4 replies to this topic

[tamooor]

In my final year project of chemical engineering, i want to designing a process to separate dimethyl ether from methanol and water. 

It should be noted that the boiling point of dimethyl ether is -24 degree C,  for methhanol is 64 degree C and that of water is 100 degree cecius.

In most of patents and books i have seen that a high pressure distillation process is being used to separate dimethyl ether. the pressure is 10 atm and temperature is 77 degree C.

My question is that why we are using high pressure distillation when we can use flash separation or any other technique because dimethyl ether exist as gas at normal temperature and pressure???

A picture  of pocess flow diagram is also uploaded  PFDFinal.jpg   58.08KB   26 downloads

[Art Montemayor]

Tariq:

 

Congratulations on getting this far into a Chem Engineering degree.  Now is the time to use all your strength and energy and finish your academic career with success by applying the most important skill required of any engineer: common sense.

 

Your question can be answered in a variety of ways.  You must furnish additional information and research into your proposed process and use common sense.  For example, does your tertiary mixture form an azeotrope?  Methanol and water do.

 

How did you arrive at the proposed distillation pressure?  Did you arrive at it yourself?  If not, who did?  If someone else did, ask how they arrived at it.

 

Common sense dictates that you make a material balance:

1. All around the process;
2. Around each of the major equipments.

Common sense also dictates that you identify and separate your Unit Operations (where physical changes occur) and Unit Processes (where chemical changes occur).  Do you now see that you have BOTH Operations in your overall process?  This is not just a simple distillation operation.  In fact, the amount of methanol in the feed is NOT COMING OUT in the products.

 

The individual boiling points are important; but what is also important is the relative volatility of each of the components during their distillation.  

 

This is a far more complex operation than just a simple distillation operation.  For some reason, which is left to your research and investigation, you are not simply just distilling out the components.  You are converting methanol into what seems to be water  --- and something else?  You should describe exactly and specifically every step of your total operation.  Then our members will be able to address your query specifically.

 

Good luck.

[tamooor]

Thanks for replying sir.  

Basically i have got a plant design project on the production of 100 tons of dimethyl ether per day from methanol.

I have applied material balance on all the components. I have taken this pfd from the following source:

"Principles and Case Studies of Simultaneous Design by William Luyben" page number 245.    

 

Now i will explain this process: 

 

1)In this process we  we are taking a feed of methanol which is pumped and send to economizer where preheated.

2)Then it passes through vaporizer and then it passes through super heater where it is further heated by the effluent coming out of reactor.

3)The reactor produces dimethyl ether by dehydration of methanol with 82.9% conversion.

4)now we have a mixture of methanol, water and dimethyl ether. The di methyl ether comes as distillate in the first distillation tower while bottom products are methanol and water.

5)In the second distillation column we are recovering methanol which is again sent with the feed to the reactor.

 

As you see the first distillation is very high pressure and is  operating at 10 atmosphere. I have searched the reason for it  but can't find. I found this site and found that it will be very helpful for the problem i encountered in my plant design project.  Can't we use flash distillation in this process which would be more energy efficient ?? what are factors that effect the relative volatility??

[PingPong]

One wants to produce nearly pure DME, so a simple flash separation is not sufficient as the vapor from that would contain a lot of methanol and even water. One needs a column with sufficient theoretical stages and sufficient reflux to obtain nearly pure DME.

 

One wants to produce liquid DME, as that is easier to store and trans[port than gas, so one has to select a condensor pressure that corresponds to the vapor pressure of DME at the indicated condensor temperature of 45 ^oC (318 K), therefor the 10 atm.

[Art Montemayor]

An excellent observation and analysis of a chemical process by PingPong.  This is the kind of quality response and consultation that our experienced members can offer when told all details and basic data.

 

PingPong's observations show not only common sense but also indicate the correct answer to Tariq's query as well as show the proper steps to take to produce a quality final year project presentation.