Solvent recovery unit

The dewaxed oil and wax phases discharged from the centrifuges are transferred through heat exchange equipment to individual solvent recovery units for continuous recovery of solvent. 

Furthermore, although most of the CO2 is recycled from the gas-liquid separators, a substanhal amount of dissolved CO2 remains in the co-solvent recovered, which flashes away and is lost during the final product recovery. Therefore, CO2 losses in large-scale SFC systems often lead to higher production costs than those in liquid chromatography systems equipped with appropriate solvent recovery units, as discussed in the next section. 

Extraction efficiency is not the only factor to be examined in the choice of solvent or reagent for a particular application. Environmental, as well as economic considerations must be taken into account. Solvents such as benzene and chloroform (which have solubilities of 0.07 and 0.82 parts per 100 parts of water) might be preferred for extractive efficiency, but their use would result in large losses to the aqueous phase. Not only would this be expensive, but it would be undesirable for reasons of health the toxic organo lead salts would be removed, but an equally toxic organic solvent would be added to the effluent. Addition of a solvent recovery unit subsequent to the extraction step might render the technique uneconomic (relative to alternative effluent treatment techniques). 

A gas stream containing acetone in air flows from a solvent recovery unit at a rate of 142 L Is at 150 C and 1.3 atm. The stream flows into a condenser which liquefies most of the acetone, and the liquid and gas outlet streams are in equilibrium at — 18 C and 5.0 atm. Shaft work is delivered to the system at a rate of 25.2 kW to achieve the compression from 1.3 atm to 5.0 atm. To determine the condenser feed stream composition, a 3.00-liter sample of the gas is taken and cooled to a temperature at which essentially all the acetone in the sample is recovered as a liquid. The liquid is poured into an empty flask with a mass of 4.017 g. The flask containing the liquid acetone is weighed and found to have a mass of 4.973 g. 

Finally, if the solvent recovery unit is operating at 67% efficiency, then it operates as though it has only four theoretical stages. This means that the fraction of phenol recovery from the toluene reduces to 0.1 (verify this for yourself using Fig. 3). 

Quantify the impact of the emission factor in the degreasing operation on the flowrates within the solvent recovery unit. 

A PCE mass balance around the solvent recovery unit is shown in Figure 105. 

Figure 105. PCE mass balance around solvent recovery unit.

If the emission factor were lower, the flowrates to the solvent recovery unit and the recycle stream would be higher. Additionally, there would be less PCE lost from the system. To determine the effect of the emissions factor on the system flow streams, the equations above were solved using three different emission factors 0.78, 0.60, and 0.40. These results are summarized below. 

Process 2 - Process Description. The impurities in the raw material form azeotropes with tetrahydrofuran and ethylacetate. All the azeotropes had to be separated by a combination of counter current extraction and rectification. The aim was to recover ethylacetate and THF. The following major problems had to be solved by a solvent recovery unit 1) separate the THF/ methanol and the THF/ ethanol azeotropes, 2) dewater the THF and ethylacetate (azeotropes), 3) separate THF (Atmospheric boiling point (Tb) = 65.7°C) from ethylacetate (Tb= 77°C) and methylacetate (Tb = 57.1°C). 

The extraction cascade in Pig. 11.6 has both extract and raffinate reflux. Raffinate reflux is not processed through the solvent recovery unit since additional solvent would have to be added in any case. It is necessary, however, to remove solvent from extract reflux. 

A reliable supply of cooling medium is the most important utility for the safe operation of a distillation unit. For a very small unit, where utility cost may be negligible, mains water provides an almost totally reliable means of cooling but the cost, both in supply and disposal, is large imless the water can be used for another purpose after passing through the condenser. One possible use on the solvent recovery unit itself is for the dilution of any effluent that would be otherwise unacceptable for disposal to the sewer (e.g. because of a low flash point). 

Even when a solvent recovery unit is being designed for a stream that is beheved to be fully specified in quantity and quafity for both feed and product, it is wise to build in spare condenser capacity. It has been shown that additional evaporation can easily be obtained with direct steam heating and extra column capacity can often be obtained with minor investment. However, additional condensation is often... 

A vessel designed to withstand full vacuum is likely to withstand the pressure at which one would wish to operate a solvent recovery unit, although this should not be taken for granted. The vessel must be designed for the relief valve pressure. 

A solution to this problem is to add to the distillation residue, either at the point of discharge from the solvent recovery unit or by adding to the feed so that it will remain in the residue at the end of recovery, a flux which keeps the residue in a form which allows it to be pumped easily. A commercial solvent recovery firm is likely to have solvents or mixtures of solvents that are of very low value or even unsaleable which can serve this purpose, since they may be used to allow more valuable solvents to be released from a crude mixture. 

The pressure drop through the plant. Reduced to the minimum of an evaporating surface and a condensing surface placed as close to each other as practicable, a solvent recovery unit can have a very low pressure drop. If any fractionation is needed (Fig. 5.10), pressure drop is inevitably introduced in pipework and column packing. 

Access to an adequately equipped and staffed laboratory is essential for the safe operation of a solvent recovery unit. Its function can be divided into various areas. 

After work which involves breaking joints on a solvent recovery unit, the plant should be pressure tested before being returned to service. This test need not be done to a pressure over that specified for the bursting disc since it is to detect gross leaks which cannot be corrected by pulling up joints while the plant is operating. 

Adsorptive solvent recovery units have at least two, but usually three or four parallel-eonneeted fix-bed adsorbers whieh pass sueeessively flirough die four stages of the operation eyele. " ... 

Adsorbers for solvent recovery " units (Figure 22.1.8) are build in several configurations ... 

Table 22.2.7. Performance data for the ACF solvent recovery unit...

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