Design membrane area, determination

The value of the parameters can be determined with only a cursory consideration of design. The area resistance, r, is composed of the membrane resistance and the stream resistance, its square root appearing in the smallest cost expression. The membrane resistance quoted by manufacturers is a static value, measured while the membrane still has its minority carriers and consequently is not yet markedly permselective. In operation the membrane has a resistance nearly twice the values quoted. A value of 25 ohms per sq. cm. per pair, measured for some thin membranes, is used in the following calculations. Because the resistance of the concentrate stream can be made arbitrarily small by an increase of concentration, a value /4 of dilute stream resistance has been used for the fluid resistance in the preparation of Figure 1. 

Several field test studies have been undertaken utilizing the SEPAREX process in a 2-in. diameter element size Due to the modular configuration of membrane systems, a full size system can be directly designed from the test results with a small pilot plant. Although the flow rates for a pilot unit are considerably lower than might be encountered in a full-size system, all process parameters such as product purities, pressure drop, product recoveries, optimum pressure and temperature, membrane area required and series/parallel arrangement of the elements can be directly determined. 

In general, every stream must be tested to determine design factors such as the specific membrane polymer, membrane element design, total membrane area, applied pressure, system recovery, flow conditions, membrane element array, and pretreatment requirements. 

So, the rate-controlling step of the BAHLM transport of the solute to the strip phase is determined by the Kf/e overall mass-transfer coefficient. In this case, at designing the BAHLM module, the main attention has to be taken to the determination of optimal feed-side membrane area. 

Pmp Pr and Pp are considered fixed or determined by conditions external to the model. The remaining variables are 0> Pi, and A, totaling 2C + 2 variables. Hence, one variable must be specified in order to define the process. This could be the fraction permeated, the membrane area, or one of the components mole fraction in the permeate or residue. For existing equipment the membrane area is known and the products flow rates and compositions can be calculated. In a design situation the membrane area can be calculated to satisfy a performance specihcation such as the fraction permeated or a component mole fraction in one of the products. In this discussion the permeances are assumed to be known, which implies given permeabilities and membrane thickness. 

It is required to design a reverse osmosis unit to process 2500 mVh of seawater at 25°C containing 3.5 wt% dissolved salts, and produce purified water with 0.05 wt% dissolved salts. The pressure will be maintained at 135 atm on the residue side and 3.5 atm on the permeate side, and the temperature on both sides at 25°C. The dissolved salts may be assumed to be NaCl. With the proposed membrane, the salt permeance is 8.0 x 10 m/h and the water permeance is 0.085 kg/rn-.h.atrn. The density of the feed seawater is 1020 kg/m ( of the permeate, 997.5 kg/nv and of the residue (with an estimated salt content of 5 wt%), 1035 kg/rnc Assuming a perfect mixing model and neglecting the mass transfer resistances, determine the required membrane area and calculate the product flow rates and compositions. 

Recently, the fluidized bed membrane reactor (FBMR) has also been examined from the scale-up and practical points of view. Key factors affecting the performance of a commercial FBMR were analysed and compared to corresponding factors in the PBMR. Challenges to the commercial viability of the FBMR were identified. A very important design parameter was determined to be the distribution of membrane area between the dense bed and the dilute phase. Key areas for commercial viability were mechanical stability of reactor internals, the durability of the membrane material, and the effect of gas withdrawal on fluidization. Thermal uniformity was identified as an advantageous property of the FBMR. 

EXAMPLE 13.4-2, Membrane Design for Separation of Air It is desired to determine the membrane area needed to separate an air stream using a membrane 1 mil thick with an oxygen permeability of P = 500 X 10 cm (STP) cmy(s cm cm Hg). An a = 10 for oxygen permeability divided by nitrogen permeability (S6) will be used. 

This furnishes a determination (i.e., estimation) for the membrane area A2 in the rectification section based on component i. Integration is from the feed location toward the more-permeable product end, designated D, with A viewed as positive. 

Design problem. The concentrate outlet composition Xa2l is provided. We have to determine the total membrane area and other necessary quantities. There can be other types of specifications instead of xa2l-... 

Consider now the rating problem where the membrane area is known the two product flow rates and compositions have to be determined. Here also assume a value of xa2l at the exit end. Then foUow the procedure described earlier for the design problem and arrive at the feed end where A+ now would be zero since is known at z = L and therefore A - dA is stiU positive at z = X - dX and so on tiU z = 0. Check now whether the value of jcaio and 9 obtained numerically satisfy the relation (8.1.438d) for the assumed x zl- If they do not, make an additional guess of Xa2l and repeat the procedure. 

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