13.4.4 Technical questions

1.  Why pressure is not reduced for the first phase separator ?

Ans: Methanol is separated out in the liquid stream by just cooling the reactor product stream. Therefore, since the separation is achieved physically, there is no need to reduce the pressure of the stream. Also, if pressure is reduced, then again so much pressure needs to be provided using the compressor.

2.  Why the pressure is reduced to 14 atms for the phase separator ?

Ans: The second phase separator is required to remove dissolved fuel gas components in the liquid stream at higher pressure. If this is not done, then methane will remain in the liquid stream and fractionators will produce methane rich ethers which don't have value. Fuel gas on the other hand has value or it can be used as a fuel to generate steam in a boiler or furnace.

3.  Why two compressors are used in the process flowsheet but not one ?

Ans: The main compressor is the feed compressor where feed is compressed to 3000 – 5000 psi. The second compressor is for the recycle stream which is brought to the reactor inlet pressure conditions by taking into account the pressure losses in the reactor, cooler and phase separator.

4.  How multiple products are obtained from a single distillation column ?

Ans: This is an important question. Any distillation column consists of liquid reflux stream. A careful simulation of a distillation column using process simulators such as ASPEN or HYSYS or PRO II will give the liquid compositions at each tray. Using this information, one can exploit whether the intermediate liquid stream is having composition of any specific product. In such case, the liquid stream from the column can be taken out (as a pump around stream in the crude distillation column) and the balance could be cooled and sent back to a section above the distillation unit. Alternatively, without pump around also we can operate the column, but the basis of keeping pump around or not is based on the desired liquid reflux flow rates on the particular tray.

5.  Can heat integration be carried out in the flowsheet ?

Ans: Yes, the reactor product is at higher temperature and can be energy integrated with the feed stream after compression. This is also due to the fact that compression usually increases the temperature and feed stream can be subjected to further heating after compression.

6. From engineering perspective, what is the most difficult part in the process flow sheet ?

Ans: The design and operation of the high pressure reactor is the most difficult. To withstand such high pressure, thick walled reactor needs to be designed. Other materials of construction need to be as well looked into for safeguarding the long term shelf life of the reactor.

 

References:

1. Dryden C. E., Outlines of Chemical Technology, East-West Press, 2008

2. Shreve R. N. ,   Austin G. T., Shreve's Chemical process industries, McGraw – Hill, 1984