Ind. Eng. Chem. Res. 1991,30, 177-185
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SEPARATIONS Adsorptive Drying of Hydrocarbon Liquids Sudhir Joshit and James R. Fair* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712
On the basis of experimental work coupled with mathematical modeling, a parametric analysis was made of the adsorptive drying of hydrocarbon liquids. The experimental work involved toluene and xylene dehydration over several desiccants. Equilibrium relationships and intraparticle diffusion coefficients were determined experimentally. The parameters required by the model are equilibrium constants, liquid film mass-transfer coefficients, intraparticle diffusion coefficients, and axial diffusivities. The model that was fitted to the experimental data was compared also with breakthrough data from the general literature; the model fit was quite good. I t was found that in general the percentage bed utilization was substantially higher for the smaller particle sizes. Compound beds of aluminas and molecular sieves were simulated to take advantage of the higher water holding capacities of aluminas near saturation. Background information on distillative drying is included in the paper, to enable comparisons between the distillative and adsorptive methods. In the hydrocarbon processing industries, it is often necessary to dry liquids before they can be processed further. The effects of moisture can be decreased catalyst activity, lowered reaction yields, accelerated corrosion, or plugging of lines by ice or hydrates. In cases where the hydrocarbon has gone through a water-oil phase separation, water may be present in saturation amounts or even greater if entrainment occurs in the separator. In some cases, the liquid to be dried exists as an emulsion. The problem facing the technologist is to determine the optimal method of water removal. Of the drying methods available, distillation and adsorption are the ones most used commercially. Other methods include inert gas purging, liquid extraction, freeze drying, and membrane permeation. The purposes of this paper are to present new data on hydrocarbon liquid drying and to provide the technologist with information that can support the design or analysis of an adsorptive drying system. Some information, for comparative purposes, will be provided for distillative drying. Only background material for the other drying methods will be included. Background Distillative Drying. Distillative drying has been used commercially for many years. The wet hydrocarbon is fed to a conventional tray or packed column, and the separation is made. For hydrocarbons forming azeotropes with water, the constant boiling mixture can serve as one of the terminal products. An example flow diagram for the azeotropic drying of toluene, taken from the paper by Brown et al. (1970), is shown in Figure 1. Clearly, the effective volatilities of all components of the feed mixture must be taken into account. While operation of a distillative drying system is normally straightforward, on the basis of mass transfer, the results of tests show that it is a relatively inefficient process. Observed tray efficiencies are low-in the range of Present address: Rohm & Haas Research Laboratories,Spring House, PA 19477.
4-18% (Gester, 1947;Aerov et al., 1966; Brown et al., 1970). The use of a simple phase separator, as shown in Figure 1,permits entrained water (sometimes as microdroplets) to be returned to the column, eventually emerging as a contaminant in the “dry” stream. Suggestions for minimizing this entrainment have been made by Clay (1961) and by Brown et al. (1970). In practice, lower limits of water in the dry product are evidently in the 10-20 ppm range. It appears to be uneconomical to attempt further drying by distillation. Advantges of distillative drying are that it is a continuous process (as opposed to cyclic adsorption), it involves a lower peak heat requirement than the regeneration step in adsorption, it can provide an opportunity for simultaneous removal of volatile contaminants in the feed mixture, and it is an established practice, with design and troubleshooting techniques well understood (Mundale et al., 1979). A key disadvantage is that it cannot produce “superdry” products. Adsorptive Drying. Ever since the development of activated aluminas and molecular sieves, adsorptive drying has been used quite extensively in industry for liquids. An early article by Derr and Willmore (1939) described drying of liquid ethyl and butyl acetates and pyridine with activated aluminas in a percolation column. Despite this early start, little research on liquid drying was reported in the following years, especially when compared with the large literature on adsorptive drying of gases. Practical plant experience has apparently served as a rough guide for determining the design and operation of the systems. The most comprehensive review of published work on liquid adsorptive drying has been provided by Basmadjian (1984). A variety of adsorbents have been employed for drying liquids. Sulfonic-type ion-exchange resins have been used in laboratory studies (Gregor et al., 1951a-c, 1952, 1955; Waxman et al., 1953; Wymore, 1962,1964). Dewey (1962) found these resins to be superior to molecular sieves for drying benzene and chlorinated hydrocarbons. Burfield and Smithers (1980) were able to dry toluene to
