7 replies to this topic

[chemeng498]

For the detailed design section of my design project I have to design a depropanizer which will remove propane from a propane/isobutane feed.

 

The problem I am having is that the feed to this unit is 99% isobutane 1% propane. As expected using shortcut methods for distillation the reflux ratio has to be very high (100+) to achieve a high purity propane top product.

 

I understand that in industry this unit may not be used due to the low propane in the feed. However, as this is for a university report I still have to design the unit (sticking with the set feed due to passed decisions).

 

Is there any setup/configuration of distillation that could make a feasible design?

 

 Are there any examples of distillation with a very low key concentration in the feed?

 

 Are there any other separation methods for propane/butane that may have the potential for a feasible design?   

[Zauberberg]

What purities of C3 and i-C4 products are required? If you are targeting 100% separation, this would be in the theory domain anyway.

[chemeng498]

What purities of C3 and i-C4 products are required? If you are targeting 100% separation, this would be in the theory domain anyway.

The initial idea of the unit was to remove the propane to sell. So a top product of around 98% propane.

 

The i-C4 is recycled back to a reactor. 

[Zauberberg]

I did a quick run in Hysys: column with 25 theoretical stages, condenser operating pressure 2,000 kPaa and feed composition 99% i-C4 and 1% C3.

 

Product specs: 98% C3 overheads, 1% C3 in the bottoms product (all mole %).

 

The model has converged easily. For 10,000 kg/h feed rate (17.8 m³/h STD) the reflux rate is 11,230 kg/h (22.1 m³/h STD). This is not an unacceptable solution for industrial design, speaking of column dimensions and energy intensity.

 

Of course, this is just a quick-step solution. Depending on available cooling utilities, operating pressure of the tower may change, and there is further work in optimizing feed conditions and number of theoretical stages vs. reflux ratio. But as said, the solution is quite workable. If you can relax the specification for bottoms product, the operation will be even less energy intensive.

[chemeng498]

I did a quick run in Hysys: column with 25 theoretical stages, condenser operating pressure 2,000 kPaa and feed composition 99% i-C4 and 1% C3.

 

Product specs: 98% C3 overheads, 1% C3 in the bottoms product (all mole %).

 

The model has converged easily. For 10,000 kg/h feed rate (17.8 m³/h STD) the reflux rate is 11,230 kg/h (22.1 m³/h STD). This is not an unacceptable solution for industrial design, speaking of column dimensions and energy intensity.

 

Of course, this is just a quick-step solution. Depending on available cooling utilities, operating pressure of the tower may change, and there is further work in optimizing feed conditions and number of theoretical stages vs. reflux ratio. But as said, the solution is quite workable. If you can relax the specification for bottoms product, the operation will be even less energy intensive.

So far I have been looking into using a condenser pressure of around 15 bar to allow the use of cooling water. My understanding is that as a rule of thumb it is almost always cheaper to operate at a higher pressure and use cooling water than a lower pressure and have to use refrigeration. Is this understanding correct or in this case is it possible that a lower pressure could be cheaper?

 

Thank you for looking into this.

[Zauberberg]

Yes, you definitely want to avoid refrigeration condenser if there is absolutely no need to do so. Available cooling water is probably the best choice (both in terms of condenser size and the maximum cooling temperature) when compared to air-cooling. Refrigeration is way too expensive and it will just save you a few tons of steel for the material of the column and accessories.

 

I have chosen 20 bar abs as condenser pressure arbitrarily, and by remembering De-C3 towers from my career. Operating pressure is selected by looking at Propane vapor pressure at the condensing temperature, plus 10-15 degC margin depending on condenser design.

[chemeng498]

Yes, you definitely want to avoid refrigeration condenser if there is absolutely no need to do so. Available cooling water is probably the best choice (both in terms of condenser size and the maximum cooling temperature) when compared to air-cooling. Refrigeration is way too expensive and it will just save you a few tons of steel for the material of the column and accessories.

 

I have chosen 20 bar abs as condenser pressure arbitrarily, and by remembering De-C3 towers from my career. Operating pressure is selected by looking at Propane vapor pressure at the condensing temperature, plus 10-15 degC margin depending on condenser design.

Thanks again. My mistake I took 2,000 kPaa as 2 Bara (school boy error).

[Pilesar]

Confirm your propane product specification. Most commercial propane is HD-5 "consumer grade". There is no higher grade of propane available to retail consumers in the US! You could meet this spec with a mixture of 90% propane and 10% isobutane. Using specs for HD-5 should make your design more reasonable.