10 replies to this topic

[ReddeerC]

Dear Colleagues,

I am now working on a 3-effect evaporator in my plant. The evaporator is mainly composed of 3 flash drums, 3 heat exchangers, and 1 final condenser.

Steam and Vapor flow direction: steam from other production process goes to the 1st effect heat exchanger; the vapor from 1st flash drum goes to the 2nd effect heat exchanger; the vapor from 2nd flash drum goes to the 3rd effect heat exchanger; and the vapor from the 3rd flash drum goes to the final condenser.

Feed direction: the feed goes to the 3rd effect heat exchanger; then 2nd; and at last to the 1st effect heat exchanger. From the 1st effect heat exchanger, the feed is pumped into final product bin.

The 3-effect evaporator is using a 2-stage ejector to vent remaining vapor/steam to the ejector itself condenser through vent lines connecting the 3 heat exchangers and the ejector, and then discharge them to the atmosphere.

The problem I am facing is that there is no vacuum in all the 3 flash drums (According to the design, the drums are supposed to have vacuums in them). I am wondering: what should result in vacuums in the 3 flash drums? The vacuums in the flash drums are generated by the 2-stage ejector? Or, the vapor condensation process in the 2nd and 3rd heat exchangers and the final condenser causes the vacuums in the drums?

Any one can recommend me some books about multiple effect evaporators?

Thanks for your time and help in advance.

Sincerely.

[Jiten_process]

hi,

Frankly speaking your question though it is apparant but not clear. I assume that your plant has already been erected and now you are trying to commission it and the first problem you are facing is you cannot get vacuum in empty plant, is it correct? Because we are supplying evaporator plants and as to my knowledge vacuum on is the first step in commissioing.

If your all lines are connected properly, all valves are set at the position they should be then you should not have any problem in getting vacuum however the level may be not as per your set point and is different issue but as you said there is no vacuum at all in drums. Try measure vacuum at various points off the ejector. First put ejector in shut off and check whether shutoff vacuum is as per specification?

Plz upload sketch or basic drg. for better answers.

to be continued...

[ReddeerC]

Hi, Jiten_process,

Here is more information about the evaporator process at my plant. The evaporator has been running at my plant for many years, and two years ago our evaporator lost all the vacuum in the 3 flash drums. Since then, our evaporator just became a flow vessel and no more solids concentration. Now I am asked to look into the problem and try to bring the evaporator back to normal.

My main confusion is that what mainly causes the vacuum generation in the flash drums: the ejector of the evaporator or the condensation process of vapor in the heat exchangers and final condenser?

In your solution, you mentioned to measure "shutoff" vacuum. You mean I shut down the ejector while keeping the evaporator running, and measure the vacuum in the 3 flash drums?

In addition, I made a process draft for reference.

Thanks

Attached Files

•  Evaporators_Process.pdf   14.97KB   187 downloads

[Ali66]

Hi ReddeerC,

Ejector gives you the highest vacuum while the condenser reduces the load on the ejector thus you are able to go deeper in vacuum.

Some questions if you can answer or comment on:
1) I'm not clear on why you have both series and parallel connections - yellow vapor lines in series (I understand that) but why are all the red lines connected directly to final condendser/ejector?

3) Why do you have the red-line between the steam heater (1st HX) and the condenser/ejector? This can be an unnecessary load on the ejector.
Perhaps red line leaving 1st HX can be directed differently/elsewhere.
Same can be said about red line leaving 2nd HX unless vapor leaving first flash drum is meant to be below 1 atm abs, i.e. under vacuum?

4) Can red line leaving 2nd HX be connected to vapor line leaving 2nd flash drum, i.e. both serve as heat supply stream to 1st flash HX.

Note that redline leaving 3rd HX is already connected to vapor line leaving 3rd flash drum, so no change suggested.

The above would make both vapor lines and red lines in series, and can result in a cascade of vacuum/pressure levels.

Ali

[ReddeerC]

Hi, Ali,

Thank you very much for your time and help. I really appreciate your valuable opinion and suggestions. Please refer to your posts for my questions and answers/comments.
=============================================================

Hi ReddeerC,

Ejector gives you the highest vacuum while the condenser reduces the load on the ejector thus you are able to go deeper in vacuum.

***Following your opinion, may I understand like this: ejector is the main source of vacuum generation for the evaporator; and the condensation process in the heat exchangers and final condenser will strengthen the evaporator vacuums (you are definitely right that the condensation process reduces the vapor load on the ejector)?***

Some questions if you can answer or comment on:
1) I'm not clear on why you have both series and parallel connections - yellow vapor lines in series (I understand that) but why are all the red lines connected directly to final condendser/ejector?

***Series lines are for the feed and energy (vapor/steam) flow directions. As to the parallel lines, according to the design, it is assumed that the remaining steam from 1st effect HX, and the remaining vapors from the 2nd and 3rd HXs and final condenser are all vented to the atmosphere (after passing through ejector's own small condenser) by the ejector.***


3) Why do you have the red-line between the steam heater (1st HX) and the condenser/ejector? This can be an unnecessary load on the ejector.
Perhaps red line leaving 1st HX can be directed differently/elsewhere.
Same can be said about red line leaving 2nd HX unless vapor leaving first flash drum is meant to be below 1 atm abs, i.e. under vacuum?

***I have the same opinion as yours that the red-line between the steam heater (1st HX) and the ejector increases the load on the ejector, especially when the steam goes to the 1st HX is in a very large flow volume (although the initial design assumed that remaining steam could be handled by the ejector and was supposed to be vented to the air by the ejector).

Your guess is correct: according to the initial design, the 1st flash drum is supposed to run under vacuum (but now there is no vacuum in the 1st flash drum), and the remaining vapor in the 2nd HX is supposed to be sucked into ejector and to be vented to the air. I have two questions here, how come you guess out 1st flash drum is working under vacuum if there is a red line leaving the 2nd HX? If there is no vacuum in the 1st flash drum, the remaining vapor from the 2nd HX should be vented to the air directly, instead of being vented to the ejector?***


4) Can red line leaving 2nd HX be connected to vapor line leaving 2nd flash drum, i.e. both serve as heat supply stream to 1st flash HX.

***You mean to combine the remaining vapor from the 2nd HX with the generated vapor from the 2nd flash drum and to supply them to the 3rd HX as its heat source? I am not sure about this suggestion, but I can see your point that you are trying to make a red lines in series.***


Note that redline leaving 3rd HX is already connected to vapor line leaving 3rd flash drum, so no change suggested.

***No, in the diagram the red line leaving the 3rd HX is not connected to the vapor line leaving the 3rd HX. Instead, the red line leaving the 3rd HX is connected to the other red lines from final condenser and the 1st and the 2nd HXs for the remaining vapors in them; and in the end all the red lines go into ejector (passing through the ejector condenser) and then to the air.***

The above would make both vapor lines and red lines in series, and can result in a cascade of vacuum/pressure levels.

***Frankly speaking, I don't know why the original design arranged a parallel vent lines (red lines) for the remaining vapors/steam in the 3 HXs and the final condenser. Maybe it is easier to control the vacuums in the 3 flash drums and 2nd and 3rd HXs by this way design?

But, I can see your point and the advantages of your arrangement. Thanks again for your thoughtful inputs.***

Ali

[Mehrdad]

hi ReddeerC

please to comment :
1-what type of condenser is used in the plant?
2-did ejectors inspected for nozzles ,scales in throat and inlet pressure?
3-what is the inlet gas of ejectors (compressed air or steam)?
4-isn't plant working in overload production condition?
5-what about condensed water?

i will try to present my answer after your comments.
sincerely
mehrdad

[pawan]

Can you please share the operating condition of P & T before & after the problem.
This will probably tell us the problem area.
Also as mentioned by Ali all red lines from HX should be removed & use steam traps at condensate outlet for these HX.

[Jiten_process]

hi

appreciate your prompt reply,

Firstly on the basis of your drawing i have a doubt, Why do you route red lines directly to your ejector, i suppose because these lines ideally should not have any vapours and generally it is given for non condensable (air or other gases...) and you must have some orifice in between to ejector and discharge nozzle. Ideally your heat exchangers should have only condensate out of it from the heating media side. As to my exp. you should route these lines through the condensers. And if at all these lines designed aiming remaining steam then this steam should be used as an heat input rather than wasting it to atm. So for this purpose these lines should be connected to the yellow lines leaving off each flash drums. Hope you are getting what i mean to say!!!

Second is, the flash drums which you are talking about, is not exactly the flash drums but more or less vapour liquid separators only. Do u have pump in between H.E. and flash drum to transfer concentrate to flash drum?? If it so then you have to have Pressure drop valve (which called as back pressure valve) since your pump discharge line shall be in positive pressure and flash drum shall be in vacuum this valve creates desired pressure drop across it.

I would not go now on design stage of your plant coz it has already been working for years and if we discuss on this then we would deviate from your question.

Now let me give u answer of your question, presently your plant must be stopped, right? shut off vacuum for ejectors means I wanted to say that you must have some isolation valve on discharge line of your ejector before the condenser, you keep your ejector on and close this valve and check how much is the vacuum your ejector shows. Likewise you keep on doing the same exercise at different parts off the ejector and see vacuum reading wherever you have instrument. And as to my understanding you should see some vacuum in your instrument on the drums when plant is stop. So if you get vacuum ok in your plant in stop condition then there is no problem in ejector. If this test is negative then Remaining there are two possibilities one is there is some problem in your condenser performance or second one is there is heavy air leakage through some point.

Each point of your plant which is directly or indirectly connected with ejector shall be in vacuum only, however since i don’t know the product and type of your plant i could not comment on the level of vacuum.

So this is what from my end trying to explain in a simple way.

[katmar]

Jiten is correct - the red lines are there to take the non-condensibles directly to the ejectors. This is very standard in this type of plant. The nozzle where the non-condensibles is extracted will be at the opposite end of the exchanger from where the steam enters. These lines must NOT be re-routed to the condenser. The condenser creates vacuum by condensing vapours, and the presence of NC gases will decreases its ability to condense the condensibles. As Jiten stated - there must be an orifice or valve to limit what is extracted from the heat exchangers via these NC lines or you will suck steam and overload the ejectors.

There are three possible reasons (at least) for the lack of vacuum. These are
1. Ejector not removing the NC gases
2. Condenser underperforming because of fouling or lack of water (or water too hot)
3. A leak of air into the system somewhere.

As already suggested - spade off various sections and test to see where you are able to generate vacuum and where you could have air leaking into the system.

I suppose there is a 4th possible reason for no vacuum. If there are large quantities of air or other NC gases dissolved in your feed that were not there in the old days when the plant did work properly. Did anything change in your process at that time that could cause large amounts of NC gases to be in the feed?

[ReddeerC]

Thank you very much, Jiten_process, katmar, Ali, Pawan and Mehrdad, for your kind help and in-depth analysis. I am very glad that I come to this forum for my production problem: so many nice and experienced professionals here!

I should have told you more information about my plant evaporator at the beginning: it is a forced recirculation 3-effect evaporator. As to some details: we collected condensate from the bottom of all the 3 HXs and the bottom of the final condenser (the condensate from ejectors' intercondenser goes to the final condenser first).

Also Jiten_Process and Katmar's analysis is correct: we do have one throttle valve on each red-line branch just out of each HX.

Based on your suggestions/solutions, I know what I should do next and I will keep you updated.

Thanks again, :-)

[Jiten_process]

I agree with forth possibilities by katmar also try check whether your throttle valve is goofing or valve before flash drums.

and do let us know what is the result of your efforts.

all the best.

to be continued....