Cooling water temperature for a Distillation unit

[advsign]

We were wanting to do a temperature test on our cooling towers. We wanted to increase the variable speed fans to 100% and see how far the temperature could decrease.

Problem:
Here we have several distillation columns and all use water cooled condensers. The distillation department would not let us decrease the cooling water temperatures. He stated it would upset the process. All of the condensers condense the vapor 100% now with cooling water at 25C. If we lowered the temperature to 20C, what would this hurt?

Thanks in Advance

 

[EmmanuelTop]

You might be subcooling the reflux excessively, for which I have seen it can cause problems in some towers (liquid flood). Otherwise nothing to be concerned about, if towers are equipped with correct and efficient pressure control scheme. At the end of story, we usually want more cooling, isn't that so?

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[katmar]

We have all become spoilt by the ability of modern control systems. In the past a swing of 5C in the cooling water temperature over 12 hours was not at all unusual. I have seen much worse, and the operating staff coped with it. If your cooling tower control is this good then I would agree with EmmanuelTop and expect the distillation tower controls to handle the variation with ease.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BlackSmoke]

With colder water temps, partial condensers are of course affected by additional condensation of light compounds, but these temperatures should be actively controlled already.

Some condenser types (such as horizontal shellside) would not show much additional subcooling, due to the fact that condensed liquid falls off the tube before it can be cooled substantially. A vertical tubeside condenser may show additional subcooling.

Of course this all depends on the process, esp. how sensitive the columns are, but I'm guessing that you could pull off a test while running. I sincerely doubt that cooling water temperatures do not fluctuate substantially through the year, so your distillation guy should be able to handle it. Ramp your fans to 100% slowly and there shouldn't be a problem.

 

[sshep]

Since this post got active again, I would point out that running colder CW could be an good economic sell depending on how your cooling water users control their duty. If you quantify the savings it could break all resistance to considering the change or test run.

This is what I call trading temperature for flow at constant duty (Q=dT*F): Consider an existing CW operating range from 40 to 30degC. Operating with a CW supply of 25degC could potentially reduce the CW circulation pump horsepower by 1/3. Although you will have to increase the fan horsepower to get the airflow higher, the fans operate at inches of water head vs the pump dP which could be as much as 100psid. Even after all the efficiencies (at pumps, HX's, cooling tower, etc) are considered, the economics will likely increase in favor of the coldest water temperature that you can maintain. If the CW flows are balanced manually, then some field rebalancing would be required, but the electrical savings could make it very worthwhile.

best wishes,
sshep