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I n d . Eng. C h e m . Res. 1988,27,500-506
An Integrated Process for Simultaneous Desulfurization, Dehydration, and Recovery of Hydrocarbon Liquids from Natural Gas Streams Steven F. Sciamanna and Scott Lynn* Department of Chemical Engineering, University of California, Berkeley, California 94720
Conventional processing schemes for desulfurizing, drying, and separating natural gas liquids from natural gas streams require treating the gas by a different process for each separation step. In a simpler process, based on the University of California, Berkeley, Sulfur Recovery Process (UCBSRP) technology, hydrogen sulfide, propane and heavier hydrocarbons, and water are absorbed simultaneously by a poly(glyco1ether) solvent containing a homogenous liquid-phase catalyst. The catalyst promotes the subsequent reaction of hydrogen sulfide with added sulfur dioxide to produce a high-quality sulfur product. Hydrocarbons are separated as two product streams with the split between propane and butane. This new process offers an overall reduction in both capital and energy costs. The University of California, Berkeley, Sulfur Recovery Process (UCBSRP) is being developed as an alternative to conventional sulfur recovery technology for removing hydrogen sulfide from gas streams and converting it to elemental sulfur. In the UCBSRP, the hydrogen sulfide is absorbed by a physical solvent, and the resulting solution of HzS is mixed with a stoichiometrically equivalent amount of sulfur dioxide dissolved in the same solvent. The reaction between the two sulfur compounds forms water, which is miscible with the solvent, and elemental sulfur, which crystallizes from solution when its solubility is exceeded. Part of the sulfur formed in the reaction is burned to make the SOz needed in the process, and the heat of combustion is recovered in a waste-heat boiler. The water content of the solvent is maintained at 3-4 wt % by stripping the excess water from the side stream of solvent that is subsequently used to absorb the SOz. Sulfur is recovered by cooling the solution, settling the additional crystals that form, and centrifuging the slurry pumped from the bottom of the crystallizer-surge tank. Patent rights to this process are held by the University. The application of the UCBSRP to treatment of recycle gas from a crude oil residuum hydrotreater and to a coal gasification stream have been discussed, respectively, by Lynn et al. (1986) and by Neumann (1986). The focus of this work is the treatment of sour natural gas for conditioning to pipeline sales specifications. The conditioning process entails dehydration, desulfurization, and recovery of natural gas liquids (propane and higher hydrocarbons). The conventional approach is sequential processing to meet each specification. The performance of the conventional processing scheme shown in Figure 1 is used as a point of reference for this study. In this process a raw natural gas is cooled and separated into sour gas and sour liquid condensate streams at available cooling temperature and pipeline pressure. The sour liquid condensate is fed to a depropanizer, producing sweet C4+ (butane and heavier) product and a sour CB-(propane and lighter) gas stream. No mercaptans are assumed present, so additional treatment of the C4+ product is unnecessary. The sour overhead product is combined with the sour gas from the “knockout drum” (gas-liquid separator) and fed to an ethanolamine absorber-stripper operation. The acid off-gas from the stripper is converted to elemental sulfur in a Claus plant, and the tail gas is sweetened to meet emission standards (Kohl and Riesenfeld, 1979) in a Beavon-Stretford plant. The sweet, water-saturated, hydrocarbon-rich natural gas from the
ethanolamine plant is fed to an absorber-stripper circulating triethylene glycol (TEG) to effect gas dehydration. Finally, propane and lighter hydrocarbons are recovered in a low-temperature separation (LTS) plant. Overall C3+ recovery for this processing scheme is typically 65-75%. The duplication of equipment in this approach suggests that an integrated processing scheme has the potential for savings in both capital and operating costs. This study does not include the sizing and costs for either the conventional processing scheme or the UCBSRP. Only the technical feasibility of applying the UCBSRP technology to natural gas conditioning is investigated. The sizing of the stream flows for a feasible process configuration and an estimation of the requirements and costs of the major process utilities have been included for their usefulness in identifying areas of potential improvement.
Design Bases The flow basis is 4986 kmol/h (100 X lo6 standard ft3/day) of a natural gas stream containing sour condensate. The composition, temperature, and pressure of the feed gas stream are shown in Table I. The gas stream is rich in propane and higher alkanes and is representative of off-shore California production. The product specifications are shown in Table I. The objective of this process synthesis is to meet these specifications while minimizing energy usage, capital equipment, and solvent losses. A further objective is to design a process that will be stable with respect to moderate variations in gas composition and flow.
Solvent and Catalyst Selection The results of gas solubility studies (Sciamanna, 1986) indicate that diglyme is the best of the poly(glyco1ethers) for the coabsorption of hydrocarbon gases. However, diglyme is not the solvent of choice because of the following considerations. Diglyme is the most volatile solvent of the glycol ethers tested in the gas solubility study. Since diglyme is miscible with water, a water-wash section at the top of an absorber is a method of minimizing the loss of solvent vapor. However, because the sales gas stream must be dry, the water-wash technique cannot be applied to the primary absorber. Thus, the efficient separation and recovery of the heavier hydrocarbons from the solvent necessitates that the solvent volatility be much less than that of the heavier hydrocarbons. With a more volatile solvent,
0888-588518812627-0500$01.50/0 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 3, 1988 501 Table 11. Selected Physical Properties Solvent: Tetraglyme molecular wt 222.28 275 at 1 atm bp, "C vapor pressure, Pa
