Solvent, recovery

Water from steam stripping in the solvent recovery section must be removed. In the furfural solvent system water is removed for process effectiveness and product quality. Water contamination in furfural reduces DWO VI and leads to furfural degradation. In NMP and phenol systems excess water is removed for process control. 

Oil in the solvent results from incomplete solvent-oil separation and may be due to entrainment from flash vessels, volatilization or stripper flooding. Characterization of the solvent contamination by GCD can be used to determine if contamination is occurring by light or heavy oil fractions. A light oil contamination suggests that the accumulation of distillate in the front end. Presence of heavy oil suggests entrainment oil in the solvent, which can reduce raffinate yield and increase the treat rate required. 

The objectives of solvent recovery sections are to Recover furfural/NMP/phenol from product streams Purify furfural/NMP/phenol for recycle Maximize energy efficiency while recovering solvent Simplified recovery sections are shown below 

Solute removal from the extracting solvent (solvent recovery) and recycle to first step. 

The process may be operated continuously. The end result of the solvent extraction process is to separate the original solution into two streams a treated stream or raffinate, and a recovered solute stream, which may contain small amounts of water and solvent. Therefore, solvent extraction is a recovery process, because the solute chemicals are generally recovered for reuse, resale, further treatment, or disposal. 

In Fig. 1 the actual extraction step occurs in the Extractor unit. In practice this unit could have one of three configurations (3)  

Regardless of its mechanical configuration, the extractor brings two liquid phases (feed and solvent) into intimate contact to allow transfer of solute from the feed to the solvent. The process yields two streams, the cleaned stream or raffinate and the extract or solute-laden solvent stream. Both streams will contain extraction solvent and may require further processing to remove and/or recover the solvent and solute. 

As Fig. 1 indicates, reuse of the solvent (following solute removal) and recovery of the solvent dissolved in the raffinate phase are usually necessary aspects of the overall 

It is highly desirable that a consideration of solvent recovery implications be made at an early stage in the selection of reaction conditions for a particular chemical transformation. In miscible systems the use of a single solvent, or, if two are necessary, solvents of widely different boiling point, is to be preferred. Use of two-phase water/solvent systems can often simplify solvent recovery. For liquid reactants it is always worth asking whether a solvent is necessary at all, as in the transformation of octyl bromide to octyl cyanide [24, 25]. This obvious question sometimes 

Superficially the simplest way to do this is to drown-out the reaction mass into water in order to recover product as a separate phase. This generates a ternary mixture of water/inorganic salt(s)/dipolar aprotic solvent from which it is not easy to recover quantitatively dry solvent. In the case of dimethylformamide a recovery option involving extraction with dichloromethane is available, but this is only viable for relatively concentrated aqueous solutions. Disposal to drain may be precluded in the case of nitrogen-containing solvents due to limits imposed on nitrogen levels in effluent to prevent eutrophication. Where possible the preferred recovery method is to filter off inorganics directly from the reaction mass and then recover dry solvent by distillation. 

Dimethylsulphoxide is a particular problem. Distillation can be hazardous. Although non-toxic, disposal via a conventional bacteriological effluent treatment plant is not recommended any anaerobic bacteria will convert the dimethylsulphoxide to dimethylsulphide, which is highly malodorous. A recent excellent textbook is available covering the subject of solvent recovery [50]. 

Solvent effects can have a profound effect on reaction rate and selectivity. 

The basic principles underlying the effects of solvent on reaction rate -solvation of reactants and transition states - are reasonably well understood and can be used in a predictive sense to aid in the selection of the optimum solvent for a particular transformation. 

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If solvent recovery is maximized by minimizing the temperature approach, the overall heat-transfer coefficient in the condenser will be reduced. This is due to the fact that a large fraction of the heat transfer area is now utilized for cooling a gas rather than condensing a Hquid. Depending on the desired temperature approach the overall heat-transfer coefficients in vent condensers usually range between 85 and 170 W/m K (ca 15 and 30 Btu/h-ft. °F). 

The extract is vacuum-distilled ia the solvent recovery column, which is operated at low bottom temperatures to minimise the formation of polymer and dimer and is designed to provide acryUc acid-free overheads for recycle as the extraction solvent. A small aqueous phase in the overheads is mixed with the raffinate from the extraction step. This aqueous material is stripped before disposal both to recover extraction solvent values and minimise waste organic disposal loads. 

It is possible to dispense with the extraction step if the oxidation section is operated at high propylene concentrations and low steam levels to give a concentrated absorber effluent. In this case, the solvent recovery column operates at total organic reflux to effect a2eotropic dehydration of the concentrated aqueous acryflc acid. This results in a reduction of aqueous waste at the cost of somewhat higher energy usage. 

The bottoms from the solvent recovery (or a2eotropic dehydration column) are fed to the foremns column where acetic acid, some acryflc acid, and final traces of water are removed overhead. The overhead mixture is sent to an acetic acid purification column where a technical grade of acetic acid suitable for ester manufacture is recovered as a by-product. The bottoms from the acetic acid recovery column are recycled to the reflux to the foremns column. The bottoms from the foremns column are fed to the product column where the glacial acryflc acid of commerce is taken overhead. Bottoms from the product column are stripped to recover acryflc acid values and the high boilers are burned. The principal losses of acryflc acid in this process are to the aqueous raffinate and to the aqueous layer from the dehydration column and to dimeri2ation of acryflc acid to 3-acryloxypropionic acid. If necessary, the product column bottoms stripper may include provision for a short-contact-time cracker to crack this dimer back to acryflc acid (60). 

The component C in the separated extract from the stage contact shown in Eigure 1 may be separated from the solvent B by distillation (qv), evaporation (qv), or other means, allowing solvent B to be reused for further extraction. Alternatively, the extract can be subjected to back-extraction (stripping) with solvent A under different conditions, eg, a different temperature again, the stripped solvent B can be reused for further extraction. Solvent recovery (qv) is an important factor in the economics of industrial extraction processes. 

Dual solvent fractional extraction (Fig. 7b) makes use of the selectivity of two solvents (A and B) with respect to consolute components C and D, as defined in equation 7. The two solvents enter the extractor at opposite ends of the cascade and the two consolute components enter at some point within the cascade. Solvent recovery is usually an important feature of dual solvent fractional extraction and provision may also be made for reflux of part of the product streams containing C or D. Simplified graphical and analytical procedures for calculation of stages for dual solvent extraction are available (5) for the cases where is constant and the two solvents A and B are not significantly miscible. In general, the accurate calculation of stages is time-consuming (28) but a computer technique has been developed (56). 

Formex pro-cess, Snam-progetti /V-formyl-morph o-line (FM) water is added to the FM to increase its se-lectivity and also to avoid high reboiler temperatures during solvent recovery by distillation 40 perforated-tray ex-tractor, FM density at 1.15 aids phase separation low corrosion allows use of carbon steel equipment... 

DifficultSepa.ra.tions, Difficult separations, characterized by separation factors in the range 0.95 to 1.05, are frequentiy expensive because these involve high operating costs. Such processes can be made economically feasible by reducing the solvent recovery load (260) this approach is effective, for example, in the separation of m- and -cresol, Hnoleic and abietic components of tall oil (qv), and the production of heavy water (see Deuteriumand TRITIUM, deuterium). 

The pot extractor is a batch extraction plant in which extraction and solvent recovery from the exhausted soHds can be carried out in a single vessel. These extractors are normally agitated vessels having capacities in the range of 2 to 10 m, beyond which the battery system becomes a preferred technical alternative. 

Benzene, toluene, and a mixed xylene stream are subsequently recovered by extractive distillation using a solvent. Recovery ofA-xylene from a mixed xylene stream requires a further process step of either crystallization and filtration or adsorption on molecular sieves. o-Xylene can be recovered from the raffinate by fractionation. In A" xylene production it is common to isomerize the / -xylene in order to maximize the production of A xylene and o-xylene. Additional benzene is commonly produced by the hydrodealkylation of toluene to benzene to balance supply and demand. Less common is the hydrodealkylation of xylenes to produce benzene and the disproportionation of toluene to produce xylenes and benzene. 

Water is continuously added to the last extraction bath and flows countercurrenfly to filament travel from bath to bath. Maximum solvent concentration of 15—30% is reached in the coagulation bath and maintained constant by continuously removing the solvent—water mixture for solvent recovery. Spinning solvent is generally recovered by a two-stage process in which the excess water is initially removed by distillation followed by transfer of cmde solvent to a second column where it is distilled and transferred for reuse in polymer manufacture. 

The Courtaulds semicommercial production system is iUustrated in Figure 8. Dissolving-grade woodpulp is mixed into a paste with NMMO and passes through a high temperature dissolving unit to yield a clear viscous solution. This is filtered and spun into dilute NMMO whereupon the ceUulose fibers precipitate. These are washed and dried, and finally baled as staple or tow products as required by the market. The spin bath and wash Uquors are passed to solvent recovery systems which concentrate the NMMO to the level required for reuse in dissolution. 

Heat/Solvent Recovery. The primary appHcation of heat pipes in the chemical industry is for combustion air preheat on various types of process furnaces which simultaneously increases furnace efficiency and throughput and conserves fuel. Advantages include modular design, isothermal tube temperature eliminating cold corner corrosion, high thermal effectiveness, high reHabiHty and options for removable tubes, alternative materials and arrangements, and replacement or add-on sections for increased performance (see Furnaces, fuel-FIREd). 

Economic Aspects. The 1992 MEK nameplate capacity for the United States, East Asia, and Western Europe is Hsted in Table 5. During the period 1980—1989 MEK achieved a negative growth rate as demand dropped from 311,000 (48) to 228, 000 t/yr (49). Stricter VOC regulations were largely responsible for the decline, and the trend will continue as solvent recovery and recycling, as well as substitution away from MEK, take effect. 

In typical processes, the gaseous effluent from the second-stage oxidation is cooled and fed to an absorber to isolate the MAA as a 20—40% aqueous solution. The MAA may then be concentrated by extraction into a suitable organic solvent such as butyl acetate, toluene, or dibutyl ketone. Azeotropic dehydration and solvent recovery, followed by fractional distillation, is used to obtain the pure product. Water, solvent, and low boiling by-products are removed in a first-stage column. The column bottoms are then fed to a second column where MAA is taken overhead. Esterification to MMA or other esters is readily achieved using acid catalysis. 

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