April 20, 2011

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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Jyoti said...

Very clearly explained in a simple Language for a static mixers

Kelebek Vana said...

Nice and clean article. Thanks !

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