6 replies to this topic

[ralf ruffel]

Good day to all! Can any one help me design a condenser capable of condensing about 30% of heavy oil from the hot pyrolysis vapor. We're thinking of using a shell and tube heat exchanger and the feed rate is 2.66m^3/hr. The inlet temperature is 200 degrees C and the pressure is 0.825 atm at the tube side... We already know some of the properties that we need but we have no idea how to design a heat exchanger which is partially condensing something.. Please help us!

[djack77494]

but we have no idea how to design a heat exchanger which is partially condensing something.. Please help us!

If you are not in the heat exchanger manufacturing business, and it certainly sounds as if you are not, then why are you trying to do a mechanical design of the heat exchanger? You are best off generating a (process) datasheet containing the information that you DO know, such as process conditions, fluid flowrates, properties, etc. and supplying this information to a competent manufacturer of heat exchangers. I have seen high quality datasheets of the type that would be suitable for your purposes readily available on the internet - I think perhaps even in this site. The manufacturer should be more than happy to supply you with a preliminary design and quote. Wouldn't that meet your requirements?
Doug

[Art Montemayor]

Ralf:

If you seriously want help in what you are trying to do, then you have to level with all of us and tell us exactly what it is that you are trying to do – and I mean ALL the details, like why would you want to fabricate a heat exchanger. Otherwise, no one is going to spend their time trying to help you while you spoon-feed your readers with a few crumbs of data and information at a time.

Doug is exactly right in what he has said. No professional engineer is going to sit down and attempt (and I mean attempt) to design and fabricate a heat exchanger unless he is in the business or has done it before. And very few of us have ever had the opportunity to do it before. I am an exception; I’ve done it before – both process design and mechanical design and fabrication. So what I’m saying is from first-hand experience. The average chemical engineer plays a direct and important role in “designing” a heat exchanger by specifying the unit in great detail using an adequate Specification Sheet and subsequently submitting it to an experienced and recognized fabricator who does the actual, real mechanical design. This is the real-world method of obtaining a correctly sized and designed heat exchanger. If you search through these Forums you will find one of my workbooks with heat exchanger specification sheets. These Spec Sheets, I can assure you, are better than most Spec Sheets issued by fabricators. I’ve designed my own and incorporated just about everything needed to describe the unit and start mechanical design and fabrication. You can download it, if you wish, and use it.

If you are located in a developing country and you need to fabricate your own exchanger because you can’t spare the hard currency or your government won’t let you import a foreign exchanger – then tell us. I've been in that situation, so I can understand. If you are just toying with the idea and want the experience and the thrills that go with designing and fabricating your own equipment (I can tell you that there are no thrills …. Just hard work!), then tell us that too. But by all means tells us all the facts and the scope of work.

[ralf ruffel]

i'm so sorry if i wasn't able to present all the details... I'm actually a chemical engineering student and our group is currently doing a plant design on manufacturing diesel from waste thermoplastics.
we're encountering a lot of problems simply because the "thing" is new, well, not so new but the properties and certain processes are mostly confidential as of now. maybe i interchanged the designing and sizing... what i actually mean is sizing a heat exchanger or multicomponent condenser for hydrocarbon vapors ( C3-C29 ). we already obtained a lot of equations but they seem so difficult to evaluate...Is there a more simpler equation that we can use? Please help us!

[djack77494]

Is there a more simpler equation that we can use? Please help us! :(

Doing a complete mechanical design of a shell & tube (or any other kind) of heat exchanger is an arduous task. You've now heard that from two experienced engineers. There is no "equation" that will size the exchanger for you. Instead there are multiple equations that must be simultaneously satisfied. When one side of the exchanger includes multicomponent phase change, these equations become very complex and difficult to solve. That is why even large companies in the business spent (sometimes) tens of thousands of dollars per seat to obtain the software capable of performing this design. Sorry, but in the engineering field there are many situations where there are no shortcuts. In the academic area, you may be permitted to make simplifying assumptions that will solve a problem that is much simpler than the "real world" problem. Since you apparently lack the specialized software that a working engineer would have who was expected to do the design you discuss, such simplifying assumptions seem reasonable to me. I'm afraid you're going to have to discuss that with your instructor.
Doug.

[ralf ruffel]

Doug,
Thank you so much for the time and idea..I guess we'll just stick to our assumptions..Can i ask one more thing??in a multicomponent system, is it possible to use a flash drum in series and narrowing the components into three fractions(the non condensible, heavy and light fractions) rather than condensing the heavy fraction and condense the remaining vapor in a fractionating column??

Again, thank you so much..

[djack77494]

in a multicomponent system, is it possible to use a flash drum in series and narrowing the components into three fractions(the non condensible, heavy and light fractions) rather than condensing the heavy fraction and condense the remaining vapor in a fractionating column??

ralf,
The short answer is "yes", it is possible to separate a multicomponent mixture into multiple fractions using flash drums in series. The longer answer would indicate that this is inefficient with poor separation between the fractions. If you have available a general purpose process simulator, try it with a mixture of interest and you'll easily see the poor performance that the flash drums will produce. A fractionation column can be thought of as a collection of vertically stacked flash drums, but you might note that there is a combination of liquid traffic moving down the column AND vapor traffic moving up the column. This is done for a reason - effective separation - which would not be achieved with a series of flash drums.