21 replies to this topic

[alicia88]

I did heat integration of my design plant and drawn the PFD after heat integration. But i have problem in determining the shell inlet and outlet temperature after heat integration. If the product to be heated is at the tube part of the HEX from 29oC to 116.7oC, how can i determine the temperature&pressure of inlet and outlet of the shell part of the HEX?
Is it i have to assume the temperature by myself as well as the flowrate of the steam in order to determine the temperature? Hope to get some advice regarding this matter.

[pavanayi]

Alicia,
Your post is a bit unclear to me. Did you apply heat integration to an already developed PFD? Or are you developing a conceptual design and then did a HX network design?
Please upload the PFDs, HX network diagram, and describe the steps you have taken in applying heat integration including a description about the utilities in the design.

Edited by pavanayi, 25 February 2011 - 11:13 AM.

[alicia88]

pavanayi

I did heat integration on E-501 (cold stream), E511 (hot stream),E512 (hot stream),E513 (hot stream)
Upon performing pinch analysis, i obtain the HEN design as per attached
For PFD before pinch, temperature for critical stream are stated as below
While for PFD after pinch, the temperature of the HEX and cooler/heater involved are stated at the stream line itself. Based on the HEN, i added 4 HEX, which is E514, 515,516 & 517.

But now i doesn't know how to determine temperature for the HEX for tube at E514, and shell of E515, E516 and E517, whereby those are stream that will be passed by steam.

Hope to get guidance. Thanks.

Attached Files

•  PFD before pinch.pdf   21.11KB   34 downloads
•  pfd after pinch.pdf   26.41KB   31 downloads
•  Heat Exchanger Network Design.pdf   11.43KB   37 downloads

[pavanayi]

Alicia,
From the attached documents, I gather that you decided min DelT as 10°C. Is that right?

Also, What are your utilities? When you are designing a plant, the maximum process to process heat exchange is facilitated with pinch study and HX network. The rest heating and cooling will be done by utilities. What are the temperature and pressure levels of those utilities? (Steam levels), cooling water (supply and return temperatures?) I can see there are mentions of HPS, LPS, CWi and CWo on the PFD, but fail to see these streams in the HX network that you uploaded?

What are the mass heat capacities (m.cp) of the streams on the HX network? You have not marked them.

As a basic minimum, tell me what are the mass heat capacities of the streams and utility levels that you have decided. If you can update the HX network, that would be better. Dont worry about the shell side. I'll tell what you need to do/where you are going wrong.

On a secondary note, why have you not included other heat exchangers in the analysis? F-501/E-502/E-503/E-504/reboilers and condensers of all the columns? If you had done that, this would have been a really good project.

[pavanayi]

Alicia,
When you have one cold stream and three hot streams, you only need three exchangers. The way you drew the PFD for the HX network after pinch is not correct. There would be no stream that gets heated up at E-514, and then get cooled in E-515/516/517.
What you should have drawn is the stream 1 heating progressively (on the shell side) of three heat exchangers, while the other streams (tube side) get cooled.
I have attached a modified PFD in the excel sheet, with the calculated shell side temperatures for the three exchangers. Have a look and see if you understand. There is no need for streams 27,27a,28,28a.

Attached Files

•  hx net.xls   101KB   51 downloads

[alicia88]

Pavanayi


-yup min dT is at 10oC.
-I didn't start yet for utilities design as i wish to complete the heat integration before starting the next part of design.
- Pinch analysis was not being done on those stream (E502,E503,E504) as those cooler operates to reduce temperature fr almost 600oC to 65oC.
In addition, if were to integrate those, there are no pinch temperature exists.
-I was thinking to supply steam fr the steam superheater (lps) to E503, then the outlet (bfw) to E-502, and hps to the inlet of E-501
-m cp of streams involved are as attached below
-Is it possible for me integrate F-501?how do heat transfer occur between the flue gas & other stream?
- E502,E503,E504 could not be integrated because no pinch temperature are obtained due to large dT for the 3 cooler. If F501 could be involved, then pinch T could be obtained.
-for reboiler & cooler, quite impossible to integrate because it does not approach to the pinch temperature which is much more higher. It has low dT

I think 3 cooler and a heater should be added as well due to this reasons.Please find attached for further info regarding my explanation
-As what i understand, stream 3 needs a heater with 523.88 kW of energy (for del H above pinch)
-While for below pinch, cooler with 5.06kW, 14.52kW & 215.83 kW need to be added to accomodate the the cooling to 30oC & 15oC respectively below the pinch temperature.

Attached Files

•  Grid network.pdf   47.22KB   26 downloads

[alicia88]

Pavayani

Actually i did some mistake in the pfd by considering 1 extra HEX. Corrected version are as attached.
I had few questions regarding to Heat Integration:
1) Is it possible for me to integrate steam superheater since only flue gas will be released by the fired heater? how do i integrate it with my exchanger then?
2) How can i determine the temperature of stream 27a & 28 of the attached PFD?
3)Is it ok if Ethylbenzene entered the shell of heat exchanger E515,516,517 or should it be entering the tube of those exchangers?

Attached Files

•  pfd after pinch.pdf   26.33KB   25 downloads

Edited by alicia88, 27 February 2011 - 10:35 AM.

[pavanayi]

Alicia,

Now you have correctly figured out how to convert the HX network into the PFD. But as per your previous HX net, stream 4 on the HX net (stream 1 on the PFD) should have first heated up against stream 3 on the HX net (E-517) rather than E-515 as per PFD. But it dosen't matter in this case as both cases, you are starting from the pinch and moving outwards.

1. Temperature of stream 27 a:
In a heat exchanger, ideally, heat released by hot stream = heat gained by the cold stream
For E-515, heat lost by the hot stream = 68.44KW , which should be the heat gained by the cold stream (27--27a)
Therefore, for stream 27, (m.cp)*delT = 68.44 KW
for stream 27, (m.cp) = 15.226
Hence, delT = 68.44/15.226 = 4.49°C
The temperature of stream 27 = 29°C
Hence, temperature of stream 27a = 29+4.49 = 33.49°C

2. Temperature of stream 28:
Similar to argument above, heat lost by hot stream = duty of exchanger E-516 = 255.13 KW
m.cp of cold stream = 15.226 (27a is the inlet)
delT of the cold stream = 255.13/15.226 = 16.76°C
Temperature of stream 28 = 33.49+16.76 = 50.25°C

3. Temperature of stream 28 a:
Similar to above, duty of the exchanger = 1011.64 KW
m.cp of cold stream = 15.226 (same cold stream)
delT of cold stream = 1011.64/15.226 = 66.44°C
Temperature of stream 28a = 66.44+50.25 = 116.69°C

4. Cross checking the results: If the above calculations were correct, then the same should apply for E-501, and we should get an outlet temp of 151.1°C.
duty of exchanger E-501 = 523.88 KW
m.cp of cold stream = 15.226
delT of cold stream = 523.88/15.226 = 34.41°C
Temperature of stream 2 calculated = 116.69+34.41 = 151.1°C (which is what it should be)
So we can be assured that our calculations are correct



In a shell and tube exchanger, which fluid should be in the shell side and which fluid should be in the tube side are selected based on the characteristics of both fluids, and you normally have to take a number of parameteres into consideration. I believe you should have gone through this part in your heat transfer course. It is also covered in a wide variety of text books. Try and refer back to that course and see if you can figure it yourselves out. This will come handy in future too.

Flue gas in a fired heater can be considerd as a hot stream. The tempearture of the flue gas as it exits the radiant section will be very high, and you can consider it as a heat source, which you need to cool before you can release it to atmosphere. (This is required by the environmental consideration too). As a guideline, you can consider the stream to have an initial temperature of 1000°C and a final temperature of 200°C. But you will have to calculate the m.cp value, by calculating the mass flow of the flue gas (Orsat analysis)

[alicia88]

Pavayani:

Can I just assume that the flue gas heat will be directly transferred to an economizer for boiler water preheat? If in this case, then it will not be involved in heat integration but still economical as well.

[pavanayi]

Alicia,
Recovering flue gas heat by preheating BFW is also heat integraton!!!. Whenever you are recovering available process heat back into the process so that you reduce your dependence on external utilities (steam, fuel gas for the heater etc) all that processes come under 'heat integration'. When you are analysing a process for pinch, and for heat integration, you ideally should include all streams for analysis.

What do you mean by saying there is no pinch temperature existing? If you analyse correctly, for any process, there always will be a pinch temperature. Sometimes you will end up with a threshold system (where either you dont require hot utility or you dont require cold utility at all). Thats all.

For reboilers and condensers, even though dT is low, dH is large (due to phase change). It will stand out on your grand composite curve. (Have you developed one?). That is why you also should have included the reactor outlet stream (7 to 10) as well as steam preheat (F-501) within pinch analysis as well.

If you had, you would have seen that you dont need E-501 to be heated by a utility to reach 151°C, because you have hot stream at 600°C (reactor effluent, stream no.7) that you had not considered within the analysis. Also, a part of the remaining heat could have been recoverd in heating up the steam (reactor feed) thereby reducing your dependency on the fired heater. You dont want to generate too much BFW and too much steam through heat recovery, to end up having no use for them!!.

I did not understand your logic regarding E-502 and 503. In the reactor effluent stream (7 to 10), as per PFD, E-502 has BFW coming in and HPS coming out (heat is recovered from effluent by generating steam). But in E-503, you have showed LP steam condensing to BFW ? LPS --> BFW ? (utility heating up the reactor effluent!!). That should have been the other way round.
Instead of the crude way of planning "you will generate steam here, superheat there and utilise it somewhere else", Pinch analysis provides you a structured way of doing the very same thing. That is why I said you should have included even that stream in the analysis.

For below pinch, your observation is correct in saying 3 coolers need to be added.

How are you going to attempt the utilities design? Particularly steam levels (temperatures and pressures) determination? Are you doing a total site analysis? I have no idea how deep/detailed your course in heat integration is.

If you have time and you want to do, try and attempt a pinch analysis for the whole process. I can help you if you are stuck somewhere. It will help in understanding the principles of pinch analysis perfectly.

Hope the information has helped you. Also, just for information, which year of university are you at the moment?

Edited by pavanayi, 28 February 2011 - 09:24 AM.

[alicia88]

Pavanayi

Sorry for the late reply as I was not being able to online for the past 2 weeks. I did integration on all utilities but I found out that 2,3,4,5,6,8 did not pass through the pinch temperature as attached. Is it possible for me to add exchangers on those streams as attached (4-8,5-8,6-8). Or Should I be doing a separate pinch analysis for those exchangers?

***I'm 3rd year student and we would have to alter the PFD upon doing pinch analysis for this project

Attached Files

•  Heat Exchanger Network Design integrated.jpg   37.79KB   17 downloads

[pavanayi]

Alicia,
In the diagram that you attached, are the three streams (1,2,3) the reactor outlet stream? If then, they should have been one single stream rather than three different ones.
Can you upload the stream data for the figure (stream no, Initial T, final T, m.cp, dH) ? I can do a quick cross check on the pinch temperature, and then tell you if you are doing correctly. I cant see the duties of the exchangers in the diagram, so I believe they all are okay.

But overall, it does seem okay. You will have two cooling water exchangers below pinch (for stream 6, as you have correctly shown, and one more for cooling 2+3. Now it makes more sense because your reactor inlet stream is much less dependent on external heating (only from 570°C to 850°C) rather than your previous condition (159°C to 850°C).

The only point of concern I am having is with your exchanger between stream no. 8 and 4. Outlet temperature of stream 8 is 151.1°C and inlet temp of 4 is 160.7°C, which means the difference is 9.6°C. Check that and rectify

There is no requirement for all streams to pass through the pinch point. This is because, particularly here, it is an endothermic process. The reactants need to be heated so much that overall, the process pinch temperature is quite high. Such situations are quite normal. As long as you do not need external heating below pinch, or external cooling above pinch, your analysis is working fine.

It should be straight forward for you to modify your pfd to incorporate all process to process heat exchangers.

Edited by pavanayi, 12 March 2011 - 10:59 AM.

[alicia88]

Pavanayi

Data involved are as below. Please be noted that the numbers (1-8) refers to the stream as per the heat exchanger network attached in previous thread respectively:

(1) mCp=69.732 H= 12551.76
(2) mCp=63.970 H= 12794

(3) mCp=56.709 H= 7655.72
(4) mCp=0.5624 H= 73.51

(5) mCp=1.6137 H= 269.65
(6) mCp=8.9844 H= 1227.27

(7) mCp=39.8398 Below pinch H= 16374.16 Above pinch H= 11155.14
(8) mCp=8.928 H= 1090.11

Please be noted that stream temp are as shown in diagram attached previously

-For the reactor product cooler, I would like to know if is it really practicable for a temp drop between 580oC to 65oC be operated by only a single exchangers in industry? May i know what is the normal range?
- I have no idea on integrating utilities on distillation column as it involves a phase change. Doesn't have a brief explanation regarding that in literature.
- 3,4,5,6,8 is totally discontinuous with 1,2,7. I doesn't know if it is correct to do pinch analysis that way? If were to do a pinch for only that stream, I think the pinch T obtained would be different and be much much lower

[pavanayi]

Alicia,
What I meant regarding the reactor outlet stream was that, in the figure you attached, it should have been represented by one line (one stream) from 580°C to 65°C, and not one exchanger.
Then, similar to your stream 8, different exchangers are to be mapped on to that stream, progressively reducing the temperature. Whatever delH is remaining after all possible process to process heat recovery will be rejected to cooling water.

I will go through the data and get back to you.

[alicia88]

pavanayi:

Sorry to make the question to be confusing. What i meant is: based on the previous PFD attached, can the temperature drop between 580 oC to 65 oC be only represented by only a single product cooler instead of 3? I was not sure of this as it involved a large T drop and hence I decided to cool the in phase (3 cooler). Really, really thanks for your help.

[pavanayi]

Alicia,
Well, now I am confused what you want!!!
Let me explain a bit about PFD before pinch and PFD after pinch.

When you have a initial PFD (before heat integration), you dont know how many exchangers/heaters/coolers your design will finally have. As an example, let us take the feed stream: ethyl benzene that needs to be heated to 151.1°C from 29°C. In the PFD, that will be represented as only one heater, with inlet and outlet temperatures and the duty. You are not concerned with the number of exchangers or split of duties because at this point you do not know how they are going to be. That stream is one among your streams within the HX net (correctly shown by one single stream, no.8). After integration, you now know there is going to be three exchangers (the ones you mapped), so you will include them in your design. In the PFD after pinch, you will show the feed stream passing through three heat exchangers, getting progressively heated to its final temperature as you have designed.

Similarly, with the reactor product cooler, before pinch, you represent the total temperature drop with only one cooler. This representation does not have any relation to industrial scenario or practises because you have not even done the integration to know in how many steps the same cooling will take place. As you said, the temperature range is quite large, and therefore, you can expect to map several exchangers to that stream. As per your representation in the HX network, the exchangers (1-7 and 2-7), it should have been one single exchanger because it is the same two streams between which you are exchanging heat, and it is only logical that it be done in one exchanger. At this stage, you do not think about industrial practise neither should you be worried about them.

You do not need to guess how many exchangers you need, before you attempt heat integration, because then it defeats the whole purpose of analysis. Heat Integration provides you with a sequential analysis method for the same.

Is that clearing any doubts you have?

Regarding integrating the distillation columns, have you leant to develop a 'Grand Composite Curve'? If you have, then you should be knowing how to.

Edited by pavanayi, 13 March 2011 - 06:19 PM.

[alicia88]

Pavanayi:

Understood about your explanation. I already altered my HEN according to your explanation and as per attached. So, can I know whether I did it correctly?

Thank you.

Attached Files

•  Heat Exchanger Network Design integrated.jpg   37.31KB   8 downloads

[pavanayi]

Alicia,
That looks much better.
But again, what is the difference between streams 1 and 2? You are cooling it totally to 65°C (10124+7655 KW). It is now that you should be thinking of generating steam for process with the remaining heat, because you have recovered maximum heat back into the process. You have a stream that has initial temperature of 345°C, and final temperature of 65°C. You might be able to use the 345°C to 100°C part for BFW preheat and steam generation (depends on the process, and the requirement), and dump the rest to cooling water (but as I said earlier, that ultimately depends on you, the designer!!!)

You might also want to consider doing the final heating of incoming feed stream against the stream 1 at 345°C, because as I pointed out earlier, your temperature difference as it stands is less than your originally decided 10°C.

Hope it helps

Edited by pavanayi, 14 March 2011 - 11:52 AM.

[alicia88]

Pavanayi:

Yup, function of stream 1 and 2 is to cool the product outlet to 65 °C. Already noted. Many thanks for your guidance.

[alicia88]

pavanayi,

Upon simulating the heat integration via HYSYS for overall plant simulations, the heat exchanger for stream 2,3,4,6 cannot be simulated. HYSYS says that those exchanger cannot be flashed. Should I eliminate all those streams?

[pavanayi]

Alicia,
Well, I am not able to understand what you did in HYSYS. Describe what you did in detail, before you got the error message.

[alicia88]

pavanayi,

Upon thorough study on the analysis, I found out that there are some mistakes with the HEX mapping. Only 3 streams can be mapped because for the design below pinch, CPhot > CPcold, and hence it violates the rule of thumb for a pinch analysis. Thats why HYSYS comments that the equipment is not able to flash; which means that there is a temperature cross. Anyway, thanks for your concerns and guidance.