Holdup solvent

The sohd can be contacted with the solvent in a number of different ways but traditionally that part of the solvent retained by the sohd is referred to as the underflow or holdup, whereas the sohd-free solute-laden solvent separated from the sohd after extraction is called the overflow. The holdup of bound hquor plays a vital role in the estimation of separation performance. In practice both static and dynamic holdup are measured in a process study, other parameters of importance being the relationship of holdup to drainage time and percolation rate. The results of such studies permit conclusions to be drawn about the feasibihty of extraction by percolation, the holdup of different bed heights of material prepared for extraction, and the relationship between solute content of the hquor and holdup. If the percolation rate is very low (in the case of oilseeds a minimum percolation rate of 3 x 10 m/s is normally required), extraction by immersion may be more effective. Percolation rate measurements and the methods of utilizing the data have been reported (8,9) these indicate that the effect of solute concentration on holdup plays an important part in determining the solute concentration in the hquor leaving the extractor. 

Figure 3.55. Correlation of dispersed phase fractional holdup values with aqueous (L ) and solvent (O ) flow rates.

The actual volume of each phase in element AV is that of the total volume of the element, multiplied by the respective fractional phase holdup. Hence considering the direction of solute transfer to occur from the aqueous or feed phase into the organic or solvent phase, the mass balance equations become ... 

In the auto-refrigerated reactor shown below, an exothermic reaction A —> B is carried out using a low boiling solvent C. The heat of reaction is removed from the reactor by vapourising the solvent, condensing the vapour in the reflux condenser and returning the condensate as saturated liquid to the reactor. The total holdup of liquid in the reactor is maintained constant, but the temperature of the reactor is controlled by regulating the mass flow of vapour to the condenser. The example is taken from the paper of Luyben (1960). 

Figur 4.31 Sequential, isocratic elution using a stepwise reduction in solvent strength to identify a binary solvent of acceptable strength for elution of a five ca x>nent mixture. In this example the column holdup time was 1 min.

The variation in the holdup of solvent dispersed phase is given by... 

For a given range of feed flow rate, L, it is assumed that the fractional holdup, h, and the solvent phase flow rate, G, can be correlated in the form... 

The program is written in array form, with Nclll )., the parameter representing the desired number of compartments in the column. The simulation starts with a uniform holdup distribution throughout the column and with the column operating at steady-state. At time, Tswitch, the organic solvent flow is suddenly increased, as effected by the use of a MADONNA IF-THEN-ELSE command. 

Kd = the solute s organic solvent water distribution coefficient. k = chromatographic capacity ratio (k — tr — t0/t0, tr and t0 being solute retention time and mobile phase holdup time, respectively), a and b = coefficients whose magnitudes depend on the LL distribution and RPLC systems. 

The types of equipment used, which range from stirred tanks and mixer-settlers to centrifugal contactors and various types of columns, affect both capital and operating costs [9]. In the decision to build a plant, the choice of the most suitable contactor for the specific situation is most important. In some systems, because of the chemistry and mass transfer rates involved, several alternative designs of contacting equipment are available. In the selection of a contactor, one must consider the capacity and stage requirements solvent type and residence time phase flow ratio physical properties direction of mass transfer phase dispersion and coalescence holdup kinetics equilibrium presence of solids overall performance and maintenance as a function of contactor complexity. This may appear very complicated, but with some experience, the choice is relatively simple. 

The objective of this paper is to illustrate the efficacy of inferring the interdroplet forces in a concentrated protein stabilized oil-in-water emulsion from the knowledge of the equilibrium profile of continuous phase liquid holdup (or, dispersed phase faction) when the emulsion is subjected to a centrifugal force field. This is accomplished by demonstrating the sensitivity of continuous phase liquid holdup profile for concentrated oil-in-water emulsions of different interdroplet forces. A Mef discussion of the structure of concentrated oil-in-water emulsion is presented in the next section. A model for centrifugal stability of concentrated emulsion is presented in the subsequent section. This is followed by the simulation of continuous phase liquid holdup profiles for concentrated oil-in-water emulsions for different centrifugal accelerations, protein concentrations, droplet sizes, pH, ionic strengths and the nature of protein-solvent interactions. 

From the above equation, the variation of equilibrium disjoining pressure and the radius of curvature of plateau border with position for a concentrated emulsion can be obtained. If the polarizabilities of the oil, water and the adsorbed protein layer (the effective Hamaker constants), the net charge of protein molecule, ionic strength, protein-solvent interaction and the thickness of the adsorbed protein layer are known, the disjoining pressure II(x/7) can be related to the film thickness using equations 9 -20. The variation of equilitnium film thickness with position in the emulsion can then be calculated. From the knowledge of r and Xp, the variation of cross sectional area of plateau border Qp and the continuous phase liquid holdup e with position can then be calculated using equations 7 and 21 respectively. The results of such calculations for different parameters are presented in the next session. 

Cons. Emulsions can be formed by severe mixing which are hard to break up, pumping of one or both phases between tanks may be required, independent agitation equipment and large floor space needs are expensive, and high holdup of valuable or hazardous solvents exists particularly in the settlers. 

Centrifugal Contactors. These devices have large capacities per unit, short residence times, and small holdup. They can handle systems that emulsify easily or have small density differences or large interfacial tensions or need large ratios of solvent to feed. Some types are employed as separators of mixtures made in other equipment, others as both mixers and settlers, and some as differential contactors. 

The dynamic behavior of the coke gas purification process has been investigated systematically (139,140,145). For instance, local perturbations of the gas load and its composition have been analyzed. A significant dynamic parameter is represented by the liquid holdup. Figure 20 demonstrates the changes of the solvent composition after a decrease of the gas-flow rate from 67 m3/h to 36.4 m3/h and a simultaneous small increase in the liquid-flow rate. 

The liquid holdup of the packing section decreases, which leads to a lower conversion of the kinetically controlled reactions of C02 and a reduction in the C02 absorption rate. As a consequence, the solvent mole fractions of HC()3 and carbamate decreases whereas the relative fraction of HS increases. The selectivity of the absorption process toward the H2S and HCN reduction is enhanced by minimizing the liquid holdup of the column. At the same time, a larger interfacial area improves the performance of the plant. Therefore, modem industrial sour gas scrubbers should be equipped with structured packings. 

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