Olefins Process Liquid Dryer Pumps 1

[Mike Drey]

Hey everyone this is my first co-op term at a olefins site producing mainly ethylene and propylene. One of my projects was to determine if our process liquid dryer pumps were needed in order for our unit to remain online. The pumps were intended to transport a liquid hydrocarbon mixture from a dryer feed knockout drum to a set of coalescers, a lead-lag set of liquid dryers, and then to a demethanizer stripper. In order to control the flow for this process, a flow control valve is used downstream before flowing into the stripper. Aside from the process, the pumps are used once a month to refill the liquid dryers after servicing and remained off for the remainder of the time. Without the pumps the time it would take to refill the liquid dryers would be a few days compared to if the pumps were used then the time would decrease to about 6 hours. I have created a simulation in SINET 9 with and without the pumps and the differential pressure across the flow valve on the outlet side of the dryers with and without the pump is 120 psi and 75 psi. Essentially there is more than enough differential pressure across this valve to control the flow between the knockout drum and demethanizer stripper.
I have had the chance to speak with the engineers on site and the number one answer I have recieved is that we need the pumps in order to transfer the mixture at a faster rate if more heavies are produced when using a different feed case. However, if the pressure between the two vessels remain how they are then there is no reason to use the pumps besides the once a month dryer refills. I was also told by another engineer that the pumps are used to keep the efficiency of the dryers high, but the pressure drop is essentially high enough when tranporting the mixture through the system. Does anyone else have a similar design at their plant or know any reason why these pumps incorporated in the plant's design?

 

[georgeverghese]

The feed liquid to the pumps is probably at or very close to its bubble point. If so, the main function of these pumps would be boost pressure in order to prevent the feed liquid from vaporising as it passes through the liquid dryer. The liquid must be maintained at single phase liquid conditions to maintain drying efficiency. Pressure drop through the dryer may be high at EOR ( end of cycle run) and the pump must develop sufficient boost pressure to maintain liquid phase conditions at dryer outlet even when dp across the dryer is high.

 

[Mike Drey]

Would this decrease the run span of the dryers if the pumps were not used during the process? Essentially, we are not using the pumps right now since the dP is high enough from one vessel to the other and there isn't any back flow. Also, we regen. these dryers once a month, so I wonder if we used these pumps for a continuous flow will the regen. time-frame change to more than one month.

 

[georgeverghese]

It is not enough that there be sufficient dp from then source vessel to the demethaniser. The liquid must be subcooled adequately to prevent vapor breakout in the dryers. I think you may be taking a risk without these booster pumps. Are you meeting the required residual water content after the dryers without these pumps? When you have a higher lightends in the liquid, vapor breakout would be much more and it may be too late by the time you realise there is too muuch residual water in the deC1 feed.
In theory, yes, it would appear you could extend run time to more than a month if you did keep these pumps running. In my past experience, toying around with some of these operating parameters to save power or heating demand, has in some cases lead to disastrous consequences. What if you toyed around with these parameters and the liquid freezed up in the cold section cryo heat exchangers in the cold box? Be very cautious when you begin changing operating parameters - you'll sleep much better I can assure you.