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Material of construction.Contents of solids or tendency for fouling.

The liquid distributor design follows four basic steps:

Selection of the distributor type depending on abovementioned criteria.

. Determination of the principle of l iquid distribution.o Determination of the liquid feed system.o Hydraulic design of drip points.

Seleclion of the dislributor typeThe selection of the distributor type is mainly influenced bythe column diameter, operating range for the l iquid and the

Figure 5. Trough with orifices at thebottom.

Figure 7. Trough type distributor withpredistributor by orifice and laddertype feeding pipe.

selected material of construction. Additionally, the follow-ing considerations are taken into account:o Gas side pressure drop should be low.o Low sensitivity to fouling or contents of solids in the

l iquid.. Low droplet entrainment.o Unequal gas distribution should be avoided.o Maldistribution of the l iquid should be avoided.

The following basic designs of liquid distributorsare suitable for a wide range of applications: pan type(Figure 1); deck type (Figure 2); trough type (Figure 3); lad-der type (Figure 4); and spray nozzle type.

Standard values for various basic designs of l iquiddistributors are given in Table 1.

Principles of distribufionSeveral key principles must be considered in terms of opti-mal l iquid distribution design. Applicable principles aredependent upon criteria such as the volume of the l iquidto be distributed, its working range, the potential for foulingand the allowable entrainment.

The number of drip points ranges between 60 - 150/m2

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Figure 8. Trough type distributor withtwo parting boxes and H type feedingpipe.

depending on the particularapplication. Frequently, ori-fices at the bottom are used.Bottom orifices (Figure 5)could cover an operatingrange of 1:2.5, which is suffi-cient for approximately 80% ofall applications.

Forwideroperating ranges.orifices, weirs or attached dis-tribution pipes on the sides ofindividual troughs and over-flow drip tubes are available.Even very low liquid loads(0.5 m3/m2h) can be achievedby using overflow drip tubes(Figure 6). lt is recommendedto test liquid distributors onsuitable test benches in termsof l iquid distribution qualityunder operating conditions.

Liquid feed syslemsUniform l iquid feed to the l iq-uid distr ibutor selected for apart icular appl icat ion is cr i t i -cal to the opt imum perform-ance of th is distr ibutor. Thefeed system must distr ibutel iquid as evenly as possibleto the ent i re l iquid distr ibutorwhi le taking into considera-t ion l iquid discharge veloci tyand l iquid pressure f luctua-t ions . A l l l i qu id feed des ignsare aiming to route the l iq-uid evenly, constant ly, andwith a calmed f low to thedr ip ho les . The l iqu id feedsystem is especial ly cr i t i -ca l in te rms o f h igh l iqu idloads and large diameterco lumns.

Fundamentaloptions are:

Table 1. Standard values for various basic distributor designs

Distr ibutor designtype

L iqu id load

(m3/m2h)

Recommended column

diameter (mm)

Pan 0.3 - 200 100 - 1600

Deck 5 - 2 0 0 > 200

Trough 0 .3 - 50 > 600

Ladder 4 - '100 al l s izes

Spray nozzle 3 - 2 0 0 a l l s izes

Figure 6. Various drip tube designs.

Figure 9. Flash gallery. Figure 10. Double wall tube.design

Ladder type feed sYstem forlow and medium l iquid loads(< 80 m3/m2h) and turndownratios of 1:3 (Figure 7).

Parting box with downcomers(Figure 8) for h igher l iquid loads

and wider oPerat ing ranges(1 :10 or g rea ter ) .

A system that distributes l iquid

feed prior to the distributor is not

required by lower l iquid loads (up to

5 m3/m2h) and smaller column diam-

eters, particularly for pan type dis-

tributors or through type distributorswith an integral Parting box'

Feed syslems for lwophose feedsl f superheated mixtures are fed

into the column, sPecial Precau-t ions must be taken in order to

obtain a thorough degasificationto the best degree Possible before

feed enters the l iquid distr ibutor or

the tray, collectivelY.Flash gal ler ies (Figure 9)

are part icular lY sui table for

larger column diameters and for

large feed volumes. The incom-

Figure 12. Deck type distributor with

covered gas chimneys.

second Packed bed below the col-

lect ing t raY.Collecting trays and redistributors

are typically applied in the following

CASES:

o An additional l iquid feed stream

needs to be introduced or

removed.. A high number of transfer stages

must be achieved, and thus bed

dePth is high.o Maldistribution of the downcom-

. ing l iquid must be avoided.

o The weight (suPPort Plate) or

the mechanical stabil itY of thepacked bed is l imited'

For less crit ical aPPlications,total height of the required collect-

ing t ray and subsequent l iquid dis-

tributor can be reduced bY the use

of a vane type collector construction(F igure 11 ) .

Typically, l iquid in the collectingtray is removed either via a circulat-ing collecting trough or a draw off.

The collecting traY can be used as

a sump. Dur ing equiPment down-

t ime, l iquid is retained in th is sump'

The height of the chimneYs deter-

mines the volume of l iquid that caning feed is introduced tangen-

t ia l ly against the column wal l . The f lash gal lery accu-

mulateJ the l iquid so that the vapour port ion can

degasi fy. The l iquid can then be routed direct ly into a

distr ibutor.D o u b | e w a l I t u b e s ( F i g u r e 1 0 ) h a v e p r o v e n S u c c e S S -

ful when relatively low feed flows of feed flows with a low

level vapour port ion have to be suppl ied whi le reducing

space at the same t ime. The incoming gas- l iquid mixture

degases in a central tube that has slots facing downwards

anä boreholes in the upward direction. Overflow spouts

are arranged in the encasing tube while the gas phase

escapes üpwards by way of boreholes arranged on the

sides.Flash boxes are suitable for low flows. These boxes are

arranged in the column near the distributor. The incoming

feed degases in the flash box where the stabil ised l iquid is

directed downwards and the vapour escapes upwards'

Hydroulic designToiricell i 's equation is uti l ised for the design of l iquid distrib-

utors. Depending on the l iquid level (h), the l iquid velocity,

and thus the l iquid flow rate (Q), is calculated:

Q = n ' 1 t ' A ' ( 2 ' S ' h ) o s

The coefficientpr must be adjusted to the relevant param-

eters. lt depends on the l iquid level, the material thickness,

the open area of the orif ice, the orif ice geometry, the l iquid

viscosity and other parameters. A minimum liquid level of

25 mm ls used. The number of drip points (n) and the drip

hole area (A), or diameter (d), respectively' could be deter-

mined. Regular drip hole diameters range approximately

1 0 m m .

Collecting troys ond redistributorscol lect ing t rays and redistr ibutors are designed accord-

ing to the same pr inciples ment ioned above. col lect-

ing t rays are used to col lect downcoming l iquid at the

boltom of a packed bed and to allow for either removal

of th is l iquid f rom the column or redistr ibute to a

be retained. ln order to prevent leakage, the collecting tray

is often welded or laminated in the vessel by the vessel

manufacturer. Figure 12 shows a deck type redistributor

w i t h c o v e r e d g a s c h i m n e y s . B y | e a v i n g o u t t h e b o t t o m o r i -f ices. the redistributor changes to a collector'

Moferiol of construclionA great variety of materials, both metals and plastics, are

pröcessed for column internals. The selection of the mate-

rial depends mainlY on:

o The oPerat ing andtemperature.

max imum (emergencY)

. The composition of gas and liquid'

o Economical considerations, i.e. investment and main-

tenance costs.o Desired l ifetime and plant availabil ity'

l n t h e c a s e o f p l a s t i c s , s t a n d a r d m a t e r i a l s a r e P P 'P P H , P V C , P V D F , b u t s p e c i a l i t i e s a r e a l s o p r o c e s s e d b yRauschert, including C-PVC, ETCFE, PTFE (TFM)' ln addi-

tion to standard metal materials, such as carbon or stain-

less steel types, alloys (Hastelloy, lnconel, Monel etc') or

titanium are also used.

ConclusionColumn internals are crucial for the proper opera-

t ion of a packed tower. The opt imum performance and

t h e d e s i r e d o p e r a t i n g r a n g e s c a n o n | y b e a c h i e v e d , i fboth tower packings and tower internals are careful ly

selected for each indiv idual case. Al though the latest

machinery and product ion tools are used, product ion of

column internals today is st i l l a 'manufactur ing' in the

basic Sense of th is word. Design, drawings and produc-

t ion for each part are indiv idual and adapted to the proc-

ess. Therefore, i t is recommended to study careful ly al l

re levant parameters when select ing column internals

Figure L1. Vane type collectingtray'

for a project. t