Solvent recovery from raffinate

From the time of inception of this process, much concern has been indicated concerning the problem of solvent recovery from the product water and raffinate. This concern stems from two important considerations first, the cost of solvent losses and second, the possible toxicity of the remaining solvent. To investigate solvent recovery... 

A detailed SMBR model was used in order to optimize the unit with the objective function of minimal eluent requirement under the constraints of full conversion and complete product separation. Due to the reduced cost of these resins, the cost of this process is in fact controlled by the cost of the solvent recovery from the extract and raffinate streams. 

However, in general, solvent recovery is an important step in the overall solvent extraction process. Solvent recovery from the raffinate (i.e., water phase) may be accomplished by stripping, distillation, or adsorption. The extract, or solute-laden solvent stream, may also be processed to recover solvent via removal of the solute. The solute removal and solvent recovery step may include reverse solvent extraction, distillation, or some other process. For example, an extraction with caustic extracts phenol from light oil, which was used as the solvent in dephenolizing coke plant wastewaters (4). The caustic changes the affinity of the solute (phenol) for the solvent (light oil) in comparison to water as will be explained in the equilibrium conditions section. Distillation is more common if there are no azeotropes. 

An important economic feature of Haifa process operation is the need to maintain good solvent recovery from the brine raffinate as well as from the washed waste solids and the extracted acid. With care, it has been found possible to maintain solvent losses at not more than about 5 kg/tonne P2O5 equivalent produced [66]. 

LLE may be more complicated to operate than a direct separation system (e.g., distillation), since LLE involves more processing steps (e.g., extraction, solute removal from the extract, solvent recycle, and solvent recovery from the raffinate). The solvent cost, and the additional steps that are required to recycle the solvent, increase the cost of LLE and can make it unattractive. 

Solvent Recovery from Raffinate. The small amount of solvent left in the raffinate portion can be removed by a water wash. The latter... 

The entire series of operations can be followed conveniently on the triangular diagram, as in Fig. 6.6. When solvent Sr is removed from raffinate R, finished raffinate /J results. Material balances on the raffinate solvent recovery ... 

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. 

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). 

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. 

In general, the sulfolane extraction unit consists of four basic parts extractor, extractive stripper, extract recovery column, and water—wash tower. The hydrocarbon feed is first contacted with sulfolane in the extractor, where the aromatics and some light nonaromatics dissolve in the sulfolane. The rich solvent then passes to the extractive stripper where the light nonaromatics are stripped. The bottom stream, which consists of sulfolane and aromatic components, and which at this point is essentiaHy free of nonaromatics, enters the recovery column where the aromatics are removed. The sulfolane is returned to the extractor. The non aromatic raffinate obtained initially from the extractor is contacted with water in the wash tower to remove dissolved sulfolane, which is subsequently recovered in the extract recovery column. Benzene and toluene recoveries in the process are routinely greater than 99%, and xylene recoveries exceed 95%. 

Figure 3 also illustrates the rapid growth of lubricating oil solvent extraction processes. From the point of view of popularity, furfural, phenol, and the Duosol processes are outstanding. Most of the recent developments have been improvements in the solvent recovery systems to minimize solvent consumption, to reduce costs, and in the treater sections, to increase raffinate yield. 

The extract is pumped from the bottom of D-l to a stripper D-2 with 35 trays. The stripped solvent is cooled with water and returned to D-l. An isoprene-acetonitrile azeotrope goes overhead, condenses, and is partly returned as top tray reflux. The net overhead proceeds to an extract wash column D-3 with 20 trays where the solvent is recovered by countercurrent washing with water. The overhead from D-3 is the finished product isoprene. The bottoms is combined with the bottoms from the raffinate wash column D-4 (20 trays) and sent to the solvent recovery column D-5 with 15 trays. 

Solvent recycle from the extract is often more complex than solvent recycle from the raffinate. In the case of metal extraction, the metal solute is typically removed by stripping (washing) the extract to transfer it into a second aqueous phase for subsequent metal recovery. This aqueous phase containing the metal may then be steam stripped similarly to the raffinate from the original extraction step. In the case of organics extraction, the extract is often treated by extractive distillation to recover both the products and the solvent for recycle. 

This may be a batch or continuous operation (refer to Figure 7.7). Feed of mass F (if batch) of F mass/time (if continuous) contains substances A and C at xCF weight fraction of C. This is contacted with mass S (or mass/time) of a solvent, principally B, containing ycs weight fraction of C, to give an equilibrium extract E and raffinate R, each measured in mass or mass/time. Solvent recovery then involves separate removal of solvent B from each product stream (not shown). 

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