Amine solutions are often used to treat LPG streams that contain acid gases. In this work, the selection of an amine and the method of contacting the amine with a 50 gpm [189 lpm] LPG stream containing 7.7 mol% as CO2 are evaluated. A packed contactor is compared to a static mixer and MEA, DEA, and MDEA are compared as potential solvents. A static mixer using 70 gpm [265 lpm] of 25 wt.% DEA is chosen for the final design. The operating data reveal 0.10 mol% CO2 in the sweet LPG compared to the design value of 0.16 mol%.
Since the inception of the gas processing industry, acid gas removal has been a very energy-intensive process. The energy requirements vary depending on the acid gas concentration in the feed, the scrubbing solution concentration, the solution loading, and the product purity specifications. Typical product purity specifications. Typical consumption ranges from about 1000 Btu/lbm (2300 kj/kg) to 10,000 Btu/lbm [23,000 kj/kg] of acid gas removed. The contaminants in hydrocarbon liquids and liquefied petroleum gases (LPG's) can include CO2, H2S, mercaptans, COS, and elemental sulfur. In general, most liquid hydrocarbon products must meet a 1A copper strip corrosion products must meet a 1A copper strip corrosion test which corresponds to less than 4 ppm H2S According to Perry (1977), the copper strip test is not sensitive to COS. However, in the presence of water, COS can react to form H2s. presence of water, COS can react to form H2s. Thus liquids containing COS will often not meet the copper strip test after storage.
The most common alternatives to sweetening liquids with amines are caustic wash and molecular sieves. For significant amounts of acid gases, a caustic wash can be quite expensive due to the caustic cost and disposal problems. Molecular sieve drawbacks are large problems. Molecular sieve drawbacks are large capital and operating costs as well as catalyzing the formation of COS if both H2S and CO2 are present. Thus amine treatment is usually the present. Thus amine treatment is usually the most cost effective choice for liquid sweetening when significant amounts of acid gases are present. present. In most liquid hydrocarbon or LPG production facilities, amine solutions are used to production facilities, amine solutions are used to sweeten the inlet gas stream before the liquids are condensed. In these cases, amine solutions are especially convenient for sweetening the liquids since both absorbers can share a common amine regenerator. In this work, the design alternatives for sweetening LPG's and liquid hydrocarbons using amine solutions are discussed. An LPG sweetening unit at the Champlin Petroleum Conroe Plant was designed using the Petroleum Conroe Plant was designed using the methodology presented in this work. Champlin constructed a cryogenic expander plant to remove approximately 85,000 gallons per day [322 m3/day] of demethanized liquids from the Conroe Oil Field. The feed gas to the expander plant contains an average 1.68 mol% CO2 and no sulfur compounds and the demethanizer bottoms product was concentrated to 7.7 mol% CO2. A CO2 removal facility was required to lower the CO2 concentration to below 0.16 mol%. This product was then transported by pipeline to fractionation and sales facilities near Conroe.
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