Two liquid phase processes

To improve productivity further, limitahon by product inhibition has also to be circumvented, but that implies a deeper modificahon of the process. A classical method is the use of a two-liquid phase process. Only small-scale experiments were carried out with moderate success [100,102,103]. Moreover, given the strong oxygen requirements of the biotransformation (cf below), the up-scaling of this method would probably be hampered. 

The use of a higher substrate concentration for a given amount of enzyme is also a very important goal for preparative scale applications. Such an achievement has been successful very recently [213]. Thus, a two-liquid-phase process allowing for the preparative scale resolution ofpara-bromo styrene oxide (6 g,... 

Fig. 23. Fungal epoxide hydrolase. Preparative scale resolution of para-bromo styrene oxide using a two-liquid-phase process [213]...

Commercial Processes.. The Oxo reaction is carried out in the liquid phase. With gaseous olefins, a suspension medium such as an inert hydrocarbon is used of a boiling range sufiSciently different from the aldehyde product to allow easy separation. No additional medium is required for hydrofonnylation of liquid olefins. Two liquid-phase processes have been developed. In the earlier one the catalyst is slurried in the feedstock or added liquid medium. Less operating difiSculty is claimed for the newer fixed-bed system because the need for filtration and solids handling is eliminated. 

Historically, the processing routes moved from the isothermal CVI process to more cost-effective techniques such as gradient-CVI and liquid polymer or liquid silicon infiltration. These routes are faster and lead to shorter manufacture cycles than isothermal CVI and, especially the two liquid phase processes LPI and LSI, use technologies already developed for polymer matrix composites (PMC). 

The separation of components by liquid-liquid extraction depends primarily on the thermodynamic equilibrium partition of those components between the two liquid phases. Knowledge of these partition relationships is essential for selecting the ratio or extraction solvent to feed that enters an extraction process and for evaluating the mass-transfer rates or theoretical stage efficiencies achieved in process equipment. Since two liquid phases that are immiscible are used, the thermodynamic equilibrium involves considerable evaluation of nonideal solutions. In the simplest case a feed solvent F contains a solute that is to be transferred into an extraction solvent S. 

Figure 8-7. System with heterogeneous azeotrope-two liquid phases in the equilibrium with one vapor phase. Used by permission. Smith, B.D., Design of Equilibrium Stage Processes, McGraw-Hiii, New York (1963), all rights reserved.

The process is similar to the catalytic liquid-phase oxidation of ethylene to acetaldehyde. The difference hetween the two processes is the presence of acetic acid. In practice, acetaldehyde is a major coproduct. The mole ratio of acetaldehyde to vinyl acetate can he varied from 0.3 1 to 2.5 1. The liquid-phase process is not used extensively due to corrosion problems and the formation of a fairly wide variety of by-products. 

This method exemplifies a broad class of processes that proceed via transfer of reacting species between two liquid phases. Such processes may require a catalyst that can combine with species present in one phase and effect their transfer in this form to the second phase where the main reaction occurs. Starks23 has termed such a process phase-transfer catalysis and has demonstrated its utility in reactions involving inorganic anions. For example, he has shown that the rates... 

The process of direct synthesis of DME includes reactions of methanol synthesis and methanol dehydration, which are catalyzed by two different catalysts. Although the technology for the production of methanol is generally considered mature, most of thran are gas phase process, and the performances of these catalysts are restricted remarkably in liquid phase process. Development of high performance bifunctional catalyst system is very... 

Adrian et al. (2000) have reported a novel high-pressure liquid-liquid extraction process with reference to processing in biotechnology the example of cardiac glycosides (digitoxin and digoxin) is cited. A completely miscible, binary system of water and a hydrophobic organic solvent like ethanol can split into two liquid phases when a near-critical gas (e.g. CO2) is added. The near-critical C02/water/l-propanol system is reported, for which possibilities for industrial exploitation exist. 

In 1982, Lilly [21] reviewed the first two-liquid phase biocatalysis. In 1987 and 1992, Lilly s group [39,40] published reviews dealing with process engineering of biphasic bioreactors. In 1993, Van Sonsbeek et al. [41] gave an overview on biocatalysis in different biphasic... 

The development of the two-liquid phase biocatalytic processes requires the knowledge of different phenomena and problems that are not necessarily encountered in monophase aqueous media [40]. 

Several experiments using different organic solvents in different biphasic media are necessary to find the adequate distribution of the reaction components. A series of experiments are essential for the choice of a process and for scaling-up. Experiments using Lewis cells [44] may yield useful results for understanding equilibrium, kinetics, and interactions between organic solvent-substrate and/or organic solvent-biocatalyst. A study of two-liquid phase biotransformation systems is detailed below in Sections II-IX. 

Separation of two liquid phases, immiscible or partially miscible liquids, is a common requirement in the process industries. For example, in the unit operation of liquid-liquid extraction the liquid contacting step must be followed by a separation stage (Chapter 11, Section 11.16). It is also frequently necessary to separate small quantities of entrained water from process streams. The simplest form of equipment used to separate liquid phases is the gravity settling tank, the decanter. Various proprietary equipment is also used to promote coalescence and improve separation in difficult systems, or where emulsions are likely to form. Centrifugal separators are also used. 

As for the former problem, the researchers of GA found that the mixed acid solution produced by the Bunsen reaction separates spontaneously into two liquid phases in the presence of excess amount of iodine [17]. The heavier phase is mainly composed of HI, I2, and H20, and is called "Hix" solution. The main components of the lighter phase are H2S04 and H20. The phenomenon (liquid-liquid (LL)-phase separation) offered an easy way of separating the hydriodic acid and the sulfuric acid. As for the HI processing, some ideas have been proposed by GA [17], RWTH Aachen [18], and JAEA. JAEA studied the utilization of membrane technologies for concentrating the Hix solution to facilitate the HI separation and also for enhancing the one-pass conversion of HI decomposition [19,20]. 

EBMax A continuous, liquid-phase process for making ethylbenzene from ethylene and benzene, using a zeolite catalyst. Developed by Raytheon Engineers and Constructors and Mobil Oil Corporation and first installed at Chiba Styrene Monomer in Japan in 1995. Generally similar to the Mobil/Badger process, but the improved catalyst permits the reactor size to be reduced by two thirds. 

To get some idea of the prices to be expected for compounds produced with these approaches, we have estimated the total cost of producing 10,000 tons per annum of 1-octanol from w-octane, based on data collected for this conversion by P. oleovorans, during growth in a two-liquid-phase system containing 15% (v/v) hexadecane as a carrier phase. n-Octane is dissolved in the carrier phase to a concentration of 5-10% (v/v), converted by the P. oleovorans cells in the aqueous phase, and the product 1 -octanol dissolves in the hexadecane phase once more. Downstream processing consists of a phase separation, followed by two distillation steps. In the first step, the C8 alkane/alkanol are separated from the hexadecane, which is recycled into the bioreactor. In the second step, the w-octane is distilled off the n-octanol the octane is recycled to the bioreactor, and the octanol is collected as the desired product. This approach leads to a very clean product stream of >98% pure 1-octanol. ... 

The previous chapters have demonstrated that liquid-liquid extraction is a mass transfer unit operation involving two liquid phases, the raffinate and the extract phase, which have very small mutual solubihty. Let us assume that the raffinate phase is wastewater from a coke plant polluted with phenol. To separate the phenol from the water, there must be close contact with the extract phase, toluene in this case. Water and toluene are not mutually soluble, but toluene is a better solvent for phenol and can extract it from water. Thus, toluene and phenol together are the extract phase. If the solvent reacts with the extracted substance during the extraction, the whole process is called reactive extraction. The reaction is usually used to alter the properties of inorganic cations and anions so they can be extracted from an aqueous solution into the nonpolar organic phase. The mechanisms for these reactions involve ion pah-formation, solvation of an ionic compound, or formation of covalent metal-extractant complexes (see Chapters 3 and 4). Often formation of these new species is a slow process and, in many cases, it is not possible to use columns for this type of extraction mixer-settlers are used instead (Chapter 8). 

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