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Distillation Pilot Plant Design, Operating Parameters and Scale-up Considerations
(Special Shared Content with Norton Chemical)

Factors To Be Considered In The Design And Operation Of A Distillation Pilot Plant

In operating an existing pilot plant distillation column, there are three fundamental issues involved:

a. Chemical test system
b. Tower packing
c. Liquid and or gas distributors

Typically, in a particular pilot test the performance data on two out of these three items are known fairly well; it is the purpose of the test to get information on the performance of the third item in the presence of the other two.

But in designing a new pilot distillation column one needs to decide ahead of time the type of chemical systems, whether corrosive or non-corrosive, high vacuum, atmospheric, or high pressure system that will be distilled in the column. As discussed earlier, the size and type of the packings and distributors to be tested will also have to be considered.

From an economic standpoint, the most important consideration is the diameter of the pilot distillation column.

A. Tower Diameter To Packing - Size Ratio

Most of the early laboratory distillation data were taken in small columns, say 150 mm (6 in.) or less. Only very small random packings, viz., 3 to 12 mm (0.12- 0.5 in.) in size could be tested in such columns. There were several reasons for this. As the diameter of the pilot plant distillation column increases, in addition to the increase in installed cost, the cost of operating utilities, viz., reboiler steam and condenser cooling water increase proportional to the square of the tower diameter. Thus there is strong economic incentive for keeping the tower diameter as small as possible without affecting the quality of the test data.

One consideration was the rule of thumb that the test tower diameter should be at least 10 times the size of the packing. The rationale behind this rule is that if larger packings were used, the wall area surrounding the packed bed would be a significant fraction of the packing area, and as such the column wall would provide a significant portion of the mass transfer area. It follows, based on this reasoning that when scaling up such data to large towers some derating would be necessary.

On the other hand, it can be argued that in a small tower, the gap between a bed of large packings and the tower wall can cause partial short-circuiting of liquid and vapor through these gaps. For structured packings, wall wipers minimize this problem. In large commercial towers the effect of such gaps will have negligible effect on packed bed hydraulics.

Most of the commercial size random packings fall in the size range of 15 mm to 90 mm (0.6-3.5 in.), and the structured packings has crimp heights in the range of 6 mm to 30 mm (0.25-l .2 in). But the majority of random packings sold commercially fall in the size range of 25 to 70 mm (1 - 2.8 in.), while the majority of commercially sold structured packings have crimp heights in the range of 8 mm to 12 mm (0.3-0.8 in.). With the 387 mm I.D. pilot distillation columns that NCPPC operates, it was found possible to test random packings in the size range of 15 mm to 70 mm (0.6-2.8 in.); the column I.D. to packing size ratio ranged from 26 to 5.5. In the case of the structured packings that were tested in these towers, the column I.D. to crimp height ratio ranged from 13 to 65. Based on experience with commercial installations, the test data taken in a 387 mm (15.25 in.) I.D. column gives reliable design data for commercial size columns. As mentioned earlier, the FRI columns have relatively large diameters, viz., 1213 mm (47.75 in.) and 2438 mm (96in.), probably because they were originally designed for testing trays. But the SRP columns which were built in 1986 have 429 mm (16.875 in.) I.D., because they were designed primarily for testing packings. Similarly another distillation pilot column operated by the Delft University of Technology in the Netherlands has an I.D. of 450 mm (17.72 in.) (Olujic et al., 1992). Thus, a distillation pilot column of approximately 400 mm (16 in.) I.D. gives reliable test data on random and structured packings. This type of test data along with reliable distribution technology can be used, without any scale-up factor, to design commercial distillation columns.

Distribution Technology