Pressure drop calculation

[koshyeng]

I have a vacuum distillation column with vacuum of 15 inHG being pulled from overheads condensate receiver. There is a pressure equalization line from overheads condenser to this receiver.

At increased vapor loads, condenser will not handle so I am installing a vent condenser on vacuum line from receiver to vacuum pump. At high vapor rates, about 3000 lbs/hr of acetone flows through equalization line ( condenser to condensed liquid receiver) How should I calculate pressure drop across this 2" line?

Does pressure in my receiver and condenser will be same due to equalization line?

Thank You
Koshy

 

[rmw]

You shall try to find a copy of Crane Technical Paper #410 and if and when you do, you will be able to get on with the calculations.

rmw

 

[katmar]

The liquid from the condenser should not flow via the equalization line to the receiver. The liquid should flow from a nozzle at the bottom of the condenser to a point below the liquid surface level in the receiver. There is often a U-bend seal in this line to ensure that it is always sealed with liquid, preventing vapor from flowing back up the line.

The equalization line should be a separate line and it should see vapor only. It should be connected from the top of the condenser (or the line to the vacuum pump) down to the vapor space at the top of the receiver.

You are correct that the pressures in the condenser and receiver are (or should be) the same. That is what the equalization line is for. The head that drives the liquid from the condenser to the receiver is the static head due to the difference in heights - the receiver is placed below the condenser.

I suppose it would be possible to combine the liquid drain line and the equalization line into one if the line was sized for self venting flow. The line would have to run from the bottom of the condenser to the vapor space of the receiver and have no low points in it. It sounds as though this is what you have and 3000 lb/h of acetone in a 2" line is just at the limit of self venting flow. In my opinion you are lucky that this system works and this type of arrangement should be avoided.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[koshyeng]

Thanks Katmar. I might have not expressed my clarifications in detail. Please look at the attachment for better explanation of my process.

Liquid flows through bottom discharge. At high rates, excess vapors will flow through equalization line to receiver and will be pulled by vacuum pump. My intention is to condense these excess vapors using a BEM type condenser installed on vacuum line between receiver and vacuum pump.

I request for how to calculate pressure drop in eqalization line( flow is 3000 lbs/hr). IS the type of configuration have success? what is momentum factor? ( used in vapor nozzle sizing)

Thanks a Lot
Koshy

 

[katmar]

OK - I misunderstood what you were trying to do. rmw gave you the right advice - calculating this pressure drop is not any different from any other. 3000 lb/h is too much for the 2" line. Why not put the new condenser in parallel with the existing unit?

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[koshyeng]

Katmar,

I agree with you on pipe size. Existing condenser nozzle is 2" and 3.5" pipe will work at 3000 lbs/hr.

Can I still use 2" condenser nozzle and upsize my pipe to 3" with a reducer? Please advice me

Thank You very much

 

[katmar]

Koshy, there is a chance that it will work but it all depends on the details of your pipe and equipment layout, your process conditions and requirements and so on. You need to do the calculations and satisfy yourself that it is adequate. I still think that putting the condensers in parallel is a better option.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[koshyeng]

Thanks Katmar.

I guess I did my home work after some inputs from this post.

Maximum vapor flow rate handled by main condenser is 23,500 lbs/hr of acetone. At high rates, ( 26,000 lb/hr) I need another small condenser ( this rate is limited by flooding in columns with high efficiency trays, superfrac) to handle this rate.

Option 1: Installing vent condenser on vacuum line to pump requires more piping and hence pressure drop.

Option 2: Parallel condenser requires vapor flow meter and control valves on chilled water and vapor side( pressure drop and cost)

Option 3: To install condenser on pressure equalization line. This seems to be better as I have to upsize only one nozzle on main condenser and ofcourse piping too.

I have attached drawing in my previous post. Please help me with your advice/ recommendations on above options.

Thank you Katmar and rmw.

Koshy

 

[koshyeng]

Can someone please help me recommend on above options (no 2 and 3).

Thank You
Koshy