Download a Printable Version Here (Adobe Acrobat Format)

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

B. Distribution Technology

Factors to be considered in selecting liquid distributors for a distillation test tower are:

Turndown ratio and height of the distributor
The number and size of distribution points (orifices) per unit tower cross-sectional area
The liquid flow variation allowed between distribution points
The layout of the liquid distributor points over the tower cross-sectional area

It is common practice, when testing a packing, to cover the complete operating range of the packing. In the authors’ experience, the typical turndown ratio is 5:l. And, it is not uncommon to have a 7:l turndown ratio. Several types of liquid distributors are used for distillation tests. Except for the notched weir-trough distributor (which happens to have high turn-down ratio), spray distributor (which is seldom used in distillation), most of the distillation distributors fall into one of the following three categories.

Orifice-pipe arm distributors
Orifice-pan distributors
Orifice-trough distributors

Let us first consider the design of orifice-plate and orifice-trough distributors. Both of these types of distributors are open at the top. In the orifice pan distributor, the gas flows through specially designed risers as well as the area between the pan and the tower wall. The rest of the pan area is available for locating liquid orifices. In the orifice-trough distributor, the liquid is held in specially designed troughs with liquid orifices at the bottom and/or on the sides of the troughs; the rest of the tower cross-sectional is available for gas flow.

For a given orifice size, the flow rate through the orifice is approximately proportional to the square root of the liquid head, when the orifice is running full of liquid. Therefore, for a given set of orifices at a fixed elevation, the required head of liquid above the orifices is proportional to the square of the liquid flow rate. Thus a 2: 1 turndown ratio in flow requires a 4: 1 ratio of liquid head. Typically the minimum liquid head required for predictable flow of liquid through the orifice is about 50 mm (2 in.). Thus the liquid head required at maximum flow rate for 2: 1 turndown is 200 mm (8 in.). For 5: 1 turndown the maximum required is 1250 mm (50 in.), and for 7: 1 turndown the maximum head required is 2450 mm (8 ft.). It follows that, unless over 2.5 m (8 ft.) of column height can be reserved for liquid distributor, one must resort to using a distributor with multiple levels of orifices or use more than one single-level orifice distributor, each with a different orifice size. The design features of many of these types of distributors are proprietary.

The pipe-arm distributors depend, for their performance, on the liquid head prevailing upstream of the orifices; this pressure is generated usually by a liquid feed pump. The turndown capability of the pipe-arm distributors are only limited by the capacity of the feed pump and the maximum allowable velocity of liquid through the orifices above which formation of liquid spray might cause entrainment. The biggest drawback of this type of distributor is that the flow variation from orifice to orifice can be excessive, especially at high flow rates due to variability of the size and shape of the orifices and the pressure drop through the pipe arms. Therefore, orifice-pan and orifice-through distributors are generally preferred for both pilot plant distillation columns and industrial distillation columns.

The number of liquid distribution points required for unit tower cross-sectional area is a function of the type and size of the packing. Based on the authors’ experience, the following general statements can be made:

Large random packings require fewer pour points than smaller random packings.
Large structured packings require fewer pour points than medium sized structured packings.
Small structured packings have better liquid spreading characteristics than larger structured packings.
Except for small random packings, most packings will operate well with pour point densities of between 40 points/m² (4/ft²) and 60 points/m² (6/ft²). Even small random packings of commercial interest perform well with 100 points/m².
The smallest size orifice used is 2-3 mm in diameter; this small orifice can only be used with clean systems.
Sufficient liquid head should be allowed to limit the individual orifice flow variation to ± 5% of the average flow rate.
The layout of liquid distributor orifices over the tower cross-sectional area is based on the method of Moore and Rukovena (1986).

Bed Depth