Of continuous-membrane columns

Earlier papers on the continuous membrane column (28,29) have discussed the separation of CO2-N2, CO2-O2 and O2-N2 (air) mixtures in stripper, enricher and total column units composed of 35 silicone rubber capillaries. A characterization of the membrane column using a membrane unit concept (analogous to transfer unit concept — HTU, NTU) has also been presented. The purpose of this paper is to present some new data and discussions on the extended study of continuous membrane column. Specifically, the topics of multicomponent separations, Inherent simulation difficulties, composition minima in the enriching section, variation of experimental parameters, and local HMU variation along the column will be covered. 

One of the next steps in developing the continuous membrane column will be to obtain extensive data on multicomponent systems. Some preliminary experiments with a C02 CH -N2 mixture using a stripper have already been conducted. The results of two such experiments are presented in Figures 3 and 4. The agreement between experiment and model is excellent. 

The performance of the continuous membrane column was characterized earlier (28) in a manner analogous to the transfer unit concept for packed columns. The expressions developed for NMU (difficulty of separation) and HMU (efficiency) were... 

Inherent difficulties accompany some simulations of the continuous membrane column. Perturbation of boundary conditions within the range of experimental error may be necessary in certain Instances to achieve a proper fit of experimental data. 

Thorman, J.M. "Engineering Aspects of Capillary Gas Perme-ators and the Continuous Membrane Column," Ph.D. Thesis, University of Iowa, Iowa City, Iowa, 1979. 

To get higher purity permeate, the product from the first stage can be compressed and sent to a second stage, as shown in Fig. 26.11c. Two or more stages could be used in this fashion to get the desired purity, but the cost of recompression and the increased complexity of the system makes this scheme generally uneconomical. A novel approach that uses two separators and one recompression step is the continuous membrane column. As shown in Fig. 26.1 Irf, part of the permeate product from the second separator is compressed and sent back to the other side of the membrane, where it flows countercurrently to the permeate. This reflux action permits very high purity permeate to be obtained. The reflux steam loses the more permeable component as it flows through the separator and is combined with the feed to the first separator. This scheme was demonstrated in pilot units but has not yet been used commercially. 

Continuous Membrane Column., Dissertation, University of Iowa (1979). 

Tsuru, T. and S. T. Hwang, Permeators and continuous membrane columns with retentate recycle. Journal of Membrane Science, 1995, 98(1 2) 57 67. 

Figure 8.1.4. (a) Type (4) systems. Countercurrent flow of feed gas mixture and permeated gas mixture in a membrane device, (b) Continuous membrane column method of gas mixture separation, (c) Countercurrent dialyzer with the feed solution and the dialyzing liquid entering the device countercurrently on two sides of the membrane. 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

Pertraction (PT) can be realized through a liquid membrane, but also through a nonporous polymeric membrane that was applied also industrially [10-12]. Apart from various types of SLM and BLM emulsion liquid membranes (ELM) were also widely studied just at the beginning of liquid membrane research. For example, an emulsion of stripping solution in organic phase, stabilized by surfactant, is dispersed in the aqueous feed. The continuous phase of emulsion forms ELM. Emulsion and feed are usually contacted in mixed column or mixer-settlers as in extraction. EML were applied industrially in zinc recovery from waste solution and in several pilot-plant trials [13,14], but the complexity of the process reduced interest in this system. More information on ELM and related processes can be found in refs. [8, 13-16]. 

Another approach to fraction collection is the use of an on-column frit structure or capillary fracture that depends on the electroosmotic flow to deposit the eluent in a continuous manner on a moving surface. Although this approach circumvents the dilution problem, the collection structures are complex and can result in the loss of some of the analyte. One commercially available fraction collection device couples CE with membrane fraction collection, without the need for frits or capillary fractures. The outlet vial holder can be removed and replaced with a wetted circular polyvinylidene difluoride (PVDF) disk, which enables the collection of eluted analytes and subsequent manipulations such as immunoblotting and microsequencing. Figure 6.13 shows a schematic diagram of the CE membrane fraction collector interface.74... 

Figure 2-5 is a schematic of apparatus for the measurement of osmotic pressure. A solution is separated from its pure solvent by a semipermeable membrane, which allows solvent molecules to pass but blocks solute. Both components arc at the same temperature, and the hydrostatic pressure on each is recorded by means of the heights of the corresponding fluids in capillary columns. The solute cannot distribute itself on both sides of the membrane. The solvent Hows initially, however, to dilute the solution, and this flow will continue until suflicient excess hydrostatic... 

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