A forced circulation evaporator is usod to concentrate dilute liquors by evapora

Question

A forced circulation evaporator is usod to concentrate dilute liquors by evaporating solvent from a feed stream. Feed is mixed with recirculating liquor and pumped into the tube side of a vertical heat exchanger. Steam enters the shell side of the vertical heat exchanger and condenses on the outside of the tubes. The liquor is partially vaporized as it passes through he tube side of the heat exchanger and passes into the vapor/liquid separator. Vapor from the separator is condensed in a cooling water exchanger. A portion of the liquid stream tant flow rate pump is withdrawn as product, and the rest of the liquid stream is recirculated to the heat exchanger. The feed rate is uncontrolled ul Cooling Water Pressure PC LC Level Steam ? Evaporator tA Feed Product Product

Explanation / Answer

A)In this forced circulation evaporator, three controllers are used. 1. Pressure controller (PC) 2.Level Controller (LC) 3.Product composition controller (CC). PC controls the pressure inside the evaporator. As we know that, boiling point of liquid increases with increase in pressure, pressure has to be kept low to decrease the BP of liquor so that it may save the energy of steam and hence steam consumption. So monitoring PC is a mandate thing. LC is used to control the level of liquor level in the separator. If the level drops below a certain point in the separator, there may not be proper product flowrate and it will decrease. If the level increases there are chances for the liquid droplets to get into condenser which might interfere with mass transfer operation. Hence an optimum level has to maintained hence LC is used. To measure the concentration of liquor in the product stream, CC is used. The ultimate aim of the evaporator is to concentrate the feed, hence the CC controls this so that it won’t drop below a certain composition. B) If the level in the separator increases beyond a particular value, to counteract and to maintain the level, more product has to be withdrawn. Hence it is recommended to withdraw product to control the liquid level in the separator.

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