16 replies to this topic

[Low]

I'm trying to design a cryogenic separation system for hydrogen and methane in a benzene plant.



Stream 11 would be the one where the hydrogen is separated from the rest.

Stream 11: 30C, 36 bar, 5991 kgmol/h
0.854 H2
0.141 CH4
0.004 Benzene
0.0005 Toluene
trace amount of Diphenyl

To be honest I'm not sure how pure the hydrogen should be but for the simulation attached the results were obtained using about 98.3% purity hydrogen recycled with 100% pure hydrogen make-up(assumed). The recycled hydrogen would have to be at 38 bar and 37.7C.

Is there any specific way I should go about designing this or cryogenic separators are just like normal separators except that they need different materials of construction?

Any books specifically regarding Cryogenic Design that is recommended?

Attached Files

•  Mass Balance.xls   28.5KB   70 downloads
•  Draft 8.png   271.28KB   69 downloads

[Art Montemayor]

Low:

Before you hear it from someone else trying to help you out, you will hear it from me: DO NOT MENTION A STREAM COMPOSITION TO ANOTHER CHEMICAL ENGINEER WITHOUT STATING THE BASIS: IS IT MOL, WEIGHT, OR VOLUME %? As a Ch.E. student, it is best you heed this advice NOW and not later, when you could be reprimanded. If you are not SPECIFIC in your basic data, you are passing on information that could be misinterpreted and used erroneously – possibly causing future errors and costly mistakes. State specifically the basis for your composition listing as well as your “standard conditions” (if you are referring to a gas or vapor).

The next piece of advice is: Don’t use Visio PFDs as uploads of your PFD. Generate a PFD on an Excel spreadsheet to enable us to review, examine, and markup the entire flow diagram on one spreadsheet. Visio does not allow downloading in a spreadsheet format and NOT EVERYONE HAS A COPY OF VISIO LOADED ON THEIR COMPUTER. But everybody has a copy of Excel loaded on their computers – especially all engineers. It is impossible to use Visio as a learning tool because I can’t use call outs, comments, calculations, or spreadsheet notes on your sketch. I need to have Visio to do that – and I am not going to go out and spend my money to obtain Visio just to help you. I will help you as much as you need it, but you must make the effort or the sacrifice to present your material in a format that I can use to help you. I am submitting your workbook as Rev 1.

Note the comments that I make on your PFD and take them seriously. Cryogenic processes do not – and cannot (on a practical basis) – tolerate solids. Why are you doing work on a cryogenic process? Have you tried an ADSORPTION process? I would recommend an adsorption process rather than the cryogenic one. Give us the reasons as well as ALL THE REST OF THE BASIC DATA AND BACKGROUND.

Stop being lazy and start to look at your work as a serious and practical engineer. We are not lazy and ill-organized chemists or physicists here. As engineers we are held liable for hard, measurable, industrial results to expensive investments made according to our recommendations. Therefore, start reporting real, practical and workable numbers – and not just a copy of what the stupid computer spit out. Organize your reported stream balances in an orderly and logical manner, using significant figures that make sense for application to a mass and heat balance. No one is interested in knowing that there are parts per billion of toluene in a stream if it is of no industrial or commercial importance for the heat and material balance. USE COMMON SENSE.

How can you NOT KNOW what purity you have to produce the hydrogen at? We cannot comment on such ill-prepared or baseless information. Once again, take this advice seriously: We are ENGINEERS. We must be specific and detailed because we will ultimately be held liable by society for what we produce, design, build, erect, or advise on. We are not scientists who can merely theorize and not be held liable. We are well above that stage in society - and society holds us responsible. Find out what degree of purity you need for the product hydrogen. Otherwise, you cannot play around with different processes without a reason or logic. You will be failing as a future engineer.

Attached Files

•  Mass Balance Rev1.xls   319.5KB   61 downloads

[Low]

Art,

My mistake. It is indeed % mol.

As for the cryogenic part, we have decided to remove the potential solids using a flash separator after stream 11( PFD in excel not updated yet), by expanding the gas from 36 bars to 3 bars and the temperature drops to -80.75C which I think will be helpful for the cryogenic separator part. The reason we had to go to such low temperature to separate the benzene and toluene (f.p. of 5.5 and -93C) is because for some reason the benzene does not liquefy even at -30C. I would like to try the PSA process but due to a lack of time my lecturer advice was to add that separator to remove the liquid that might potentially freeze, and proceed with the current design.

Another reason would be with the cryogenic design we could recover 99.99% of the hydrogen at about 92% purity.

I've done the calculations for the dimensions for both the benzene/toluene separator (V-104) as well as the cryogenic separator (V-103).

The part I am stuck with is how do I proceed with the mechanical design of the cryogenic system. Do I have to design a cold box? I read in other thread about the need for a vapour trap. Do all cryogenic systems need a cold box? And does the vapour trap have to be within the cold box?

Attached Files

•  Mass Balance_rev2.xls   337.5KB   41 downloads
•  Flash.xlsx   11.88KB   42 downloads

[riven]

Separation of benzene and toluene should be done in a packed distillation column. Here is a reference from 1952
http://smartech.gate...ndle/1853/11963

Cryogenic separation CH4 and H2 does not make any sense to me. Pressure swing adsorption or a membrane process (unlikely at this scale) could be useful. Why you would want to separate the two is in question also? It is a valuable feedstock.

Consider http://www.greencarc...10323.html#more
(The first thing to pop into my head was steam reforming)

[Art Montemayor]

Low:

Thank you for responding – although it took you 10 days.

I have given your effort and candor importance and for that reason, have done a detailed review of your revised workbook. I am submitting Rev3 for your review and serious study. I hope you learn and apply what I have expounded in the revision. Pay particular attention to the type of Calculation Pad that is used to generate Engineering Calculations. There is a practical and useful reason for this. As explained, engineering calculations need to be fully documented and controlled. I would expect all engineering students reading this to quickly download and use this form of calculation as soon as possible to become accustomed to what awaits you in the real world.

If you are going to have equipment operating below -50 ^oC, then in my opinion you should be applying a cold box construction.

As a student you probably have not been taught the theory and design of a vapor trap as it applies to a cryogenic system. All a “vapor” trap is, is an inverted “U” made up of cryogenic piping or tubing. A vapor trap is a practical and useful way to avoid heat leaks into a cold box. ALL piping (and/or tubing) that is connected to the external, ambient atmosphere of the cold box must be protected from the relatively “HOT “ temperatures found outside the cold box. For example, all liquid drain lines that lead to the outside for drainage would be subject to continuously vaporizing their cold liquid content at the junction where they exit the cold box. In order to avoid these heat leaks, an excess of piping or tubing is used to make an inverted “U” before the same piping is connected to the external system. The inverted “U” is a natural, static system that initially allows vaporization of its contents; but once it builds a static gas inventory, it acts as a natural insulator because gases are notoriously bad conductors (look up the thermal conductivity of any gas). If you are interested, I can generate a sketch of such a typical system – but that will take a week or so at the moment.

Yes, of course the vapor trap has to be within the cold box. By what I tried to explain above, that should be common sense.

Attached Files

•  Mass Balance Rev3.xls   486KB   70 downloads

[Low]

Riven:

We are separating hydrogen and methane because of the main reaction: Toluene + Hydrogen -> Benzene + Methane.
So by removing the methane we hoped to achieve a higher overall production of benzene.

Art:

Thank you for the Calculation Pad. I will make sure I put it and your advice to good use from now on.
It took me quite some time to respond because I was thinking whether I should change my design after hearing everyone's comment. In the end time is of the essence and I have already wasted 10 days without proceeding with my designing.

I've attached the drawing of how I think the Coldbox might look like, but it is mainly just guesswork at the moment.

By the way, heat exchanger design for the cryogenic systems is the same as those for "normal" temperatures? Just different materials of construction? Any recommended materials for construction?

Attached Files

•  Mass Balance Rev4.xls   763KB   28 downloads

[riven]

Riven:

We are separating hydrogen and methane because of the main reaction: Toluene + Hydrogen -> Benzene + Methane.
So by removing the methane we hoped to achieve a higher overall production of benzene.
[font="Arial"][size="3"]

Toluene hydrodealkylation is the process you are doing. This process runs in the region of 550 to 600 degrees C. It also has high conversions of over 80%. Typically the components (unreacted material and products) are separated by phase separation and distillation with ensueing recycle.
To enact a change on your equilibrium, you will need to removbe methane in situ; i.e. while it is being produced in the reactor itself. The temperature swing from the reactor to the ctryogenic process will make this impossible.

[Low]

Toluene hydrodealkylation is the process you are doing. This process runs in the region of 550 to 600 degrees C. It also has high conversions of over 80%. Typically the components (unreacted material and products) are separated by phase separation and distillation with ensueing recycle.
To enact a change on your equilibrium, you will need to removbe methane in situ; i.e. while it is being produced in the reactor itself. The temperature swing from the reactor to the ctryogenic process will make this impossible.

Yes. We are current setting the conversion at 75% because as I've read in some textbooks it says this improves selectivity of the main benzene reaction. What you mentioned makes a lot of sense, regarding removing the methane in-situ. But if I'm not mistaken, reducing the amount of methane that is being recycled back along with the hydrogen should also help a little?

By the way, what kind of heat exchanger should be used for cryogenic conditions? From my calculation, using 20mm tubes at 7.32m long gives 5600 required number of tubes. at 38mm it's around 3000 tubes. It is too high a number right?

[Art Montemayor]

Low:

OK, now look at Rev 5 of your workbook and read and study the comments and recommendations I show on your Separator sketch Tab.

I hope this engineering detail starts to give you a strong idea of what it is that you have to apply to any engineering design and proposal - especially that for a cold box. I also hope that you can see the practical application of a vapor lock in this type of design. Today, the material of construction of choice in cryogenics is stainless steel.

Attached Files

•  Mass Balance Rev5.xls   779KB   59 downloads

[Low]

Dear Art:

I have studied your comments carefully and came up with a simple diagram of how I think the Coldbox would look like implementing the necessary improvements that you stated in Excel. Apparently there are some problems with the version so I hope the Diagram comes up looking the way I intend it to. But just in case it doesn't, I attached a .png file with a screenshot of the diagram.

I really can't thank you enough, Art. This tutoring has really, really improved my understanding of the coldbox and some cryogenic construction requirements, and hopefully what is required of me when I graduate in a few months time.

Now, if you could just spare a little more time for me and take a look at the heat exchanger for the cryogenic system. I seemed to have chosen a wrong overall heat transfer coefficient. Are shell and tube exchangers are common for cryogenic systems and in a coldbox? And also what are the usual number a tubes a shell-and-tube heat exchanger contains?

Attached Files

•  Mass Balance Rev6.xls   674.5KB   36 downloads
•  Coldbox.png   51.37KB   32 downloads

[Art Montemayor]

Low:

Attached find Rev 7 of the Workbook with answers to all the issues you have recently raised. I have endeavored to address all your concerns and those areas where I obviously have failed to communicate with you correctly. Apparently you have not understood some of my writings and it has cost me to repeat myself and also to re-sketch some illustrations. The language problem is probably part of my problem because your written English is practically impeccable. In the future, do not remove worksheets or prior work from the revised workbook. Leave it there for reference to what has been done before and modified. This leaves a trail of calculation "footprints" for others to follow.

I believe your lack of knowledge in cryogenics is what has held you down. I have given you a reference to a book that can help you tremendously in cryogenics. I also give you some insights to what you normally do not find in the textbooks regarding cryogenic valves and heat exchangers.

The cryogenic exchanger follows a different sizing technique that your standard shell & tube TEMA type of heat exchanger. I have tried to show you the highlights of what and how it is and how I would attack the sizing estimate. I believe you have obtained a grossly oversized estimate on the area because you assigned an LMTD correction factor to the calculation and your U is pessimistic. I have given you the rationale of why I would assign a nominal U value as I have to my estimate. Follow the common sense approach and you can’t go wrong – even if we know very little of how actually these exchangers are really sized. What we do know is that they are inherently very efficient and compact. Because of the assumed U, allow a generous pressure drop through the exchanger to support your assumption of a good U. 50 Btu/hr-ft²-^oF is as good as you can obtain with a gas-to-gas service, so try to establish complete turbulence to get good forced convection and a good U.

I hope this resolves all the support you need to resolve your project assignment. Note that the cryogenic separator's level indication is nothing more than an old-fashioned manometer reading converted to the actual height of the fluid. This is very basic stuff.

Let us know how this all turns out.

Attached Files

•  Mass Balance Rev7.xls   917KB   85 downloads

[Low]

Art:

Thanks for the information. I searched the library for the book you recommended, "Cryogenic Systems" by Randall Barron, but to no avail. The only place I found it is on Amazon.com and on other used books online book store.


Just met with my professor today and he wants us to design our major equipments to as detailed as possible. He especially stressed the mechanical design part as most of us can do the process/chemical design part fairly well. I've chosen the plate-fin heat exchanger (PFHE) since it is cheaper compared to the CWHE. My problem is that like you mentioned, the detailed design and sizing of a PFHE. I double-confirmed with my lecturer and he wants a detailed design of the PFHE, pretty much everything that has to be in a detailed equipment design.

Besides "Cryogenic Systems", are there any other books regarding this? Right now I'm referring to "Compact Heat Exchangers: Selection, Design and Operation" by J.E. Hesselgreaves and "THE STANDARDS OF THE BRAZED ALUMINIUM PLATE-FIN HEAT EXCHANGER MANUFACTURERS' ASSOCIATION (ALPEMA )" Handbook.

Just to be sure, all the cryogenic equipments should be in the coldbox right? Including the PFHE and the cooler with external cooling utilities (from -130 to -180C).

I went through all the replies and workbook revisions up to Rev 7 to make sure I don't miss anything this time.

allow a generous pressure drop through the exchanger to support your assumption of a good U. 50 Btu/hr-ft²-^oF is as good as you can obtain with a gas-to-gas service, so try to establish complete turbulence to get good forced convection and a good U.

I allowed a 1 bar pressure drop through the heat exchanger for the 'hot' stream. I hope that is considered generous. Do I get turbulence by increasing the fluid velocity using perhaps a smaller pipe or and orifice?

And how can I check if my U selected is correct? Same method as checking for Shell and Tube?

Appreciate the help. Thank you.

[Art Montemayor]

Low:

I can give you the following advice regarding how you prepare for and put together your project for final grading:

Your professor wants you to design your major equipment as detailed as possible. Therefore, do as I have been stressing on previous comments: use your common sense. I am sure that your prof is looking to see if you have the makings of a future engineer – making use of your ingenuity and common sense when faced with a complex problem. Mention and point out in your design that a cryogenic process has to work safely and consistently on stream for long periods. Therefore, stress that ALL cryogenic equipment and its related piping, valves, and fittings must be protected inside a cold box from the ambient conditions. You must protect your equipment against

• Ambient heat and moisture. You do this with the cold box and you continuously purge it with inert, dry nitrogen (obtained from an air separation unit or purchased from outside battery limits. The continuous nitrogen purge ensures that no ambient moisture will migrate into the insulated cold box and ruin the insulating effect. It also ensures a positive method to test for whatever leaks may develop inside the cold box. This is an important safety step – especially when processing organic compounds that are flammable and explosive.
• The demanding stress conditions generated by the extreme temperature changes in the equipment and related piping and fittings. Mechanically allow for easy and natural expansion-contraction of all the equipment and piping by assembling and installing it in such a manner that it can expand and contract without generating dangerous stresses and forces inside the cold box. For example, make generous use of “omega” loops in all your piping runs inside the cold box or use “offsets” (lots of elbows) in the piping to allow the piping to “flex” and absorb its own expansion and contractions without putting a strain on the equipment or itself. Do not “anchor” everything inside the cold box down tight. Always allow for self-expansion/contraction. Do not use expansion bellows (they have a limited life span and can leak). Use insulation blocks (simple wood blocks work well) to support and segregate the equipment from the outside cold box panels. You must insulate the conducting legs and supports of the equipment and the piping. Use wood or other insulation materials to do this.
• Leaks. These have to be mitigated at all costs. The use of a cold box makes it impossible to see or repair leaks as they occur. The best and safest solution is to NOT ALLOW LEAKS to occur. You simply weld (or solder) every joint and piece of equipment. Flanges are prohibitive inside a cold box. Gaskets simply cannot be relied upon in cryogenic service. In order to understand what you have to do here, study the next subject of material of construction.
• Plugs or freeze-ups. You must avoid any contaminant that could possibly produce this. You must provide for purging and “de-riming” your system in the event it gets plugged up with water ice or some other freeze-up. Provide for convenient insertion of dry, hot nitrogen in one part of your equipment and a positive purge out in a position downstream of the hot insertion. Design the de-riming piping with correctly placed cryogenic block valves that will allow you to do this. You have to have the process totally shutdown before proceeding with a de-riming operation. It is common sense and AN EXPECTED FEATURE of a cryogenic process to have de-riming piping and ability designed into the equipment. Be sure to make this clear up-front to your professor. This is how ALL cryogenic equipment is designed. Make sure that you design your piping and valving installation to allow for 120 oC nitrogen to enter and heat up the equipment. There will be a LARGE expansion taking place in the equipment between -180 ^oC and 120 ^oC.

The material of construction for cryogenic service is a material that will sustain its design allowable stress while allowing for some practical method of assembling it together and to its piping. Materials that are normally “soft” and ductile at ambient conditions often result being very strong and dependable at cryogenic temperatures. Materials such as copper, brass, bronze, lead, aluminum, and the stainless steels (yes, stainless is a relatively soft material) result in making for a good material of construction (MOC). Your cryogenic fluid should first be looked at for its characteristics before looking at a MOC. For example, is your fluid corrosive? Does it have solids? Does it decompose?
Today, Stainless is the material of choice because it is compatible with most fluids and it can be welded easily with stainless welding rods. Stainless can also be silver-soldered – something some engineers fail to know. Copper is easily silver-soldered – as well as brass. Aluminum is a headache to weld and doesn’t take to solder. If I could apply copper, I would use it. Stainless steel seems to easily meet your conditions, so I would recommend it.

Do not forget the golden rule of cryogenics: your fluid must be totally free of solids and of any contaminant that can freeze. You must state (or ensure) that your fluids have been dried and/or filtered and not going to solidify or decompose. You are dealing with totally encapsulated equipment and cannot tolerate a plug-up or a freeze-up. Any of these two incidents will shut down your system. Your fluids must remain 100% liquids and/or gases as they go through the entire cryogenic process.

To maximize your heat exchanger efficiency you increase your velocity in the tubes and propagate more turbulence. The tradeoff for getting more efficient is that you suffer more pressure drop – but if you allow and prepare for this you can achieve it. You should explain this in your presentation and admit that the pressure drop means more energy consumption and less energy efficiency – but you achieve the desired higher U in the exchanger. That’s a price you have to pay.

You should be solving for your expected film coefficients in your exchanger. In such specialized heat exchangers you do not know the physical and mechanical construction so you have to make several assumptions. There is nothing you can do to avoid this situation. Your prof should know this and accordingly accede to it. However, you have several advantages going for you in this respect:

• By ensuring that your cryogenic fluid is clean and pure, you are in fact ensuring that you will have ZERO FOULING. This reinforces your assumption that your U will be relatively high - but no higher than approximately 50 Btu/hr-ft²-^oF.
• By using a special cryogenic design (a sealed, efficient PFHE) with a generous pressure drop, you are also reinforcing your assumption that your U will be relatively high.
• You can safely assume that you can obtain a counter-current flow in your PFHE – and thereby also a very close temperature approach. There is NO TEMPERATURE CROSS to fear or a LMTD correction factor to apply.

As I requested before, let us know how this all turns out for you and tell us what you learned from the process. This would be helpful to others that follow you.

[Low]

Thanks a lot Art. I wlll return with more questions and if not hopefully it turns out well for me. I will keep you guys updated then. Definitely learnt quite a bit already from this discussion itself. You have my sincerest gratitude.

[Low]

• By ensuring that your cryogenic fluid is clean and pure, you are in fact ensuring that you will have ZERO FOULING. This reinforces your assumption that your U will be relatively high - but no higher than approximately 50 Btu/hr-ft²-^oF.
• By using a special cryogenic design (a sealed, efficient PFHE) with a generous pressure drop, you are also reinforcing your assumption that your U will be relatively high.

From my calculations of Re, Nu, Pr and Colburn factor, I got a overall U that is about 2000 W/m2.K and Re averages in the 10 thousands if I use 100 plates each. If I use 200 plates each I get a slightly lower 1200 W/m2.K. Anything wrong here?

attached my sample calculations for vetting.

p.s.
"DESIGN OF COMPACT PLATE FIN HEAT EXCHANGER"
http://www.google.co...hQdAGDQ&cad=rja

Attached Files

•  Plate Fin.xlsx   1.24MB   59 downloads

[Low]

Appreciate any help I can get.

[Low]

I learnt quite a lot from this experience and all of it regarding cryogenic systems that I know are in the excel files uploaded by Art.

I did not do quite so well for my project but I suspect it is more due to my presentation of work. I really do need to work on time management and I think this is a problem for quite a number of students.

So good luck to the students as I leave this student forum and hopefully will next contribute as a employed, earnest, young engineer,

-Low