2 replies to this topic

[TCC736]

Hello all.  I am not a chemical engineer, so forgive my ignorance on the topic. 

 

I work in a small industry that often has the need to gas free chlorinated solvents --EDC, TCE, Choloroform, etc.  The physical location where these gas free operations take place has a bulk nitrogen storage tank, and would like to use it as a refrigerant in a unit to condense the vapors from the storage tanks.

 

As I understand it, the Nitrogen gas, must be compressed back into a liquid, ran through a condenser where the chlorinated solvent vapors are drawn in and condensed to a state of liquid --the nitrogen gas is then vented to atmosphere --since it is a non-pollutant.  Is this basic approach achievable?   

 

If anyone has any thoughts on this process, please let me know.  Or if anyone would just like to engineer a unit similar to this and save me the headache, I would be open to that too. (send me a quote) Ha.

 

Thanks.

[Art Montemayor]

TCC736:

 

In the past, I operated and managed a lot of compressed gas and cryogenic plants, producing among many gases: liquid and gaseous nitrogen among some.  I can tell you from practical and personal experience that while your idea of using gaseous, saturated, cryogenic nitrogen vapor may sound as a neat and economical idea, it probably won’t work for you on a practical basis.

 

The basic principles of process heat transfer apply in trying to use the cold (approx. -340 ^oF) to condense what you identify as “chlorinated solvent vapors”.  Unfortunately, the rules of heat transfer also apply in that the heat transfer film coefficients in question are those of a gas and that makes the rate of heat transfer from the  solvent vapors to the cold nitrogen gas to be very, very slow.  Nature (and probably God) has deemed gas heat transfer coefficients as very poor and undesirable in carrying out a heat transfer operation.

 

To aggravate your proposal even further, bear in mind that the cryogenic temperature of the nitrogen gas is so low that the solvent vapor condensed in a coil or other heat transfer apparatus would not only condense (as you desire it to) - but also probably solidify and clog or plug up your coil or tubular heat exchanger.  The bottom line would be that your heat transfer application would not work economically nor practically.  Sorry for the bad news.

 

To give you an example of how inefficient and poor gas heat transfer coefficients are, look at how a polar bear is able to withstand extremely cold temperatures with just a coat of hair to protect its warm, blood-circulating body.  The bear’s hair acts as a cushion of static atmospheric air trapped within its fibers and that same air acts as an efficient insulator that isolates the polar temperatures from his blood stream.  A wool sweater does a similar thing (but not as efficient as the Polar bear) for a human being in the wintertime.  Additionally, your process insulation material used on pipes and vessels as well as even your house insulation works on the same principle: gases are notoriously poor transmitters of heat and are used as insulation to prevent efficient heat transfer.

 

In your proposed case, it would be much more efficient to use LIQUID nitrogen as your heat sink - but, again, this would really cause your solvent vapors to solidify due to the extremely low temperatures.  The best heat sink to use in order to condense solvent vapors is an engineered refrigerant (such as a Freon, ammonia, or better yet, a refrigerated glycol solution).

 

I hope this helps you out.  I won’t apologize for the long response, because I sensed this Heat Transfer Forum is a good opportunity to air out this topic of heat transfer basics and perhaps draw some more comments and opinions from our veteran Forum members.

[Padmakar Katre]

 

Use of N2 as a refrigerant is not so common because of the lower temp level. Use of gas as Art stated, film coefficients would be low and hence the size of condenser, in addition compression and expansion would energy intensive process.

Alternate refrigerants like C3R, Freon, NH3 can be explored based on the process side temp requirements.