Phase inversion development

Flooding and Loading Since flooding or phase inversion normally represents the maximum capacity condition for a packed column, it is desirable to predict its value for new designs. The first generalized correlation of packed-column flood points was developed by Sherwood, Shipley, and Holloway [Ind. Eng. Chem., 30, 768 (1938)] on the basis of laboratory measurements primarily on the air-water system. 

The methods developed by EBC and others in the late 1990s using hydrocyclones and phase-inversion techniques may be sufficient for separation of the treated oil from the aqueous phase and biocatalyst. However, a cost analysis of such methods may be necessary to determine the economic feasibility. Recent work using hydrophobic membranes, magnetically separable immobilized biocatalysts and other techniques may provide alternate methods for separation of oil and recycling biocatalyst. A comparison of these techniques with each other and the previously investigated hydrocyclone techniques is needed to demonstrate improvements in the separation efficiency. 

Quantities useful for predicting phase continuity and inversion in a stirred, sheared, or mechanically blended two-phased system include the viscosities of phases 1 and 2, and and the volume fractions of phases 1 and 2, and ij. (Note These are phase characteristics, not necessarily polymer characteristics.) A theory was developed predicated on the assumption that the phase with the lower viscosity or higher volume fraction will tend to be the continuous phase and vice versa (23,27). An idealized line or region of dual phase continuity must be crossed if phase inversion occurs. Omitted from this theory are interfacial tension and shear rate. Actually, low shear rates are implicitly assumed. 

The first, and currently only, successful solvent-permeable hyperfiltration membrane is the Starmem series of solvent-resistant membranes developed by W.R. Grace [40]. These are asymmetric polyimide phase-inversion membranes prepared from Matrimid (Ciba-Geigy) and related materials. The Matrimid polyimide structure is extremely rigid with a Tg of 305 °C and the polymer remains glassy and unswollen even in aggressive solvents. These membranes found their first large-scale commercial use in Mobil Oil s processes to separate lube oil from methyl ethyl ketone-toluene solvent mixtures [41-43], Scarpello et al. [44] have also achieved rejections of >99 % when using these membranes to separate dissolved phase transfer catalysts (MW 600) from tetrahydrofuran and ethyl acetate solutions. 

Development of W/O/W Emulsion during Phase Inversion. Sherman et al. (2lQ and Dokic et al.(25) emphasized that the development of a W/O/W type dispersion precedes the thermal induced phase inversion of 0/W emulsions. This suggest that the state of multiple structure in emulsions may be generalized as one of the mesophase between 0/W and W/0 emulsions, and also that there is a possibility of more simplifying the method for preparing W/O/W emulsions. 

Figure 6. Development of the W/O/W-type dispersion during phase inversion of W/0 emulsions. "Reproduced with permission from Ref. 21. Copyright 1983, Academic Press. "...

Phase inversion is a process in which a polymer is transformed from a liquid to a solid state. There are a number of methods to achieve phase inversion. Among others, the dry-wet phase inversion technique and the temperature induced phase separation (TIPS) are most commonly used in the industrial membrane manufacturing. The dry-wet phase inversion technique was applied by Loeb and Sourirajan in their development... 

Relying heavily on their established expertise In the field of polymer solution thermodynamics, Ramlde and Manabe (12) take a more detailed look at the mechanism of phase-separation which occurs during membrane formation. In this context, phase separation Is the process which takes place during phase Inversion. A theory Is developed which allows for the prediction of pore characteristics... 

The reader may find it helpful at this junction to consider the phenomenological model originally developed by Cahn (.2) to describe two phase metal alloys and more recently used in conjunction with polymer alloys. This model explains the appearance of isotropic, interdispersed domains in terms of spinodal decomposition. This may yield some insight into the reasons why "uphill" diffusion (that is, diffusion against the concentration gradient) occurs in phase inversion. The reader is also referred to the contribution of Strathmann in the present volume (.3). ... 

The literature describes numerous manufacturing methods for synthetic membranes. A recent review by Pusch and Walch (1) considers membranes from a number of techniques for manufacturing membranes and discusses applications ranging from microfiltration to desalination to gas separation. In this paper, a thermal phase-separation technique of preparing membranes Is presented. The method Is a development of an Invention described In US Patent 4,247,498 by Anthony J. Castro (,2). This technique Is similar In many respects to the classical phase-inversion methods however, the additional consideration of thermal solubility characteristics of the poly-mer/solvent pair offers new possibilities to membrane production. 

Figure 8.8 Principle of the PIT method an o/w-emulsion changes into a w/o-emulsion above a certain temperature. In the phase inversion range a microemulsion develops, which becomes a blue o/w-emulsion after cooling down. (From Ref. [48], reprinted with permission of Elsevier.)...

Figure 11 shows the theoretical permeabilities that are expected for a two-phase blend of polymers. The two solid curves represent calculations based upon Maxwell s equation (24) for an aspect ratio of 1 for the discontinuous phase. The dotted line is a prediction of the permeability using Nielsen s model (25) when a barrier polymer with an aspect ratio of 8 is discontinuous in a nonbarrier matrix. Figure 12 shows the expected result of a phase inversion for a two-polymer blend. The discontinuous phase is assumed to have an aspect ratio of 1. At some critical composition, the composite switches from being continuous in one polymer to being continuous in the other. Figure 12 is really a special case of Figure 11. Selar RB is a blend of polyethylene and nylon-6. Polyethylene is the majority constituent and forms the continuous phase. The product has its best barrier when it can be used in processes that impart orientation to the product. This gives a high aspect ratio to the nylon-6 and enhanced barrier to the article. Blends of polyethylene and EVOH are being developed.

This work ably illustrates the importance of the surface selection rule. Unfortunately, the phase inversion technique described in this paper has not been further developed by Pons and co-workers and there are some difficulties associated with it it is evident, for example, that exact balancing of the positive and negative phases will lead to complete cancellation of the cen-treburst. This is of significance as the spectrometer software may well rely on the location of this centreburst to allow the Fourier transform to take place. It is, therefore, essential to build in mis-match into the phase-inversion amplifiers, though this in turn makes the technique very difficult to use quantitatively. Suffice to say that the authors of this report have not found it easy to use in practice and have relied on the subtraction of spectra already transformed as described above. 

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