Using Steam Injectors to Pull Full Vacuum

Hello Guest,

What you are looking for is called a steam ejector.

Steam is the "motive fluid" - the vacuum is pulled at the "Inlet Gas, Liquid or Other" port. I have used a unit made up of 5 of these in series configuration. We were able to get very low vacuum (sub-micron if I remember correctly).

Remember that any vapors that are given off by the reactor mix are being carried away in the steam exhaust. This stream must be dealt with accordingly. That is you may not want it going to the steam return because it may crap up your steam system. Additionally, if toxic/restricted materials are involved, you don't want to release them to the environment.

There is more info on Wiki - just Google "Steam ejector"

Hope this helps!

Mike

Beware when operating under a vacuum as you can pull air into the reactor head space which can lead to an explosive atmosphere within the vessel. I have had cases in the past where the vessel would not be allowed to operate in a vacuum for fear of letting oxygen into the system. A nitrogen purge is the preferred way of inerting a vessel with hazardous/combustible vapors.

Hi Guest,

I would endorse the caution advised by Healybj. You need to be clear on whether you want vacuum in the reactor or an inert atmosphere. If the reaction warrants vacuum then an ejector is the way. To keep oxygen out of the reactor Nitrogen is the best way. Do keep in mind that operating ejectors can be pretty energy intensive too - motive Steam and Cooling water for interstage condensers. Any fumes sucked out will create an effluent stream that needs to be tackled.

If the reactor has been charged with water, it is not possible to achieve 'full vacuum'. It is only possible to achieve that vacuum which corresponds to the boiling pressure of water at that temperature. A set of steam tables can give more insight, for example:

• Mayhew & Rogers - "Thermodynamic and Transport Properties of Fluids", Oxford Basil Blackwell, 1976 ISBN 0 631 96400 2
• "Chemical Engineers' Handbook", a.k.a. 'Perry', any edition
• "Kempe's Engineers Year Book" 1988, by Morgan Grampian Book Publishing Co. Ltd., section F10.

In order to avoid explosion it is necessary to remove the oxidant in the vessel, which is usually done by displacing the oxygen in air with something inert. It is usually done by nitrogen purge, and widely practiced in bulk pharmaceutical manufacturing operations.

It sounds like the services of an experienced Chemical Engineer would be of value at that location, perhaps?

PWSlack makes an excellent point about the vapor pressure of the water. In addition, it sounds like this is an exothermic reaction which will heat the water and increase its vapor pressure. In effect the harder you pull the faster the water will evaporate and you will just ride on the P-T curve as your absolute pressure in the vessel.

Nitrogen/ Inert gas blanket at positive pressure seems a more reasonable approach!

Sorry to nit-pick, but I believe you mean "-100 kPa gauge". Absolute pressures are referenced to 0 (total vacuum), so there cannot be negative numbers.

So, if 101 kPa = 1 atm =760 mm Hg

Then:

-100 kPa gauge = 1 kPa absolute

At least this is the convention I have always seen. Your meaning is clear in this case, but it could be a problem in other cases if you have the terms confused.

The other listers are correct, you cannot pull a vacuum greater than the vapor pressure of water. At 25C, this would be about 3 kPa absolute.

Tad

Using Steam ejectors to clear equipment of oxygen and "inerts" is very common in the chemical industry. The method usually involves having water available in the equipment heated to a temperature where the vapor pressure of the water is above the pressure available from the ejector (or what ever the vacuum source is). As the ejector removes gas from the top, the water boils in the equipment and the "steam" acts like a carrier gas and the vapor space is purged and exhausted through the ejector. The resulting vacuum is broken with process fluids or reactants.

In the older days a barometric leg was required to seal and balance the system.

The beauty of the system is that, if properly designed, the steam ejector exhaust can be condensed at atmospheric pressure to maintain water in the barometric leg and the incondensable vapor can be sampled for oxygen, nitrogen or other "inerts / incondensable".

Often the vapor stream (and liquid) went to a recovery system to capture other components before exhausting to atmosphere.

The technology of oxygen sensors is also important. The explosive hazard is not just explosive limits of the vapor space. A more benign hazard is the formation of peroxides in the process equipment. These nasty contaminates can become unstable and can detonate when conditions are right. I recall vinylperoxide species and polymerization initiation limits were formed on the order of 10 parts per million oxygen. At the time the only sensor that could measure it on line were Paramagnetic Detection technology, there may be others now.

Good luck

Scan Mechanical threads. There was a similar case and there was success.

Del the Cat helped him out. Contact him/ read all these in his thread.