Question

Consider the following process in which liquid feed material A (normal BP of 110°C) is reacted with gaseous feed material G to produce main product C and byproducts R and S via the following reactions:

 

Both feeds enter the process at ambient temperature and pressure. Both reactions occur in the gas phase at moderate temperature and pressure (250°C and 10 bar). The normal boiling points of G, S, and C are less than −120°C. Byproduct R has a normal boiling point of 75°C and is highly soluble in water. Product C is very soluble in water but G and S are insoluble. The single-pass conversion through the reactor is low for feed A, and the ratio of G to A in the feed to the reactor should be maintained in excess of 4 to minimize the chance of other unwanted side reactions. Using this  nformation, and assuming that both A and G are expensive, do the following:

1. Draw a preliminary process flow diagram identifying the main unit operations (reactors, compressors, pumps, heat exchangers, and separators), and identify the recycle structure of the process.

2. Justify the methods used to recycle A and G.

3. What unit operations do you suggest for your separators? Justify your choices.

4. How would your PFD change if the price of feed material G were very low?

Answer 1

• A and R are both condensable and may be separated via distillation 

• C may be separated by absorption into water 

• R will be absorbed into water 

• G and S cannot be separated except at very high pressure or low temperature

• After reaction, cool and condense A and R from other components. 

• Separate A from R using distillation and recycle purified liquid A to the front end of the process 

• Treat remaining gas stream in a water absorber to remove product C 

• Separate C and from water via distillation 

• Recycle unused G containing S since S does not react further. But a purge must be added to prevent accumulation of S in the system. This stream must be recycled as a gas using a recycle gas compressor.

 

If the value of G was very low, then consider not recycling G (and S.)