2 replies to this topic

[fai1994chemng]

Hi,

 

I am designing a demethanizer column as a part of my senior project. I am struggling to have a start at the chemical design part of it.

 

I know that the mechanical design of the demethanizer is similar to the distillation column.

 

However, in terms of chemical designing, the demethanizer column removes NGL's from the natural gas to produce LNG. First, the natural gas is cooled and enters a phase separator (Cold separator), then the liquid product is fed straight to the demethanizer and the vapour product (top product) goes to a turbo-expander where temperature is dropped to around -110C (Most C2+ condensed while C1 remains gas) and then the 2 phase mixture is fed to the top of the column.

 

I have attached a diagram of the process.

 

My question is, can McCabe thiele method be used in this case knowing that there is no condenser in the column.

 

also, does not having a condenser mean there is no reflux in the system? I have thought of the top feed liquid fraction as a reflux provided to the system but I do not see an equilibrium between the vapour leaving at the top and the reflux.

 

Can the shortcut method be used? knowing also there is no condenser.

 

What is the theoretical way of carrying out a chemical design calculation on such case to find the number of stages? I am not supposed to use Hysys or any simulation software at this stage and will only be used at the end to compare and optimise.

Attached Files

•  CheDemethanizer.jpg   14.45KB   10 downloads

[Pilesar]

First you need a workable flow sheet. Were you given the process flow diagram or is this your work? Were you assigned a column pressure? What is the missing stream in your overhead exchanger?

Consider taking a portion of the vapor stream from the separator to condense against the column overhead so you can feed a liquid stream at the top of the column. That way you will not need an external refrigeration system. The compressor and expander capacities are linked by a shaft and will have approximately equal work. The flow split to the expander is an initial optimization exercise to scope the expander and compressor case size. The demethanizer column pressure will be a variable in this optimization exercise. I am not sure how you would do this without a simulator. Perhaps you can rough out the overhead product rate based on the top temperature and pressure. This will definitely be an iterative procedure.

 

Will you be considering economics? Normally NGL plants operate to recover ethane. Depending on economics, there may be a better profit to reject ethane from NGL product. The results are not intuitive. Increased residue gas affects the compressor.

 

The big capital items are the residue compressor, the expander, the tower, and the brazed aluminum (plate-fin) heat exchangers. Your initial scoping should help determine the size of these pieces of equipment.

[Bobby Strain]

It's not a chemical design. It is thermodynamics and VLE. Pilesar has good advice. But your flowsheet is simpler to analyze, it just won't recover much ethane and propane loss will be high, too. It would not be used in a real operation. Your principal objective is to learn about distillation. The process is not that important.

 

Bobby