Flash Vessel Design and Specifications

[anj3]

Hi,

I know this question has been asked several times but I really don't know where to begin. I am given several scenerios where the flow rate of the incoming stream is constant. I know the incoming stream conditions (P and T) but the composition is not given. The incoming stream has heavy boiler material, HTF and low boiler material and the flash drum is used to separate the HTF and low boiler material from the heavy boilers. How do I chose the operating temperature and pressure for the flash vessel? after proceeding, how do i find the specifications without knowledge of streams' compositions?

Thank you for help in my first engineering design job.

 

[chemebabak]

anj3,

With respect I tell you that your question can be rewritten as, "I want to flash a stream that I don't know what it is." Who could answer that?!

First you must define the fraction of flash drum feed that will be flashed as vapor. Since you know that it is

heavy boiler material, HTF and low boiler material

then you should at least be able to use your engineering judgement to determine flash drum pressure. Then you can use Rachford-Rice procedure for multicomponent flash calculations.

For example, gasoline is about 22% aromatics, 4% olefins and the remainder is paraffins. You could assume that the aromatics are benzene, the olefins are propylene and the remainder is octane. Assume the flash vessel is at the stream temperature. Calculate K-values of each using the Antoine's equation. Rachford-Rice procedure is iterative, so you should plug the values into an excel spreadheet.

anj3, do not worry. The first engineering job is the most difficult. I graduated valedictorian and I could not even do a simple hydraulics calculation in my first job. Flash drum calculations are much more difficult. Just grab your old textbook from Separation Processes 401 and be confident that those 4 years in college did not go to waste.

 

[MJCronin]

Is there software available that can do flash calculations ?

 

[PaoloPemi]

almost all simulators and process libraries can do that, problem being that you need to characterize the components however there are models as black oil (which do not consider compositions) and require less data, for additional information
google
"rachford-rice excel"
"vapor liquid excel"
etc.