7 replies to this topic

[ors1237]

Hello all,

 

I have been set the task of designing a heat exchanger for use in a cumene production process and am unsure if my method of carrying out the task is correct. The problem statement outlines how the conversion reactor is cooled using dowtherm, which enters the reactor at 200C and exits at 210C. The dowtherm is then itself cooled by generating 15 bar gauge steam using a shell and tube heat exchanger and a waste heat boiler. After carrying out an energy balance over the reactor, along with modelling the process in Aspen HYSYS, i determined the heat to be removed from the reactor (in order to maintain a constant outlet temperature) is -1.386 E+6 kJ/h. There are little design parameters for the heat exchanger other than a tube velocity of between 1 and 3 m/s is to be achieved. I have assumed a total heat transfer coefficient, U, of 200 kJ/h.m^2.C for a shell and tube using dowtherm from Coulson's. I have determined the steam outlet temperature, 201.41C, from saturation tables, however am unsure what the inlet temperature would be? I have assumed the dowtherm to be flowing tube side however am not sure if this would be best? Should i be getting a positive or negative LMTD for the arrangement? I am to choose my own shell and tube size, arrangement length etc...

 

Any help would be much appreciated.

 

Regards, Oliver

[Bobby Strain]

Sounds strange that you are using such a reactor for producing cumene by alkylating benzene with propylene. It's unlike any process that I am aware of.

 

Bobby

[ors1237]

It's just a hypothetical scenario for a group project, a conversion reactor seemed like the simplest one to use in HYSYS so we could set 100% conversion, which obviously wouldn't be achieved in a real world scenario. The majority of the project is regarding the heat exchanger design.

 

Oliver

[srfish]

What is the waste heat boiler to be used for?

 

Yes, I would put the Dowtherm in the tubes. What kind of Dowtherm is it?

 

There will be very little cooling of the Dowtherm if it enters the heat exchanger at 210 C and the steam is to be generated at a temperature of 201.4 C. 

[Pingue2008]

ors1237,

 

A sketch of your system could be very helpful. LMTD should always be positive.I agree with "Srfish" I would put the dowtherm in the tubeside.

Thanks

Attached Files

•  HX-101.xlsx   13.13KB   396 downloads

Edited by Pingue2008, 18 March 2013 - 03:47 PM.

[ors1237]

It is Dowtherm A to be used, as for the use of the waste heat boiler i am unsure, we were just told 15 barg steam would be generated to cool the Dowtherm, however i have chosen to reduce the pressure to 12 barg in order to give me a higher LMTD, now around 20C. What is a reasonable number of tubes to have? I have chosen 4 passes in order to reach a tubeside velocity of 1.5 m/s and there is around 200 tubes.

 

Thanks

[srfish]

ors1237

 

I would put the heat exchanger into a type BKU to generate the steam. The tube count of 200 tubes (100 u-tubes) is reasonable if the straight tube length is 16 ft.

[ors1237]

Thank you for the help