4 replies to this topic

[kinetic123]

Hi

 

I have one query please.

 

We have a reactor with an internal coil and outer jacket. Coil and jackets are used for cooling [with cooling water] and heating [using steam].

It is a batch process - and steps are:

1) add raw materials

2) Heat up to 125 deg C [use 3barg steam in coil and jacket both]

3) Exothermic reaction will bring temperature from 125 to 200 degC

4) Cool down batch from 200 to 125deg C [use cooling water in the jacket ONLY to avoid thermal sock to the coil]

5) Cool down batch from 125 to 40degC [use cooling water in a jacket + coil both]

 

Please attached drawing for valves' air fail close/open positions as it stays at the moment.

 

As both valves all the valves on coil inlet and outlet are 'air fail close', there would be the situation of trapped fluid within the coil - below is my understanding.

 

condition 1) Liquid expansion in the coil - batch is below 100deg C, coil is full of water and air fail close valve at coil outlet will cause trapped in water to expand.

condition 2) Vaporisation in the coil - batch is above 100deg C [let's say maximum 125deg C] , coil is full of water and air fail close valve at coil outlet will cause trapped in water to vaporise.

 

My queries:

 

1. Do you think Condition 2 would happen at all? If not, then why?
2. Also, I believe, either of AV04 or AV05 should be air fail open as no pressure relief protection on coil especially, AV05?

 

Would appreciate input from members.

Thanks

Attached Files

•  Drawing 1.PDF   192.32KB   23 downloads

Edited by kinetic123, 04 April 2014 - 07:39 AM.

[TS1979]

I tend to agree with you that one of the outlet valves should be fail open.

 

There is another situation which may damage the coil as well. Thinking about that you are heating the material inside the tank and suddenly there is an air failure. All the valves are closed. Steam will continue to be condensed inside the coil - you create a vacuum case. If the coil is not designed for full vacuum, potentially you will damage the coil.

 

During the cooling stage, if all the valves are closed, thermo-expansion is a concern. Therefore, the cooling water outlet valve seems to be fail open. However, there is a risk as you mentioned during heating stage if all the valves but the cooling water outlet valve are closed, cooling water will back to the coil and result in thermo-shock of the coil.

 

Let's see how about set the condensate outlet valve fail open. In the cooling stage, you have an air failure, then all the valves but the condensate valve are closed. If your cooling water inlet valve has some leaking, then untreated water will enter the condensate system.

 

You probably need to think about which way you want.

 

Answer to your first question: Yes vaporization inside the coil is possible. However, this scenario is not a concern because vapor produced is compressible and the final pressure will hardly be over the coil design pressure if there is vapor space inside the coil.

 

The best solution for the outlet valves is to put the outlet valves "fail last"

 

Thanks

Edited by TS1979, 04 April 2014 - 11:03 AM.

[Dazzler]

Regarding the heating /cooling coil and jacket on a batch reactor, I would have thought it typical design to have a relief valve on the coil and jacket at least for the relief scenario of liquid thermal expansion.

 

The jacket would need to be rated for full vacuum, and the coil also (although pipe of suitable design pressure would typically be fine for vacuum).

 

I've seen reactors with actuated double block and bleed arrangements on steam and cooling water, to prevent the chance of cross contamination, also an actuated vent on the coil and jacket to vent off steam when finished with, and to allow air in so the coil can be drained.

 

Coil and jacket are normally emptied (drained or vented) of the steam or cooling water when not in use, not left full with no flow and blocked in.

 

Also, if there is a chance of exothermic reaction runaway, the cooling water could be fail open, otherwise at least have manual override handwheels on a couple of valves so that cooling can be put on manually to prevent an exotherm and overpressure.

[kinetic123]

TS1979, Dazzler,

 

Thank you very much for your reply.

 

@TS1979,

I think if we have both, coil inlet and outlet valves fail close, coil will be full of water and no evaporation takes place but it would be rather a liquid and metal thermal expansion scenario? Or am I missing something here?

 

Thanks

[TS1979]

Kinetic123,

 

When you close the both inlet and outlet valves, the main concern is liquid and metal thermal expansion. Liquid thermal expansion will be greater than metal thermal expansion. As liquid is essentially incompressible, the extra liquid would create high pressure over the design pressure and rupture the coil. This is called thermal relief. There will be no space for evaporation. Water will have minimum volume at 4C.

 

Thanks