semi-batch

[rika kose]

A semi- batch process. Reactant A is dosed by a metering pump to the reactor. Reactant B is dissolved in the solvent in the reactor and heated to 9 barg.
Dosing nozzle has no dip-pipe of J-pipe. So, it is splash loading.

Will there be any problems with splash? Reactor does have agitator and reaction is temperature controlled.


With dosing, liquid level in the reactor will be increased. Because product has lower vapor pressure than reactant B, in my opinion as long as the temperature is controlled, there should be no issue with over pressure.

Thanks.

 

[TiCl4]

Rika,

It depends on the chemicals being used. Can you be more specific? Splash entry can cause foaming issues, depending on the contents of the vessel. If the material that is being fed via splash entry is flammable, you might want to reconsider the entry arrangement. Additionally, some applications can be feed-point sensitive, especially if reaction rate is fairly quick or particle size/ molecular weight distribution in important (in polymeric systems, for example).

 

[rika kose]

Thanks TiCL,

This is a hydrolysis reaction. reaction is not very quick. single phase. reactant A's boiling point is 180ºC, flash point is also high. reactant B is just water. the solvent is Acetonitrile. exothermic reaction but very mild. The hydrolyzed product dissolves in acetonitrile. ACN and water are miscible. I assume the feed point doesn't impact a lot.

I am wondering the physical process. When the pump pushes cold A into hot vapor phase, what will happen? I think pressure will drop dramatically.

Reactor is 18 m3. Reactant B and ACN are 8m3, total A will be 6 m3. Reactor will be filled 80% full at end of the reaction. Will there be over pressure issue?

 

[TiCl4]

Rika,

Okay, so you will have to figure out the energy balance yourself - compare the heat of hydrolysis to the incoming A enthalpy. You say that the reactor is “heated to 9 barg”. What does that mean? Heating is typically done to a temperature setpoint, not a pressure setpoint.

If you have a heated reactor, then you typically don’t have to worry too much about keeping temperature up - the heating jacket/coils/whatever should be more than sufficient to maintain bulk temperature. Still, you should perform the overall energy balance to ensure that is the case, as it really does depend on jacket heat transfer, heat of reaction, and feed enthalpy.

Dropping a cold feed through the vapor space won’t really have any discernible impact on pressure. As long as the process is maintained isothermal (meaning your temperature control loop is maintained at the same temp), your pressure will actually increase as the process continues. A bottled-up reactor that runs semi-batch is essentially an isothermal compression. You can calculate final pressure fairly easily from an isothermal compression of the acetonitrile/water mix. You’ll need to accept the higher pressure in the reactor or bleed the pressure as the batch continues if pressure starts approaching the MAWP.

This does bring to mind a question though - why is this semi-batch? Oftentimes a process is converted to semi-batch from full batch because of the difficulties in controlling exothermic batch reactions.