7 replies to this topic

[kim lee]

Hello! I am trying to design a heat exchanger. My hot fluid is a vapor mixture (a mixture of styrene, benzene, toluene, ethylbenzene, ethylene, h2O, methane, CO2, H2) and my 'cold' fluid is also a vapor mixture. (mostly composed of ethylbenzene and has relatively small amounts of styrene, benzene and toluene). The inlet and outlet temp of the 'hot fluid' is 883 K and 526 K, respectively. And the inlet and outlet temp for the 'cold' fluid are 420 K and 845 K, respectively. The pressure is at 1 atm.

 

What should be a suitable type of heat exchanger to be used for these conditions? Should I change the operating pressure? I tried using shell and tube but then the pressure drop is too high. Probably because of the gas' small density? I am only familiar with shell and tube. Please drop in your recommendations or solutions. Thanks a lot.

[thorium90]

It is just hypothetical right? Just for calculations sake? Since you even mention you can change your own operating pressures? Wouldnt an AES type work well? Perhaps you can post your calculations here, im sure there are many people eager to check your spreadsheet

Edited by thorium90, 11 March 2013 - 09:27 AM.

[kim lee]

Yup! It's only for calculations sake and just hypothetical. Yes, I was actually working on the AES type of S/T Heat exchanger. But as I've mentioned, I calculated a very high tube pressure drop. It was around 1million plus Pa. Should I change the S/T specs instead? or is there another type of heat exchanger more suitable for gas-gas? I read one source that Plate fin HE is an option for gas-gas. But I've also read another source that it can only work within limited temperature ranges.. 

[Art Montemayor]

I would not recommend an internal floating head TEMA type exchanger for gas-to-gas heat exchange.  There is no justification for the floating head when a simple BEU will do the job.

 

Temperatures in the range of 600 ^oC are extraordinarily hot for a shell & tube unit in the first place and this puts exceptional stress on any internal gadgets like internal floating heads with screwed fittings and gasketed connections.  The internal joints won't stay sealed for very long before leaking starts to take place.    Even if the gas streams contain some fines or solids, I would rather put filters on the streams and still only use a simple, cost-effective BEU with the highest pressure in the tube side.

 

Keep it simple!   Why resort to an internal floating head if you don't have to?   No mention is made of any solids or having to clean out the exchanger on routine basis.  So why even use an "A" cover plate?    Internal floating heads were developed for refinery applications involving solids, sludges, muds, salts, and other trash mixed in with the crude oil.  This is not the case described here.  All internal floating heads should be justified from a process, operating, maintenance, and economic point of view.

 

You are not a real plant engineer until you have spent the weekend with your maintenance crew trying to extract a TEMA internal floating tube bundle.  After surviving that type of experience, you have arrived.

[srfish]

With that much of a temperature cross where the colder side outlet temperature is thaqt much higher than the hot side outlet temperature, you will need for economies sake a counterflow type heat exchanger.    

[kim lee]

@ Art Montemayor -  Thanks a lot sir. I might have mixed up the applications and usages of the different types of TEMA HEs. My apologies. I am a student and my group is doing plant design on production of styrene. I am tasked to design the HEs involved in the process. The outlet temperature from the reactor is about 883 K and since it is desired to separate styrene from the gas stream, we need to cool it down to a temperature where it is efficient enough for the gas stream to be condensed. So that's why from 883 K, we decided to lower it down to just around 526 K. Then afer, a condenser then liquifies the aromatics (including styrene). The heat that is released as the gas products stream is cooled down is then used as to heat up the vaporized ethylbenzene feed (EB is our raw material). With the high temperature conditions, I am at lost as to how I should design the HE. But gladly, I am now a bit enlightened to how I should go about my design. I'll be using BEU and I'll operate at higher pressures. I understand that at higher pressure, the density of the gas mixture increases and thus decreasing pressure drop. Right? 

[kim lee]

@srfish - Yes sir, I am employing counterflow type heat exchanger. The problem is that I am not sure of what type of HE to be used; if it should be even Shell and Tube, in the first place since the temperature conditions are high and the pressure drop of the gas streams are calculated to be much higher than are allowable. I was considering plate and fin since I've read sources that it can be used in gas-gas but I am not familiar with how to go about the calculations. 

[Art Montemayor]

Kim Lee:

 

Try to use engineering units for your temperature readings.  Conventional industrial thermal instruments are furnished in Celsius or Fahrenheit scales – not absolute scales (Kelvin, Rankine).  Perhaps you are a chemistry student.  Absolute temperatures are more popular with the pure science majors, but engineering only uses absolute temperatures for special equations and identifying properties.

 

You have failed to tell us your flow rates and the estimated heat transfer area you are proposing.  The heat transfer area is an important data to consider when selecting a proper type of exchanger.

 

Try to apply what Srfish has recommended.  That is the proper and practical logic when considering heat exchange that may involve a critical temperature cross.  Pure counter flow is the way to go – but you must have a clear estimate of the size of heat transfer area you are dealing with.  There is a way you can employ a basic BEU type of exchanger and convert it to a counter flow type – but first estimate the heat transfer area because you might have a rather small size where you can simply used a double-pipe type of unit instead of a formal TEMA design.  Because of the high temperatures you may want to avoid any fixed tube sheet design due to a need for expansion joints – which I always try to avoid because of the inherent weak points the expansion joints represent.

 

With one stream at relatively high pressure, that stream should normally go in the tube side.  Using a double-pipe or U-tube bundle makes the application a simpler one because the tubes are allowed to expand freely and no stresses are built up at high temperatures.   Spend some time studying your TEMA design options and learn the advantages/disadvantages of each type.