8 replies to this topic

[boringppl]

Hi,

I am now doing the distillation column design for my final year project.
However, I have encountered some problems that made me can't proceed further.
This is basically an benzene recovery column used to separate benzene from isopropylbenzene (DIPB) and cumene. The data are as following:

Distillate (D):
T: 78 degC
P: 100kPa
Xbenzene: 0.9999
Xcumene: 0.0001

Bottom (:
T: 169 degC
P: 150kPa
X DIPB: 0.0219
X cumene: 0.9781

Feed (F):
T: 160 deg C
P: 1500kPa
X benzene: 0.83
X DIPB: 0.0037
X cumene: 0.1663

As highlighted, the pressure of feed into the column is too high since the pressure at top and bottom is only around the atmospheric pressure. This is because the feed is from the previous reactor that generate high pressure stream and feed into distillation column.So, is it this design is not logic? If not, what can I do to make it make more sense? Thank you very much.

boringppl

[katmar]

This is not an unusual situation. You will have a valve in the feed line to regulate the feed flow, and this valve must be sized for this large pressure drop across it.

The important factor for you to consider is what the phase condition of the feed is after it has passed through this valve. It is possible for an all-liquid high pressure feed to partially flash as it goes through a valve like this, and if vapor is generated it will have an effect on your column's operating lines and on your tray hydraulics.

[boringppl]

Thank you for the reply.
Ya, I am worry about the serious flashing problem that might effect the tray hydraulic and hence column operations.Is it possible if an flash vessel is installed prior to the distillation column for pressure reduction to approximately within the range of top and bottom? Or is tere any better and more practical method to reduce the feed pressure before enter the column? Thank you very much~~

Boringppl

[katmar]

The quantity of flash generated by the pressure drop would likely be much less than the vapor flow up the column. It's not going to damage your trays or anything like that. You just need to take into account that the vapor traffic up the column will be higher above the feed point than below it. I doubt whether you would need a dedicated flash vessel, but you might have to use a larger pipe size downstream of the valve. Until you have run the numbers you can't really say what the situation would be.

[boringppl]

Thank you for reply.
So it is more recommended to install a valve for the feed before enter the distillation column?
May I know which kind of valve that is usually being used in industry and more dedicated to used for high pressure reduction?
Thank you.

Boringppl

[katmar]

Yes, you must have a valve in the feed line to control the flow rate, or if you are designing your system such that the feed rate is controlled further upstream (before the high pressure column) then this valve would be controlling the reflux (or some other parameter) on the high pressure column. Whatever the case, the valve would be sized with the pressure drop you have given above. The valve would most likely be a globe valve, but V-ball and offset butterfly valves are used as well. It depends on the size. The pressure you have is not very high by industrial standards.

Depending on the quantity of flash relative to the vapor traffic in the bottom of the column you may need to have a larger tray spacing for the feed tray to allow the vapor velocity to dissipate. The problem you need to avoid is known as "vapor cross flow channeling" (which you can Google if you need to know more).

[boringppl]

Thank you for reply.
May I know whether J-T valve is suitable to be used in this case?


Boringppl

[katmar]

You will have to explain to me what a J-T valve is. Be careful that you are not confusing a valve function with a valve type. By function I mean that a valve can be a pressure control device, or flow control, or bypass or a whole lot of other functions. By type I mean ball or gate or globe. There is not a one to one mapping between type and function.

[boringppl]

Oops..sory...what I mean the Joule Thomson valve for pressure reducing function.
By the way, after passing the valve and reduce the pressure to ~130kPa, I found that vapor phase is exist hence the feed into distillation column is no more sub-cooled liquid, but a liquid vapor mixture. By the way, this is not an issue as long as the tray hydraulic is ok rite?
Thank you very much.

Boringppl