In our plants we use structured packed columns for gas-liquid operations, such as distillation, absorption, stripping, degassing, etc. As a process engineer we are required to estimate the packed column diameter during design calculation. Or, many times we need to check the maximum possible throughput capacity from the existing installed column. Apart from this sometimes we are looking to change the existing column internals (i.e., trays or random packings) with high efficiency structured packings. In all these circumstances, we will be looking for the packings characteristics to estimate the superficial velocity through column. And, for different type of packings we can afford different superficial velocities.
Different Types of Structured Packings
For example, Mellapak 250Y is a less denser structured packing and mostly used where we are looking for highest vapour or gas throughput rates and low theoretical plates per meter of packed height. While, Mellapak 750Y or gauge packing like BX & CY are used where we are looking for highest number of theoretical stages in per meter of packed height. Here, we can get around 8-10 stages. But these packing packings are good for low liquid flow rates and gas throughput rates.
Once we get the superficial velocity through column for a given structured packing, using the velocity and volumetric flow rate we can calculate column diameter.
Flow Area (m2) = Volumetric Flow Rate (m3/s)/ Superficial Velocity (m/s)
Diameter (mm) = 1128.4*(Flow Area^0.5)
Superficial Velocities for Different Structured Packings
In below table there are the values of superficial velocities which we can use to estimate the packed column diameter for given vapour load. In my experience I found these values are very close to actual plant operation. These values are good for atmospheric column operation. For vacuum operation we can use higher superficial velocities than this, around 3-4 times of the given values.
Conclusion
Column diameter depends on vapor or gas volumetric flow rate through the column. And, volumetric flow rate is the function of column pressure. At higher pressure, vapour density will be higher and volumetric flow rates will be lesser. On other hand, in vacuum while density is low therefore we can afford higher superficial velocities through the column. In summary, we can say if we operate a column in vacuum, which was designed for atmospheric conditions, the capacity of that column will reduce. As, at higher gas velocities column will lead to flooding conditions.
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