. A third layer is included in such construction, which is a porous feed spacer. The high-pressure gas (feed) flows through the feed spacer and is in contact with the membrane. The permeate dissolves in and/or diffuses through the membrane layer into the permeate spacer. It then flows through the permeate spacer into the perforations in the steel tube, which forms the shaft of the spirally wound layers. The high-pressure gas passes through the feed spacer to the other end, with very little pressure drop.<br/><br/>In the natural gas industry, membrane technology is used mainly to reduce the concentration of CO₂ from very high concentrations to acceptable levels, such as less than 2%.<ref name="r45">Lee, A.L., Feldkirchen, H.L., and Gomez, J. 1994. Membrane Process for CO2 Removal Tested At Texas Plant. Oil & Gas J. (31 January): 90.</ref> If H₂S is present, most of the H₂S will separate. Because the sales gas specification for H₂S can be as low as 4 ppm, membrane separation is usually not sufficient to meet the specification for this compound. This process may meet the stringent specification for H₂S only if the concentration of H₂S is very low to begin with, such as perhaps 50 ppm or less.<br/><br/>While CO₂ is the main target for removal in natural gas processing with membranes, hydrocarbons also diffuse through the membranes. Because hydrocarbons are the valuable sales products, various process schemes are used to minimize the loss of hydrocarbons. The amount of hydrocarbon in the permeate after one pass through a permeable membrane facility depends on gas composition; system pressures (high and low); gas flow rate; and total surface area of membrane exposed to high-pressure gas.<br/><br/>While the permeation and potential loss of hydrocarbon is a disadvantage of membrane separation processes, there are many advantages over the alternatives for reducing CO₂ content. The advantages are low capital investment, as compared with regenerative solvent processes; ease of installation; simple operation, as the feed gas simply flows through the facility with little pressure loss; low weight and space requirements, which are important in offshore installations; low environmental impact; and no utilities requirement.<br/><br/>The loss of hydrocarbon in the permeate can be reduced by compressing the permeate to a high pressure and subjecting it to a second stage of separation with a membrane.<ref name="r46">Cook, P.J. and Losin, M.S. 1995. Membranes Provide Cost-Effective Natural Gas Processing. Hydrocarbon Processing (April): 79.fckLR</ref> The resulting high-pressure stream from the second stage is then added to the high-pressure feed gas to the first stage, as shown in '''Fig. 5.23'''. Other permeable membrane schemes use the membrane process for bulk removal of CO₂, followed by an amine system for final cleanup.<br/><br/><gallery widths="300px" heights="200px">

Conversion factor is exact.</div></div>[[Category:PEH]] [[Category:Volume III – Facilities and Construction Engineering]]  [[Category:4.1.2 Separation and treating]]