D.A. White

Numerical modelling of sedimentation processes

This paper considers the well-documented theory for the design of sedimentation tanks. The difference in this treatment is that a numerical model is used to do the necessary calculations for the limiting flux and the equations to do this are put in a dimensionless format. The model for a cylindrical settler is extended to the case of a conical settling geometry. It is shown that the conical settler can handle a wider range of solids loading. Finally, calculations are reported to estimate the characteristics of batch settling experiments for conical settlers and the results are compared with experimental data.

Permeation of ethanol and methanol vapours through a porous alumina membrane

Abstract The permeation rates of ethanol and methanol vapours through alumina membranes with pore diameter of 5 and 10 nm were measured in the temperature range 150–200°C. The permeability of ethanol and methanol vapours is less than that predicted from a model which combines bulk viscous and Knudsen flow. A new model based on the assumption that the contribution of the adsorbed phase of ethanol and methanol on the membrane pore wall is small compared to that in the gas phase is proposed. Experiments with nitrogen-ethanol and nitrogen-methanol mixtures were conducted to verify this assumption. The proposed model describes the behaviour of the mixtures well, though low temperature data with the 5 nm membrane do not agree so well with the model.

The use of cascade theory to formulate fluid carrier mass balances for multi-stage plants connected in a simple countercurrent mode

Abstract Countercurrent separation processes which do not use recycle schemes are described by cascade theory. This theory is used to prove that the stages connected in a simple countercurrent mode must produce two effluent streams equal in size. This concept is used to show how a real plant is connected to produce products through a number of stages. This theory is extended for the case of multiple-feed and interstage products to provide a general mass balance solution for a multi-stage separation plant.

Freeze Decontamination Process: Modeling in a Simplified Case of Completely Mixed Aqueous Phase and Observations with Ultrasonic Agitation in the Liquid during Freezing

Abstract Correlations among variables of the freeze decontamination process have not been developed to precisely predict experimental observations of purified solid ice out of a given aqueous solution on a macroscale. A lack of such information has been one of the main reasons for the very small practical application of the process so far. A simplified model has been presented in an effort to take into account the inhomogeneity of the solid phase. Pure ice spikes scattered on a cold surface have been supposed to grow opposite to the direction of heat dissipation, leaving unfrozen liquid channels among them along their length. The ice phase has thus been supposed to consist of pure ice spikes and a quantity of liquid entrapped among them. The liquid at the freezing front and in bulk is supposed to be completely mixed. Volume changes on freezing have also been taken into account. The model has been compared with experimental observations on a specially fabricated apparatus. Temperatures of a cold freezing s...

A generalised algorithm for cascade mass balances

Cascades are multistage plants such as those used for the enrichment of uranium by gaseous diffusion. In a paper (White, 1989) a simple algorithm was given to enable cascade mass balance to be worked out. The algorithm assumes that the flow rates of the two product streams are exactly the same. In this mode of operation the product split (θ), the ratio of enrichment product flow to the feed flow, is exactly one-half and, assuming that the feed flow rates of the units in the stage is fixed, the size of the unit varies exactly with the flow rate. However, the restriction that θ should be exactly equal to one-half is somewhat limiting in designing processes, for gas separation for example. We outline a more flexible procedure which is still fairly simple to implement

Cascade Design for Uranium Enrichment Employing Chemical Exchange and Solvent Extraction

Abstract This paper describes the optimization of a solvent extraction plant used for the enrichment of uranium by redox chemistry and chemical exchange. The basic unit of the plant is a standard mixer-settler where the organic and aqueous phases are in chemical equilibrium. Repeated contact of the phases transfers small amounts of 235U into the organic phase and equally small amounts of 238U into the aqueous phase. The mixer-settler units, all of the same size, are grouped into a countercurrent train. The plant or cascade as a whole consists of a number of trains. Each of these consists of mixer-settlers of different sizes, and the number of stages in the trains varies. The optimization procedure is a four-stage process designed to minimize the whole plant size.

Simulation of the Errors in Equilibrium Correlations Using the Langmuir and BET Isotherms

This paper considers methods of processing sorption equilibrium data so that the errors of derived information, such as maximum sorption capacity, can be calculated. The models considered in this study are the Langmuir and BET isotherms. In both cases the data are rearranged and correlated, on a standard spreadsheet, with a linear and quadratic function, respectively. Several estimates of the constants in the functions can be obtained by omitting data points in a consistent manner. These estimates can be statistically analyzed to give the mean and standard deviations of the constants. In obtaining the correlations on a spreadsheet, the coefficient of correlation (R 2) is obtained. Analysis of a large series of data would indicate that the value of R 2 does not give any information as to the variance of the coefficients in the correlating polynomial. The statistical analysis is finally illustrated with some examples.

Gas diffusion cascades—properties and optimization

Abstract This paper considers the optimization of gas diffusion cascades in which the pressures on both sides of the diffusion membrane are considered. This leads to a modified form of the equilibrium relationship which depends on the ratio of feed pressure to transmembrane pressure. This ratio has to be varied to obtain on optimal design for the cascade. An example of the optimization procedure is given.

Optimization of precipitate production by co-current washing

Abstract The optimization of co-current washing on a simple daily cycle basis is considered in this paper. The process requirements specify that soluble species in contact with the precipitate should be reduced by successive dilution to a preset level. This level determines the number of daily cycles required and the optimum volume ratio of wash water to settled precipitate for maximum production to be maintained. Where the plant is idle over the weekend more care has to be taken with the optimization calculations.