Volatile solvent recovery

Figure 83 Tray absorber with volatile-solvent recovery section.

Enclosed agitated filters are useful when volatile solvents are in use or when the solvent gives off toxic vapor or fume. Another significant advantage is that their operation does not require any manual labor. Control can be manual or automatic, usually by timers or by specific measurements of the product. Most filters are made of mild steel, with the exposed surfaces protected by lead, tile, mbber lining, or by coating or spraying with other substances as necessary. Filtration areas up to 10 m are available and the maximum cake thickness is 1 m. Apphcations are mainly in the chemical industry for the recovery of solvents. 

Solvent Recovery. Most of the activated carbon used in gas-phase applications is employed to prevent the release of volatile organic compounds into the atmosphere. Much of this use has been in response to environmental regulations, but recovery and recycling of solvents from a range of industrial processes such as printing, coating, and extmsion of fibers also provides substantial economic benefits. 

In recent years, the use of solvent-borne adhesives has been seriously restricted. Solvents are, in general, volatile, flammable and toxic. Further, solvent may react with other airborne contaminants contributing to smog formation and workplace exposure. These arguments have limited the use of solvent-bome adhesives by different national and European regulations. Although solvent recovery systems and afterburners can be effectively attached to ventilation equipment, many factories are switching to the use of water-borne rubber adhesives, hot melts or 100% solids reactive systems, often at the expense of product performance or labour efficiency. 

Solvent Recovery We now focus attention on the operation of recovery. Volatile solvents vaporized during a manufacturing process may be recovered and used again. From the mixture of air and vapor, which is generally the form in which the solvent must be sought, the latter may be condensed to a liquid and trapped by the application of cold and moderate pressure the vapor-laden air may be passed... 

The volatile solvents recoverable by the activated carbon system or any other system are nearly all organic, and many of them form flammable or explosive mixtures with air. Such mixtures may lie between upper and lower explosive limits. The activated carbon system can avoid the explosive range by staying well below the lowest percentage of vapor which is still explosive it functions well at very low concentrations. The system also recovers solvents efficiently even in the presence of water the recovery efficiency is high (98 percent and 99 percent are not unusual) it may be fully automatic. The annual maintenance charge rarely exceeds 5 percent of the cost of equipment. The recovery expense may be as low as 0.2 cent per pound in some installations it rarely exceeds 1 cent per pound. 

Polyad [Polymer adsorbent] Also written POLYAD. A family of processes for removing volatile organic compounds from air streams by continuous adsorption on an adsorbent and desorption with hot air. Several adsorbents are used, depending on the sorbate, including a macroporous polymer ( Bonopore ). Used for control of emissions and for solvent recovery. COMBI-AD is a variant for simultaneously removing several solvents, using two different adsorbents. Developed and offered by Chematur Engineering, Sweden. Twelve units had been installed, in several countries, by 1995. 

Organic solvents such as methanol, ethanol, and other alcohols are used for pulping. This process is economical for small- to medium-scale mills with significant recovery of chemicals for reuse. However, pulping must be conducted in enclosed containers to prevent the loss of volatile solvents and for workers safety. Additionally, some of these processes are more energy intensive than traditional methods. Major benefits include the elimination of odorous sulfur-containing compounds in the effluents and air. 

Recovery Constraints Distillation is a primary means of solvent recovery. In a separation process the chosen solvent must be easily distilled from the extracted solute. The relative volatility should be large to ensure easy separation. The enthalpy of vaporization should be low for reduced energy consumption. 

Easy solvent recovery (wide difference in volatility between solvent and oil)... 

Extractive Distillation Recovery of Isoprene. A typical flowsketch and material balance of distillation and solvent recovery towers for extracting isoprene from a mixture of cracked products with aqueous acetonitrile appears in Figure 13.26. A description of the flowsheet of a complete plant is given in Example 2.10. In spite of the fact that several trays for washing by reflux are provided, some volatilization of solvent still occurs so that the complete plant... 

The manuf of US solvent double-base propel-lants is similar to that described for US singlebase propints, except that the mixt of ale acet is used as the volatile solvent, and the solvent recovery procedure is omitted because of the hazard involved in handling the vapors contg... 

I) 4ir-dry treatment (batch process). Here the proplnt, transferred directly from the solvent recovery house, is placed in narrow bins of the dry house where air, heated to 43° is circulated during at least 30 days. The temp may then be raised to 550. The time required to drive off the volatile matter to the desired content varies from 30 days for 73mm cannon proplnts to 90 or more days for lfrinch cannon proplnts. For proplnts having a web thickness of more than 0.095 inch, a period of at least 2 weeks in the dry house without heating must procede any heating... 

Solvent recovery. The green powder contains a large amount of ether and alcohol which presents a twofold problem (1) the recovery of as much of the valuable volatile solvent as is economically feasible, and (2) the removal of the solvent to such an extent that the finished powder will not be disposed either to give off or to take up much volatile matter or moisture under changing atmospheric conditions. For the recovery of the solvent, the pow-... 

Once all fractions are collected, we can take them to our analytical apparatus to ensure purity, then combine odd fractions—1,3,5, and so on—for recovery for peak A, and even fractions—2,4,6—for recovery of peak B. Volatile, organic solvents can be rotary evaporated for sample and solvent recovery. 

Further considerations pertain to the recovery of the reaction products after its completion. Crystallization of the reaction product may be induced if to the reaction medium, in which it is well soluble, a co-solvent is added in which the product is insoluble. Such combinations may also be used for the recrystallization of the crude product. Since for the latter purification method the solubility should be high at high temperatures but much lower at low temperatures, the temperature coefficient of the solubility becomes an important criterion for the employment of a solvent. Little general guidance on this point can be given, in view of the temperature T appearing in the denominators of both terms of opposite signs in expressions such as (2.15). The impurities need to be retained in the solution at all temperatures. The solvent should be either well volatile, so that it is readily removed from the purified crystals, or else washable away by means of a further, volatile, solvent, in which the crystals remain insoluble. None... 

The extractive distillation profits from the capacity of an entrainer (solvent) to modify selectively the relative volatility of species. Normally, the entrainer is the highest boiler, while the component to be separated becomes heavier, being carried out in bottoms. For this reason, this operation may be regarded as an extractive absorption. Extractive distillation can be used for separating both zeo-tropic and azeotropic mixtures. The entrainer is fed near the top for a zeotropic mixture or a minimum-boiling azeotrope, or mixed with the feed for a maximumboiling azeotrope. The separation sequence normally has two columns, for extraction and solvent recovery [5]. 

Description Extractive distillation is used to separate close-boiling components using a solvent that alters the volatility between the components. An ED Sulfolane unit consists of two primary columns they are the ED column and the solvent recovery column. Aromatic feed is preheated with lean solvent and enters a central stage of the ED column (1). The lean solvent is introduced near the top of the ED column. Nonaromatics are separated from the top of this column and sent to storage. The ED column bottoms contain solvent and highly purified aromatics that are sent to the solvent recovery column (2). In this column, aromatics are separated from solvent under vacuum with steam stripping. The overhead aromatics product is sent to the BT fractionation section. Lean solvent is separated from the bottom of the column and recirculated back to the ED column. 

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