tray columns 20design

7/28/2019 Tray Columns 20Design

1/6

Figure 1 (A) Sieve ray with downcomer in a 30 cm diameter

column. (B) Dual-flow tray in a 30 cm diameter column.

Tray Columns: Design

K. T. Chuang and K. Nandakumar,

University of Alberta, Edmonton, Alberta, Canada

Copyright^ 2000 Academic Press

Distillation has remained an important separationtechnology for the chemical process industries. In1997 it was reported in the journal Chemical Engin-eering that about 95% of all worldwide separationprocesses use this technology. In the USA alone, some40 000 distillation columns represent a capital invest-ment of about US $8 billion. They consume the en-ergy equivalent of approximately 1 billion barrels ofcrude oil per day. Such columns are used in reRneries,petrochemical plants, gas processing plants and or-ganic chemical plants to purify natural gas, improvegasoline, produce petrochemicals and organic prod-ucts, recover pollulant species, etc.

Distillation can be carried out in a tray or a packedcolumn. The major considerations involved in thechoice of the column type are operating pressure anddesign reliability. As pressure increases, tray coulmnsbecome more efRcient for mass transfer and can oftentolerate the pressure drop across the trays. The designprocedure for the large diameter tray column is alsomore reliable than that for the packed column. Thus,trays are usually selected for large pressurized columnapplications.

Distillation trays can be classiRed as:

1. cross-Sow trays with downcomers (see Figure 1A);2. countercurrent trays without downcomers (also

known as dual-Sow trays) (see Figure 1B).

The-dual Sow tray allows the gas and liquid to passthrough the same tray openings. This results in a lim-ited operating range because the dispersion height isvery sensitive to the gas/liquid Sow rates. In general,dual-Sow trays are employed only in cases where highcapacity or high resistance to fouling are required.

However, because of its narrow operating range, themarket share is small and such trays will not bediscussed further.

The cross-Sow tray utilizes a weir on the down-comer to control the spray height on the tray, andthus provides a stable gas}liquid dispersion overa wide range of gas/liquid Sows. A tray is the combi-nation of a tray deck, where froth is generated toprovide vapour}liquid contact, and a downcomer,where the vapour}liquid mixture is separated. Thebulk of the vapour rises from the aerated liquidthrough the vapour disengagement space to the tray

above. However, the passage of the liquid from the

top to the bottom of the column occurs mainly viadowncomers.

There are three types of cross-Sow trays: (1) sieve,(2) valve and (3) bubble cap. Among them, sieve traysoffer high capacity and efRciency, low pressure drop,ease of cleaning, and low capital cost, but smallerturndown ratio. Although the design procedure issimilar for all three types of trays, only sieve trayperformance data are readily available in the publicdomain. The valve and bubble cap designs are oftenprotected by patents, and thus the performance dataare supplied by the vendors. This article describes theprocedure for designing an optimum sieve tray.A similar procedure can be applied in principle to thevalve and bubble cap trays, provided critical perfor-

mance data are available.The cost of a tray column is determined by twofactors:

1. column diameter, which determines the through-put;

II/DISTILLATION/Tray Columns: Design 1135


2/6

Figure 2 Tray layouts.

2. column height, which delivers the number of equi-librium stages required for the separation.

The minimum cost is generally achieved when thecolumn volume is minimized. The Rnal selection ofthe tray design is based on the combined cost of thecolumn shell, internals and installation.

It should be noted that the fraction of the cross-sectional area available for vapour}liquid disengage-ment decreases when the downcomer area is in-creased. Thus, optimum design of the tray involvesa balance between the tray area and the downcomerarea (i.e. the capacity for the tray deck should matchthe capacity of the downcomer). The correlations forsizing trays are implicit in column diameter, trayspacing and tray geometry, thus requiring trial-and-error calculations to arrive at the Rnal selection.

Characteristics of Tray OperationTypical tray layout is shown in Figure 2, and trayoperation is shown in Figure 3. High speed photogra-phy of a large operating tray indicates that the vapourerupts through the liquid sporadically. The holes thatare not erupting do not weep appreciably at a vapourrate above the weep point, although the supporting ofthe liquid by the vapour is not absolutely complete.The interaction of vapour and liquid on a properlydesigned tray results in a highly turbulent two-phasemixture of a high speciRc interfacial area with net

liquid movement in a crossSow direction to the risingvapour stream. The aerated liquid may be eitherliquid-continuous (froth) at relatively low vapour vel-ocities or vapour-continuous (spray) at high vapourvelocities.

The maximum capacity of a sieve tray is reachedwhen the tray isSooded. This may be due to excessive

spraying (entrainment) taking place in the intertrayspace or the froth in the downcomer backing-up toreach the top of the outlet weir. The onset ofSoodingis accompanied by a sharp increase in tray pressureand a sharp decrease in tray efRciency.

As vapour rates decrease to the point that thevapour Sow cannot totally support the liquid on thetray, some liquid will weep through the holes. Ifthe weepage is so severe that no liquid Sows over theoutlet weir, the tray cannot operate stably under thesedumping conditions. The minimum capacity of thetray is normally reached when moderate weepage isencountered. Ideally, a sieve tray should operate inthe shaded area shown in Figure 4 to ensure properoperation.

Tray efRciency can be divided into two compo-nents:

1. point efRciency as determined by the vertical Sowof vapour through the froth;

2. tray efRciency enhancement by the crossSow ofliquid.

The physical properties of the vapour}liquid mix-ture determine the point efRciency, although frothheight, which inSuences the gas residence time, alsohas a signiRcant effect, especially for low efRciencysystems. LiquidSow pathlength determines the liquidresidence time and the extent of crossSow tray efR-ciency enhancement. Entrainment and weeping de-press tray efRciency by disrupting the concentrationproRle in the column. The froth height and the liquidSow path are two parameters that are optimized togive maximum tray efRciency. Other geometric vari-ables, such as open hole area, hole diameter anddowncomer arrangement, also affect tray hydraulics

and efR

ciency. The goal for a tray design is to reachmaximum tray efRciency without compromising hy-draulic stability.

The steps required for tray column design areshown in Figure 5; a detailed discussion of each stepis given below.

Input Data

Once sieve trays are selected for a given application,the input data that are required in the designcalculations include density, viscosity, surface ten-

sion, diffusivity and Sow rate of the liquid stream, as

1136 II/DISTILLATION/Tray Columns: Design


3/6

Figure 3 Tray operation schematic diagram.

Figure 4 Sieve tray performance diagram.

well as density, diffusivity andSow rate of the vapourstream. This information can be obtained by per-forming tray-to-tray distillation calculations; severalcommercial computer packages are available for thispurpose (e.g. PRO II, ASPEN PLUS, HYSIM).

As the physical properties and the vapour andliquid Sow rates vary throughout a given column, it isdifRcult to provide a single design for the entire col-umn. Instead, the column is divided into a number ofsections. Within each section, trays are designed withthe same layout. Normally the section is a setof trays bounded by two column penetrations (feedand/or drawoff). Tray design calculationsshould be performed to ensure that trays at the topand bottom of the section meet the design require-ments.

Preliminary Speci\cations

Tray Spacing

Tray spacing is set by maintenance requirements, andalso by support structure design in large-diametercolumns. SufRcient crawl space must be provided fortray cleaning and repair. From these considerations,the minimum tray spacing is about 12 in (30 cm) forcolumn diameter less than 5 ft, and (150cm) and18 in (45 cm) for a column diameter greater than10 ft (300 cm). In general, it is best to keep trayspacing to a minimum, which is often the most econ-omical.

Downcomer Area

The downcomer area at the top is sized such that thevelocity of the ascending vapour bubbles exceeds the

downSow velocity of the liquid. The size is related to

the stability of the froth in the downcomer and deter-mined by the residence time required for achievingthe separation of the two-phase mixture. For non-foaming systems, such as lower alcohols, a residencetime of 3 s is sufRcient, whereas for extremely highfoaming systems such as caustic regenerators, 9 s isrequired.

To prevent the liquid coming off the bubbling areafrom splashing against the column wall, the minimumdowncomer width is 5 in (12.7 cm). Also, the min-imum side chord length should be 60% of the columndiameter. This is required to maintain good liquiddistribution on the tray.

Since the separation of the vapour}liquid mixtureis complete at the bottom of the downcomer, a slopeddowncomer can be used to maximize the active trayarea. In this case, the downcomer area at the bottomshould be about 60% of that at the top.



4/6

Figure 5 Sieve tray design procedure.

It should be noted that the downcomer area occu-pies only a small fraction of the cross-sectional area.Thus, a small overdesign does not result in a signiR-cant economic penalty.

Column Diameter

The column diameter can be calculated once the trayspacing and downcomer area have been speciRed.The Fair correlation, based on the Souders and Browncriterion, is recommended by most designers. Thevapour Sooding velocity can be calculated fromeqn [1].

UN,f"CSBL!V

V 0.5

L

200.2

[1]

In eqn [1] CSB is the Souders}Brown coefRcient,L and L (dyne cm\

1) are liquid density and surfacetension, respectively, and V is the vapour density in

the same units as L . UN,f is based on the net area,AN(ft

2), which is the active area plus one downcomerarea. The unit for UN,f is ft s\

1. The most popularempirical formula for calculating CSB is given ineqn [2].

CSB(ft s\1)"0.04232#0.1674TS

#(0.0063!0.2686TS)FlV

#(0.1448TS!0.008)F2lV [2]

In this equation FlV"(L/V)(V/L)0.5, TS is tray spac-

ing in feet, and L and V are mass Sow rates of theliquid and vapour. The CSB is valid for trays witha fractional hole area greater than 10%. For areas of8% and 6%, CSB should be multiplied by 0.9 and 0.8,respectively.

Knowing UN,f and the total vapour Sow rate, thecolumn diameter can be calculated by assuming thatthe column will be operated at a lower vapour velo-city, say 80% of the Sood point.

Number of Flow Passes

The number ofS

ow passes is set to allow the tray tooperate at a weir loading that does not result inexcessive weir crest. The weir loading can be cal-culated once the column diameter and the down-comer area are determined. The optimum weir load-ing is 4}6 US gallons per minute and the maximumloading is about 20. Downcomer choking, whichcauses liquid build-up on the tray, may occur if themaximum value is exceeded. Increasing the numberofSow passes provides a solution to this problem (seeFigure 2). However, shorter liquidSow path and pos-sible maldistribution of liquid and vapour streams in

multipass trays may result in lower tray efRciency.

1138 II/DISTILLATION/Tray Columns: Design


5/6

As a rule of thumb, the liquid and vapour handlingcapacity are a direct function of weir loading andcolumn area, respectively. Since weir length and col-umn area are proportional to column diameterand diameter squared, respectively, the use ofmultipass trays is often necessary for large-diametercolumns.

Tray Geometry

Tray geometry should be chosen so that hydraulicand efRciency calculations can be performed to arriveat the optimum design. The following parametersmust be speciRed for tray design calculations.

Tray Thickness

The choice of material for the fabrication of trays isdependent mainly on the corrosion properties of the

process Suids. In general, tray thickness is aboutgauge 10 (0.134 in; 3.40 mm) for carbon steel andgauge 12 (0.109 in; 2.77 mm) for stainless steel. Foreconomic reasons the holes are punched, which dic-tates that the thickness must be less than the holediameter.

Hole Diameter

Small holes with a diameter in the range of 316 to14 in

(4.76}6.35 mm) give better hydraulic and mass trans-fer performance than the large ones in the range of 12

to 34 in (12.7}

19.0 mm). However, large-hole traysare cheaper and show more resistance to fouling.Choose the hole size according to design require-ments.

Hole Area

The hole area is normally in the range of 5}16% ofthe bubbling area. Lower hole area allows the tray tooperate at higher efRciency and turndown ratio, butat the expense of higher pressure drop. Since theoperating pressure of the column dictates the max-

imum allowable pressure drop, the hole area is se-lected according to the type of service. Recommendedvalues are 5}10% for pressure and 10}16% for vac-uum operations.

Hole areas below 5% are not used because thedistance between holes becomes excessive and liquidchannelling may occur. However, the distance canalso be adjusted by changing the hole diameter. Ingeneral, the hole pitch should not be larger than2.5 in (6.35 cm). On the other hand, if the hole areasare greater than 16%, signiRcant weeping and en-trainment may coexist and the design equations may

not apply under these conditions.

Weir Design

Outlet weirs are used to control the froth height onthe tray. For most trays, the outlet weir height isabout 1}4 in (2.5}10 cm) and the downcomer clear-ance, where the liquid is discharged from the bottomof the downcomer onto the tray below, should be

0.5 in (1.25 cm) smaller than the outlet weir height toensure a positive downcomer seal.From the above discussion, it may be concluded

that the object of tray design is to obtain the optimumcombination of the following parameters:

1. column diameter2. tray spacing3. top and bottom downcomer area4. hole diameter and hole area5. outlet weir height and downcomer clearance.

Design CriteriaThe trays should be designed for maximum through-put. However, owing to inaccuracies in the designequations and Suctuation of process conditions (e.g.Sow rates, temperature and pressure), safety factorsare needed to ensure stable column operation at alltimes (see Figure 4).

Jet Flood Safety Factor

The jet Sood safety factor (JFSF) is deRned as the ratioof vapour velocity required to entrain the entire liquid

Sow (Umax) to the operating velocity (Uop). It is a use-ful measure of entrainment and hydraulic stability.The typical JFSF value is 1.2.

Turndown Ratio

For various reasons, the column may be operated ata reduced throughput. Weeping is encountered if thevapour velocity can no longer support the liquid onthe tray. Although Sow dynamics permit stable op-eration as long as dumping is avoided, tray efRciencysuffers because weeping reduces the vapour}liquidcontact. The turndown ratio is the ratio of the design

vapour Sow rate to the Sow rate that permits someweeping without seriously affecting the tray efRcien-cy. Recommended weepages at turndown conditionsfor vacuum and pressure operations are 3% and 7%,respectively.

Downcomer Area Safety Factor (DCASF)

and Downcomer Backup Safety Factor (DCBUSF)

The liquid handling capacity of a tray is determinedby downcomer design and tray spacing. The DCASF

determines the approach of the top downcomer area



6/6

to the minimum area to the minimum area requiredfor vapour}liquid disengagement. The DCBUSF de-termines the approach of the downcomer froth heightto the downcomer depth ("tray spacing#outletweir height). Safety factors in the range of 1.5}2.0 arerecommended.

Pressure Drop

The pressure drop across an operating tray should bespeciRed if it affects the number of equilibrium stagerequirements for the separation. This is often the casefor vacuum applications. Stable operation can beobtained at a pressure drop of 1}3 in (2.5}7.6 cm) ofliquid per tray for vacuum and 2}5 in (5.1}12.7 cm)for pressure operations.

Design Calculations

Tray Hydraulics

The hydraulic performance of a sieve tray for a givenlayout may be calculated using the methods presentedin Distillation/Tray Columns: Performance.

Tray Ef\ciency

Tray efRciency is a strong function of the physicalproperties of the vapour and liquid streams. It is alsoaffected, to a lesser extent, by the Sow rates and traylayout. In the latter case, only hole diameter, holearea and weir height have a small inSuence on thetray efRciency. The optimum design, which gives themaximum number of equilibrium stages in a column,is often obtained at minimum tray spacing and min-imum number ofSow paths that satisfy the hydraulicdesign criteria.

Conclusions

A well-designed tray should be economical whilemeeting all process design requirements. Economic

considerations suggest that it is best to use thesmallest column diameter and height that satisfy theprocess requirements within reasonable safety allow-ances. Process requirements include accommodationof the expected liquid and vapour Sow ranges and theoptimization of tray efRciency.

See also: II/Distillation: Packed Columns: Design and

Performance; Theory of Distillation; Tray Columns: Perfor-mance.

Further Reading

Billet R (1979) Distillation Engineering. New York: Chem-ical Publishing Co.

Fair JR (1963) In: Smith BD (ed.) Design of EquilibriumStage Processes. New York: McGraw-Hill.

Fair JR (1987) In: Rousseau RW (ed.) Handbook of

Separation Process Technology, ch. 5. New York: JohnWiley.

Fair JR, Steinmeyer DE, Peuney WR and Crocker BB(1997). In: Perry RH and Green D (eds) Perrys Chem-ical Engineers Handbook, 7th edn, sect. 14. New York:McGraw-Hill.

Humphrey JL and Keller GE II (1997) Separation ProcessTechnology. New York: McGraw-Hill.

Kister HZ, (1992) Distillation Design. New YorkMcGraw-Hill.

Lockett MJ (1986) Distillation Tray Fundamentals. Cam-bridge: Cambridge University Press.

Lygeros AI and Magoulas KG (1986) Column Sooding

and entrainment. Hydrocarbon Processing 65:43}44.

McCabe WL, Smith JC and Harriott P (1993) Unit Opera-tions of Chemical Engineering, 5th edn. New York:McGraw-Hill.

Ogboja O and Kuye A (1991) A procedure for the designand optimization of sieve trays. Transactions of theInstitution of Chemical Engineers 445.

Rose LM (1985) Distillation Design in Practice. Amster-dam: Elsevier.

Tray Columns: Performance

K. Nandakumar and K. T. Chuang, University of Alberta,

Edmonton, Alberta, Canada

Copyright^ 2000 Academic Press

Introduction

As pointed out in the article entitled distillation tray

columns: design, a sieve tray is designed with a num-

ber of objectives in mind. They include: (i) achievinghigh efRciency of contact between the liquid and thevapour so that the phases leaving a tray are as close toequilibrium conditions as possible; (ii) balancing thetray deck area provided for vapour/liquid contactwith the downcomer area provided for disengage-ment of the two phases so that neither limits thecapacity of the column to process large amounts

of feed; and (iii) avoiding detrimental operating

1140 II/DISTILLATION/Tray Columns: Performance

tray columns 20design

Documents