Continuous stirred-tank extraction

Figure 4.5 Enzymatic resolution synthesis of phenylalanine from phenylalanine isopropylester applying a continuous stirred-tank reactor with continuous extraction of the unconverted enantiomer...

Straightforward. We have therefore employed XAD-4 to combine biocatalytic synthesis with simultaneous product extraction. The system (Figure 15.8) comprises a continuously stirred tank reactor, a starting material feed pump, a product recovery loop with a (semi-) fluidized bed of XAD-4, and a pump to circulate the entire reaction mixture through the loop." ° Preliminary studies indicated that XAD-4 had no detrimental effects on E. coli JMlOl (pHBP461), hence, separation of biomass and reaction liquid prior to catechol extraction was not required. The biocatalytic reaction was carried out at very low concentrations of the toxic substrate and product. This was achieved by feeding the substrate at a rate lower than the potential bioconversion rate in the reactor. 

Laboratory studies of the rearrangement process began with semi-continuous operation in a single, 200-mL, glass reactor, feeding 1 as a liquid and simultaneous distillation of 2,5-DHF, crotonaldehyde and unreacted 1. Catalyst recovery was performed as needed in a separatory funnel with n-octane as the extraction solvent. Further laboratory development was performed with one or more 1000-mL continuous reactors in series and catalyst recovery used a laboratory-scale, reciprocating-plate, counter-current, continuous extractor (Karr extractor). Final scale-up was to a semiworks plant (capacity ca. 4500 kg/day) using three, stainless steel, continuous stirred tank reactors (CSTR). 

Batch-, stirred-tank-, extractive semibatch-, recirculating batch-, semicontinuous flow-, continuous packed-bed-, and continuous-membrane reactors have been used as enzyme reactors, with dense gases used as solvents. 

When, however, an extraction, or an extraction combined with a chemical reaction, is carried out between two phases in a continuous stirred tank reactor in which there is no interaction occurring between the dispersed particles (complete segregation), the dispersed particles will have different concentrations because of the spread in residence time. Any kind of interaction between the dispersed particles (e.g., by diffusion or by continuous coalescing and redispersion) then tends to eliminate these concentration differences. 

In this chapter some effects of segregation on the kinetics of a chemical reaction between two liquid phases carried out in a continuous stirred tank reactor (CSTR) will be discussed. In the derivations of these effects it will be assumed that during the reaction the dispersed phase is maintained (e.g., in the case of extraction combined with chemical reaction) and that all dispersed drops have the same size. This means that when there is segregation it is only the age distribution which causes a concentration distribution in the dispersed phase. 

General conclusions In series reactions, as the concentration of the desired intermediate P builds up, so the rate of degradation to the second product Q increases. The best course would be to remove P continuously as soon as it was formed by distillation, extraction or a similar operation. If continuous removal is not feasible, the conversion attained in the reactor should be low if a high relative yield is required. As the results for the continuous stirred-tank reactor show, backmixing of a partially reacted mixture with fresh reactants should be avoided. 

The reactor system may consist of a number of reactors which can be continuous stirred tank reactors, plug flow reactors, or any representation between the two above extremes, and they may operate isothermally, adiabatically or nonisothermally. The separation system depending on the reactor system effluent may involve only liquid separation, only vapor separation or both liquid and vapor separation schemes. The liquid separation scheme may include flash units, distillation columns or trains of distillation columns, extraction units, or crystallization units. If distillation is employed, then we may have simple sharp columns, nonsharp columns, or even single complex distillation columns and complex column sequences. Also, depending on the reactor effluent characteristics, extractive distillation, azeotropic distillation, or reactive distillation may be employed. The vapor separation scheme may involve absorption columns, adsorption units,... 

It may also be economical to remove the inhibitory product directly from the ongoing fermentation by extraction, membranes, or sorption. The use of sorption with simultaneous fermentation and separation for succinic acid has not been investigated. Separation has been used to enhance other organic acid fermentations through in situ separation or separation from a recycled side stream. Solid sorbents have been added directly to batch fermentations (18,19). Seevarantnam et al. (20) tested a sorbent in the solvent phase to enhance recovery of lactic acid from free cell batch culture. A sorption column was also used to remove lactate from a recycled side stream in a free-cell continuously stirred tank reactor (21). Continuous sorption for in situ separation in a biparticle fermentor was successful in enhancing the production of lactic acid (16,22). Recovery in this system was tested with hot water (16). 

If a calculation of the mass transfer in a continuously operating device (stirred tank, extraction column, etc.) is required, a range of information is necessary, which... 

The Eastman process has operated in Texas (USA) with three continuous, stirred-tank reactors, a wiped-film evaporator, a distillation train and a continuous, counter-current, liquid-liquid extractor for recovery of the catalysts since 1996 (1400 metric tons per year capacity, e.g., semiworks facility), but full commercial capacity has not yet been reached. The main problem is still the formation of oligomers. Another answer to the problem of IL loss to the organic phase consists in the extraction of the products from the IL using a supercritical fluid [101], but operational costs are high. 

In most of the ELM studies to date, the LMs and the wastewaters have been contacted in continuously stirred tanks. The problems with this design in liquid-liquid extractions originate from the fact that most of the volumetric energy dissipation occurs in a limited part of the total volume near the impeUer tips [74]. 

Forney et al. [85] were among the first authors to study the potential advantages of the liquid-liquid extraction using a Taylor-vortex column. In the column, the power input is evenly distributed throughout the entire volume of the contactor, and the rotor and tank stirrers are roughly equal in diameter [74]. The maximum shear is one to two orders of magnitude lower than in a continuously stirred contactor. This leads to a between 10-fold and 100-fold increase in the area inside the continuously stirred tank that is exposed to constant maximum shear. 

A simplified scheme of the RCH/RP unit is presented in Figure 2 [1, 10, 11], The reactor (1) is essentially a continuous stirred tank reactor equipped with a gas inlet, a stirrer, a heat exchanger and a catalyst recycle line. Catalyst and reactants are introduced at the bottom of the reactor. Vent gas is taken from the head of the reactor and from the phase separator. Control of the liquid volume inside the reactor is simple the liquid mixture composed of catalyst solution and aldehydes leaves via an overflow and is transferred to a phase separator (2), where it is partially degassed. The separation of the aqueous catalyst solution (density of the catalyst solution 1100 g/L) and the aldehydes occurs rapidly and completely, favored by the difference in densities (density of aldehyde layer 600 g/L due to dissolved gases). The catalyst solution passes a heat exchanger and produces process steam that is consumed in downstream operations. Some water is extracted from the catalyst solution by its physical solubility in the aldehydes (about 1.3% w/w) which may be replaced before the catalyst solution re-enters the reactor. 

Batch, recirculating batch, extractive semibatch, semicontinuous flow, continuously stirred tank (CSTR) and continuous packed bed reactors have alt been succesfully tested as enzyme reactors for SCFs (Figure 4.9-1). References to helpful descriptions for designing small-scale reactors for enzymatic studies are collected in Table 4.9-1. 

A one-liter continuous stirred tank reactor (CSTR) was used in this study. A sintered metal filter was installed to remove the wax samples from the catalyst slurry. The wax sample was extracted through the internal filter and collected in the hot trap held at... 

Hatton and coworkers have also analyzed processes involving ELMs. Using their advancing-front model as a basis, they have studied staged operations (10 1), continuous stirred tank reactors (105), and mixer cascades (106). One interesting aspect of their analysis is the effect of emulsion recycle. They analyzed the effect on extraction rate of recycling used emulsion and combining this with new emulsion. 

Nitric acid removal from an aqueous stream was accomplished by continuously passing the fluid through a hollow fiber supported liquid membrane (SLM). The nitric acid was extracted through the membrane wall by coupled transport. The system was modeled as a series of (SLM)-continuous stirred tank reactor (CSTR) pairs. An approximate technique was used to predict the steady state nitric acid concentration in the system. The comparison with experimental data was very good. 

Recently, Bunge and Noble (39) have extended the approach of Ho et al. ( ) to Include reversibility of reaction 1. Batch extractions and calculations from this reversible reaction model demonstrate that reaction reversibility significantly affects extraction performance in some cases (39,40). In this paper, we extend these batch extraction calculations to a continuous stirred-tank extractor. We show that a single, dimensionless parameter can be used to assess the likely contribution of reversibility for a given set of conditions. 

Owing to the high catholyte flow rate necessary to avoid side reactions the conversion of acrylonitrile to adiponitrile per pass of the catholyte through the cell is only 0.2%. Hence the catholyte streams in each stack were coupled to a reservoir tank and the catholyte was continuously recirculated through the cell stack. A fraction of the solution in the reservoir passed into an extraction plant and hence the reservoir combined with the cell stack operated in the same way as a continuous stirred tank reactor. 

Feed purification generally involves absorption, adsorption, extraction, and/or distillation. Reaction involves agitated batch, agitated semibatch, continuous stirred tank, or continuous flow reactors. The continuous flow reactors may be empty or contain a mass of solid catalyst. Product separation and purification involves distillation in the petrochemical industry or extraction and crystallization in the extractive metallurgy and pharmaceutical industries absorption is used to a lesser extent. 

Section 6.4 covers continuous stirred tank separators. Section 6.4.1 studies equilibrium separation processes most of this section is devoted to crystallization, with additional coverage of liquid extraction. Membrane separation processes/devices are sometimes modeled as CSTRs. Section 6.4.2 touches upon a few of these examples, encountered, for example, in ultrafllUation and gas permeation. There are brief treatments of batch systems that are well-stirred in Sections 6.4.1 and 6.4.2 for both equilibrium based and membrane separation processes. 

In this section, the flow vs. force configuration of a continuous stirred tank separator (CSTS) will be illustrated with a few examples. The examples cover crystallization, solvent extraction, ultrafiltration and gas permeation. 

Most chemical processes involve two important operations (reaction and separalion) that are typically carried out in different sections of the plant and use different equipment. The reaction section of the process can use several types of reactors [continuous stirred-tank reactor (CSTR), tubular, or batch] and operate under a wide variety of conditions (catalyzed, adiabatic, cooled or heated, single phase, multiple phases, etc.). The separation section can have several types of operations (distillation, extraction, crystallization, adsorption, etc.), with distillation being by far the most commonly used method. Recycle streams between the two sections of these conventional multiunit flowsheets are often incorporated in the process for a variety of reasons to improve conversion and yield, to minimize the production of undesirable byproducts, to improve energy efficiency, and to improve dynamic controllability. 

Figure 4.3 Continuous stripping synthesis of (R)-ethyl-3-hydroxybutyrate from ethyl acetoacetate applying a stirred-tank reactor, stripping module, extraction module and distillation...

The principle of the perfectly-mixed stirred tank has been discussed previously in Section 1.2.2, and this provides an essential building block for modelling applications. In this section, the concept is applied to tank type reactor systems and stagewise mass transfer applications, such that the resulting model equations often appear in the form of linked sets of first-order difference differential equations. Solution by digital simulation works well for small problems, in which the number of equations are relatively small and where the problem is not compounded by stiffness or by the need for iterative procedures. For these reasons, the dynamic modelling of the continuous distillation columns in this section is intended only as a demonstration of method, rather than as a realistic attempt at solution. For the solution of complex distillation and extraction problems, the reader is referred to commercial dynamic simulation packages. 

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