Product separation, membrane

If product inhibition occurs, either a stirred-tank reactor in batch or a plug-flow reactor should be used. In these two reactors, the product concentration increases with time. Alternatively a reactor with integrated product separation (membrane, solvent, etc.) is preferable. 

Product Recovery. Comparison of the electrochemical cell to a chemical reactor shows the electrochemical cell to have two general features that impact product recovery. CeU product is usuaUy Uquid, can be aqueous, and is likely to contain electrolyte. In addition, there is a second product from the counter electrode, even if this is only a gas. Electrolyte conservation and purity are usual requirements. Because product separation from the starting material may be difficult, use of reaction to completion is desirable ceUs would be mn batch or plug flow. The water balance over the whole flow sheet needs to be considered, especiaUy for divided ceUs where membranes transport a number of moles of water per Earaday. At the inception of a proposed electroorganic process, the product recovery and refining should be included in the evaluation to determine tme viabUity. Thus early ceU work needs to be carried out with the preferred electrolyte/solvent and conversion. The economic aspects of product recovery strategies have been discussed (89). Some process flow sheets are also available (61). 

Membrane reactors, where the enzyme is adsorbed or kept in solution on one side of an ultrafHtration membrane, provides a form of immobilized enzyme and the possibiHty of product separation. 

Selective gas permeation has been known for generations, and the early use of p adium silver-alloy membranes achieved sporadic industrial use. Gas separation on a massive scale was used to separate from using porous (Knudsen flow) membranes. An upgrade of the membranes at Oak Ridge cost 1.5 billion. Polymeric membranes became economically viable about 1980, introducing the modern era of gas-separation membranes. Hg recoveiy was the first major apphcation, followed quickly by acid gas separation (CO9/CH4) and the production of No from air. 

Refinery product separation falls into a number of common classes namely Main fractionators gas plants classical distillation, extraction (liquid-liquid), precipitation (solvent deasphalting), solid facilitated (Parex(TM), PSA), and Membrane (PRSIM(TM)). This list has been ordered from most common to least common. Main fractionators are required in every refinery. Nearly every refinery has some type of gas plant. Most refineries have classical distillation columns. Liquid-liquid extraction is in a few places. Precipitation, solid facilitated and membrane separations are used in specific applications. 

In gas separation with membranes, a gas mixture at an elevated pressure is passed across the surface of a membrane that is selectively permeable to one component of the mixture. The basic process is illustrated in Figure 16.4. Major current applications of gas separation membranes include the separation of hydrogen from nitrogen, argon and methane in ammonia plants the production of nitrogen from ah and the separation of carbon dioxide from methane in natural gas operations. Membrane gas separation is an area of considerable research interest and the number of applications is expanding rapidly. 

In addition to the insoluble polymers described above, soluble polymers, such as non-cross-linked PS and PEG have proven useful for synthetic applications. However, since synthesis on soluble supports is more difficult to automate, these polymers are not used as extensively as insoluble beads. Soluble polymers offer most of the advantages of both homogeneous-phase chemistry (lack of diffusion phenomena and easy monitoring) and solid-phase techniques (use of excess reagents and ease of isolation and purification of products). Separation of the functionalized matrix is achieved by either precipitation (solvent or heat), membrane filtration, or size-exclusion chromatography [98,99]. 

There are comparable incentives to develop new process-related materials that are more selective as catalysts, extractants, or separation membranes and more effective in controlling flow in porous media. In addition, the development of materials that are less energy intensive in terms of production and use is a goal equivalent to other means of energy conservation. 

The production of membranes with specific pore sizes is now relatively easy, as membrane separation processes have become increasingly com-... 

As mentioned earlier, a major cause of high costs in fine chemicals manufacturing is the complexity of the processes. Hence, the key to more economical processes is reduction of the number of unit operations by judicious process integration. This pertains to the successful integration of, for example, chemical and biocatalytic steps, or of reaction steps with (catalyst) separations. A recurring problem in the batch-wise production of fine chemicals is the (perceived) necessity for solvent switches from one reaction step to another or from the reaction to the product separation. Process simplification, e.g. by integration of reaction and separation steps into a single unit operation, will provide obvious economic and environmental benefits. Examples include catalytic distillation, and the use of (catalytic) membranes to facilitate separation of products from catalysts. 

The more permeable component is called the fast gas, so it is the one enriched in the permeate stream. Permeabihly through polymers is the product of solubility and diffusivity. The diffusivity of a gas in a membrane is inversely proportional to its kinetic diameter, a value determined from zeolite cage exclusion data (see Table 20-26 after Breck, Zeolite Molecular Sieves, Wiley New York, 1974, p. 636). Tables 20-27, 20-28, and 20-29 provide units conversion factors useful for calculations related to gas-separation membrane systems. 

Halogenatlon. Poly(2,6-dimethyl- and 2,6-diphenyl-l,4-phenylene ether) can be aryl-brominated simply by exposure to a bromine solution no catalyst is required.6 In fact, the use of Lewis acid catalysts to promote the chlorination of poly(2,6-dimethy1-1,4-phenylene ether) leads to substantial degradation of the molecular weight of the chlorinated products.7 Membranes produced from ring brominated PPO (40% wt Br) exhibited enhanced permeability to CHi and CO2 and proved to be more selective in separating CH4/CO2 mixtures.8... 

Recent scattered reports on use of immobilized bioreactors [38,197,302,303], membrane separation techniques [271] provide initial results and possible ways to employ these techniques to achieve product separation however, the limitations posed by each of these such as reduced rates due to immobilization, or limited yield using membranes are issues which have not been completely addressed. 

Pex, P.P.A.C. and Y.C. van Delft, Silica membranes for hydrogen fuel production by membrane water gas shift reaction and development of a mathematical model for a membrane reactor, in Carbon Dioxide Capture for Storage in Deep Geologic Formations—Results from the C02 Capture Project Capture and Separation of Carbon Dioxide from Combustion Sources, eds., D. Thomas, and B. Sally, Vol. 1, Chapter 17, 2005. 

The great advantage of the absorption process over the synthesis of a by-product was its direct recovery of chlorine. Such a process or one that uses chlorine in another on-site process with steady demand is the ideal. More vigorous liquefaction is one approach to reducing the amount of chlorine value to be disposed of, and it has usually been chosen as the substitute for absorption. In this chapter, we discuss the use of gas-separation membranes as an alternative. 

Hall et alP discuss the use of cross-corrugated membrane modules, illustrated in Figure 20.24, as offering the potential for developing multi-functional units for performing both reactions and product separation in one miniaturised module. The use of microporous... 

The development and mass production of membranes for the separation of uranium isotopes by the process of gaseous diffusion applied to UF. ... 

Gavalas, G. R., C. Megiris and S. W. Nam. 1989. A novel composite inorganic membrane for combined catalytic reaction and product separation. Chem. Eng. Sci. 44(9) 1825L-35. 

It has been mentioned earlier that using porous membranes for product separation during the course of an equilibrium reaction, maximum attainable conversions are limited because of reactant permeation. This is the case where the membrane forms the wall of the reactor in which a catalyst is packed. It has also been mentioned that in this mode equilibrium conversions for some slow reactions could be increased by factors ranging between 1.3 and 2.3. Another important operation mode arises when the membrane is inherently catalytic or when the catalytically active species are placed within the membrane pores (catalytically active membrane as shown in Figure 7.2b and 7.2c). In this case, reaction and separation take place simultaneously and are combined in parallel rather than in series as was the case in the previous mode. 

Apart from hydrocarbons and gasoline, other possible fuels include hydrazine, ammonia, and methanol, to mention just a few. Fuel cells powered by direct conversion of liquid methanol have promise as a possible alternative to batteries for portable electronic devices (cf. below). These considerations already indicate that fuel cells are not stand-alone devices, but need many supporting accessories, which consume current produced by the cell and thus lower the overall electrical efficiencies. The schematic of the major components of a so-called fuel cell system is shown in Figure 22. Fuel cell systems require sophisticated control systems to provide accurate metering of the fuel and air and to exhaust the reaction products. Important operational factors include stoichiometry of the reactants, pressure balance across the separator membrane, and freedom from impurities that shorten life (i.e., poison the catalysts). Depending on the application, a power-conditioning unit may be added to convert the direct current from the fuel cell into alternating current. 

Dendritic catalysts can be recycled by using techniques similar to those applied with their monomeric analogues, such as precipitation, two-phase catalysis, and immobilization on insoluble supports. Furthermore, the large size and the globular structure of the dendrimer can be utilized to facilitate catalyst-product separation by means of nanofiltration. Nanofiltration can be performed batch wise or in a continuous-flow membrane reactor (CFMR). The latter offers significant advantages the conditions such as reactant concentrations and reactant residence time can be controlled accurately. These advantages are especially important in reactions in which the product can react further with the catalytically active center to form side products. 

In the synthesis of A-acetyllactosamin from lactose and A-acetylglucosamine with (3-galactosidase (289,290), the addition of 25 vol% of the water-miscible ionic liquid [MMIM][MeS04] to an aqueous system was found to effectively suppress the side reaction of secondary hydrolysis of the desired product. As a result, the product yield was increased from 30 to 60%. Product separation was improved, and the reuse of the enzymatic catalyst became possible. A kinetics investigation showed that the enzyme activity was not influenced by the presence of the ionic liquids. The enzyme was stable under the conditions employed, allowing its repeated use after filtration with a commercially available ultrafiltration membrane. 

The way in which the active microzone is retained also depends on its relationship to the detector (Fig. 2.6) and the type of interaction with the analyte or its reaction product. If the microzone is an integral part of the probe, an additional support (usually a membrane) is often required, so contact with the sample is hindered to some extent. On the other hand, a microzone located in a flow-cell can be retained in various ways. Thus, if the microzone consists of a porous solid or particle, the flow-cell is simply packed with two filters in order to avoid washing out (e.g. see [21]). Too finely divided solids (viz. particle sizes below 30-40 pm) should be avoided as they require pressures above atmospheric level, which complicates system design and precludes use of microzones with a high specific surface. Placing a separation membrane in a flow-cell is... 

Figure 12-4 Membrane reactor in wJiicIi a cataJyst promotes reaction in the membrane and maintains reactants and products separate. Examples sliown are CH4 oxidation to syngas and C2H6 reduction to C2H4. Tlie membrane eliminates N2 from the syngas and produces C2H4 beyond equilibrium by removing H2 in these apphcations.

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