Thread: Cyclohexane Production Unit

good day,

i am a final year(4th year) university student from malaysia currently enrolled in 'universiti malaysia pahang'. in our final year, we are required to take up 'process and plant design' subject. my group has been assigned with the task of producing 100,000MTA of cyclohexane. our process of acquiring cyclohexane is through the hydrogenation of benzene. i would love it if someone in this forum can help us with a few questions that have been playing in our mind for quite some time already.

Q1. is it possible for a mixture of hydrocarbons, in our case a mixture of hydrogen, n-hexane, cyclohexane, benzene and methylcyclopentane, to have a pressure of 40atm with a temperature of only 306.24K? in the case of a supercritical fluid(a fluid having pressure and temperature above its critical pressure and temperature), we have definitely exceeded the critical pressure, but we are not sure about the critical temperature of the hydrocarbon mixtures.

Q2. can the pressure of 40atm be maintained throughout the pipeline using a pump and a compressor?

A1. What we would do (in an engineering firm) is model the mixture in a process simulator such as Hysys or VMGThermo. This would generate properties for the mixture at a range of temperatures and pressures.

Otherwise, you could look at each component at its partial pressure to determine individually if it's in the gaseous, liquid or dense (i.e. supercritical) phase. You mention that the mixture is definitely above its critical pressure, but that must surely depend on the composition. For instance, hydrogen has a critical pressure below 40 atm., but if its partial pressure in the mixture is much lower than 40 atm it could still be in the gas phase. You can find properties on the NIST website.
http://webbook.nist.gov/chemistry/fluid/ although I didn't see cyclohexane there.

A2. It depends on the pressure drop through the pipeline, which in turn depends on length, diameter and flow rate. Is it a short pipeline inside a refinery, or is it hundreds of miles long? You might need recompression stations if it's hundreds of miles, but you shouldn't need them in a refinery or chemical plant.

i have two more questions. sorry to disturb.

1. can a heat exchanger handle/withstand a mixture of hydrocarbons with a pressure of 40atm?

2. what exactly are a high pressure separator and a low pressure separator? in petroleum refining, these two are always mentioned.

Originally Posted by calebdanvers

i have two more questions. sorry to disturb.

1. can a heat exchanger handle/withstand a mixture of hydrocarbons with a pressure of 40atm?

Yes, heat exchangers in chemical plants and refineries are custom designed. 40 bar is not very challenging at all. Howver the pressure is not the only variable you need to specify. Temperature and pressure must be considered together with metallurgy, because the ability of metals to handle pressure is reduced as the temperature increases.

2. what exactly are a high pressure separator and a low pressure separator? in petroleum refining, these two are always mentioned.

Look here for a start:
http://en.wikipedia.org/wiki/Natural_gas_condensate

. thx a lot for the web address. really appreciate it. i finally understand what's the use of high pressure and low pressure separators.

can a valve be used to reduce a a pressure of 39.6atm to 1atm? what kind of valve is that if there's one?

I'm not a valve expert. I suggest you google Fisher Control Valves and then look for pressure reduction valves or J-T (Joule Thompson) Valves. There should be some info on limits of pressure reduction. You will have to consider the fact that in a single stage the valve will be operating at sonic velocity, and the J-T cooling effect will depend on the actual composition, and you might end up having to use multiple stages either within a single valve body or in a series of valves. As I said I'm not an expert and manufacturer websites might be your best resource. Another one is Masoneilan.