Distillation column Pressure/Temperature 1

[22082002]

Thanks in advance for your help.
It's about distillation column design. I need to seperate multicomponent mixture of hydrocarbons. I just have feed, expected products (Top & bottom).I would like to kow how to start the design.
First, we need to have vapor liquid equilibrium, this is to be generated. No previous information about alpha, relative volatility, vapor pressure, temperature. How will we fix top temperature, pressure, etc.
If you can guide me/let me know to basics of these steps, I would be glad.

Thanks & Regards,
SAA

 

[TD2K]

Typically, you want to keep the column pressure low as that increases relative volatility. On the other hand, lower pressures means larger diameter columns to handle the vapor load so it's a bit of a trade-off.

That said, overhead product temperature is typically set by what you plan to use for condensing. If you were using air coolers in a place where summer temperatures can reach 100F, minimum temperature you'd likely want to design the overhead product leaving the condenser is 120F. An air coolers' minimum approach temperature is typically about 20F (you can go lower, but you need proportionally a LOT more area). You can go through a similar exercise if you are using cooling water (minimum approach temperature is likely about 10F for shell and tube, down to less than 5F for plate and frame). If you need lower temperatures, you have to look at refrigeration.

As a starting point, I'd take the overhead product as a bubblepoint liquid leaving the condenser to estimate my overhead pressure.

The pressure at the bottom of the column is the top pressure plus the dP across the trays. You'll need an estimate of the number of trays and try a dP of 3" of liquid per tray as a starting point. Bottom temperature is the bubble point temperature for the product at your pressure.

 

[25362]

TD2K gave you the tools.

Besides, if the HC mixture has a narrow boiling range, an external "heat pump" type of arrangement may be advisable. Such a system includes compressing the OVHD vapours, which upon been re-heated by compression, can be used as reboiler heat source for the bottom product. Upon condensing in the reboiler, and trim cooled under column pressure control, part of the stream is returned as top reflux.

 

[StoneCold]

25362
I have never seen the "heat pump" type of araingment in use. Have you seen an actual unit running this way? How big was it? What kind of service was it?

Just curious

Thanks

 

[22082002]

Thank you gentlemen, but look forward to more elaboration.

Say, as first estimate, I will assume 100°C as top temperature. I will use antoinne equation to find out vapor pressure and then relative volatilty. Say I will use air cooler and outlet temperature of air cooler I fix, say 50°C (Max air temperature = 45°C)
This for each component and then I will get total pressure.

This of course will be iterative, but say, I hit some pressure and temparature, should I take it as good point? (I am sorry, I cannot express exactly what to say)

Thanks & Regards,
Satish