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Rotary vacuum drier operation

Q: I request your guidance in improving operation of a rotary vacuum drier. We have a horizontal RVD with a central shaft for heating with 3 kg steam pressure and a ribbon blender. Our input is spray dried material with a bound moisture of 10% to 12% and output as dried product has less than 0.5% moisture. We face agglomeration problems. Typically, what should the loading capacity for a rotary Vacuum drier (RVD) be? Should we have a high vacuum to begin with? Also, should we stop heating once material has dried?

A: The first thing that you need to determine for your material is the "sticky point." The spray dried material may have an average moisture of 10-12%, but the moisture may not be well distributed in the particles and some interior spots may be much higher in moisture content. Sticky point is where the solids become most cohesive and it is not unusual for it to be at around 8-12% for many materials. It can be determined by observation. Look at the material as it dries and see where it starts to ball up or clump rapidly. The sticky point should really be determined by testing it in a shear cell. Material is put into a cylinder with a center agitator (a drill press works well). The cylinder has a jacket and it heated. Look at the current draw on the motor and when it hits its peak, take a sample and determine the moisture content. That is the "sticky point."

Next, check that the moisture is uniform. If the particles are very small, this may not be an issue, but larger particles can have low surface moisture (determined by infra-red measurements) and higher internal moisture (determined by RF measurements or evaporative loss, gravimetric). You need to check the total moisture.

If the total moisture of the spray dried material is higher than the sticky point, you will need to back-mix with dried material to lower the starting moisture of the RVD. If the sticky point is 8%, you would need to recycle at least 22% of the 0.5% material with the 10% spray dried material to avoid the clumping or agglomeration. While this will reduce the capacity of the RVD, it may shorten the drying time due to the lack of agglomeration. Overall capacity has gone up on many dryers with this approach.

High vacuum at the beginning will slow the overall drying process and may increase the moisture content in parts of the RVD. This is because the evaporation causes the solids to cool, but the moisture in the air can condense on the colder material and cause clumping (raise the moisture content above the sticky point). A slight vacuum is good, but save the high vacuum operation until the end of the drying process. Sometimes the vacuum has to be increased on temperature sensitive materials to avoid product damage, so the vacuum and heating need to be coordinated. The central heating shaft with a "ribbon" blender is rather unusual (normally a paddle is used), and may be making your heating problem bigger than it needs to be. If you can turn off the shaft heating at the early stages of drying, it will lessen the effect of the clumping due to the vacuum. The shaft heat can be turned on later in the cycle. Unless there is a heat sensitive issue, don't stop heating at the end. It will increase drying time.

The loading is another tricky issue. If the bulk density becomes larger during the drying, 80 to 90% loading can work. However, if the bulk density decreases, you can only load the dryer in such a way that the fill is less than 80% at the end of drying. The limit on loading for very fine particles is the superficial velocity of the vapor. As the material dries, the vapor velocity can reach the minimum fluidization velocity and entrain solids. You need to determine the freeboard that is required for that velocity to allow the solids to settle back into the RVD. You may only be able to fill the RVD 1/2 full. At which point the central shaft heating could damage your heat sensitive product.

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