39 replies to this topic

[panagiotis]

Thank you so much all of you! For your time and the knowledge you shared with me.

[daraj]

I wanted to add a question to this..if the condenser is the one that creates the vacuum, then these kind of condensers are present in most other types of columns also which operate above atmospheric pressure. so what is special about the condensers in the vacuum columns alone that help create vacuum?

[daraj]

Reason iam askingh the above question is one of the earlier responses mentioned that a vacuum pump etc. may not be actually needed if non-condensibles were not present. so how is an initial vacuum created? also does the feed to the column need to be lowered close to the vacuum pressure before being fed to the column? if i have a vapor coming in at say 5 bar, do i need to expand it before sending it to the column which is operating at say 100mbar?

[SilverShaded]

A barometric condenser works because of the reduction in volume of the vapour as it condenses, so if there are no non-condensibles the condenser is pulling all the vacuum. 

 

Typically though, there will some non-condensibles due to air-leakage, if for no other reason, so a pump will be required.

 

If the vapour was arriving at 5 bar, you would most likely need a control valve to control the flow or the condenser would not be able to condense all the vapour and you would lose some or all of the vacuum and potentially pressurise the column to 5 bar.

 

 

[daraj]

silver when iam designing a condenser like that for condensing organic vapors(say maleic/phthalic anhydride) using aspen plus and EDR, what TEMA type the vacuum condenser tends to be? BEM?

[SilverShaded]

I should add Barometric condensers are specifically direct contact so fairly specific in use, im from a refining background so not going to comment on condenser design for pulling a vacuum because they are extremely uncommon in refinery service.

Generally speaking though you want low pressure drop designs.

Edited by SilverShaded, 31 March 2022 - 07:58 AM.

[katmar]

Daraj - there is nothing special about the physical design of a condenser for vacuum service, other than of course the shell that must withstand the low pressure.  What distinguishes a vacuum condenser from atmospheric or pressure condensers is the temperature of the coolant.  If you want to condense a vapor you must bring its temperature down to below its saturation temperature at the operating pressure, which means that the coolant must be even colder than that.

 

SilverShaded - barometric condensers are relatively cheap to build, but they suffer the major drawback that the condensed vapor is mixed in with the coolant.  I have seen many barometric condensers used on evaporation pans in the sugar industry but that is because it is just water that is being condensed and it is not part of the product that must be recovered. But this is an evaporation duty and barometric condensers are rarely used in distillation these days.

[breizh]

Hi,

Attached a document about vacuum condenser supplied by Graham . 

I encourage everyone to visit their website .

Breizh 

[daraj]

thanks all for inputs.

 

I have similar question with regard to pumps as well for these columns. i mean vacuum pumps. if i know the volume of the column and then i use some correlation to calculate leakage rate, i need to know how to arrive at the duty of the liquid seal vacuum pump. a lot of sites give info on calculatingleakgerate, but a quick conceptual design of vacuum pump duty(or power) is needed for costing purposes. are there any thumb rules or correlations or info that provide duties based on column dimensions, pressure specifications (or leakge rates)?

[daraj]

katmar, while vacuum condensers in principle may not be different from regular condensers, because of the necessaity to maintain vvery low pressure drops across these condensers i havew sometimes seen different type of condensers being used, not your usual AEM or BEM type but something like welded plate exchangers etc. which cause less pressure drop. wanted to know your thoughts on it. simulating and designing such condensers using aspen EDR may be  achallenge. pressure in the column top is about 100-200mmHg

[breizh]

Hi,

a document is attached about air leakage in vacuum system

Breizh

[daraj]

thanks, will look into this

[breizh]

Hi,

another document about air leakage:

 

Breizh

[daraj]

silvershaded and others are there any quick thumbrules or steps to sizing a vacum pump for conceptual cost estimates? it is for a distillation column. i know the dimensions of the column. most cost simulators ask for the ACFM or actual flow per minute at the inlet of the pump and/or duty of the pump. so i need to get it without too many rigorous calculations, using quick thumbrules or shortcuts. please let know

[breizh]

Hi,

Consider studying the documents attached, the purpose of the vacuum pump is to remove non condensable material.

Attached several documents including one from Art Montemayor (Producing and maintaining vacuum)

Good luck

Breizh