Membranes Raffinate

Membrane instability results in partial mixing of feed and stripping phases, which deteriorates the selectivity. In addition, raffinate and product are contaminated by the extractant, leading also to extractant losses. Economy of separation and hence industrial application of LM for separation of cephalosporins are strongly dependent on membrane stabilization. 

Partitioning of components between two immiscible or partially miscible phases is the basis of classical solvent extraction widely used in numerous separations of industrial interest. Extraction is mostly realized in systems with dispergation of one phase into the second phase. Dispergation could be one origin of problems in many systems of interest, like entrainment of organic solvent into aqueous raffinate, formation of stable, difficult-to-separate emulsions, and so on. To solve these problems new ways of contacting of liquids have been developed. An idea to perform separations in three-phase systems with a liquid membrane is relatively new. The first papers on supported liquid membranes (SLM) appeared in 1967 [1, 2] and the first patent on emulsion liquid membrane was issued in 1968 [3], If two miscible fluids are separated by a liquid, which is immiscible with them, but enables a mass transport between the fluids, a liquid membrane (LM) is formed. A liquid membrane enables transport of components between two fluids at different rates and in this way to perform separation. When all three phases are liquid this process is called pertraction (PT). In most processes with liquids membrane contact of phases is realized without dispergation of phases. 

The methods and literature are briefly reviewed for solid-suspension separations, solution-phase separations, liquid-phase separations, and gas-phase separations. In the terminology used, the objective is to separate a feed stream (or streams) into a permeate phase and a reject phase, either of which may contain the compon-ent(s) of more interest. For a single membrane, say, the permeate phase remains on the feed side or high-pressure side of the membrane, and is subsequently discharged, whereas the reject or raffinate phase builds up on the opposite or low-pressure side of the membrane, and is then discharged. 

It is helpful to think of a simple membrane process as shown in Figure 8.1. A hydrogen-selective membrane is sealed within a housing (pressure vessel) to make a membrane module. The feed stream enters the membrane module, and hydrogen selectively permeates the membrane. The hydrogen-depleted raffinate stream exits the membrane module as does the permeate stream (enriched in hydrogen). The hydrogen partial pressure in each stream is denoted by PU2 where the subscripts f,... 

There are several processes for the separation of liquid mixtures using porous membranes or asymmetric polymer membranes. With porous membranes, separation may depend just on differences in diffusivity, as is the case with dialysis, where aqueous solutions at atmospheric pressure are on both sides of the membrane. For liquid-liquid extraction using porous membranes, the immiscible raffinate and extract phases are separated by the membrane, and differences in the equilibrium solute distribution as well as differences in diffusivity determine the extract composition. 

Cobalt and Nickel Recovery. Cobalt and nickel are relatively valuable metals often found in complex ores such as laterites or deep sea nodules. The metals can only be extracted from these ores by hydrometallurgy. A proposed recovery scheme based on coupled transport is shown in Figure 9.29. The first membrane contains LIX 54, which produces a nickel and copper concentrate and a cobalt raffinate stream. The concentrate stream is then passed to a second Kelex 100 membrane, which produces a copper and nickel stream. The cobalt III raffinate stream is neutralized and reduced to cobalt II, which can then be concentrated by a LIX 51 membrane. 

Figure 9.41 Variation of copper concentration in the raffinate during extraction with a liquid emulsion-membrane complexing agent system.43...

H-permeable membrane converts reformate to pure hydrogen and produces raffinate (membrane reject stream) for combustor Pure hydrogen, raffinate (H, CO2, CO)... 

A radial concept is used for the integration of the combustor and catalytic reactor where concentric functional chambers build outward from a central core (Figure 4). The innermost sections comprise the combustor unit, where air is carried to the interior and raffinate gas from the membrane penetrates through the wall to form a combustion flame along the itmer wall of the reaction chamber. Preheated and mixed steam and fuel enter the reaction chamber and are catalytically converted to reformate. Reformate exits the reactor and is then sent to the... 

The systems that have been studied show that most, if not all, of the liquid-liquid extraction processes can operate successfully in membranes, often with better overall performance. The reason for this is that these membrane processes are dynamic and the extraction is not limited by loading of the organic phase. Also, the chemistry allows the metal to be transported uphill against the concentration gradient. Thus, it is possible to reach very, very low concentrations of metals in the raffinate (parts per billion)—well below the legal discharge limits for most metals. Currently, to reach such levels does take some time (i.e., days), but this is where future work could help. 

The direct expenditure of energy in the separation process of solvent extraction is due to the pumping costs and costs of agitation/dispersion in conventional solvent extraction devices (membrane solvent extraction devices do not have dispersion only pumping costs are relevant). Let us ignore the small amount of energy required for these steps. Instead, focus on the extract stream from which the extracted product and the solvent have to be separated to reuse the solvent and recover the product. Usually this is achieved by distillation. This cost must be less than that required to separate the product from the feed solution being subjected to extraction by distillation. This is the reason to choose solvent extraction to start with, unless the thermal stability of the product is in question, as is true for many pharmaceutical molecules. Solvent extraction is a necessity in such cases, e.g. penicillins. A small amount of extraction solvent will also be present in the raffinate. This solvent should be removed and recovered also (usually by distillation) (NuU, 1980). 

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