Question

Separation Process Problem 1 [30 marks] A double-effect forward-feed evaporator is used to concentrate a 5 wt% aqueous NaCl solution to a final concentration of 25 wt% The feed enters the first effect at a temperature of 40°C. Saturated steam at a pressure of 2 bar (absolute) is fed to the first effect at a rate of 1500 kg/h, and the vapour from this effect is used to supply heat to the second effect. The overall heat transfer coefficient in both effects are 1500 W m2 K·The heat transfer area is 20 m2 in each effect. The boiling point rise and specific heat of the solution may be estimated from the following expressions: BPRIC P.Lig Sol N-4.2-4x[kg NaC/kg solution P.NaCISol a) Draw a fully labeled flowchart for the process described above. Calculate the pressure in each effect, the flow rate of the feed, and the flow rates of all vapour and liquid streams. You may use a spreadsheet for your calculations, but each step must be clearly described below b) Suggest a configuration for a single-effect evaporator with mechanical vapour compression that would accomplish the same task as the multi-effect system described in part (a). Assume that such an evaporator would operate at a pressure of 25 kPa. The minimum requirement for answering this question is to draw a diagram to describe the proposed configuration and to describe the procedure that you would use to perform sizing calculations (including any assumptions that you would make). You may choose to implement some or all of the calculations, but this is not strictly required to receive full marks for the question. Hint: the new single effect evaporator should handle the same amount of feed as the multiple effect in part (a) and generate the same amount of concentrated final product that is produced in part a (the liquid product that exits effect 2 in part a). This part is optional: if you want to implement all the calculation using numbers, assume that you obtained the following results from, part a). Also assume efficiency of the compressor-75% F-3022.30 kg/h v,=1 183.61 kg/h ˇ 1234.23 kg/h L1= 1838.69 kg/h L2-604.46 kg/h

Answer 1

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