Solvent Recovery Solvent recovery is a form of waste reduction. In–process solvent recovery still is widely used as an alternative to solvent replacement to reduce waste generation. It is attractive, like end–of–pipe pollution control, since it requires little change in existing processes. There is widespread commercial availability of solvent recovery equipment which is another attraction. Availability of equipment suitable for small operations, especially batch operations, make in–process recovery of solvents economically preferable to raw materials substitution. Commercially available solvent recovery equipment include: Carbon adsorption of solvent, removal of the solvent by steam, and separation of the solvent for reuse in the operation. Carbon must be regenerated, two or more units are required to keep the operations continuous. Chloric acid formation from chlorinated solvents, carbon bed plugging by particulates, and buildup of certain volatile organics on the carbon and corrosion can be a problem. Distillation and condensation can be used to separate and recover solvent from other liquids. Removal efficiency can be very high using this process and can be used for solvent mixtures as well as single solvents. Dissolving the solvent in another material such as scrubbing. Solvents must be then recovered from the resulting solution, through distillation but efficiency of removal is often not high using this method. Adsorption processes are useful and versatile tools when it comes to waste solvent recovery unit as they can be applied with high efficiency at relatively low cost in cases in which the desired component presents either a fairly small or a fairly high proportion of the stream. The applicable adsorbents vary according to different purposes.108,109 Adsorbents with low polarity (activated carbon, etc.) tend to adsorb nonpolar compounds, whereas ones with high polarity (e.g., silica, alumina) have higher affinity to adsorb polar substances. However, some adsorbents operate via specific binding sites (e.g., molecular sieves, molecularly imprinted polymers) rather than simple hydrophilic-hydrophobic interactions. It is worth mentioning that adsorption cannot easily be installed in a continuous configuration and is usually either a one-bed batch process or a twin-bed process with one bed in the adsorption, whereas the other one in the regeneration phase. In organic solvent recycling, the most frequent issue is the removal of water content. Even traces of water can cause unexpected solubility problems, side reactions, or the decomposition of a reactant. There are various processes to recover wet solvents such as distillation methods or fractional freezing, whereas adsorptive methods are advantageous due to their low energy consumption. Molecular sieves (with pore size 3 or 4 ?), silica, and alumina are widely used for solvent drying.110,111 The polarity of the solvent affects the efficiency of water removal, which decreases with increasing polarity of the solvent. With the proper choice of adsorption technique, residual water content between 1 and 100 ppm is usually a realistic target. In the regeneration stage of adsorption, high volumes of gas containing organic solvent are produced. Other processes in the chemical industry, such as paint drying or the drying of solid pharmaceutical intermediates or products, also generate a significant amount of solvent vapor.112 This raises another issue, as the recovery of this solvent is highly desired to minimize solvent loss and the environmental burden, as urged by the increasingly strict regulatory environment. For example, the recycling of chlorofluorocarbons has gained a lot of attention since the Montreal protocol.113–115 Incineration of solvent vapors is a widely used solution since it makes use of the solvent's latent heat. However, incineration likely needs supporting fuel to reach the required efficiency and needs continuous solvent vapor feed, not to mention that nonflammable halogenated solvent cannot be eliminated in this manner. Adsorptive systems have proved to be good alternatives. This field of adsorption is dominated by activated carbon adsorbents,116 but molecular sieve zeolites are also employed.117 Polymeric adsorbents are seldom employed in such processes, mainly because of their high price compared with activated carbon and zeolites.118 The choice of adsorbent regeneration technique has a significant effect on the quality of the recovered solvent. Examining the efficiency and applicability of various regeneration processes has been the aim of several studies.112,119 A typical system utilizing activated carbon adsorption to recover solvents from air emissions is shown in Fig. 3.15.11. Steam regeneration is employed to strip solvents from the activated carbon followed by condensation of the steam/solvent mixture through cooling. Eventually the solvent layer is separated by simple decantation. The integrated production and recovery of ABE using glucose as a substrate and gas stripping as a means of solvent recovery distillation equipment has been reported by Groot et al. , Mollah and Stuckey , Park et al. , and Ezeji et al. . Groot et al. produced butanol in a free cell (not immobilized) continuous reactor and removed the product in a separate stripper . As a result of simultaneous product recovery, glucose utilization was improved by threefold, but the selectivity of butanol removal was low at 4 as compared to 19, which is the selectivity at equilibrium, suggesting that the stripper was not efficient. Also solvent productivity in the integrated system was 0.18 g/L h, as compared to 0.17 g/L h in the nonintegrated batch system . Mollah and Stuckey used immobilized cells of C. acetobutylicum to improve productivity and recover butanol by gas stripping . The cells were immobilized in calcium alginate gel and used in a fluidized bed bioreactor. This integrated system achieved a productivity of 0.58 g/L h, which is considered low for an immobilized cell continuous reactor. Ezeji et al. tested the use of a hyper-butanol-producing strain, Clostridium beijerinckii BA101, in an integrated system with butanol produced in a free cell fed-batch reactor coupled with in situ product recovery . As a result of simultaneous product recovery, the rates of fermentation (productivity) and glucose utilization improved. To compensate for the utilized glucose, a concentrated sugar solution (500 g/L) was intermittently fed into the reactor to maintain a solventogenic substrate concentration. This reactor was operated for 207 h before the culture stopped fermentation due to the accumulation of unknown inhibitory products. In this system 500 g/L glucose was used to produce 232.8 g/L ABE. ABE productivity was also improved from 0.29 g/L h in a nonintegrated batch system to 1.16 g/L h in the integrated system, a 400% increase. Given that the fed-batch fermentation stopped due to the accumulation of unknown inhibitory products, the authors devised another system in which a semicontinuous bleed was withdrawn from the reactor to eliminate or reduce the accumulation of unknown toxic by-products. As a result, the continuous reactor was operated for 21 days (504 h) before it was intentionally stopped . Results from this continuous reactor suggest that ABE fermentation can be operated indefinitely in continuous mode, provided that toxic butanol is removed by gas stripping and unknown toxic products are removed by a bleed. In a 1-L culture volume, the system produced 461.3 g ABE from 1125.0 g glucose, with an ABE productivity of 0.92 g/L h, compared to 0.28 g/L h productivity in the nonintegrated batch system. Adsorption is a physical process in which organic species are transferred onto the surface of a solid adsorbent. Adsorption is a particularly attractive control method as it can handle large volumes of gases of low pollutant concentrations. It is capable of removing contaminants down to very low levels.1 Removal efficiency is typically greater than 95%. The most frequently used adsorbent in the organic compound applications is activated carbon, although zeolites and resins are also used. Adsorption is the most widely used solvent-recovery technique and is also used for odor control. The latter application is necessary to meet statutory air pollution control requirements. Depending on the application, adsorption can be used alone or with other techniques such as incineration.14 Solvent recovery with adsorption is most feasible when the reusable solvent is valuable and is readily separated from the regeneration agent. When steam-regenerated activated-carbon adsorption is employed, the solvent should be immiscible with water. If more than one compound is to be recycled, the compounds should be easily separated or reused as a mixture.9 Only very large solvent users can afford the cost of solvent purification by distillation.’ The advantages include the availability of long-term operating data. In addition, adsorbers can handle varying flow rates or varying concentrations of organic compounds. The main disadvantage of adsorption is the formation of a secondary waste, such as the spent adsorbent, unusable recovered organic compounds, and organics in the waste water if steam is used for regeneration. Secondary waste may require off-site treatment or specialist disposal.12 (see Table 13.12) In addition to air, moisture and photochemical stability, the thermal stability is an important aspect of improving the economy of the process. The occurrence of thermally induced polymerization or decomposition reactions results in a loss of solvent recovery potential, specialized facilities for the treatment and post-purification of solvents and product streams and poor flexibility in the optimization of the thermal profile of the process (solvent extraction and extractive distillation steps). N-Methyl pyrrolidone has been shown to be chemically and thermally stable in the Arosolvan process. Sulpholane is reported to be stable to 493 K and undergoes some decomposition at 558 K . In the sulpholane process, the influence of oxygen on solvent stability in the form of minor oxidative degradation has been observed under normal operating conditions. Consequently, the exclusion of air in the feed to the extraction unit has been advocated for this process together with the inclusion of a solvent regenerator unit. The latter operates by removing oxidized solvent from a small side-stream of the circulating solvent that is directed towards the solvent regenerator unit . Ionic liquids exhibit excellent thermal stability and lack of sensitivity to oxygen would be advantageous with respect to the processing and recovery of the solvent. Pfizer has redesigned the synthesis of several of its pharmaceutical products to reduce generation of hazardous waste. Changes were made in the synthetic route to sildenafil citrate (see Fig. 9.7), the active ingredient in Viagra? (Dunn et al., 2004), which resulted in a more efficient process that required no extraction and recovery system for solvent steps (see Fig. 9.8). The E-factor (Sheldon, 1992) for the process is 6 kg waste/kg product, which is substantially lower than an E-factor of 25–100, which is typical of pharmaceutical processes. Furthermore, all chlorinated solvents had been eliminated from the commercial process. During the medicinal chemistry stage in 1990, the solvent usage was 1816 L/kg, and the optimized process used 139 L/kg solvent, which was reduced to 31 L/kg during commercial production in 1997 and to 10 L/kg with solvent recoveries. Pfizer plans to replace t-butanol/t-butoxide cyclization with an ethanol/ethoxide cyclization. Combined with other proposed improvements, this is expected to increase the overall yield from 76–80% and further reduce solvent usage and organic waste. A first point of economic comparison is the variable cost requirements of each process. Here, variable costs are defined as the sum of all raw materials costs plus the utilities cost for conversion of raw materials to product. All labor, overheads and depreciation costs are not included. On a variable cost basis, both the diacetate and diphenate routes show a distinct advantage over the acid chloride route. The largest component of the cost differential results from the high cost of the acid chloride monomers relative to the free acids. The second largest component arises because the acid chloride process inherently uses greater solvent volumes than the other two routes. Solvent losses which invariably occur contribute to increased variable cost as the solvent recovery processes are not completely efficient. Variable cost differences between the diacetate and diphenate processes are not very large. Both processes can be thought of as variations to reacting free diphenol with the free diacids. In the diacetate variation, acetic anhydride is consumed in forming the diacetate, but some of this cost is recouped by selling acetic acid — the process by-product. In the diphenate route, phenol is first consumed in monomer preparation, then recovered during the polymerization. The variable cost of the diacetate route may be slightly higher than that of the diphenate route due to the conversion of anhydride to acetic acid, but this disadvantage can be mitigated depending on the phenol recovery/recycle efficiencies in the diphenate process. Secondly, the capital investment requirement required to construct facilities to practice each of the three process technologies can be compared. The acid chloride process is a low temperature, atmospheric pressure process and process fluid viscosities are low. Thus, standard design reaction equipment with low cost supporting utilities are used in the reaction area. However, polymer recovery would generally be accomplished by precipitation, washing and drying followed by extruder pelletization — operations which are capital intensive. Also, extensive used solvent recycler for sale is required in the acid chloride process, again leading to increased capital cost. Both the melt or solution diacetate and diphenate processes on the other hand are high temperature, high vacuum processes where process fluid viscosities reach very high values. For these processes, polymer reactors will require some special design features particularly with respect to agitation and heat transfer. Supporting utilities will be rather capital intensive. To balance these costs, however, product recovery is expected to be relatively simple, requiring only one or two melt processing operations most likely using a thin film polymer processor followed by an extruder. Solvent recovery requirements would be modest for the diacetate process but somewhat more costly for the diphenate process where large quantities of phenol (especially from monomer production) will require purification prior to recycle. Some difference in capital investment required for monomer production in the diacetate and diphenate processes is also expected. Diphenyl ester production is less attractive due to the more extreme reaction conditions required and the large phenol recycle streams. However, even with the noted differences, it is estimated that any of the three described processes could be built for approximately the same dollar amount per annual pound of polymer capacity at the 15 Mlb year?1 scale (1 kg = 2.2 lb).
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