Spray Drier - Another view

Hello Friends, I am late again due to shifting to some very interesting industry this time making very useful product Furfural from Bagasse.

The incident of yesterday's interaction forced me to come back to the blog for sharing.

Yesterday I met vice president of a growing mid size company in Gujarat. He told me that spray driers can not be counter current and I should learn this fact. The reason he explained is that dried powder shall be carried away by hot incoming air and therefore you need a cyclone separator at the top for separation of solids & air.

Quite interesting ......This forcefully made me to think about fundamentals of drying......but later on. Before that I thought about creativity of Human mind. its brilliant......it's unlimited......it's really really much much bigger than what we can think of.

Yes dear, all of you should take this as a challenge on this blog itself that how to prove this wrong & how to prove it right.

In the beginning I thought I should re-think on the basics of spray driers and then searched on net also. So I found so many picture / images here.

He may not be fully wrong also, but the word 'Spray driers can not be countercurrent' was not right I am sure.

Really its very difficult for me to write any thing on this topic now. The basic reason he mentioned was that if flow is counter current solid particles will go up with the air & you need a cyclone separator for recovery. Is it really so??

I think it is always so whether it is counter current or co-current. Isn't it?? Most of the pictures on the above link are having cyclone separator. So this argument was also not so good for making spray dries co-current.

In fact, If I see the principle of drying and nature of most of the solids where spray drying is essential or most suitable, it is always better / preferable (Few Exceptions depending on nature of solids) to go for counter current as driving force is always higher and moisture content in dried product from the same size is less.

In fact, those applications where spray drier is generally used, I will not prefer to go for it as it is highly energy intensive & capital intensive compared to thin film drying which I have already done for a very highly hygroscopic material. I will try to put the data for that installation in one of the next articles.

I will prefer spray driers mostly for hygroscopic material with sticky nature in wet condition otherwise its better to go for other options.

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Easy way to find altitude of the Plant

Altitude of a plant location is very important in terms of designing of your vacuum systems, identifying boiling points for correct calculations & design of distillation system, boil-off systems etc.

It is also important in terms of correct sizing of condensers, reboilers due to change in enthalpies, significant change in LMTD will cause errors in design if percent variation is large specially when temperature difference are lower.

Therefore, it should be identified correctly.

Though the best method is to measure it at site location using barometers, however this is not always possible due to absence of such meters. The major problem in it is that normally designers are actually away from the location.

So here are two quick methods to understand the actual barometric pressure.

Method-1
Use any vacuum pump which is reliable & relatively new. if you are installing a new pump or have recently installed a new pump then run it on shut off condition with a pressure transmitter in the suction line. The shut off reading will practically be equal to the barometric pressure at site.

But you need to be sure in this case that pump should be designed for 1 torr or lesser shutoff pressure. if it is designed for higher shut off pressure, you need to subtract the design shut off from actual reading of pressure transmitter during test mentioned above.

So, if pump is designed for shut off pressure of 720 Torr and pressure reading during shut off test is 10 torr then barometric pressure will be 730 Torr at that site.

Method-2
Another method is use of google map service. For this you need to download the file altitude.html from the link Here.

Now you need to just identify your plant location and you will quickly come to know the altitude of your plant from MSL (Mean Sea Level).

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When to Use PSD system?

Today there was a comment on Name of specific Industries where we can use PSD (Pressure Swing Distillation) system on my previous article. Out of interest of few visitors I would like to suggest few points.

Also this post onwards, I will try to post very short posts but may be many on the same topic.

Request feedback from all of you on this kind of posts.

See, the pressure swing distillation means - More than one distillation columns in series but at different pressures to achieve the desired purity of one component due to shift in Azeo condition with respect to pressure.

Let me explain again.....with an Example of Ethanol & Ethyl Acetate mixture.
As per literature data, the composition at two different pressures are as below

Pressure Ethanol Ethyl Acetate
760 mm Hg 31% 69%
1500 mmHg 39% 61%
300 mmHg 23% 77%
25 mmHg 13% 87%

Considering the wide difference in Azeo composition in the given system, If I choose say 1500 mmHg pressure of first column & 25 mmHg pressure of second column then the first column top will be richer (Feed is 69% EA & 31% Ethanol at ambient condition which is generally expected from any system) in Ethanol.

This is because top composition will be azeo composition at 1500 mmHg pressure i.e. 39% Ethanol while feed is having 31% Ethanol, so bottom shall be More EA, thus it is possible to get pure EA at the bottom......Wait if you are not clear.....read more below.

Now top of first column which is 39% Ethanol & 61% EA is fed to second column which is operating at say 25 mmHg. Now at this pressure Azeo composition is 13% Ethanol. So most of the Ethanol will settle at Bottom & EA rich top will be (13% Ethanol + 87% EA) coming out from top.

Now this top stream of second column is fed back to first column along with feed. So feed will further get rich in EA. So more EA will settle at the bottom of second column. BUT top will remain same at Azeo of 1500 mmHG & will go to second column again.

Thus with this kind of cycling eventually you will end up with pure EA at bottom of first column and Pure Ethanol at the bottom of second column. All tops will be fed to next column through recycling. This is how PSD works for any system.

However, the limitation is if difference in the Azeo compostion is small then you may need more such columns OR it may not be possible to use PSD at all.

Hope it is more clear now....

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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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