Reboiler with a hot oil jacket 2

[Mike4chemic]

Hi,
We are designing a pilot batch distillation unit, which consists of a small 40L jacketed reboiler.
We are considering to install the electrical heating element below the bottom of the reboiler in a sealed jacket filled with a thermal oil.(the simplified sketch is attached).In order to increase heat-transfer,the oil into the jacket is continuous circulate through the jacket.
The reboiler's design pressure is 3 barg and it will be designed based on the ASME Code.
I would like to ask you following questions regarding the jacket's process design:
1. How can the oil's thermal expansion inside the jacket be considered. If the jacket's total volume must include the additional oil thermal expansion volume? Is it feasible to combine in the jacket both oil operation volume + expansion volume,given the fact the oil is heated and circulated through the same jacket?
2. How the max.operation/design pressure of the jacket can be defined?
3. If the jacket should be defined as a pressurized vessel and therefore protected by a PSV? Or the jacket can be defined as atmospheric vessel equipped with a breather valve?
Thanks in advance, Mike

 

[TiCl4]

Mike:

1. I would advise against the design as you have drawn it. The only way for the total volume of the system as you've drawn it to include room for thermal expansion is to have some gas (air, N2, etc) remaining in the line. If you leave air in the system, the pressure rise will be dictated by the volume change of the oil that compresses your available air volume - you can run that as an isothermal compression of a gas to determine end pressure.

However, having entrained air will likely sit at the top of the jacket and/or will reduce heat transfer surface availability. Air pockets may also increase the possibility of localized corrosion. Many liquid filled jackets typically fill from the bottom and exit at the top to remove any possibility of the jacket trapping air in it - I would suggest at least reversing your intended pump direction to help avoid air pockets.

I suggest you consider a hot oil receiver tank that has some gas in the headspace to allow for thermal expansion. This tank would also function as a a gas/liquid disengagement vessel when filling the jacket. The receiver tank could also have the heater installed in it - it would probably be easier and safer than inserting a heater inside a jacket. If you have valves on the inlet/outlet of the jacket, removing the heat source from inside the jacket also eliminates the "block inlets/outlets, heat addition" case for jacket over-pressure.

2. If you eliminate the possibility of thermal expansion generating pressure, your jacket maximum allowable working pressure will likely be dictated by the pressure drop of the recirculating fluid. If you are using an air-operated diaphragm pump as you've stated, that pressure is dictated by your compression ratio (1:1 is typical) and supply air pressure. The pressure drop calcuations are difficult to do on AODPs due to the transients in flow rate. The safest assumption is that max operating pressure = deadhead pressure of the pump.

3. It depends. ASME Section VIII, Div 1, U-1, provides exemptions for vessels that meet certain conditions, including:
(-1) vessels having an internal or external pressure not exceeding 15 psi (100 kPa);
(-i) vessels having an inside diameter, width,
height, or cross section diagonal not exceeding 6 in.
(152 mm), with no limitation on length of vessel or
pressure;

Traditionally, conventional jackets like you've drawn would need to be considered pressure vessels if operated above 15 psig (See ASME Section VIII, Div 1, Mandatory Appendix 9). I am guessing your reboiler is >6 inches diameter - otherwise it would need to be >7 feet long to reach the 40L volume. I am also guessing your pump deadhead pressure is above 15 psig.

If my assumptions are true, your jacket is an ASME pressure vessel and should be stamped and provided with a PSV.

 

[Compositepro]

Look at steam kettle designs. They are similar to yours, and work much better without the need for a pump and external piping. There is a vacuum in the jacket below 100C. The water level stays below the bottom of the inner vessel. The air gets vented once by heating to above 100C and opening a vent valve, then leaving the jacket sealed from then on. Heat transfer is excellent and as is temperature uniformity. These are very common in the food and pharmaceutical industry. I have been able to buy working used kettles for 5 to 10% of the price of a new one. The jackets are pressure vessels and do require a relief valve.

https://www.google.com/search?clien...Sc98KHTbECEkQgIoDKAJ6BAgDEAg&biw=1304&bih=644

https://unifiedbrands.net/wp-content/uploads/2018/10/ret_om_tdb7.pdf

 

[Mike4chemic]

Gentlemen,

A lot of thanks for you recommendations.


Mike