Separation raffinates

Extract flow in cascade V Raffinate flow in cascade L Extract product D Raffinate product B Solvent separator Raffinate solvent stripper Extract solvent stripper Solvent to cascade s ... 

Phenol is sometimes used in solvent extractions to separate raffinate from petroleum, resulting in aromatic process oil. 

The same fundamental development as presented here for vapor-liquid flash calculations can be applied to liquid-liquid equilibrium separations. In this case, the feed splits into an extract at rate E and a raffinate at rate R, which are in equilibrium with each other. The compositions of these phases are... 

UOP raffinate process UOP Sarex process UOP Sorbex process UOP Sorbex processes UOP Sorbex separation... 

MX of >99% purity can be obtained with the MGCC process with <1% MX left in the raffinate by phase separation of hydrocarbon layer from the complex-HF layer. The latter undergoes thermal decomposition, which Hberates the components of the complex. 

A schematic of the MGCC process is shown in Figure 9. The mixed Cg aromatic feed is sent to an extractor (unit A) where it is in contact with HF—BF and hexane. The MX—HF—BF complex is sent to the decomposer (unit B) or the isomerization section (unit D). In the decomposer, BF is stripped and taken overhead from a condensor—separator (unit C), whereas HF in hexane is recycled from the bottom of C. Recovered MX is sent to column E for further purification. The remaining Cg aromatic compounds and hexane are sent to raffinate column E where residual BE and HE are separated, as well as hexane for recycle. Higher boiling materials are rejected in column H, and EB and OX are recovered in columns I and J. The overhead from J is fed to unit K for PX separation. The raffinate or mother Hquor is then recycled for isomerization. 

Fig. 9. Xylenes separation via Mitsubishi Gas—Chemical Co. HF-BF extraction—isomerization process (107). A = extractor B = decomposer C = separator D = isomerization reactor E = heavy ends tower F = raffinate tower G = separator H = light ends fractionator ...

Xylene Isomerization. After separation of the preferred xylenes, ie, PX or OX, using the adsorption or crystallization processes discussed herein, the remaining raffinate stream, which tends to be rich in MX, is typically fed to a xylenes isomerization unit in order to further produce the preferred xylenes. Isomerization units are fixed-bed catalytic processes that are used to produce a close-to-equiUbrium mixture of the xylenes. To prevent the buildup of EB in the recycle loop, the catalysts are also designed to convert EB to either xylenes, benzene and lights, or benzene and diethylbenzene. 

Fig. 8. UOP Sorbex simulated moving bed for adsorptive separation. AC = adsorbent chamber, RV = rotary valve, EC = extract column, RC = raffinate...

Relatively new methods for separating helium from natural gas use pressure swing adsorption (PSA) processes to recover helium at better than 99.99% purity. This type of process is probably less costiy for the production of gaseous helium but might be uneconomical for liquefied helium production. The PSA process is widely used to produce specification pure helium from 85+% cmde helium in conjunction with cryogenic enrichment of the ca 50% helium raffinate. 

Xylene Isomeri tion. The objective of C-8-aromatics processing is the conversion of the usual four-component feedstream (ethylbenzene and the three xylenes) into an isomerically pure xylene. Although the bulk of current demand is for xylene isomer for polyester fiber manufacture, significant markets for the other isomers exist. The primary problem is separation of the 8—40% ethylbenzene that is present in the usual feedstocks, a task that is compHcated by the closeness of the boiling points of ethylbenzene and -xylene. In addition, the equiUbrium concentrations of the xylenes present in the isomer separation train raffinate have to be reestabUshed to maximize the yield of the desired isomer. 

The polyamide copolymer of dodecanoic acid with methylenedi(cyclohexylamine) (MDCHA, PACM) was sold as continuous filament yam fiber under the tradename QIANA. As late as 1981, over 145,000 t was produced using high percentages, typically 80%, of trans, trans MDCHA isomer. The low melting raffinate coproduct left after t,t isomer separation by fractional crystallisation was phosgenated to produce a Hquid aUphatic diisocyanate marketed by Du Pont as Hylene W. Upon terrnination of their QIANA commitment, Du Pont sold the urethane intermediate product rights to Mobay, who now markets the 20% trans, trans—50% cis, trans—30% cis, cis diisocyanate isomer mixture as Desmodur W. In addition to its use in polyamides and as an isocyanate precursor, methylenedi (cyclohexyl amine) is used directiy as an epoxy curative. The Hquid diamine mixture identified historically as PACM-20 is marketed as AMICURE PACM by Anchor Chemical for performance epoxies. 

After mixing, the solvent and waste are separated. The solvent with dissolved organics is called the extract. The waste remaining after extraction is called the raffinate. The extract may be sent to a distillation or steam stripping unit to separate the dissolved organics from the solvent and the solvent can be recycled back to the extraction process. The raffinate may require additional treatment or may be disposed or incinerated. 

The feed to a liquid-liquid extraction process is the solution that contains the components to be separated. The major liquid component in the feed can be referred to as the feed solvent. Minor components in solution are often referred to as solutes. The extraction solvent, or just plain solvent, is the immiscible liquid added to a process for the purpose of extracting a solute or solutes from the feed. The extraction-solvent phase leaving a liquid-liquid contactor is called the extract. The raffinate is the liquid phase left from the feed after being contacted by the second phase. A wash solvent is a hquid added to a liquid-liquid fractionation process to wash or enrich the purity of a solute in the extract phase. 

A theoretical or equihbrium stage is a device or combination of devices that accomplishes the effect of intimately mixing two immiscible liquids until equilibrium concentrations are reached, then physically separating the two phases into clear layers. Crosscurrent extraction (Fig. 15-4) is a cascade, or series of stages, in which the raffinate R from one extraction stage is contacted with additional fresh solvent S in a subsequent stage. 

Solvent solubility. A low solubility of extrac tion solvent in the raffinate generally leads to a high relative volatihty in a raffinate stripper or a low solvent loss if the raffinate is not desolventized. A low solubility of feed solvent in the extract leads to a high relative separation and, generally, to low solute-recovery costs. 

The ratio of wash solvent to extraction solvent is the same in the enriching section as in the stripping section if no solvent is added in the feed. The degree of separation to be achieved can be chosen for the process design, such as 99 percent of component b into the extrac-t stream and 99 percent of component c into the raffinate stream. Then the feed rate can be chosen so that the solute loadings in the extrac-t stream and... 

Lube oil extraction plants often use phenol as solvent. Phenol is used because of its solvent power with a wide range of feed stocks and its ease of recovery. Phenol preferentially dissolves aromatic-type hydrocarbons from the feed stock and improves its oxidation stability and to some extent its color. Phenol extraction can be used over the entire viscosity range of lube distillates and deasphalted oils. The phenol solvent extraction separation is primarily by molecular type or composition. In order to accomplish a separation by solvent extraction, it is necessary that two liquid phases be present. In phenol solvent extraction of lubricating oils these two phases are an oil-rich phase and a phenol-rich phase. Tne oil-rich phase or raffinate solution consists of the "treated" oil from which undesirable naphthenic and aromatic components have been removed plus some dissolved phenol. The phenol-rich phase or extract solution consists mainly of the bulk of the phenol plus the undesirable components removed from the oil feed. The oil materials remaining... 

Nevertheless, near the feed point, there is a difference between TMB and all SMB cases due to the fact that the internal flow rates in the TMB are smaller than in the SMB, leading to a small dilution of the feed stream. As a consequence, near the feed inlet, TMB concentrations will be higher than in the SMB operation. The raffinate and extract purities in SMB units with four (95.2 % and 89.5 %), eight (98.7 % and 95.9%) and 12 columns (99.1 % and 96.8%) are increasing towards the one obtained in the equivalent TMB unit (99.3 % and 97.7 %). The optimum degree of subdivision of the SMB unit will depend of the difficulty of the separation and the product purity requirements. Typically, systems for the pharmaceutical industry have six to 16 columns. 

The design problem of a TMB consists on setting the flow rates in each section to obtain the desired separation. Some constraints have to be met to recover the less-adsorbed component A in the raffinate and the more retained component B in the extract. These constraints are expressed in terms of the net fluxes of components in each section (see Fig. 9-1). In section I, both species must move upwards, in sections II and III the light species must move upwards, while the net flux of the more retained component must be downwards, and in section IV the net flux of both species have to be downwards, i.e.. 

Figure 9-13 shows the Ynryn P ° obtained for the first case where four regions are defined a region of complete separation, two regions where only one outlet stream is 100 % pure and a last region where neither of them is 100 % pure. The closed circles are numerical results based on the equivalence between the TMB and the SMB the thick lines connect those results. The thin line in Fig. 9-13 has two branches. The diagonal 7 -7 corresponds to zero feed flow rate therefore, 7 must be higher than Yn- The horizontal branch Ym corresponds to zero raffinate flow rate in this case, the extract flow rate is 25.09 mL min k... 

The case with k = 0.4 s (open squares) is close to the situation where mass transfer resistance is negligible. These differences are due to mass transfer resistances as we can easily conclude by comparing the separation regions obtained for the cases with k = 0.4 and k = 1 s k If mass transfer resistance is important, the region of complete separation can be significantly reduced from the one obtained by the Equilibrium Theory. For example, for a mass transfer coefficient of k = 0.1 s there is no separation region where extract and raffinate are 99.5 % pure. 

The affinity of A and B for the solid phase are different, B is more retained than A. Thus, it is possible to choose flowrates in order to make A move upward and B to move downward, leading to a spatial separation. This system requires inlet lines for the feed and eluent and outlet lines for the raffinate A and extract B. 

In order to illustrate the critical process parameters of SMB process validation, we will consider the separation of the racemic drug as described in Process design. The study represents the effect of the influence of feed concentration, number of plates and retention factor on the second eluting enantiomer. The simulation of the process for different values of feed concentration is performed and the variations of the extract and raffinate purities are shown in Fig. 10.10. 

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