Which is best? Vaporization in last exchanger or Control Valve? 10

[reena1957]

Hi,
I am in a dilema: Where should I put my Flow Control valve controlling feed flow to the column? At the outlet of a preheat exchanger or at the inlet of exchanger. The liquid is getting heated close to its bubble point in the exchanger and might start vaporizing if there is no CV downstream and the vaporization might occur in the flow orifice/CV if it is downstream of Exchanger. Which is a better alternative? Exchanger outlet temperature is controlled by the flow of Thermic Fluid which I feel will work only if there is no vaporization at the outlet of exchanger. However there is a danger of cavitation at the orifice and CV in case of high flows. Also thereis a danger of slug flow in a 15 meterlong vertical line before the column inlet. Thanks

 

[BigInch]

If that's the situation you have, won't the fluid vaporize immediately downstream of the CV anyway? Sounds to me like you need a higher column pressure too.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[pleckner]

You get this all the time. In my past life when designing Ethylbenzene systems, we would heat the feed and then control, but the feed to the column is way up there so these is enough head on the control valve to keep the fluid liquid until it starts its travels uphill.

 

[BigInch]

Thanks pleckner. I was curious about that.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[reena1957]

Thanks Pleckner. A Bit Curious.... Where are you in your present life or rebirth( Pardon my curiosity!)?

 

[sshep]

I think there have been similar control valve location question posts in this forum, but last I can find was: thread124-148096

best wishes,
sshep

 

[djack77494]

I would suggest that the control valve be mounted on the column's feed nozzle. This will maintain everything in the liquid phase right up to the column avoiding many concerns. You will get flashing in the valve, so buy one capable of withstanding it. If you feel the need to have a downstream isolation valve, then still mount the control valve as close to the column and as high as practical. If the maintenance folks insist on a more accessible location, oh well.
Doug

 

[pleckner]

reena1957: I was involved with inorganic systems design for my first 10 years of industrial life. I spent the next 10 years doing EB/Styrene and other petrochemical systems design, and process safety work and the last 10 years has been in process safety, project and process engineering design in the specialty chemical, beverage and bio-pharmaceutical industries.

 

[CJKruger]

reena1957,

I agree with the replies from sshep.

You can use either approach. In general (but there are exceptions), it is better to have the flow meter and control valve before the exchanger.

If you move the control valve to downstream of the exchanger, you can/should leave the flow meter upstream of the exchanger.

The two main reasons for sometimes putting a control valve downstream of the exchanger are:
- To prevent slug flow in the vertical line.
- The flashed mixture is very corrosive (rich amine).

In these cases, elevate and put the control valve at the tower inlet. Make sure the valve upstream pressure is always above bubble point by at least a certain margin (can't remember off hand what margin I used in the past).

Refer to sshep's replies for the impact on relief cases, etc.

 

[Montemayor]

In my opinion, both Doug and Phil have nailed it. The flashing is going to take place - regardless. And the best place to have it happen is at the control valve orifice because that's where you are controlling it.

I've often employed this application in the manner I first saw it resolved: in a Girdler Corporation MEA Stripper feed inlet. Hot, flashing MEA feed into the top of a stripper is a classical application that borders on potential big-time problems. The hot MEA is potentially corrosive, the feed is flashing with both MEA solution vapors and hot, corrosive CO₂ + H₂S gases - everything you want to avoid. However, you've got to maintain a back pressure on the MEA feed and still control the Stripper pressure. What Girdler did more than 50 years ago was to install a 90^o control valve right on the inlet nozzle (just as Doug states), with the feed entering the valve top-down (entering the valve orifice from the stem-plug side and exiting down through the open orifice). When you see the CV, it looks rather awkward: the actuator diaphragm is in the vertical orientation and the valve stem is moving horizontally instead of vertically. However, this orientation does it all. It totally eliminates cavitation (defined as the creation of gas bubbles with subsequent bubble implosion immediately downstream) because the flashing mixture exits the CV and immediately enters the Stripper vessel itself for immediate disengagement. It avoids downstream errosion and corrosion. I did this several times and it worked on every application I did on Amine Strippers. I had so much success with it that I applied it to the manufacture of Dry Ice where I had to flash a saturated liquid CO₂ feed into a Dry Ice Hydraulic Press chamber for subsequent compression. On this latter application I did one better: I fabricated and used a "horn" (a 3:1 expander nozzle directly under the CV outlet and leading into the chamber. This reduced the outlet velocity and totally avoided any errosion and turbulence. It worked like a charm and I would repeat the application until I find something better - which I haven't in the last 46 years.

I hope this experience helps.

 

[25362]

Kindly note that if the valve is located upstream the exchanger, this exchanger would be designed as a vaporizer, not just a liquid heater, a fact that involves bi-phase flow conditions that have to be considered in addition to the physico-chemical properties of the fluid.

In this case temperatures may peak at higher levels just prior to the start of vaporization. In short, a vaporizer needs particular attention in design, and not only o/a of friction drops. The outlet temperature of the vaporizing fluid would be dictated by the back-pressure from the column and the characteristics of the fluid being vaporized.

If the vaporizing liquid is a solvent to be removed and recovered by flashing, the usual procedure is to put the FCV prior to the vaporizer, while the changes in "hot oil" enthalpy (mass and enthalpy balance) serve as control of heat input.

Please note that the amount of heat that can be obtained in vaporization (upstream CV) is usually larger than when supplying just sensible heat to the liquid (downstream CV), which for equal enthalpy would mean a much higher temperature, sometimes limited by the thermal stability of the vaporizing liquid, or just by the the temperature of the hot (thermal) fluid.

 

[reena1957]

Thanks to all of you who have taken time to reply. I am going to follow Phil, Jack and Montemayor. Once again thanks to all.

 

[reena1957]

Dear Phil(Plekner, I presume), WOW! that is an astounding list of Lives! Curiosity satisfied! Thanks. Would you pl look at the thread 124-169875 on Pressure control of distillation columns and give your views please?

 

[shoffman83]

Y'all have nailed it. We have this situation in a pressure swing amine unit, and the control valve is the place to take the brunt. Some thoughts:

1. Using a well type choke valve which is designed for handling multi-phase flows? (Right angle control valve with a bean).

2. Considered using a downstream orifice (downstream of exchanger => valve combo) so it takes allot of the brunt of the flashing? Cheaper to change an orifice than a valve stem... (Thats what fails on the control valve I am speaking of due to cavitation of the valve).

Scott Hoffman
Mustang Engineering