jaceted vessel - heating and cooling with the same jacked

[rob5377]

Hello
I have been reading this forum for ages and I ve found it very helpful many times. This time I would like to ask a question.

I have to prepare basic idea for preparation of solution (powder + solvent) in vessel with agitator. The volume is ca. 150 litres. First the batch have to be heated up to 85 C, stirred and then cooled down to 20 C.
I have only water at 7 C, 3 barg and electricity. So my idea is to use additional small tank with electic coil + pump to circulate hot water between jacket and the vessel. When the batch need to be cool down I will drain hot water and use cold water instead. Cold water would be supplied to the jacked and drained to the sewer.
Is this a good idea. Any sugestions?
Thank you very much

 

[jmw]

This is common practice in pharmaceutical applications, for example, and many other reactions with similar dynamics.
Of course, in pharmaceuticals we are now talking about steam heating and refrigerant (ethylene glycol and water) cooling.
For efficiency some form of interface detection is used to control the three-way valves (can be based on temperature).
But: a complication is that the repeated temperature step changes in temperature accelerate valve seal failures and then there are leaks between the steam and refrigerant. This leads to the refrigerant becoming more dilute which raises the freezing point up to the flowing temperature and that then causes some problems in the refrigeration plant. It is thus appropriate to monitor the "brine" concentration using density meters in each batch of reactors to help identify when and where such problems arise and to use density meters in the refrigerant plant to monitor the quality of the "brine" returned from the whole plant so that ethylene glycol can be added back in to protect the plant and ensure maximum coolant efficiency.
As to drainage, your local regs. will apply and these are sometimes more strenuous that the particular application warrants. It may be clean water but if it comes from a particular plant you may have to run it via a water treatment plant along with everything else. Worth checking but I'd see what the others say on this.

JMW

http://www.ViscoAnalyser.com

 

[Compositepro]

JMW made some very good points. I would add that if you change temperature a lot it is is usually best to keep the volume of fluid in the jacket circulation loop as small as possible. That is have a circulation pump that pumps to an electric heater (no or very small tank) to the jacket and back to pump. System is kept pressurized by your cooling water loop. When cooling is required a valve opens that drains water from the jacket loop. This allows cold water into the loop. There are several companies that make prepackged units. They are often refered to as tempering units and are common in plastics processing.

If you require very fast heat-up and don't have the power capacity for it you can store heat in a tank for when you need it. Be aware, however, that very rapid temperature changes on tanks and piping can cause severe mechanical stresses.

 

[quark]

That is a general idea and works well if you look into some details. Your batch temperature requirement is on higher side of what practically possible with hot water. Batch heating can be designed upto 10C TD, to optimise pumping power but then the hot fluid temperature goes upto 95C in your case. So, I will go with only 5C TD and approach a rather slow heating. You would require about 22.3kW if you want to heat the batch in 30 minutes and 11.15kW for 1hour.

The cooling load is about 3TR if the duration is 30minutes. I don't see any point in draining the fluids. My preference would be (as this is a smaller batch) to use a separate water bath for heating and cooling, as well. They are commonly used for small scale fermentors.

 

[Compositepro]

Let me clarify that what I meant was draining the jacket to the suction side of the cooling water loop. This allows cooling water to enter the jacket from the suuply side of the cooling water loop. The main point is that the design starts to get much more complicated if you have fluid storage tanks that are not completely filled and fluid levels have to change during operation.

 

[rob5377]

Thanks for all answers. First of all I wanted to make sure if my idea makes sense.
I havent known that in line heaters exists. It is good idea because I will not have to worry about to low pressure in the system and level in the tank. I will use heater then pump, and jacketed vessel. + four on-off valve before heater, after pump, on cooling water supply line and on drain. Hot water loop will be closed. To cool down batch I will have cold water connected to the loop with on-off valve. I will have drain line with on-off valve connected to the loop after vessel jacket as well. During the heating both valve (drain and cooling water) will be closed. During cooling down I will open the valve on the cooling water supply and on the drain and close the valve before the heater and after pump to drain hot water and to feed loop with cold water. My question is: Is it better to connect cooling water downstream or upstream the circulation pump. In my opinion it is better to connect it downstream (after) the pump. Water would circulate due to the water pressure and pump could be stopped. On the other hand if I connected cooling water line upstream the pump, circulation loop would be supplied with the pump and would cause more turbulent flow (higher heat transfer coef.)
Thanks

 

[Compositepro]

The document at the following link should tell you all you need to know.

http://www.chromalox.com/products/productinfo/en/pdf/sysintro.pdf

A temperature contoller is usually used to operate solenoid valve. These days manual control seems pretty primitive, but it will work.

Heater is on discharge of pump to reduce boiling problems. Cooling water is input to suction side of pump because there is less back-pressure there.