8 replies to this topic

[nikhil.sharma]

Hi everyone,

In our plant, we have following 3 types of vacuum pumps:

1. Water ring vac pumps
2. Oil ring vac pumps
3. Dry/Oil lubricated vac pumps

We have vacuum traps engaged with all the pumps. The trap sytem works with continuous circulation of 10% NaOH solution as the vapors generated are acidic in nature. A lot of care and effort is required to keep these systems working efficiently. This adds up some cost as well viz. of NaOH and pumping.

I am thinking of joining 2 vacuum pumps viz. a water ring vacuum pump as the first stage and a dry vacuum pump as the 2nd stage.
The water ring vacuum pump has contiuous cooling water flow in it which will carry away most of the solvent vapors with it.
This way the water ring vacuum pump will be acting as the primary stage as well as the solvent trap.
The 2nd stge i.e. the dry vacuum pump will be acting as a booster for this.

But the problem is of water vapor carry over from water ring vacuum pump to the dry vacuum pump which will severely affect the performance of the of dry vacuum pump.
If the water carry over can be avoided somehow, this system can work well as planned.
Any suggestions to avoid the same will be highly appreciated.

Regards
Nikhil Sharma

[ankur2061]

Nikhil,

Study the option of employing a high efficiency gas-liquid separator or gas-liquid cyclone between the inter-connecting piping from the water ring vacuum pump and the dry vacuum pump. While evaluating this option also consider liquid collection and disposal from the cyclone / separator as well as the impact of the extra pressure drop across the high efficiency separator / cyclone unit.

This evaluation / feasibility has to be made in context with the fact that your entire system is operating under vacuum and any liquid collection and disposal has to ensure that the system vacuum is not disturbed.

Hope this helps.

Regards,
Ankur.

[nikhil.sharma]

Ankur,

Thanks a lot.
I'll work on the suggestions you provided and reply with any further queries.

Regards
Nikhil Sharma

[nikhil.sharma]

Ankur,

After considering your suggestion, I did some calculations and from those, I thought of another idea based on your post.
Plz help me on this one:

Can we put up a Barometric Leg with an interstage condenser in this system?
This will be similar to one working on a Steam ejector where we are trapping the vapor carry over to the next stage.

Also, suggest to me whether the idea of employing a Water Ring Vac Pump as a solvent vapor removing system is fine?
I ask this because this idea will affect the pump to some extent.

Regards,
Nikhil Sharma

[ankur2061]

Nikhil,

A barometric intercondenser may not be such an effective option for ensuring the complete removal of vapor to the dry vacuum pump for the following reasons:

1. Barometric intercondensers are essentially designed to reduce the load to the next stage and not for liquid mist removal to the next stage (in your case the dry vacuum pump). If your dry vacuum pump can tolerate ppm levels of liquid mist then you could pursue this option.

2. Barometric intercondenser performance is directly a function of the water temperature and flowrate. Even slight changes in water temperature and flowrate effect the performance of the intercondenser and this can be seen in the vacuum fluctuations that are normally seen when barometric condensers are running unstable due to fluctuations in water temperature and flowrate to the condenser.

3. In steam ejector systems with intercondensers handling organic vapor evacuation, water treatment of the water supplied to the intercondensers is an absolute must due to the potential of fouling by build-up of anaerobic algae and slime. I have been an unfortunate witness to the complete blockage of intercondensers and the barometric leg pipe due to algae / slime buildup due to the fact that the water treatment was not up to the mark.

Water ring vacuum pumps have been used for applications of organic vapor evacuation since ages. Again the question arises whether the seal liquid (water in your case) is compatible with the organic vapor being evacuated and whether there is a potential of fouling due to the build-up of algae and slime if the water is being recycled to the vacuum pump system.

Hope this helps.

Regards,
Ankur.

[katmar]

What would the interstage condenser condense? All that I can see it doing is removing the heat of compression from the vapour exiting the liquid ring pump. This vapour would be saturated with water and a condenser would cool the vapour and cause some condensation, but I cannot see it having a major effect. Interstage condensers are more common on steam ejector systems where you need to condense the motive steam before passing the incondensible gas to the next stage.

Also be careful of using a water sealed liquid ring pump as the first stage. The first stage sees the deepest vacuum (lowest absolute pressure) and you may get boiling of the seal water and cavitation. It is more common to have the liquid ring pump as the second stage, after an ejector (possibly with an after condenser) as the first stage.

[nikhil.sharma]

I'm attaching a diagram(not a P&ID) of the system we are using right now.
there's a problem with the system that it's not giving vacuum level desired for the distillation operation we are performing.
If it had not been a process requirement, i'd have raised the temperature a bit but the process restricts me from pursuing this option.
We are suffering a loss in production as the rate of recovery of acidic water in the process is quite low on 660 mm Hg with the new system as compared to 690 mm Hg with Steam Ejector.

I'm attaching another file which I found while searching on the internet.
They have shown a system similar to ours but the only difference being the position of the 2 pumps.
My problem is that the vapors are highly acidic in nature(pH ~ 1.0) and I can't take the risk of letting the vapors into the Dry Vacuum Pump.

Kindly take out some time to check the 2 files and suggest me accordingly on this issue.

Thanks & Regards
Nikhil Sharma

Attached Files

•  dry-techanical-boosters -replace-steam-ejectors.pdf   230.59KB   79 downloads
•  Vacuum system Diagram.doc   35KB   63 downloads

[bmk]

Dear Mr.Nikhil Sharma,
I suggest you to use Once through lubrication vacuum pump for your application. The performance of this pump will not be affected by uncondensed vapors of acetic acid. You can achieve a vacuum of -750 mm Hg to -759 mm Hg. I am associated with the manufacturer of this pump in Maharashtra. Contact Mr.V.M.Patkar,Tel NO.09892244796 for more details.---B.M.Khare,(M.Chem.Engg.) 9869343775

[Elizabeth_I]

Interesting suggestion. Thank you:)