4 replies to this topic

[asm8930]

Hi...everybody

I have one doubt in mind, I need to really understand the concept (pros and cons) which may arise by this situation..

The Problem is "If I want to tranfer solvent(Dimethylcarbonate) from one vessel (0 level) to other vessel, located at elevation - say 10 mtrs - by using Nitrogen (I don't want to use a pump).

A) What is the Nitrogen Pressure required.
B) Effect of that pressure and corrosponding temp. on the solvent while transfering."

pl help me clarifying this doubt.

Thanks

[fallah]

A) P (of Nitrogen in kpag)>SG (of Dimethylcarbonate)*1000*9.81*10 provided that upstream/downstream vessels would be at atmospheric pressure

B) No undesired effect

[Art Montemayor]

ASM:

Your query is very basic – and a very important one for all first-year Chemical Engineering students. You state you “need to really understand the concept (pros and cons) which may arise by this situation” and you are very correct. I will go to any lengths to try to show a student a basic concept that he/she can use in later life. However, I will not do the calculations or the research work involved. That should be up to you and if you want to discuss the work and calculations, you should show them in a detailed, professional workmanship manner so that we can analyze them and make the appropriate comments.

The concept is as follows:

Lifting or pumping liquids by using gas pressure is a very old and proven technique. The operation was once known as an “Acid Egg” procedure. This is derived from the fact that a century or more ago, some very nasty and corrosive chemicals such as Sulfuric Acid, Hydrochloric Acid, etc. were being produced industrially and had to be pumped without the existence of the exotic metallurgy and materials of today (such as stainless steels, Incoloy, etc.). Neither did our grandfathers have access to the techniques of welding and high technology as we have today. They had to make-do with very simple and practical methods to accomplish what they needed to do. They used a vessel that was somewhat like an ellipsoid and employed air pressure to "push" the liquid up and out of the vessel through a dip pipe by exerting the air pressure on the liquid's surface. In this way, they avoided a mechanical pump.

By using the Acid Egg to pump or transport corrosive or toxic liquids, they employed the simple technique of hydraulic lift – using gas or vapor to lift a column of liquid. This is very similar to the concept of Pascal´s law that states that "pressure exerted anywhere in a confined fluid is transmitted equally in all directions throughout the fluid."

ΔP = ρ g Δh

Where,
ΔP = the hydrostatic pressure (given in pascals in the SI system), or the difference in pressure at two points within a fluid column, due to the weight of the fluid;

ρ = the fluid density (in kilograms per cubic meter in the SI system);

g = acceleration due to gravity (normally using the sea level acceleration due to Earth's gravity in meters per second squared);

Δh = the height of fluid above the point of measurement, or the difference in elevation between the two points within the fluid column (in meters in SI).

The intuitive explanation of this formula is that the change in pressure between two elevations is due to the weight of the fluid between the elevations.

For example, 1 bar of gas pressure will raise a column of water 401.463 inches.

By applying the same law or equation, you can also lift Dimethyl Carbonate or any other liquid. All you have to do is find the density of the liquid and apply the law to find the nitrogen pressure required to lift the liquid.

My comment to your part B) is that you have to state what the temperature of the nitrogen is that is applied. I have to assume that the nitrogen is at room temperature and the required pressure. Therefore, there should be no effect on the liquid. Possibly the only effect a gas or vapor could have on the liquid would be that it would dissolve into the liquid or, if at a high temperature, it could cause the liquid to vaporize – which is not our case if the nitrogen is at room temperature.

This is a very important concept and use in engineering and you should be taking steps to master it completely.

[asm8930]

Thanks Art, for your quick reply.

Here actually I have one more doubt, if the N2 purging is done by 1 inch line (suppose 3 bar pressure):

A) wht abt this pressure on DMC(low boiler).
B) what will be the effect on the volume of DMC to be transfered.
C) After particular level the N2 will also start flowing along with DMC, this means some pool of DMC to be maintained in the vessel.

Hope you will not mind to reply this query.

Thanks again

[Art Montemayor]

DSM:

Actually, you have 3 doubts, not one. But I don't mind.

A) A pressure of 3 bar on a "low boiler" (a compound that boils - or evaporates - at a low temperature should have no perceivable effect on the liquid.

B) The liquid (as most liquids) is considered as non-compressible; therefore, the volumetric flow is not noticeably affected.

C) Yes, just as in the case of a centrifugal pump, there will always be some liquid residual left in the source tank if you try to "pump" all the liquid out. One bad feature about an acid egg is that you will always have a gas surge going through at the end of the liquid transfer if you try to empty the source tank. This could cause you some potential problems at the other end - the target tank or vessel. You must always consider that you will have a gas "hammer" coming through at the end unless you stop the liquid transfer before you reach a liquid level in the source tank that allows high pressure gas to get into the transfer pipe. I once designed and installed a phenol transfer from a rail tankcar into an API heated tank and I had to design a safety system that avoided the high pressure nitrogen gas from reaching the storage tank and causing an over-pressure there.