3 replies to this topic

[pawanpartap]

Dear all,
We are facing a problem that our Cng gas dehydration unit is not working.
It is based on refrigeration principle.
Gas is entering at 19 bar pressure. Gas basically consists of Methane(85%), Butane(7%),others are residual gases like Nitrogen etc.
Gas is coming out of the unit at 0 to -3 degree celcius.
But still water is not coming out...

So my question is basically what can be the possible causes.Also pls provide if someone knows dew point temperatures of Methane.
Pls guide us the solutions.

[Art Montemayor]

Pawanpartap:

 

You may be having a problem with your refrigeration-type of dryer, but you don't give us all the basic data required to know, identify, and comment on.  All you tell us is that you have a gas mixture of methane and butane and other gases and that you are cooling this mixture down to 0 to -3 ^oC.  You don't tell us:

• The detailed sketch of the system and the various expected temperatures and pressures;
• Who designed and fabricated the refrigeration unit?
• What are the design compositions, flow rate, temperatures, and pressures for the unit?
• What are the actual operating compositions, flow rate, temperatures, and pressures for the unit?
• What is the water content of the feed gas?
• What is the desired water content (atmospheric dew point of the product gas?
• What is the design water content (atmospheric dew point) of the exit product gas?
• Has the unit operated as per design in the past?  For how long?
• Have there been any changes in the gas flow stream and the mechanical equipment in the recent past since the unit started to malfunction?

Expect your reply.

[pawanpartap]

Sir, thanks for ur reply.
here I m giving the remaining available specifications.
This unit is designed by ultrafilter.
It is designed for 1500SCMH flow,50degree celcius temperature,50bar pressure.
Operating conditions are 1200SCMH flow,25degree celcius temperature,22 barpressure.
Sir, Actually this is the first filteration unit for removal of water from gas installed in our organization.So, it is having no previous records.
Now,actual problem is it is not removing water from gas.
A new problem has occured now sir.
From internet we come to knew that Below 70 bar pressure HC gets condensed first than water.e.g. At 30 bar Hydrocarbon gets condensed at (-10degree celcius),where as water gets condensed at( -30degree celcius).
To condense water first we will have to apply refrigerator unit after compressor i.e. at 200, which is not possible because unit is designed for 50 bar only.
Now we are going to decrease temperature of gas to -12 to -13 degree Celsius.We will see what will get condensed first.
Please give us right suggestions sir... Are we moving is right direction ?what else we can do to make this unit work ?

[Art Montemayor]

Pawanpartap:

 

What I am now able to decipher from your information is that you bought a new refrigeration dehydration unit from a firm called ultrafilter - presumably in India.  We still don't know the feed gas water content and the required product gas water content (or atmospheric dew point).

Is the manufacturer of the unit the same as the one in the document attached?  If so, is this a special unit designed specifically for compressed natural gas (CNG)?  Ultrafilter India designs and builds units for 7 barg pressure maximum on air service.

 

When you specified your application to the dehydrator's vendor did you specify the composition of the natural gas as containing 7% butane?

 

You did not furnish the requested sketch, so I am forced to assume you are taking low pressure natural gas and compressing it in a multistage compressor for filling high pressure cylinders at approximately 20 bargs that will be used as fuel in internal combustion engines - such as cars, golf carts, fork lift trucks, etc.  I also assume that the natural gas feed to the compressor is saturated with water vapor (a normal natural gas pipeline would have approximately maximum 7 lb of water per million cubic feet of gas).  If you have any hydrocarbons higher in molecular weight than methane in your gas feed (such as butane), you run the risk of condensing them if you compress the gas vapor mixture and you cannot expect to drop out liquid water that is also mixed in.  In order to employ a refrigerated dryer to a gas or vapor stream that has water vapor in it you must refrigerate the gas mixture at a pressure where the gas mixture is in the pure vapor state - and without liquids in it.  That is why I requested your flow sketch.

 

Have you consulted with ultrafilter India about your application and the inability to drop out the condensed water content after refrigerating the gas mixture?  If they have no experience with natural gas mixtures and hydrocarbons they may not be able to calculate the equilibrium phase of the mixture at the refrigerated and compressed conditions.  You should make the equilibrium calculations to determine the phase(s) or have an experienced chemical engineer do it for you.  One solution to your problem may be to do the refrigeration at the lower gas pressures (presumably between compressor stages).  You fail to tell us your product water specs, so I can't guess the temperature you should have to cool the gas down to.

 

I have designed and used CNG units in the field and I always used adsorption type of dryers because they are simpler, more efficient, and more economical in operation and maintenance.  They also give you a very, very dry product.

 

 

 Refrigerated Air Dryer - Ultrafilter India (1).pdf   2.48MB   8 downloads