7 replies to this topic

[Nabila]

Hello,
I'm a final year student and these days i'm working on my final year project entitled production of butyl acetate to complete. Currently I'm working on the process integration. I have a problem in finding the minimum temperature difference, ΔTmin or Approach temperature for butyl acetate production. If anybody could help me I would be very very grateful.

Reaction: Esterification of butanol and acetic acid catalyzed by acetic acid

Edited by Nabila, 21 March 2011 - 11:27 AM.

[pavanayi]

Nabila,
What exactly is your problem?
Describe what steps you have done till the point where you are doubtful how to proceed.

Edited by pavanayi, 21 March 2011 - 03:33 PM.

[Nabila]

Nabila,
What exactly is your problem?
Describe what steps you have done till the point where you are doubtful how to proceed.

Thanks for the respond,

The first step is to convert the actual stream temperatures T_act into interval temperatures T_int by subtracting half the minimum temperature difference from the hot stream temperatures, and by adding half to the cold stream temperatures such as follows:

hot stream T_int = T_act - (ΔTmin / 2 )

cold stream T_int = T_act + (ΔTmin / 2 )

Based on Coulson & Richardson's Chemical Engineering Volume 6 the value of ΔTmin should not be less than 10°C and some of the examples in the book take 10°C as the ΔTmin. Does ΔTmin = 10°C can be applied to all problems? Or there are different value for different industry, in my case the production of butyl acetate?

[pavanayi]

Nabila,
There are methods/softwares to estimate what is the best ΔTmin for a given process. This is based on optimising the total heat exchange area required.
And yes, for different industries, different ΔTmin are used. For example, in plants using refrigeration, the value might be as small as possible (and it is below 10°C). In cases where there are lots of heat transfer involving gases, ΔTmin is usually high.
There are several papers available online which focuses on this issue.

If you have the software capability, you might attempt it, but you would need meaningful cost estimation factors for that.

But, as far as a final year project is considered, I dont think you are expected to find the optimum ΔTmin. If a value is not given to you by your supervisor, you can as well assume a value (10°C is more than sufficient).

[riven]

As pointed out already ΔTmin can vary. Mine below is only a very general guide.

Consider in very simple terms the same service where water is on both sides of the exchanger with one side cood and one side heot in the range of 20-100C.
A double pipe exchange might get a ΔTmin of 15C.
A shell and tube (depending on design) might get 10C.
A compact exchanger (plate and frame or spiral) might get to 2C.
(I have seen very small printed exchanger (area in cm2) have a ΔT of 0.5C)

For your final project what is required probably is for you to mainly consider your exchanger type and service i.e gas liquid temperature (cryogenic, furnace etc). Once you do that and pick a ΔT with justification, you will be fine. That is not to say you have to go through every exchanger. Time is short so generalise.
For the first stage of design I will typically use for a bioethanol plant 10C on every exchanger unless there is solids loading and on the cooling tower.

[Nabila]

pavanayi:

I think i should just use 10°C. However, there must justification for that right? Can you suggest me some references other than Coulson & Richardson's Chemical Engineering Volume 6 so that I can provide a concrete justification in my report.

[Nabila]

riven:


Thanks for reply,

Did you get this general guide from books or articles? If so, can you send me a soft copy of that so that i can read the details and put it in the references of my report?

[pavanayi]

Nabila,
As a starting point, you can refer to page 361 of Chemical Process Design and Integration by Robin Smith.

You might also want to check some research papers (Y Li, 1986)