Degradation reaction in evaporator -- How to design for?

[hollerg]

I am renovating an existing evaporator to run at a lower pressure, but at a higher level of moisture removal from an organic acid, resulting in both higher bulk temperature, and higher tube wall temperature. I have a minor component that forms from thermal degradation with first order kinetics. I want to ensure I have essentially the same concentration of the minor at the higher temperatures. The minor is not volatile

I know I need to minimize the liquid inventory but once that is minimized how do I factor in the contribution of the tube wall temperature? The degradation doubles for every 10 C rise in T. I think I need a relationship to apportion the relative contribution of the damage the heat exchanger surface temperature causes and the degradation in the bulk @ the flash temperature.

The evaporator uses an external heat exchanger, with a 100 to one circulation to product discharge rate and a 50 to one inventory to product discharge rate. Temperature rise is only one degree C in each pass. Back-pressure suppresses boiling in the tubes to prevent salts from fouling the surface.

I am considering setting up the reaction model to be two parts: 1) A mixed-flow reactor (CSTR) to account for the reaction at bulk T. 2) A second mixed-flow reactor at the temperature of the wall and limiting the volume of the second reactor to the thickness of 1/64th of an inch to represent the reaction. Only @ ~ 6000 Re#, 14 cP fluid.

Thanks in advance for your constructive comments and suggestions.
(I would lower the vacuum level further and operate colder, but precipitation would occur.)
Gary

 

[StoneCold]

hollerg
I don't have a simulator package, so I can't help you there but here are two practical things you can do to help with the drying and the degradation.
The first is sparge nitrogen into the evaporator to help strip the water off. I know you are under vacuum and that sounds crazy but small amounts of nitrogen will help push a lot of your water vapor off.

The second thing is to use water or hot oil as your heating medium instead of steam. This helps control the wall temperature better and reduces the heat flux.