54 Gas PuriJcation

Table 2-4 UCARSOL HS 101 Solvent vs. DIPA in Claus Tail Gas Cleanup Unit I

UCARSOL HS 27% DIPA

Circulation Rate, gpm 76 28 CO-, Slippage, 8 84 95 H2S Content in Recycle Stream to Claus Unit, 76 66 Reboiler Steam Consumption, Mlbs/Hr 4,469 2,284 Steam Cost @ $5.50/Mlbs, $/Year 212,300 108,500 Savings with UCARSOL HS, $/Year - 103,900 Source: Dibble (1983)

35

h4DEA-based selective solvent formulation offered by Dow Chemical, USA. The papers by Katti and Langfitt also relate to the Dow GAS/SPEC selective solvents and report on the development and use of an absorber simulator to predict plant performance.

The data from many studies indicate that, with proper design, selective solvents can yield H2S concentrations as low as 4 ppmv in the treated gas while permitting a major fraction of the C02 to pass through unabsorbed. Because of its low vapor pressure, MDEA can be used in concentrations up to 60 wt% in aqueous solutions without appreciable evaporation losses. Furthermore. MDEA is highly resistant to thermal and chemical de,pdation. is essentially noncorrosive (see Chapter 3): has low specific heat and heats of reaction with H2S and CO-, and, finally, is only sparingly miscible with hydrocarbons.

Mixed Amines

MDEA is also rapidly increasing in importance as a nonselective solvent for the removal of high concentrations of acid gas, particularly CO2, because of its low energy requirements, high capacity, excellent stability, and other favorable attributes. Its principal disadvantage is a low rate of reaction with (and therefore absorption of) CO-,. The addition of primary or sec- ondary amines, such as MEA and DEA, has been found to increase the rate of COS absorp- tion significantly without diminishing MDEA’s many advantages (Polasek et al., 1990; Campbell and Weiland, 1989: Katti and Wolcott, 1987). The kinetics of COz absorption into mixtures of MDEA and DEA has been studied by Mshewa and Rochelle (1994). They mea- sured the rates of absorption and desorption of C02 in a 50 wt%, solution of MDEA over a wide range of temperatures and partial pressures. The results were used with literature values of DEA reactions to develop a model for C02 absorption in DEA and mixtures of DEA and MDEA. The model predicts that the overall gas phase coefficient for CO-, absorption in a solution containing 40% MDEA and 10% DEA is 1.7 to 3.4 times greater than that for C02 absorption in a 50% MDEA solution under typical absorption column conditions. A com- mercial process using this phenomenon was disclosed by BASF Aktiengesellschaft and described by Meissner (1983) and by Meissner and Wagner (1983). Mixed amine processes containing MDEA are now offered by several licensors.

The BASF Activated MDEA process employs a 2.5 to 4.5 M MDEA solution containing 0.1 to 0.4 M monomethylmonoethanolamine or up to 0.8 M piperazine as absorption activa-

dlkanolamines fir HJldrogen .%&de and Carbon Dioxide Removal 55

tors (Bartholome et al., 1971; Appl et al., 1980). The activators apparently increase the rate of hydration of CO, in a manner analogous to the activators used in hot potassium carbonate solutions (see Chapter 5 ) and thus increase the rate of absorption. The process can be operat- ed with one or two absorption stages, depending on the required gas purity. In one single- absorption stage version, which is suitable for bulk COz removal from high pressure gases, the rich MDEA solution is regenerated by simple flashing at reduced pressure. In a two-stage version, when essentially complete C02 removal is required, a small stream of steam- stripped MDEA solution is used in the second stage. The comparative capacities of MDEA and MEA for COI recove137 in an absorptiodflash

process are illustrated by Figure 2-8 (Meissner and Wagner, 1983). If it is assumed that equilibrium is attained in both the absorption and strimku steps and that isothamd condi- tions are maintained, the maximum net capacity is simply the difference between equitibri- urn concentrations at the absorption and smpping partial pressures. A net CO2 pickup of 30 volhol (0.297 moldmole) is indicated for a 4.5 molar MDEA solution by flashing from a C02 partial pressure of 5 bar (72.5 psia) to one bar (14.5 psia) at 70°C (158%). By compari- son, a 4.1 molar MEA solution pv ides a nef pickup of only 5 volh.01 for the same pressure change at a scmewhat lower temperature of 60°C (140°F). This promoted h!DEA process is particularly useful when CO, is present at high partial pressures, as either no steam or only a small amount of steam is required for regeneration.

Another useful feature of the MDm-based, mixed amine systems is that the formulation can be varied to meet specific site requirements. Vickery et al. (1988j describe how the

I I I I 1 I 1 2 4 6 0 10 12 14

Cq--=(BAR)

Figure 2-8. C02 solution isotherms in MEA and MDEA (Meissner and Wagner, 7983). Reprinted with permission Il’otn Oil & Gas Journal, Feb. 7,1983. Copyright Pennwell Publishing Company.