Charles H. Gooding

Measurement errors in water vapor permeability of highly permeable, hydrophilic edible films

Abstract Water vapor transmission WVT of films is commonly measured using modifications of the ASTM E 96 Standard Method (‘cup method’). A stagnant air layer exists between the underside of the film mounted on the cup and the surface of the desiccant, saturated salt solution or distilled water contained in the cup. The method considers the air gap resistance to be negligible to water vapor transport. When high water vapor-transmitting hydrophilic edible films are measured with the cup method, the resistance of the stagnant air layer can be significant and, if neglected, can lead to underestimation of water vapor transmission rates. Equations were presented in this study to correct WVT data for the air gap resistance. For both a methylcellulose and a corn zein film, water vapor permeabilities measured with air gaps of 1·0 and 1·5 cm were statistically significantly ( α = 0·05) different. Values corrected to account for air gap resistance were not statistically significantly ( α = 0·05) different. Literature data on water vapor permeability of other hydrophilic edible films were corrected to account for the air layer resistance. Underestimation of actual values ranged between 5 and 46%.

Mass transfer in spiral wound pervaporation modules

Abstract Mass transfer performance correlation are needed for spiral wound membrane module feed spacers to determine the optimal flow velocity for each spacer, to find the most cost efficient spacer, and to compare spiral wound to other modules. The mass transfer characteristics of 15 feed spacers were determined experimentally. The mass transfer correlations derived from the data indicate that a transitional flow regime exists in spacer-filled channels at the flow velocities evaluated. The use of a spacer increased the mass transfer coefficient to 1.5 to 4.0 times the coefficient obtained with an empty channel. There was only a modest increase in the performance of the spacer with the highest mass transfer coefficient when compared to the spacer with the lowest despite the fact that the friction loss characteristics of the spacers varied greatly. Attempts to define a characteristic dimension that would allow the consolidation of the mass transfer results from many spacers into a single equation were unsuccessful. Even spacers of a common design, i.e., diamond-shaped, required a unique equation to describe the mass transfer characteristics of each spacer. Ultraflo spacers from Nalle Plastics, Inc. provided the highest mass transfer coefficient per unit of pressure drop per unit length. However, the determination of the most cost efficient spacer will require the use of a detailed economic analysis.

The influence of the porous support layer of composite membranes on the separation of binary gas mixtures

Abstract Thin film composite (TFC) membranes exhibit a high flux for gas and vapor permeation and are viable for a wide range of applications. The high flux may also increase the importance of the resistance of the porous support structure depending on the application and process conditions. A comprehensive modeling approach for TFC membranes is introduced, which considers boundary layer resistances near the membrane surface, solution-diffusion through the coating, and the influence of the porous sublayer. Permeation through the support structure is described by the dusty gas model (DGM) with the support treated as a two-layered structure with a dense but porous skin and a more open substructure. The model accurately describes experimental data on TCE/nitrogen separation using a sweep gas on the permeate side very well. The main resistance towards TCE permeation through two different membranes tested is the porous support. It is shown that changes in the support morphology can greatly enhance the performance of the composite membranes. Model calculations were also performed for vacuum assisted permeation. The pressure drop across the support is considerable depending on the coating thickness. The TCE permeation is again dominated by the resistance of the support layer, which can be reduced by altering the morphological parameters of the structure. The proposed model is able to describe the performance of the composite membrane and to identify optimum process conditions for given performance characteristics. It can be used to aid in the development of membrane structures for enhanced performance.

Characterization of the porous support layer of composite gas permeation membranes

The porous support layer of a high-flux composite membrane can have an influence on gas or vapor separation performance. In order to quantify that influence, the theory of gaseous flow through porous media was reviewed and applied to describe transport through the porous layer. The flux equations may be used to characterize the morphology of the support structure. An experimental technique was developed to determine morphological parameters based on single gas permeation experiments. Flat sheet and hollow fiber support membranes were tested and compared. Due to the axial pressure drop inside the hollow fibers, an appropriate average lumen pressure had to be determined. As a by-product, the inside diameter of the porous hollow fibers may be estimated from the permeation experiments. The calculated morphological parameters varied slightly with six different gases used, but these differences could be reconciled when the asymmetry of the porous structure was taken into account. The support membranes could be modeled accurately by a two-layered structure and a resistance in series approach. By combining the description of gaseous flow in the porous support presented in this paper with standard transport equations for the dense coating, it should be possible to describe binary gas transport through a composite membrane and, ultimately, to determine the influence of the support layer.

The permeation of binary gas mixtures through support structures of composite membranes

Abstract The dusty gas model (DGM) is used to describe transport of binary gas mixtures through porous membrane supports to quantify the resistance towards permeation. The model equations account for three different transport mechanisms for the permeating components: conventional viscous pore flow, Knudsen diffusion, and binary diffusion. Experimental data obtained with the uncoated membrane supports are used to determine the morphological parameters needed in the DGM equations. Flat sheet and hollow fiber membrane supports are characterized by the permeation of a TCE/nitrogen vapor. The DGM shows an excellent fit to experimental data when the asymmetric structure of the membrane supports is taken into account, but the morphological parameters cannot necessarily be related to precise physical structure parameters such as pore size, porosity, and tortuosity. The DGM works well even when the membrane supports are modeled as a single homogenous structure. The membrane supports exhibit different resistances towards the various transport mechanisms that occur within the porous support and the resistances vary with process conditions so that support optimization is not straightforward. With the analysis presented in this paper and transport equations specific to the dense coating and module geometries, the influence of the support layer on gas or vapor separation can be quantified.

Modeling spiral wound membrane modules for the pervaporative removal of volatile organic compounds from water

Abstract A model was developed to simulate the performance of spiral wound membrane modules for the pervaporative removal of volatile organic compounds (VOCs) from water. Differential mass and momentum balances were solved simultaneously by numerical integration to model both feed and permeate streams. With input of the initial feed concentration, the feed side mass transfer coefficient, and physical parameters to describe the membrane and the module, the model predicted the permeate pressure and velocity, the composition and average molecular weight of the permeate, the total and component fluxes, and the feed concentration, all as a function of the position in the module. Dimensionless groups that characterize the physical properties of the VOC and the design and operating parameters of the membrane and module were varied to demonstrate their effect on the overall module performance, which relates directly to process economics. The results showed that assuming the permeate pressure drop to be negligible may lead to a significant overestimation of the module performance. Permeate flow in spiral wound and hollow fiber modules is similar, and equations are provided for both.

Spiral wound, hollow fiber membrane modules: A new approach to higher mass transfer efficiency

Abstract A new type of transverse flow, hollow fiber module was evaluated for membrane separation. First, small sections of fabric were woven using silicone rubber hollow fiber membranes and monofilament nylon. Water deoxygenation experiments were conducted on each fabric in a flat cell, yielding mass transfer coefficient ( k ) values. An optimal fabric construction was identified based on k values exceeding 0.01 cm/s at moderate velocities, low pressure drop, and high membrane packing density. Spiral wound prototype modules were made and tested, each with a fabric wrapped in layers around a central permeate tube. For O 2 removal from water, k values for the prototypes were slightly below those obserbved in the flat cell tests. Wrapping the fabric tightly around the permeate tube was less effective than looser wrappings. Pervaporation experiments with trichloroethylene were attempted, but the performance was reduced, apparently due to leaching of petroleum jelly used in module construction and redeposition on the membrane. The k values observed for oxygen transfer were at least 20% higher than those achieved with traditional spiral wound modules, and the membrane packing densities achieved in the prototypes were 300 to 400% higher. This module design could prove to be practical and advantageous for membrane separation processes in which the mass transfer coefficient on the feed side of the membrane limits flux.

The economic optimization of spiral wound membrane modules for the pervaporative removal of VOCs from water

Abstract A model was developed to estimate the cost of removing relatively hydrophobic VOCs from water with a pervaporation-based separation process. Evaluation of the economic model showed that the choice f a feed-side spacer in a spiral wound module has a strong effect on the total separation cost. Several spacers that yield relatively low cost separations have been identified. The choice of permeate spacer was found to have only a small effect on the process economics, though all the permeate spacers evaluated were relatively thick and open. The most economical operation is predicted to occur with moderate permeate pressures (10–12 mmHg) and membranes ∼22 μm thick. The specific VOC to be removed from the feed water has a significant effect on the process economics. For VOCs with relatively high Henrys law constants, the model shows that the development of lower cost membranes with performance qualities similar to silicone rubber will be the most effective means of reducing the separation costs of this process.

Estimation of parameters in a sorption-diffusion model of pervaporation

Abstract In previous papers, several groups have developed and presented pervaporation models based on a sorption-diffusion mechanism. The most realistic versions of these models have several empirical parameters, most of which arise from the concentration dependence of the diffusivities. All of the parameters can be simply fit to pure component and mixture pervaporation data, but this paper shows that such a practice renders the model useless for interpreting the sorption-diffusion behavior on a fundamental level. Suitable fits can be obtained to a given set of data using many different sets of parameters, leading to totally different physical interpretations of the thermodynamic and transport behavior. However, calculations show that much of the uncertainty in the diffusion parameters can be removed by making independent sorption measurements on the membrane material. This procedure is illustrated using pervaporation data obtained with ethanol-water solutions and a thin-film composite polyether urea membrane, and sorption data determined independently by liquid chromatography.

Membrane-aided distillation of azeotropic solutions

A new process is described in which pervaporation is used in tandem with conventional distillation to separate azeotropic solutions. The pervaporator features a moderately selective, high-flux membrane and a flow arrangement that minimizes temperature drop. Preliminary data and calculations support the feasibility of the process.