Question

While designing a reactor for a highly exothermic reacton, how can I figure out whether I use a jacket around the reactor or coils to cool it down ?

Is there any mathematical relationship between the reactor dimensions and the type of cooling method used ( jacket or coils )?

How can i know which one I should choose ( jacket or coils ) ?

Answer 1

Here I am explaining the various parameters and effects associated with Jacket cooling or Coils cooling used for reactor. Which will help to understand that which one should be used while designing a reactor.

Cooling is needed for exothermic reactions (reactions that product heat), vapors that are being condensed, and bring a reactor down to operating temperature. The choice between a jacket and coils is based on a number of considerations.

Advantages of Jacket cooling:

1. For highly corrosive or highly reactive materials, a jacket has the advantage that there are no extra materials of construction and no extra metal surface in contact with the process other than the normal vessel wall.

2. In case of Jackect cooling, there is also less risk of cooling fluid coming into contact with the reaction mass.

3. For the manufacture of pharmaceuticals, fine chemicals and performance products, a jacket minimizes contamination as there are no extra surfaces to clean. Hence it can be preffered.

4. Jackets offer a more even heat transfer distribution.

Disadvantages of Jacket Cooling:

1. A jacketed vessel has the disadvantage that the area/volume ratio decreases with increasing scale.

2. The use of a greater height/diameter ratio at larger scale can help to reduce this problem, but only to a limited extent.

3.  Jackets have a lower utility velocity at the heat transfer surface that can increase fouling rate and require good mixing to ensure that the jacket has a uniform effect on the entire liquid contents.

Advantages of coil cooling:

1. A coil has the advantage that a large surface area can be provided.

2. Coils generally offer a faster temperature response than a jacket by a decrease in the volume and an increase in the velocity.

3. Both of these work to decrease the process deadtime that is the coil volume divided by the utility flow rate.

Problems with Coil Cooling:

1. The increase in velocity increases the heat transfer coefficient but this is partially offset by a decrease in the surface area.

2. An increase in the product of the heat transfer coefficient and surface area (UA) will decrease the secondary process lag in the thermal response.

3. Coils offer excellent heat transfer to fluids in the interior but generally not in a dished bottom. At low liquid levels, the process liquid may not be in contact with the coils.

So we can say that the jacket temperature control schemes are suitable for batch besides continuous. For highly exothermic plug flow reactors such as gas reactors, coils provide more effective cooling from direct process contact in the core and high fluid velocities inside and outside the coils.

For coils and jackets, the Overall Heat Transfer Coefficient can be calculated which will help to determine its usefullness in a particular reactor.

b.) Yes, reactor dimension has considerable effects on the type of cooling method used. As I have mentioned earlier that a jacketed vessel has the disadvantage that the area/volume ratio decreases with increasing scale. The use of a greater height/diameter ratio at larger scale can help to reduce this problem, but only to a limited extent.

A coil has the advantage that a large surface area can be provided,However, it is important not to pack the coil so tightly as to form a false wall.

By considering all the relevent information regarding these processes as mentioned above you can choose a suitable cooling process for reactor.