8 replies to this topic

[PhillipLow]

Hi Guys,

 

I am a student from Monash University and I have a question regarding to my design project.

Currently my team is building a Oleochemical Plant that major in C16 and C18 fatty alcohol.
I am responsible for the part after hydrogenation reactor.

According to the information, two phase mixture which consist of H2,C16 and C18 fatty alcohol, hydrocarbon, methanol and trace amount of water is transferred to a separator to remove H2.

The two phase mixture is flowing in the pipeline with 200barg and 250 degree C. 

Questions

What is the device to reduce the pressure and temperature of the two phase mixture? 

I am planning to use a Pressure Reducing Valve to reduce the pressure to 10 barg in order to enter my knock out drum but do anyone know the calculation of PRV? or should I just follow the specification of vendor from the websites.

I also planned to use a heat exchanger to reduce the pressure but what is the formula to calculate the energy consumption for two phase mixture in the  heat exchanger?

[Saml]

Phillip, 

 

As far as I see (I've been following these forums for serveral years, and just recently decided to suscribe), people is willing to help grade students with their projects if they show they've done  their own research and are stuck in a difficult detail.

 

So please, let us know what you have done, what is your process scheme, the books you have looked at and specifically what is the detail you are stuck with.

[Pilesar]

If you remove the hydrogen, what will you do with it? Will it be recycled to the high-pressure reactor? If so, you want the separation to be at high pressure also to reduce recompression costs. So consider a high pressure separator for the major separation service. You might then need to reduce the liquid pressure further into a low pressure separator from which you will probably purge some of the vapor to control the separation pressure. Heat exchangers are not generally used as pressure control devices due to economics, reliability, and predictability. While you will have some pressure drop across the exchanger, use valves to control pressure.

[PhillipLow]

Hi Saml,

Basically, I am still struggling to enhance the purity of my H2 stream to recycle back to the Hydrogenation of Fatty Acid Methyl Ester in order to manufacture the C16 and C18 Fatty Alcohol.

Firstly, there is a Two phase mixture with C16 and C18 fatty alcohol, methanol, excess H2 and trace amount of water and impurities from the Hydrogenation Reactor.
Secondly, I will use the knock out drum to separate as much H2 from the stream as possible. The problem that encountered by me is there are certain amount of H2 that entrained in my liquid stream which I do not want. Besides that, according to Hysys Simulation with NRTL package, I realized that I will need to set my operating condition to pressure = 75barg and temperature = -60degree C which is ridiculous.
In addition, I also doubt that how my cooler can withstand the 75barg pressure in order to decrease temperature to -60 degree C.

Then, as mentioned above, the feed to the cooler will be a two phase mixture also, do anyone know how to calculate energy balance for two phase mixture?
Thirdly, what is the Maximum Allowable impurities that enter back to Hydrogenator? I couldn't find any source or book to cite this.
 

[PhillipLow]

Hi Pilesar,

Yes, the H2 is going to recycle back to my High pressure and High Temperature Hydrogenation Reactor. I modeled my PFD in Hysys Simulation but I realized that I need to set my temperature to -60 degree C and 75bar in order to remove all the methanol from the recycle H2 stream.
(I know this operating condition is ridiculous ! )
I am wonder why there is still a lot of Vapour Methanol in my vapour stream from my knock out drum even if I set the Operating condition at 1atm and 60 degree C which below the boiling point of methanol. Theoretically, the methanol should be in liquid phase and only H2 will be in Vapour form, but the Hysys shows me there are still a lot of Methanol in my vapour stream.
(Hysys v8.4,  Package = NRTL, Vapor Model = PR)
 

Btw, I would like to know what is the maximum allowable methanol can enter a hydrogenation reactor that manufacture Fatty Alcohol with the catalyst is Copper Chromium.

[Pilesar]

A few thoughts on your process in general... 1) The recycle H2 stream may not need to be pure. It would be great to remove all the methanol from the recycle, but what happens if you recycle some of the methanol also? It sounds like you already have methanol vapor in the reactor, so you would just be increasing the concentration. There would be some loss of efficiency, but since the reactor vapor can stand some methanol, then this may not be a process killer. 2) Your design team seems to be split into separate groups. That is okay for detailed design, but you MUST get the plant material balance correct. This can be a fundamental flaw in your design process that will haunt your team. If you are expected to supply only purified hydrogen as a recycle and cannot achieve that, then the reactor designers will have to redo their design basis. That does not necessarily mean that you failed, but possibly that the overall material balance was flawed to begin with. Spend some time up front to get the overall plant design correct. It may be that you have the critical piece of the plant and will have to dictate changes to the material balance for the other team members. If so, it is in their best interest to help you get the best design possible and not just expect you to provide results which violate laws of thermodynamics. Recycles are by their nature iterative. You can waste a lot of time if you begin detail design of equipment before your overall recycle iterations converge. 3) I don't know why you see a lot of methanol in the vapor. You must remember the laws of partial pressure. Each component acts according to the partial pressure of that component in the vapor phase. The partial pressure of the methanol is probably much lower than the total stream pressure. 4) Check your thermodynamic methods against what the rest of your project team is using. I am not saying you are wrong, but NRTL would not have been my first choice over an equation of state at high pressure. 5) Methanol may act differently from ideal depending on how dry your stream is. Water and methanol are both polar molecules and the combination can be tricky if you have to be very accurate. 6) I do not know the allowable concentration of methanol in your reactor. You must find out if it behaves as an inert or as a poison to the main reaction. 7) Your project team needs a plan. It may seem like you are wasting time in the planning stage, but additional effort here pays dividends later. At the level of experience of your team, do not be surprised if you have to revise the heat and material balance causing rework for everyone. It is part of the learning process, so document your calcs thoroughly so you can understand them when you need to revise them later. 8) There may be other ways to separate methanol vapor from hydrogen than by reducing the stream temperature below the dewpoint.

[PhillipLow]

Hi Pilesar,

Thank you very much for all the precious information from you.
So, what will be your first choice if you need to choose a fluid package for this case?
I checked and adjusted my Hysys Simulation, during 20 degree C and 90bar, I able to remove most of the methanol.
(Mass fraction of H2 in Vapour Outlet is 98%) .. 
but I have a question here, the what is the maximum pressure that a heat exchanger can withstand? This might affect my decision on equipment selection.

I still can't find any source to find what is the maximum allowable Methanol that enter Hydrogenation Reactor. 

[Pilesar]

Well, I don't know much about your process or plant location or scale of production, but I am suspicious of your plant design. Your pressure is extremely high and I am surprised that such high pressure is required to make some high molecular weight alcohol. Then you are using a 20 C temperature that will probably require refrigeration. Maybe you need refrigeration for something else, but refrigeration is an expensive utility to use on a recycle stream that will be heated to 250 C. I suspect you are missing something critical to a successful design, but I really do not know. Can you sit down with your professor and explain your approach? If you are sincerely floundering, he may be able to offer useful hints on what you should do next.

As for heat exchanger maximum pressure, that just depends on materials and design parameters. A heat exchanger is a pressure vessel as is a distillation column as is a reactor. There are many different styles of heat exchangers and some are selected for high pressure service more than others. But just because you can design a process for high pressure does not mean you should. There are lots of problems that can be avoided if you can use equipment at lower pressures. When you get to the detailed design, I think you will find it easier if your equipment max pressure is below 40 bar or so and much lower if possible. But I do not know your process and what is optimal.

I get the impression you are working hard on your project. Getting there is not just about running fast -- it is also finding the right road to take. I think you need more help than you can get from this forum. If you can find someone with experience to advise you in person, I suggest you do that as early as possible in your project.

To represent your reactor you should have equations that determine how the reactions behave. To see the result of additional methanol in the feed, calculate it using those equations.

[Saml]

Pillip:

 

If I followed the conversation correctly, you were setting the separator pressure first to 1 atm and then to 90 bar.

 

As PIlesar said in his first post: you should separate at high pressure (that is the reactor outlet pressure minus the piping and cooler loss). Recycle the H2, and then reduce the pressure only for the liquid.

 

Also take his recommendation about temperature. Unless you are in a cold area, you cannot count on cooling water always being below 30°C, and for practical purpuses, a temperature approach of 5-10 °C is what you can get. So don't calculate the separation below 35°C, or you will have to add a refrigeration service, at least in summertime.