Designing solvent recovery systems

Different designed solvent recovery systems are used. As an example there is the solvent system that consists of fixed bed adsorbers containing activated carbon and a distillation system. The carbon adsorbs the solvent vapors. Then the beds are steamed in sequence to remove the solvent. The solvent and steam are condensed into a large tank. The distillation system is then used to distill the solvent from the water to a purity of 99.99% so that it can be reused. Because of the high cost of solvent, complex monitoring equipment is used to insure a high rate of recover. 

The lack of satisfactory solvent recovery methods prior to 1930 prevented the use of selective solvents more suitable for lubricating oils. The major part of any solvent extraction plant is its complex solvent recovery system. Chemical engineering s contributions to distillation theory and process design resulted in the development of efficient solvent recovery techniques. In 1933, as illustrated by Figure 3, large commercial plants were... 

Solvent recovery systems would also necessitate the specification of condenser duties, distillation tower sizes, holding tanks, piping, and valves. It is important to note that the engineering design of an adsorption system should be based on pilot data for the particular system. Information can usually be obtained directly from the adsorbent manufacturer. The overall size of the unit is determined primarily by economic considerations, balancing the operating costs against the capital costs. 

Results of this study have been used to design a solvent recovery system capable of separating each solvent into its original pure state. If separation of the THF-MEK mixture is unnecessary or if purity requirements are less demanding, the proposed system could be appropriately simplified. 

Ternary Separation By Distillation. To design a recovery system, a starting composition of 85 mole % water, 7.5 mole % tetrahydrofuran, and 7.5 mole % methyl ethyl ketone was chosen. This assumes 1.5 pounds of steam per pound of solvent are used for regeneration and a blend of equal amounts of the solvents for the polyvinyl chloride processing. 

Solvent recovery systems are often very complex systems to design, as they need to separate very different molecules. 

The maximum allowable working concentration of the solvent in air to which employees may be exposed is regulated by law. Solvents with small toxic potential and health risk have high exposure values. The toxicity and other stability and reactivity aspects are important in terms of environmental relevance, e.g. the amount of solvent that is permitted to be vented into the atmosphere. As such the working concentration of the solvent has an impact on the investment in and operational costs of the solvent recovery system. It determines whether the process needs official permission and to what extent regular inspections are necessary. If the amount of solvent to be vented is not restricted this simplifies very much the design of the whole process, as the different steps do not need to be sealed completely. 

The design of the still kettle must allow it to take full vacuum since it is possible, if the system vent is blocked or closed, for almost all the vapour within the solvent recovery system to be condensed once the heating medium is turned off. Since all air will have been displaced from the system in the early stages of a batch, a vacuum will form progressively as the vapour condenses. 

Griswold, Chu, and Winsauer [Ind, Eng, Chem, 41, 2352 (1949)] provide very complete data on the liquid-liquid and vapor-liquid equilibria in the system ethanol-ethyl acetate-water. Design a plant and solvent-recovery system for the recovery of 99.8% ethanol from 50,000 gal./day of a 5% solution in water, using ethyl acetate as extracting solvent. Solvent concentration in the raffinate phase is to be no greater than 0.001%. 

In many solvent recovery systems, adsorption represents only one step in a complex series of chemical engineering operations. The design of a eomplete system for recovering methylene chloride and methanol from air emitted from a dryer in a resin processing plant has been described by Drew (1975). The overall solvent recovery system includes a water scrubber to remove resins and cool the air to 100°-110°F a standard 2-bed carbon adsorber unit designed for 95% solvent removal efficiency a condenser and decanter to handle the vapors that are stripped from the carbon by steam an extraction column in which water is used to remove the water soluble methanol from the methylene chloride phase a stripping column to remove dissolved methanol and methylene chloride from the waste water and a drying column to remove water from the recovered methylene chloride. These items of equipment and operations are representative of those required for complete solvent recovery systems however, each system must, of course, be tailored to the profierties of the specific solvent involved. 

The removal of aromatics and relatively heavy hydrocarbons from gas streams with fixed beds of activated carbon is essentially the same process as. solvent recovery, and similar adsorbents and equipment are used. The principal differences are that in hydrocarbon recovery the feed is typically a natural gas or other combustible gas stream rather than air, and adsorption is usually (but not always) conducted at elevated pressure. The basic design approach for hydrocarbon recovery systems follows the same general logic as that described for solvent recovery systems. A brief outline of the key design steps is given in the Calgon Carbon Corporation bulletin, Heavy Hydrocarbon Removal or Recovery from Gas Streams (1987). 

Engineering Considerations To effect the good engineering design of an activated carbon adsorption system, it is first necessary to obtain information on the following the actual cubic feet per minute (ACFM) of air to be processed by the adsorber, the temperature of gas stream, the material(s) to be absorbed, the concentration of the material to be adsorbed, and if the intended application is air pollution control such as odor control - then the odor threshold of the material to be adsorbed. In addition, data is needed on the presence of other constituents in the gas stream, and whether or not solvent recovery is economical. 

Skladany, G.J., J.M. Thomas, G. Fisher and R. Ramachandran. The Design, Economics and Operation of a Biological Treatment System for Ketone Contaminated Ground and Solvent Recovery Process Waters. Presented at the 42nd Annual Purdue Industrial Waste Conference, Purdue University, West Lafayette, Indiana, 1987. 

Design of the membrane module system involves selection of the membrane material the module geometry, eg, spiral-wound or hollow-fiber product flow rate and concentration solvent recovery operating pressure and the minimum tolerable flux (9,11). The effects of these variables can be obtained from laboratory or pilot experiments using different membranes and modules. The membrane module as well as the solvent recovery can be chosen to minimize fouling. Spiral-wound modules are widely used because these offer both high surface area as well as a lower fouling potential. 

Recovery of the solvent, sometimes by chemical means but more often by distillation, is almost always required, and the recovery system ordinarily is considered an integral part of the absorption-system process design. A more efficient solvent-stripping operation normally will result in a less costly absorber because ofa smaller concentration of residual dissolved solute in the regenerated solvent however, this may increase the overall cost of solvent recovery. A more detailed discussion of these and other economic considerations is presented later in this section. 

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