Physical Solvent Processes

These processes are based on the solubility of the H2S and/or GO2 within the solvent, instead of on chemical reactions between the acid gas and the solvent. Solubility depends first and foremost on partial pressure and. secondarily on temperature. Higher acid-gas partial pressures and lower temperatures increase the solubility of H2S and CO2 in the solvent and thus decrease the acid-gas components. 

Physical solvent processes have a high affinity for heavy hydrocarbons. If the namral gas stream is rich in C3+ hydrocarbons, then the use of a physical solvent process may result in a significant loss of the heavier molecular weight hydrocarbons. These hydrocarbons are lost because they a c released from the solvent with the acid gases and cannot be economically recovered. 

The partial pressure of the acid gases in the feed is 50 psi or higher. 

The concentration of heavy hydrocarbons in the feed is low. That is, the gas stream is lean in propane-plus. 

Most physical solvent processes are proprietary and are licensed by the company that developed the process. 

Selexol , licensed by UOP, uses the dimethyl ether of polyethylene glycol. A Selexol plant can be designed to provide some selectivity for H2S. For example, the plant can be designed to provide pipeline quality gas (0.25 grains H2S/IOO scf) while slipping 85% of the CO2.  

The Fluor solvent, propylene carbonate, is used primarily for removal of CO2 from high pressure gas streams. The author is familiar with a plant using propylene carbonate with 15-20% CO2 in the feed at about 800 psi. The CO2 off gas stream was used for enhanced oil recovery. Propylene carbonate loses economic incentive below about 12% acid gas in the feed.  

Rectisol is licensed by the two companies that developed it, Lurgi and Linde. It uses cold methanol as a solvent and can remove total sulfur down to below 100 ppb. It will also remove other impurities present in synthesis gas such as cyanides and carbonyls. Although not cheap, the gas purity achievable with this process makes it a prime candidate for applications processing syngas to chemicals such as ammonia or methanol. 

GPSA Engineering Data Book, Gas Processors Suppliers Association, Vol. II, 10 Ed., 1990. 

Maddox, R. N., Gas and Liquid Sweetening, Campbell Petroleum Series, 1977. 

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A physical solvent process is shown in Figure 7-6. The sour gas contacts the solvent using counter-current flow in the absorber. Rich solvent from the absorber bottom is flashed in stages to a pressure near atmos... 

LOCAT units can be used for tail-gas clean-up from chemical or physical solvent processes. They can also be used directly as a gas sweetening unit by separating the absorber/oxidizer into two vessels. The regenerated solution is pumped to a high-pres.sure absorber to contact the gas. A light slurry of rich solution comes off the bottom of the absorber and flows to an atmospheric oxidizer tank where it is regenerated. A dense slurry is pumped off the base of the oxidizer to the melter and sulfur separator. 

Since pyrolizing gasifiers yield olefins and aromatics in the raw gas, these compounds tend also to occur in the acid gas stream. Physical solvent processes for acid gas extraction, such as cold methanol wash, especially tend to take hydrocarbons into the acid gas stream water solutions have less tendency to do so. 

In the physical absorption process, the CO2 is absorbed in a solvent according to Henry s Law and then regenerated using heat, pressure reduction or both heat and pressure reduction. Typical solvents are Selexol (dimethylether of polyethylene glycol) and Rectisol (cold methanol) which are applied at high pressure. At lower pressures, the chemical absorption processes are more economical. The Selexol physical solvent process is frequently specified for coal gasification applications199. 

The Sulfinol process [719]-[722] cannot strictly be classified as a physical solvent process, as the solvent is a mixture of sulfolane (tetrahydrothiophene 1,1-dioxide), DIPA (diisopropanolamine), and water in a ratio of about 45 40 10. Sulfolane is a true physical solvent, whereas the DIPA is a chemically acting component. To achieve a higher C02/H2S selectivity, DIPA may be substituted by MDEA (methyldiethanola-mine) in the M-Sulfinol process. In partial oxidation process a concept with two separate Sulfinol units is preferred the first ahead of the shift conversion (gas composition for example C02 5.4 vol%, H2S 0.5 vol%), and the second one for recovery of sulfur-free C02affer the shift conversion (feed gas for example C02 33vol%, H2S 0 vol %). 

Physical solvent processes give some, but not all, ofthe above qualities. The Selexol process has several industrial applications, most of them for synthesis gas deacidification and some for natural gas treatment [6-8]. A methanol-based refrigerated solvent process such as the Ifpex-2 process from the Ifpexol technology matrix ofIFP is also a good contender [9]. However, physical solvents have a high affinity for hydrocarbons and the separated acid gas stream contains large quantities of valuable hydrocarbon products. 

Hybrid solvent processes use a mixture of a physical solvent with a chemical solvent and combine some ofthe advantages of physical solvent processes with those of chemical solvent processes. The Sulfinol process has numerous industrial applications in sour gas de-acidification. Its energy requirement is relatively low, but hydrocarbon co-absorption is higher than that of an amine process. 

Shah, V.A. and Huurdeman, T.L. (1990) Syndiesis gas treating with physical solvent process using Selexol process technology. Ammonia Plant Safety, AIChE, 86. 279. 

Hybrid processes utilize a physical solvent and an amine to combine the benefits of both chemical and physical solvent processes [1,2]. Typically, hybrid processes are employed at high acid gas partial pressures [1,2], However, it is possible to implement a hybrid process for low-pressure applications when the hybrid process is competitive with aqueous amine processes. A well-known hybrid process is Sulfinol , which combines the organic solvent sulfolane with MDEA or DIPA and varying amounts of water to achieve selective removal of H S from CO [1 ]. Potential disadvantages of hybrid processes relative to aqueous amine processes include increased overall solvent costs and the loss of valuable product gas (i.e., hydrocarbons) dne to the likelihood for increased absorption in the physical solvent [1,2]. 

The authors have stated that in the IGCC process, it is desirable to treat the fuel gas streams at higher temperatures than typical of current processes so as to maximize process efficiency and minimize the impact of precombustion CO capture [3]. Commercial aqueous amine and physical solvent processes respectively require the gas stream to be cooled to 40°C and to -40°C or lower [3]. Minimization of the cooling requirement through warm treating ( 200°C) is an optimal temperature range which enables not only the removal of CO and H S but other contaminants such as sulfur, ammonia, chlorides, and heavy metals (Hg, Cd, etc.) whose emissions are... 

Huurdeman, T. L., and Vinod A. Shah. 1989. Synthesis Gas Treating with Physical Solvent Process using Selexol Process Technology, Ammonia Plant Safety AIChE Symposium. 

Early efforts to employ water as a physical solvent met with limited success (see Chapter 6), but the solubilities of CO2 and H2S in water are too low for water wash to be a practical commercial process. The earliest commercial process based on an organic physical solvent, methanol, was the Rectisol Process, which has been used for synthesis gas applications where the removal of other impurities in addition to CO2 and H2S and the production of treated gas containing only ppm levels of CO2 and H2S is required. This process operates at very low temperatures (to minus 100°F) and is quite complex compared to other physical solvent processes. As a result, the Rectisol process is not considered applicable to most gas treating services, although it continues to find application in purifying synthesis gases derived from the gasification of heavy oil and coal. 

This trend to physical solvents accelerated in 1960 with the introduction of the Fluor Solvent Process, which was followed by several other physical solvent processes. More recently, a new class of process based on the use of a mixed absorbent, containing both a physical and a chemical solvent, has been commercialized. Both simple physical solvent and mixed solvent processes are described in this chapter. 

A listing of the major physical solvent gas purification processes that have been or are currently offered for commercial use and the solvents used by each is provided in Table 14-1. Many more solvents have been proposed and evaluated in the past, and the search for superior solvents is continuing. A screening study to optimize physical solvent processes for the purification of gases at high pressure has been pre.sented by Zawacki el al. (1981). In this work a large number of physical solvents were screened and, after selection of two. solvents (the dimethyl ether of tetraethylene glycol and N-formyl morpholine), process schemes were proposed for a variety of applications. 

In their simplest form, physical solvent processes require little more than an absorber, an atmospheric flash vessel, and a recycle pump. No steam or other heat source is required. After the absorbed gases are desorbed from the solution by flashing at atmospheric pressure, the lean solution contains acid gas in an amount corresponding to equilibrium at I atm acid-gas partial pressure and this, therefore, represents the theoretical minimum partial pressure of acid gas in the purifled-gas stream. To obtain a higher degree of purification, vacuum or inert gas stripping or heating of the solvent must be employed. Other process modifications are used to minimize loss of valuable gas components, provide a relatively low temperature of operation, and otherwise improve process economics. 

Figure 14-1. Simplified flow diagrams of physical solvent processes showing three basic methods of solvent regeneration.

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