Process Intensification - 3, Micro Reactors

In the series, the next most interesting & commonly known equipment is the micro reactors, the buzz word of the current times in process engineering. First article on introduction of PI is Here & Second article on Static Mixers as PI equipment is Here.

As name suggests - Micro - means mini - reactor is a device in which chemical reactions take place in a confinement with typical lateral dimensions below 1 mm; the most typical form of such confinement are microchannels or capillaries. The microreactor is usually a continuous flow reactor. Microreactors offer many advantages over conventional scale reactors, including vast improvements in energy efficiency, reaction speed and yield, safety, reliability, scalability.



I am not going in to too much theory & I am directly putting up examples for quick understanding of underlying principles & benefits due to the changes.

So, Let us first see the advantages & disadvantages of Micro Reactor Technology (MRT).

Benefits

1. Very high surface to volume ratio

To see this more clearly, I am using one example from Sigmaaldrich website. Let us understand it by comparing a case of CSTR & MRT.

Case-1 CSTR
This is an example of CSTR of 1 KL at 500 rpm for an exothermic case with outside cooling.



In this case you can see that temperature distribution os very uneven & hence the reaction control, quality etc are not uniform.

Case-2 MRT

In this case, the temperature distribution is so uniform that quality is controlled in a much better way.

Therefore, because of very high surface area / unit volume of reaction mass, the control on heat transfer is very very good compared to CSTR. This is very important factor, if you can realize the importance of this aspect of any reactor. This factor alone can result into many significant differences e.g. very good conversion, lower level of impurities, higher yield etc.

Just to see the impact Do you Know, what are the typical values of surface area to per unit volume - This is

- 6 Sq Meter / cubic Meter of reactor volume for CSTR
- 100 Sq Meter / cubic Meter of reactor volume for 100 mL glass flask
- 20000 Sq Meter / cubic Meter of reactor volume for Micro Reactor.

Yes, it is almost 15 times more in Laboratory & this is the main area where most of the lab scale & bigger scale differences occur. AND Oh! my GOD - 3000 times more in MRT. My suggestion - Buddy forget Micro reactors, invent something which can just give you say 1000 Sq Meter / Cubic meter of volume, It can do wonders - believe me


2. Better mixing

Since, the micro volumes are mixed in a capillary, the mixing is instant & uniform. Due to very high length of the mixing path per unit volume compared to any other reactor the design is very effective. Therefore, the unformity of mass is almost same in the entire reactor and hence more uniformity in quality can be achieved.

This is very important aspect in some of the industries related to performance chemicals e.g. ethoxylates where more & more unformity (peak ethoxylates) is desirable.

Ultimately this results in very good purity of product by suppressing the impurities formation and very high yields due to the same factor.


3. Instant removal of the material out of the reaction zone

The holding volumes or system volumes are so low that they do not affect the reactions & therefore even equilibrium reactions can achieve higher per pass conversions due to very quick removal of products as soon as they are formed.

Therefore, this is very important in case of equilibrium driven reactions or where yield are impacted significantly due to side reactions and where the product is sequentially converted to other byproducts

Suppose you are conducting one reaction A -> B -> C & your final product is B. Also consider the case where A ->B & A -> C reactions are happening. Then in a CSTR you will either reduce the conversion of A to limit concentration of C or you will optimize parameters to suppress C in second case. But in MRT, there is very little formation of C in either case due to very low volumes especially in first reaction.

Hence, you can have very high conversion saving recycles of A and higher yield due to lesser formation of C.


4. Improved Safety

Since the volumes handled are very less - micro litres - the hazardous reactions can safely be carried out in MRT and very little efforts & investment is required in case of hazardous / run away reactions e.g. polymerisation or ethoxylation or explosive materials handling.

Therefore, investments required on safety systems, instrumentations, losses due to release of chemicals / pressures etc & hence they are environment friendly also.


5. Very High Heat Flux

Microreactors typically have heat exchange coefficients of at least 1000 kW / M3 / K to 500000 kW / M3 / K vs. a few kilowatts in conventional glassware (1 l flask ~10 kW / M3 / K). Thus, microreactors can remove heat much more efficiently than vessels and even critical reactions where exotherms are very high & instantaneous, can be performed safely at high temperatures.

This factor is also very important for example nitration is a very quick & highly exothermic reaction in general which can be safely carried out in MRT.


6. Continuous Processes

Microreactors are normally operated continuously. This allows no batch hold ups & higher time cycle before workup. Important aspect in case of unstable compunds where holding time results in more byproducts or side products formation & hence higher selectivity.

Mixing in a batch process causes a very different concentration profile compared to Continuous process. In a batch, reagent A is filled in and reagent B is slowly added. Thus, B encounters initially a high excess of A. In a microreactor, A and B are mixed nearly instantly and B won't be exposed to a large excess of A. This may be an advantage or disadvantage depending on the reaction mechanism.

So many reactions where batch is a forced neccesity due to full conversion desired, the MRT can be used as continuous option. In such cases there is no other technology available till date other than PI which can help in converting batch to continuous with 100% conversion.


7. No Scale Up Required

Since the MRT is just a module technology no further scale up is required form lab to commercial scale. Only addition of similar micro reactors will add up the capacity. This reduces product development cycle time & also losses in scale up during piloting, manpower, efforts & Energy is also saved.

Now let us understand disadvantages or rather problems in using MRT.
Concerns

1. Only Liquid Systems

MRT is still struggling in terms of handling gases & solids and only liquid liquid reactions are well performed. I see this as a area for improvement required in MRT rather than a problem. May be why solids in MRT is my first question OR when we are talking about Micro why not nano particles if any solid catalyst is required.

Yes, reactions with solids as reactants will still be a problem.

Regarding gases handling, the problem is not so critical its a matter of pressure handling but yes it is a concern as of now.

Hope it is useful for all of you & now you will start thinking on using them.