Physical solvent gas treating

Selexol. licensed by the Norton Company, 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 H S/IOOscf) while slipping 85% of the COi.  

The Fluor solvent, propylene carbonate, is used primarily for removal of COi from high pressure gas 

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

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

Tennyson. R. N., and Schaap, R. P, Guidelines Can Help Choose Proper Process for Gas-Treating Plants, Oil and Gas Journal. January 10, 1977. 

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The Rectisol process, developed by Lurgi, is the most widely used physical solvent gas treating process in the world. More than 100 Rectisol units are in operation or under construction worldwide. Its most prevalent application is for deep sulfur removal from syngas that subsequently undergoes catalytic conversion to such products as ammonia, hydrogen, and Fischer-Tropsch liquids. 

Freireich, E., and Tennyson, R. N., 1977, Increased Natural Gas Recovery from Physical Solvent Gas Treating Systems, Proceedings Gas Conditioning Conference, University of Oklahoma, Norman, OK, March 7-9. 

A necessary prerequisite to understanding the subject of absorption with chemical reaction is the development of a thorough understanding of the principles involved in physical absorption, as discussed earlier in this section and in Section 5. There are a number of excellent references the subject, such as the book by Danckwerts Gas-Liquid Reactions, McGraw-Hill, New York, 1970) and Astarita et al. Gas Treating with Chemical Solvents, Wiley, New York, 1983). 

This process uses propylene carbonate as a physical solvent to remove CO2 and H2S. Propylene carbonate also removes C2+ hydrocarbons, COS, SO2, CS2, and H2O from the natural gas stream. Thus, in one step the natural gas can be sweetened and dehydrated to pipeline quality. In general, this process is used for bulk removal of CO2 and is not used to treat to less than 3% CO2, as may be required for pipeline quality gas. The system requires special design features, larger absorbers, and higher circulation rates to obtain pipeline quality and usually is not economically applicable for these outlet requirements. 

Figure 1 shows how acid-gas-bearing process gases can be generally treated in industrial processes. The sulfur compounds and CO2 may be absorbed in a liquid medium, such as amines, alkali salts (NaOH, K2CO3), physical solvents (methanol, propylene carbonate), or water (3). The absorbed acid gases are released by reduction of pressure and/or by application of heat. Alternatively, the H2S and CO2 may chemically combine with the absorbent (as in NaOH scrubbing) to form salts which are removed in a liquid treatment unit. This requires continual and expensive makeup of sodium to the system. 

Mixed solvents are combinations of physical and chemical solvents which increase the flexibility of treating (1). The chemical solvent allows for treatment of lower-pressure streams while the physical solvent allows for bulk removal of the acid gas. 

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. 

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... 

While Fig. 6.6a-c indicates the potential process conditions where ILs might find applicability as physical solvents for H S removal, these ranges represent only a fraction of the conditions that must be addressed by industrial gas treating processes [1-4]. In view of the unique physical and thermodynamic properties of ILs and the... 

The mixed solvent of Sulfinol process consists of a chemical reacting al-kanolamine, water and the physical solvent Sulfolane (tetrahydrothiophene dioxide). The actual formulation is customized for the species of application. The process flow scheme is similar to that of other amine processes. In most applications co-absorbed hydrocarbons from the absorber are flashed from the solvent and used as fuel gas after treating in a fuel gas absorber. The solvent is regenerated in the process. Over 180 units are either in operation or under construction as of April 1994 [38]. 

Solvent loss [physical losses] /[entrainment] /[solubility] /[vaporization] / [degradation] /[solvent loss elsewhere] /for glycol dehydration typical losses = 0.015 mL m gas treated. 

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

A note on partial molar properties In case you are beginning to wonder why there are so many questions and problems about concentrations I will answer by telling you that you need concentrations in about four out of every five problems in physical chemistry. The matter of fact is that a lot of chemistry and all of biochemistry takes place in solutions. Then there are problems inherent to solutions. Solutions are considered simple physical mixtures of two or more different kinds of molecules, with no chemical bonds made or broken. For a really well-behaved solution physical chemists have a name, by analogy with the gas laws an ideal solution. Yet solutions are actually complicated systems whose molecular nature we are only now beginning to understand [1, 2, 3, 4]. Two solvents, when mixed, often release heat (or absorb heat) and undergo change in volume. Think of a water sulfuric acid (caution]) mixture or a water DMSO (dimethyl sulfoxide) mixture. After the solvent mixture equilibrates you will find that its volume is not equal to the sum of the volumes of the pure solvents (it is usually smaller). In physical chemistry we treat these problems by using the concept of molar volume, V. Molar volumes are empirical numbers - they are determined by experimental measurements for different solvent compositions. Read the next problem. 

A compulw model of a spiral-wound permeator similar to the Separex design shown in Fig. 20.2-6 was developed by the Shell group to predict performance of the module. Field tests of actual modules were found to agree well with simulation results for conditions where concentration polarization and nonideal flow problems were not encountered. For commercially important flow rates, these conditions were found to be always well satisfied, and the nnodel performed well for all flow rates above 1000 SCFH to the 8 in. diameter spiral-wound modules containing about 12(X) of area. A basis of 30 x Kf SCFD of feed gas was taken in all the evaluations, and the system performances were reported on the basis of S/IO SCF of treated feed gas. Credit was taken for heavy hydrocarbons that are retained in the residual gas by membrane processes and that typically are lost along with the CO in physical solvent systems. This benefit can be substantial in some cases for these valuable components. 

Bucklin RW, Schnedel RL. Comparison of physical solvents used for gas processing. In Newman SA, editor. Acid and sour gas treating processes. Houston, TX, USA Gulf Publishing Company 1985. 

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. 

In another class of process usually referred to as mixed solvent processes, an amine is blended with a physical solvent so that the bulk removal capabilities of the physical solvent are combined with the amine s ability to achieve very low residual acid-gas specifications in a single treating step. These processes are typified by Shell s Sulfinol Process. 

The Estasoivan process was based on the use of tri-n-butyl phosphate as a solvent. Selected physical properties of this material are listed in Table 14-20. The process development work focused primarily upon the selective removal of H2S from natural gas that also contained C02- The solubility of CO2 >n the solvent is significantly lower than in most other physical solvents used for gas treating, but this, of course, is beneficial in accomplishing selective removal of H2S. 

Buckiin, R. W., and Schendel, R. L., 1985, Comparison of Physical Solvents Used for Gas Processing, in Acid and Sour Gas Treating Processes, S. A. Newman, editor. Gulf Publishing Co., Houston, TX, pp. 42-79. 

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