Process Intensification - 4, Loop Reactor

Process Intensification has been our focus since last few posts and I have already covered two most common and most discussed equipments under this topic. Static mixer - which are already in use by the industry and Micro Reactors which are still waiting for good response from industry.

Similarly the third one which is specifically used in few of the industries is Jet Loop reactors which also qualifies to PI category - One particular configuration developed by BUSS Chem Tech

So we are going to discuss loop reactors with different variations in the configuration e.g. simple circulation loop reactors, venturi reactors etc. & then finally Jet reactors which are derived from above systems.

Now let us start from the beginning on these reactors. In the beginning only CSTR were there & at that time need was felt of increasing the mass transfer as well as heat transfer but the invention of putting a circulation loop with one heat exchanger was only based on the objective of increase in heat transfer area so as to reduce the batch cycle time & this is how the concepts of various loops came into picture.

CSTR

The above CSTR configurations were developed around 1930s.

Simple Loop Reactor
So simple configuration was similar to the following diagram for a general circulation reactor. They were developed around 1950s.


Now as you see, the exchanger in the loop is for providing faster heat transfer outside the reactor due to limitation of heat transfer area in the given volume of CSTR type vessel. So basically it was due to very low surface area to volume ratio of CSTR (As shown in Micro Reactors article).

Another version was developed with static mixer in the upstream of exchanger for very quick reactions to remove the heat as soon as reactants react. They were developed around 1960s.



Loop With Spray at Top
The next generation of loop reactors was with spray type nozzle at the top as shown in the picture. In fact, they are designed for enhanced GAS LIQUID interaction by providing a top spray in the gas zone.




Loop Reactor with Venturi
Further improvements in the loop reactors were done by introducing a venturi in the system. This was slightly different than first picture of this post. The difference was in the discharge zone of venturi, the discharge in this type of reactor was at the inlet of reactor - no mixing in liquid zone as shown in the picture. This was mainly developed to enhance the circulation of gas from the top headspace of the reactor to minimize the safety issues with hazardous & explosive gases.

Jet Loop Reactor
The Next generation loop reactors are Jet Loop reactors as shown in the first picture from BUSS reactor. The difference here is that the Jet mixes with liquid with a very high speed & therefore the reaction speed goes very high in the mixing zone of the Ejector. For example, the reaction rate can be increased by a factor of say 25 - 40 times in a jet reactor compared to CSTR.

So if you think you need higher capacity from your existing system of CSTR etc. OR if you are planning to design a reactor for slow gas liquid reactions then think on considering a loop reactor with circulation exchanger for faster heat transfer + plan for either a SM in the piping or plan for a eductor at the bottom of reactor which will thoroughly agitate your liquid mass.

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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.

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ISO - Standard or Business - 5

Further, is it true that ISO is for people, their safety, their interest as consumer etc...etc.???

ISO Claims - "They are useful to ...........and, ultimately, to people in general in their roles as consumers and end users"

Ha...Ha...Ha....Consumers, who do not even know what for this company is having ISO. is it for product or for finance or for purchase or ......?????

HAVE YOU EVER FELT ANY PROTECTION DUE TO ISO.........

See my previous articles on ISO - Part-1, Part-2, Part-3 & Part-4.

Consumers do not know anything about ISO coverage & even for a multi product company it is not clear If all the products of this company are covered under ISO or not. ISO is safeguarding them against harmful content in the product or not. The raw material which is used in this product is having or not having any quality issue which ISO can not guarantee as purchase process is not covered under ISO. All these facts are not known to any consumer in general & still ISO claim to safeguard & to be beneficial to the consumer & end user.

Company can enjoy good image in the market becasue they only publish "ISO:9000 Certified"........other info is missing. Why???? Because blindly every one is asking for only ISO certificate.........it doesn't matter if it is only for your HR (There is no link to product in this case but this is also sufficient).

I also know multi product / multi location companies where only one certificate is used for the entire company and therefore question comes - is everything covered?

The biggest flaw, still ISO claims for safeguard of consumers...HOW???? can anyone justify this?

Fed up man...Im impatient to write more on this topic. So will stop here & may come back later if I go through this subject in future.

Learning -4 It cannot be beneficial to consumers unless it is product & value chain focused. Currently it is process focused only that too with many exclusions permitted.

Disclaimer - This is a series of my personal views on ISO systems, its implementation, Effectiveness etc. They do not carry any legal issue related to my personal views under the human right of "Freedom of Speech".

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Process Intensification - 2, Static Mixers

My last post was focussed on general & brief overview of process intensification abbreviated PI. The term may be new to almost half of the population but we use it somewhere or somehow knowingly or unknowingly in our current life also.

As explained earlier, the miniaturisation is the key word for PI as best & smallest explanation. However, it can be simple on one side like static mixers and very complicated like immobilized solvents for absorption & desorption OR SDR (spinning disc reactor) etc. on the other side.

So instead of discussing it in general, let us go through these specific examples of PI equipments & how can we utilize them in our life...CHEMICAL life...; our first item is - Static Mixers.



According to wikipedia, the static mixers were invented by the Arthur D. Little Company in 1965. This device was licensed to the Kenics Corporation and marketed as the Kenics Motionless Mixer.

Static Mixers
A very simple piece of equipment & very easy to understand yet fully misunderstood by many of us. The basic image of an static mixer is the take a piece of pipe & fill it with any packing and your static mixer is ready. YES.....& ......NO.

YES.....because of its general construction......NO because improper design can lead to many failures and because of that you are not using it for real applications where it is more beneficial.

These mixers can be utilized for various purposes e.g. For slow Gas & Liquid Reactions where the BCT is very high & therefore, many benefits can be achieved by using SM for this type of slow reactions. Another example where we can use them effectively are homogeneous & quick mixing of liquid liquid systems.

In such cases, the benefits are many like BCT is reduced so the capacity can be enhanced from the same size of system OR the system volume can be reduced and hence the inherent safety is increased. The yields will be better due to complete or higher utilization of GAS, more uniform product due to lesser side product formation etc.

Can you think of any other good use of static mixers???

Now Let us classify SM in broad categories & how to estimate pressure drop.

1. LPD - Low pressure drop mixers for low viscosity applications.
2. ISG - Interfacial Surface Generator for high viscosity applications.

Now to estimate the pressure drop in a SM, following is the process.
First Assume the Mixer Diameter.

1. First Calculate the Reynold No as Nre = 6.31 w / mu D = 3161 Q sg / mu D


2. If Nre is < 500 the flow is laminar. Use either Fig.1 for LPD or Fig.3 for LLPD or Fig.5 for ISG mixers to estimate the pressure drop per element.









3. If Nre > 500 the flow is turbulent. The pressure drop per element is estimated from Fig.2 for LPD or Fig.4 for LLPD & Fig.6 for ISG mixers.









4. Multipy the pressure drop per element by the number of elements to obtain the estimated pressure drop through the mixer.


5. For turbulent flow, multiply the estimated pressure drop by the specific gravity and the correction factor K to obtain the actual pressure drop as below -
   
   K = 1.1 for mu upto 10 cp.
   K = 1.3 for mu from 10 to 100 cp.
   K = 1.5 for mu from 100 to 1000 cp.
   K = 1.6 for mu from 1000 to 5000 cp.



6. For laminar flow, the pressure drop is proportional to the viscosity. So for Figures 1, 3, & 5 which are based on a fluid having a viscosity of 10,000 cps, the actual pressure drop is calculated as follows
   
   Actual DP = Cal DP x Actual Viscosity / 10000


7. Now if calculated total pressure drop is more than allowable pressure drop use next higher diameter.

Some of the information & Graphs are from Staticmixers.com website.

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