Any liquid distributor gives some mal-distribution, because for practical reasons, liquid can only be divided into a limited number of streams. From these point sources the liquid spreads. The main considerations in selecting a distributor for a given service are compatibility with the service and avoiding large-scale mal-distribution.

Most of the common types of commercial liquid distributors are compared in Table 1.

Table 1 - Liquid Distributors


Ladder
("spider pipe")
Perforated ring Multiple spray Orifice pan Tunnel orifice Notch trough Weir riser
Driving force PR PR PR G G G G
Type PP PP S O O W W
Materials available M, P M, P M M, P, C M M, P, C
M, P, C
Tower diameter, in >18 >36 Any Any usually <48 Any usually >48
Any usually >24
Any usually <48
Plugging tendency Med Med L-Med H H L L
Resistance to gas flow L L L H Med L H
Prone to uneven levelness N N N Mainly at low rates Mainly at low rates
Y Y
Affected by corrosion Y Y Some Y Y N N
Prone to liquid surface agitation N N N Y Y Y Y
Likely to cause entrainment Y Y Y N N N N
Turndown L L L Med L H Med
Approximate range of liquid rates for standard design (gpm/ft2) 1-10 1-10 Wide 1-30 1.5-70 1-50 1-10
Weight L L L H Med Med Med
Quality of distribution Med Med L-Med H H Med Med

Legend:
C: ceramic
G: gravity
H: high
L: low
M: metal
Med: medium
N: no
O: orifice
P: plastic
PP: perforated pipe
PR: pressure
S: spray
W: weir
Y: yes


Several modern designs, often referred to as high-performance distributors are sophisticated versions of the common types as described in Table 1. They incorporate features for minimizing large-scale mal-distribution and for improving distributor compatibility with the service. These high-performance distributors are usually proprietary custom-designed devices and can be expected to perform better than standard distributors when properly designed, fabricated, and installed. Industry has been reported of substantial column efficiency enhancements resulting from replacement of standard distributors by their high-performance counterparts. However, the nonstandard nature of high-performance distributors makes them more expensive, more complex, and more susceptible to errors. Many of the unique features incorporated in high-performance distributors will be discussed in other articles.

Liquid distributors are usually classified into pressure distributors and gravity distributors (Table 1). In general, pressure distributors provide more open area for vapor flow and tend to be less expensive, lighter, less robust, and to require smaller lead-up piping than gravity distributors. Their disadvantages are high operating cost (because of the liquid pressure drop), susceptibility to plugging and corrosion, entrainment, and a relatively inferior quality of liquid distribution. The common pressure distributors are the perforated-pipe type and the spray type.

The common gravity distributors are the weir type and the orifice type. Both types can handle large liquid flow rates. The weir type is generally one of the least troublesome distributors and has an excellent turndown, but it can usually provide only a limited number of drip points and is extremely sensitive to levelness and liquid surface agitation. The orifice type may suffer from corrosion and plugging, but it can be designed with a large number of drip points to provide superior liquid distribution.

Each of the common distributor types is discussed in separate articles.


* Reproduced from Distillation Operation by Henry Kister