Cooling of Exothermic liquid reaction

Is direct quenching to be excluded ?

CMO:

I've worked in the Natural Gas Processing business, from field gathering to LNG, and most of the operations involved are UNIT OPERATIONS. Since you're a ChE, you're aware that this means that most of the operations don't involve a chemical reaction. You haven't told us what it is that you are doing in a liquid reaction. My point here is not curiosity, but we've got to know the nature of the type of operation before we can come up with comments or recommendations. For example, is solid catalyst involved? Is any catalyst involved? Is the reaction batch or continuous? Are gases involved?

More detailed information is needed.

Art Montemayor

Sorry I'll try to add a little more detail. To sets of liquids are contacted via a static mixer from separate tanks, cooled and returned to a third separate tank. From the heat of reaction ~600 kJ/gmole we are experiencing a high temperature increase when we are attempting to keep the bulk heat of the combined mixture under 70 C. Currently we have an arial cooler however on hot days or simply days above 0C blend times increase exponentially. This ends up being the bottleneck in the process.
So simply it is two liquids mixed via a static mixer through an arial cooler and off to tank. From the heat of reaction and the volumes we need to throw through the cooler works out to a duty (theoretical) of 8 mmbtu/hr.
Hope this helps. I can try to elaborate further if required.

Cmo:

Thanks for the needed info. It looks like a very simple solution (if you have cooling water available) is called for. However, you still haven't identified the heat load and I can't estimate the size of a cooling coil - to get an idea of the practical size of the unit. You give the exotherm release as heat quantity/gm mole, but fail to state the gm mole rate. Basic data requirements for a heat removal operation call for a heat rate identification - we need Btu/hr or other units - but the heat basis must be time, not mass.

A cooling coil would be my first preference. However, since I lack basic information like the availability of a CWS (cooling water supply), I would suspect that you don't have access to this utility were you are at. That would explain the air-cooled unit. Air-cooled units are, as you have probably found out, very inefficient when compared to water-cooled units. A gas film heat transfer coefficient can't compare with a liquid film heat transfer coefficient. If you're having trouble with 0 oC air, you must really have a high heat accumulation or a very dirty air-cooled unit. I trust your common sense to have the aerial cooler working as efficiently as you possibly can, so I suspect it has been undersized or you are operating above design capacities. No matter what the cause is, you need an efficient heat sink - and a water-cooled coil can get you out of this dilemma. Can you obtain a CWS? Can you obtain a self-contained evaporative cooler? Perhaps where you're at, it is arid and dry - which would make for excellent evaporative cooler conditions (although you still need a make-up water supply).

Your access to CWS or generating one is the answer to getting a heat sink that you can reject the exotherm to; your air heat sink is not sufficient. I hope this analysis is going in the right direction. I regret I can't give any more details, but the scarcity of basic data limits my observations.

Good luck.
Art Montemayor

Wow,
Thanks Art, good information to absorb. In regards to the heat rate, the 8 MMBtu/hr quoted above is not enough?
I can try to track down more info.
You are correct that we don't have access to CWS. Also our ambient temps have the potential to reach -40C at some points in the winter which may also be concern with freezing. Although I know they exist at some of the bigger polyethylene such as Dow and Shell in the area. On a side note the air cooler works well at these temps!
Can you elaborate on your comment above for me "A gas film heat transfer coefficient can't compare with a liquid film heat transfer coefficient"? More specifically then we need to change the exchanger internals?
Thanks for your help in this there is a big learning or should I say re-learning curve on this matter.
Regards,
CMo

Cmo:

Thanks for the quick reply. I apologize for requesting info that I overlooked by focusing on the 600 kJ/gmole figure and missed your 8MM Btu/hr figure. This is a healthy and significant heat load. I did not expect this size of heat load from a liquid mixing operation. I can visualize several banks of Marley or Hudson aerial coolers being dedicated to this service, if the 8 Million Btuh figure is a design, and not a peak value. If this is a consistant and steady load, this calls for a serious and detailed engineered design - not just a utility cooling coil.

The -40 oC (-40 oF) doesn't bother me as much as the total heat rejection quantity and the fact that you find yourself in a situation where you don't have normal plant utilities. I would suspect you're in field gathering or in a remote location. The low temperature levels can be easily handled with glycol solutions. However, I'm coming up with an estimated 600 to 800 gpm of cooling fluid recirculated - depending on the design conditions. This is a formidable cooling service. Aerial coolers this size are expensive to temperature-control. Additionally, as I stated, they are very inefficient compared to liquid-cooled units.

All heat transfer operations dependent on a transfering heat through a gas film are hampered by the notoriously low values of the gas film heat transfer coefficient. This is the trade off for using a quick, simple heat sink like the free air atmosphere. Gas are great thermal insulators - especially when they are static. This is why the good insulators, like expanded polyurethane, use air or Freon as gas-trapped bubbles to do the actual insulation (or heat transfer prevention). You have to move a lot of air and use a lot of fins (extended tube surface) to try to make up for the relatively low heat transfer coefficient on the air side - which is the controlling factor in an aerial cooler. This doesn't make the aerial cooler a bad application, it just paints it in the true light of its shortcomings. While a liquid-cooled exchanger is more efficient, it may also have its respective trade offs also. However, in this application I suspect the answer may be a refrigeration unit (which I always prefer not to place out in the field) servicing this need. A cooling water tower or evaporative cooler is probably not practical due to the low environment design temperature.

There isn't much you can do about the aerial coolers. They will work best in the colder climes (due to the larger delta T), but they still require enclosures to control the outlet process temperature. I don't know how important or economically driven this application is in your operations, but I kind of suspect that if you're accumulating heat in the magnitude of 8 MM Btuh and you're trying to make money by processing natural gas, this is costing you profits.

Do you have a potential for refrigeration in your NG processing unit? Perhaps some of your process fluids offer this potential - much like an NGL unit works. Just a thought since I don't know enough.

Good Luck.
Art Montemayor