1 reply to this topic

[Octavius7]

Ok say we have a feed stream with some %NaOH in water coming into a vessel in which it is heated and two streams are taken off, an evaporate stream and a concentrated NaOH stream.. The pressure in the vessel is controlled by changing the flow rate of cooling water to a heat exchanger which thus condenses more or less steam that leaves as condensate.

Anyways we had a test question regarding this and I said that you could control the % NaOH in the product stream leaving the bottom by controlling the liquid level in the tank and pressure in the tank. I am correct in saying that at some liquid level (and thus the liquid and vapor volumes in the vessel) and some pressure the %NaOH can be calculated (how would you go about calculating this though? what type of model to use?).

Now obviously this wouldn't be the best way to control the process but it is what I put down on a test and the professor said he didn't think it was possible but I still believe it is and would like to come back with proof. (was with regards to a question on final exam)

[isbarqi]

dear friend Octavius7.

anyway, the way to control the desired concentration of product in evaporator is strongly depend on the ENERGY BALANCE within that evaporator system. Of course the operation pressure is highly affect the energy balance since the pressure affect boiling point of the solution being concentrated. Usually as far as I know, the evaporators operational pressure is designed since early construction of that evaporator, since that pressure must be matched with heating area of evaporator (the lower the operational pressure the smaller area can be obtained). It also depend on the steam condition as the heating media nonetheless (pressure and/or temperature of steam being used). Since the heating surface area is constant and usually in the real plant steam condition is also constant (usually all steam needed equipment use the same type steam from central boiler, commonly low pressure steam), the two things that have to be controlled is the operational pressure and the amount of steam used and it depend each other, once you change the pressure you must to change the amount of steam as long as you want to get a constant product concentration. for the calculation, it depend on the number of effect of evaporator. it's quite simple for single effect evaporator (I guess yours is single effect), you can check on various textbook, for instance you can find it in "Unit Operations of Chemical Engineering" by McCabe, Smith, Harriot, McGraw-Hill. Chapter 16 (in 5th edition). one more possible source is http://www.nzifst.or...evaporation.htm.

For the liquid level problem, I don't know exactly the relation to product concentration but as far as I know, determination liquid level of the evaporator is strongly dependent of the type of evaporator. for instance, heat transfer of short-tube vertical evaporator type is affected by the liquid level. Highest heat transfer are achieved when the level is only about halfway up the tube. (you can check this information in Perry's Chemical Engineering Handbook). different type of evaporator give different optimal liquid level. I didn't get any information for the calculation relating with this liquid level so-called problem.

Hope this can help you....