Selecting the Best Heating and Cooling Solutions for Stirred Reactors - Part 2
This is Part 2 of the blog post on the topic; refer here Heating & Cooling Blog - Part 1 for Part 1
Oftentimes we notice customers struggle to specific the best heating and cooling options when ordering reactors. We thought it may be useful to review some of the options and share our experience about which option works best in what scenarios.
In the previous post in this series we described several basic heating and cooling arrangements. In this post we continue to expand on possibilities.
Let’s move on to a modification of the jacketed configuration is a split jacket. This is usually used for larger sized reactors and provides more control over the heating / cooling arrangements. If used properly better Heat Transfer Coefficients can also be attained. In some challenging systems two different heating / cooling fluids can be used in both jackets for quick switchover and also to ensure positive segregation of both utility media. Often the differences may not be large at pilot scale but huge at the plant scale equipment. Hence this configuration is sometimes requested by customers to make sure that the pilot-plant rig remains similar in configuration to the final equipment characteristics in production plant.
Sketch of a larger reactor with split external jackets.
Next in terms of cooling configurations are internal cooling coils. These can be used with both heating and cooling media. These can also be relatively cheap to fabricate and we recommend customers always consider these as an option. Especially when the reactor pressures are high internal cooling / heating coils can be a great option since thick reactor walls otherwise impede heat transfer via a jacket. In general the coil configuration leads to faster velocities on the utility side and hence better HTCs with no dead pockets. Another advantage of coils is that they can be very easily removed or replaced by our customers.
One situation to be careful about is corrosive systems. Coils are relatively thin and in a corrosive system can be subject to pitting or other modes of failure. In systems which are in corrosive duty and need exotic MOCs e.g. Hastealloy the coils may also have to be fabricated from the expensive MOC. This can add to costs and in these cases customers should carefully compare the costs versus an external jacket made from SS316 or a lower cost material.
Sketch of reactor with single internal coil
For applications which are more demanding customers sometimes as for twin coils of different diameters. i.e. an inner coil and an outer coil. For larger reactions this is an option. Both coils can be used on the same colling fluid in which case we get greater cooling capacities. Sometimes this also helps quick switchover from heating to cooling duties since the two fluids run on different coil circuits. This also helps avoid cooling fluid contamination where different utility fluids (e.g. glycol vs oil or glycol vs steam) must be used for the cooling vs heating duties usually due to a large temperature range. One caution is that this arrangement requires large amount of space on the top head for nozzles and can only be feasible for reactors above a certain size.
And now consider a situation which is specific to pharma or other industries requiring GMP conditions. Cleaning with coiled internals may be more difficult especially if particulates or sticky solids are involved. For most demanding applications we also see customers demand very unique cooling configurations e.g. multiple coils and split jackets. In most situations these are not demands of scale up but a requirement to maintain similarity with the production configuration. As the reactor increases in size, surface area available for cooling reduces and a prudent customer will design the scale up rig keeping this in mind. The sketch below shows one such arrangement with complicated, multiple systems for heating / cooling
Sketch with external jacket and twin internal coils.
Oftentimes the limiting constraint for such demands is the availability of nozzle space on the top head. Larger reactors chosen for scale up stage make this more feasible. Very often good practice is to keep the scale up reactors as close in geometric similarity to the final in plant reactors. Hence features essential on the large reactors (e.g. split jackets or twin cooling coils) must also be provided at the scale up stage. This makes it imperative to chose a scale up reactor that is not too small lest it becomes impractical to fabricate such features.
We hope this series of two blog posts will give our customers some guidance in exploring the various options available for heating and cooling duties on lab and pilot plant reactors. Stay tuned for further blog posts that offer more technical information and correlations for actually estimating the heat transfer performance of such configurations.