Cellulose acetate membranes separation factors

The process shown in Figure 9.21 was first developed by Separex, using cellulose acetate membranes. The separation factor for methanol from MTBE is high (>1000) because the membrane material, cellulose acetate, is relatively glassy and hydrophilic. Thus, both the mobility selectivity term and the sorption term in Equation (9.5) significantly favor permeation of the smaller molecule, methanol, because methanol is more polar than MTBE or isobutene, the other feed components. These membranes are reported to work well for feed methanol concentrations up to 6%. Above this concentration, the membrane is plasticized, and selectivity is lost. More recently, Sulzer (GFT) has also studied this separation using their plasma-polymerized membrane [56],... 

The development of asymmetric membrane technology in the 1960 s was a critical point in the history of gas separations. These asymmetric structures consist of a thin (0.1 utol n) dense skin supported on a coarse open-cell foam stmcture. A mmetric membranes composed of the polyimides discussed above can provide extremely high fluxes throuj the thin dense skin, and still possess the inherently hij separation factors of the basic glassy polymers from which they are made. In the early 1960 s, Loeb and Sourirajan described techniques for producing asymmetric cellulose acetate membranes suitable for separation operations. The processes involved in membrane formation are complex. It is believed that the thin dense skin forms at the... 

The ideal separation factors and the mobility and solubility contributions comprising this parameter for the various polymers shown in Fig. 20.3-9 are presented in Table 20.3-1 for a case corresponding to a 600 psia total feed pressure with 3(X) psia of CO2 upstream and veiy low downstream pressures of both components. The data for preparation of Table 20.3-1 were estimated from a number of sources, and the absolute value of the entries for cellulose acetate may be in error up to 10-15%. - The data for selectivities reported in the table are higher than those normally report for asymmetric polysulfone and cellulose acetate membranes since the dense-film forms of the samples tend to eliminate flow through pores that are present in asymmetric membranes and decrease their selectivity (see Table 20.1-2). 

The concept of mixed-matrix membranes has been demonstrated at UOP ° in the mid-1980s using silicalite-cellulose acetate mixed-matrix membranes for CO2/H2 separation. In the demonstration, a feed mixture of 50/50 CO2/H2 with a differential pressure of 50 psi was used. The calculated separation factor for CO2/H2 was found to be 5.15 + 2.2. In contrast, a CO2/H2 separation factor of 0.77 + 0.06 was found for cellulose acetate membrane. This indicates that silicalite in the membrane phase reversed the selectivity from H2 to CO2. Experimental results and modeling predictions indicate that mixed-matrix membranes with the incorporation of fillers within polymeric substrates provide potential possibilities to achieve enhanced membrane performance, which will open up new opportunities for the separation and purification processes. Highlighted applications for mixed-matrix membranes include separation and purification of gas mixtures such as separation of N2 from CO2 removal from natural and separation... 

Kulprathipanja and coworkers reported the preparation of integrally skinned siUcaUte-1/cellulose acetate flat sheet asymmetric mixed-matrix membranes via phase inversion technique in 1992 [73]. The O2/N2 separation performance of these membranes was investigated. It was demonstrated that the separation factor of... 

Other system variables that will have an effect on the separation process are temperature and relative humidity of the gas. Increasing the temperature raises most permeabilities by about 10 to 15% per 10°C and has little effect on separation factors. The effect of relative humidity is variable depending upon the membrane used. High relative humidities, greater than 95%, are generally detrimental due to membrane plasticization. Contamination with liquid water has been found to dramatically reduce membrane performance for cellulose acetate ... 

In oil processing, separation of aromatic isomers Cg (ethylbenzene 7b= 136°C,p-xylene 7b= 138.3°C, m-xylene Ty, = 139.1°C, >-xylene T], = 144.4°C) is required. According to the literary data, the following isomers of hydrocarbons are separated p-xylene/m-xylene, p-xylene/o-xylene, -hexane/2,2-dimethylbutane, -hexane/3-methylpentane, and n-butane/f-butane [8,83,130-137]. Pervaporation method is the most effective for this purpose. To separate the isomers, membranes based on various polymers were used. Good separation for aU isomer mixtures was attained by the polyimide Kapton film (fip = 1.43-2.18) but parylene films and cellulose acetate also exhibited a relatively high separation factor (fip = 1.22-1.56 and /3p = 1.23-1.56, respectively). Temperatures >200°C were required to obtain a reasonable flux through the polyimide film and a pressure of about 20 atm was necessary to keep the feed stream liquid [8]. 

---