3 replies to this topic

[Kakashi-01]

I understand that in practice, crude methanol is first fed to a topping column to remove compounds lighter than methanol. The bottoms stream of the topping column is then pumped into the main refining column, where the methanol is purified to the required grade. However, the process flow diagram I am designing for methanol production using the natural gas steam reforming process does not include this step, and I am required to simulate a simple distillation column.

I am simulating a distillation column for separating a methanol/water mixture, imposing a product specification of 99.9% purity and a mass flow rate of 10,500 kg/h for the methanol stream, with 1% methanol in the bottom stream. The software I am using allows the number of stages and the feed stage location to be set as parameters that the user can change. The reflux ratio, boilup ratio, and the column condenser vapor fraction can either be specified or calculated by the program.

Using initial values of 13 stages and a feed stage located at the 7th stage, and specifying the purity specifications (1% methanol in the bottom stream and 99.9% in the overhead liquid), I let the program calculate the reflux ratio, boilup ratio, and condenser vapor fraction. The results were 14.3951, 10.5097, and 0.00137, respectively.

I understand that the separation specifications must be met regardless of the reflux ratio, but increasing the reflux ratio incurs additional reboiler and condenser duties, at the expense of saving column stages. The simulation program sets the column height per stage at 0.59 m and the diameter at 1 m.

I am considering that increasing the number of stages might be a better long-term investment than the ongoing costs of operating the column with a higher reflux ratio. How can one decide the optimal trade-off between capital and operational costs in this case? Additionally, is there a practical limit to column height due to crane lifting or other considerations?

 

 

Attached Files

•  Methanol.JPG   61.03KB   1 downloads

Edited by Dumpmeadrenaline, 30 January 2025 - 02:09 AM.

[Dacs]

How can one decide the optimal trade-off between capital and operational costs in this case?

 

 

By performing a Life Cycle Cost analysis. At the end of the day, it's all about the moolah.

 

Additionally, is there a practical limit to column height due to crane lifting or other considerations?

 

 

While I can understand your question, we can't answer this as Process Engineers. Engage with your Mechanical/Civil counterpart for this.

Edited by Dacs, 27 January 2025 - 11:41 PM.

[Pilesar]

There is a minimum number of stages required to meet the specifications even with the reflux ratio approaching the infinite. There is a minimum reflux ratio required to meet the specs even when the stages approach the infinite. Consider rules of thumb for initial sizing such as 'twice the minimum number of stages' or '1.2 times the minimum reflux ratio' to get in the general vicinity of a reasonable design. 

[breizh]

Hi,

You may want to use this Xcel sheet to simulate your process.

R= 1.2 to 1.3 Rm is a standard value.

Breizh

Attached Files

•  separation tower design.pdf   1.41MB   21 downloads
•  Multicomponent distillation.xlsx   6.07MB   33 downloads