Actual exam questions for the Chemical Engineering major, 2011 – Case study, question 13 in the afternoon session
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Hello everyone. It’s question 13 from the afternoon exam questions on cases related to the Chemical Engineering and Technology major from 2011: the calculation of the cross-sectional area of a distillation tower. The formula is as follows – Cross-sectional area = bubbling area on the tray + 2 * area of the downcomer pipes. Where can this formula be found? I couldn’t find it in any textbooks on principles of chemical engineering or in any relevant examination materials
There’s a problem with this question; in my opinion, it cannot be used, as such calculations do not even meet the requirements regarding the speed of point spread. I really haven’t been able to find a formula for this
I would like to ask about questions 1 and 2 of the case study from the afternoon of year 11 Where does the relationship between the recovery rate and m in question 2 come from? Do we need to subtract 1 for the first question? Also, the 17.5 figure shown online isn’t correct, right?
For the first question, 1 needs to be subtracted, as the reboiler itself is also considered a theoretical plate. The approach used to solve this question is correct; according to the answer, the content of n-butane at the top of the tower should be 0.1； The second question is no problem; just use the given formulae, and plug in the values to get the answer.
Does the theoretical plate not include a reboiler? The question asks for the minimum theoretical number of plates for this tower; no information such as details about the tower itself is provided, and I’ve always found this confusing. There is also a cooling condenser at the top of the tower, which can be considered a fractional condenser – so shouldn’t the value be reduced by 2? How do I determine this?
The minimum number of theoretical plates does not include the reboiler and the degasifier; in some questions it is explicitly stated that both the degasifier and the reboiler are included, and in such cases they should be deducted. Let me ask you another question. There are 32 questions on professional knowledge in the morning session of the 12th exam; take a look at them
For the 32 questions related to 12 years of professional knowledge in the morning session, my calculated score was 77 points
How was it calculated? Could you show me the calculation process?
I’ll get back to you tomorrow; I’ll do it tomorrow morning. I won’t look at it for now
This question is rather unconventional; it requires the use of the heat transfer film coefficient for convective heat transfer, as explained in section 4.3.2.1 of the guide, on page P265 (old edition). In other words, the heat transfer rate in a single layer is related to the fluid temperature and the wall temperature. Now that we know the heat transfer rate, the film coefficient, as well as the inlet and outlet temperatures of the fluid, that’s sufficient. Using the logarithmic mean temperature, I calculated it to be 91°C; if the arithmetic mean is used, it seems to be 82°C
The formula you mentioned isn’t stated explicitly, but it’s mentioned indirectly in the book. For specific details, refer to Chen Minheng’s Principles of Chemical Engineering, Volume 2, page 123. Regarding the plate arrangement in sieve tray towers. But I really don’t understand; this should refer to the flooding gas velocity, and if we follow that logic, it should have nothing to do with these areas. The answers written there are also brief; I couldn’t understand them. If you can solve it, could you tell me the detailed solution?