sieve tray extractor

Sieve Extractor Column DesignCris-Anne A. Juangco IIIBS Chemical Engineering2009-10339Engr. Elaine Mission

Table of ContentsEquipment Description...............................................................................................3

This paper will discuss the design principles, consideration and criteria that are inputted in designing a typical sieve extractor column.

SCOPE

Design Principles........................................................................................................3Design Considerations................................................................................................3Design Criteria............................................................................................................6Design Calculations....................................................................................................7

Tray Hydraulics.......................................................................................................7Point Efficiency Calculation.....................................................................................9Murphree Tray Efficiency Calculation......................................................................9Column Efficiency..................................................................................................10

References:..............................................................................................................12

SIEVE TRAY EXTRACTOR COLUMN DESIGN 2

Equipment Description

The sieve tray extractor resembles a sieve tray distillation column. Downcomers and slots are provided to guide the vapor and liquid phase and enables the two phase to come in contact thus aiding mass transfer process.

Design Principles

The approach to 100% efficiency is governed by three (3) mechanisms, occurring in sequence:

1. Formation of drops at the sieve tray perforations. As stated, a significant amount of total mass transfer occurs during this process. Factors governing transfer rate include surface created (drop size), interfacial tension, wettability of the tray material,, and rate of drop formation (flow rate through the perforations).

2. Rise of drops through the crossflowing phases. Mass transfer resistances inside and outside of the drops must be considered, and the approach to free rise velocity of the drops must be taken into account. Distance of drop travel, a function of tray spacing, influences the amount of mass transferred.

3. Coalescence of drops under the tray. The contribution of this mechanism to the total mass transfer is usually quite small.

Design Considerations

The input data required in the design calculations include density, viscosity, surface tension, diffusivity, and flow rate of the liquid stream, as well as density, diffusivity and flow rate of the vapor stream, this information can be obtained by performing tray-to-tray distillation calculations; several commercial computer packages are available for this purpose (e.g. PRO II, ASPEN PLUS, HYSIM).

1. Tray Spacing. This is set by maintenance requirements and also by support structure design in large-diameter columns. Sufficient crawl space must be provided for tray cleaning and repair. From these considerations, the minimum tray spacing is about 12 in (30 cm) for column diameter


less than 5 ft (150 cm), and 18 in (45 cm) for column diameter greater than 10 ft (300 cm).

2. Downcomer Area. The downcomer area at the top is sized such that the velocity of the ascending vapor bubbles exceeds the downflow velocity of the liquid. The size is related to the stability of the froth in the downcomer and determined by the residence time required for achieving the separation of the two-phase mixture. For non-foaming systems, such as lower alcohols, a residence time of 3 s is sufficient, where for extremely high foaming systems, such as caustic regenerators, 9 s is required.

To prevent the liquid coming off the bubbling area from splashing against the column wall, the minimum downcomer width is 5 in (12.7 cm). Also the minimum side chord length should be 60% of the column diameter. This is required to maintain good liquid distribution on the tray.

Since the separation of the vapor-liquid mixture is complete at the bottom of the downcomer, a sloped downcomer can be used to maximize the active tray area. In this case, the downcomer area at the bottom should be about 60% of that at the top.

3. Column Diameter. The column diameter can be calculated once the tray spacing and downcomer area have been specified. The Fair correlation is recommended. The vapor flooding velocity can be calculated using equation:

U N , f=CSB( ρL−ρvρv )0.5

( σL20 )0.2

Where: CSB is the Souders-Brown coefficient, ρL and σL are liquid density and surface tension, respectively, and ρv is the vapor density. UN,f is based on the net area, AN, which is the active area plus one downcomer area.

CSB can be calculated using:CSB( fts )=0.04232+0.1674T S+(0.0063−0.2686T S ) F IV+(0.1448T S−0.008 )F IV

2

Where: FIV=(L/V)(ρv/ρL)0.5, TS is tray spacing in feet, L and V are mass flow rates of the liquid and vapor. The CSB is valid for trays with a fractional hole area greater than 10%. For areas of 8% and 6%, CSB should be multiplied by 0.9 and 0.8, respectively.

Knowing UN,f and the total vapor flow rate, the column diameter can be calculated by assuming that the column will be operated at lower vapor velocity, ,say 80% of the flood point.


4. Number of Flow Passes. This is set to allow the tray to operate at weir loading that does not result in excessive weir crest. The weir loading can be calculated once the column diameter and downcomer are determined. The optimum weir loading is 4-6 US gallons per minute and the mazimum loading is about 20. Downcomer choking, which cause liquid build-up on tray, may occur if the maximum value is exceeded. Increasing the number of flow passes provides a solution to this problem (see fig.2). however, shorter liquid flow path and possible maldistribution of liquid and vapor streams in multipass trays which results to low

efficiency. As a rule of thumb, the use of multipass tray is often necessary for large-diameter columns.

5. Tray Geometry. This is chosen so that hydraulic and efficiency calculations can be performed to arrive at optimum design. The following parameters must be specified for tray design calculations.

Tray Thickness. The choice of material for the fabrication of trays is dependent mainly on the corrosion properties of the process fluids. In general, tray thickness is about gauge 10 (0.134 in; 3.40 mm) for carbon steel and gauge 12 (0.109 in; 2.77 mm) for stainless steel. For economic reasons, the

holes are punched, which dictates that the thickness must be less than the hole diameter.

Hole diameter. Small holes with diameter in the range of 3/16 to ¼ in (4.76-6.35 mm) give better hydraulic and mass transfer performance than the large ones in the range of ½ to ¾ in (12.7-19.0 mm). However, large-hole trays are cheaper and show more resistance to fouling.

Hole area. The area is normally in the range of 5-16% of the bubble area. Lower hole area allows the tray to operate at higher efficiency and turndown ratio, but at the expense of higher pressure drop. Since the operating pressure of the column dictates the maximum allowable pressure drop, the hole area is selected according


Figure 1. Tray Layout

to the type of service. Recommended values are 5-10% for pressure and 10-16% for vacuum operations,

Hole areas below 5% are not used because the distance between holes becomes excessive and liquid channeling may occur. However, the distance can also be adjusted by changing the hole diameter. In general, the hole pitch should not be larger than 2.5 in (6.35 cm). On the other hand, if the hole areas are greater than 16%, significant weeping and entrainment may coexist and the design equations may not apply under these conditions.

Weir design. Outlet weirs are used to control the froth height on the tray. For most trays, the outlet weir height is about 1-4 in (2.5-10 cm) and the downcomer clearance is 0.5 in (1.25 cm) smaller than outlet weir height to ensure a positive downcomer seal.

Design Criteria

The tray should be designed for maximum throughput. However, owing to inaccuracies in the design equations and fluctuation of process conditions (e.g. flow rates, temperature and pressure), safety factors are needed to ensure stable column operation at all times.

1. Jet Flood Safety Factor (JFSF). This is defined as the ratio of vapor velocity required to entrain the entire liquid flow (Umax) to the operating velocity (Uop). Typical JFSF value is 1.2.

2. Turndown Ratio. For various reasons, the column may be operated at a reduced throughput. Weeping is encountered if the vapor velocity can no longer support the liquid in the tray. Although flow dynamics permit stable operation as long as dumping is avoided, tray efficiency suffers because weeping reduces the vapor-liquid contact. The turndown ratio is the ratio of the design vapor flow rate to the flow rate that permits some weeping without seriously affecting the tray efficiency. Recommended weepages at turndown conditions for vacuum and pressure operations are 3% and 7%, respectively.

3. Downcomer Area Safety Factor (DCASF) and Downcomer Backup Safety Factor (DCBUSF). The liquid handling capacity of a tray is determined by downcomer design and tray spacing. The DCASF determines the approach of the top downcomer area to the minimum area required for vapor-liquid disengagement. The DCBUSF determines the approach of the downcomer froth height to the downcomer depth that is equal to the sum of


tray spacing and outlet weir height. Safety factor in the range of 1.5-2.0 are recommended.

4. Pressure Drop. The pressure drop across an operating tray should be specified if it affects the number of equilibrium stage requirements for the separation. This is often the case of vacuum operation. Stable operation can be obtained at pressure drop of 1-3 in (2.5-7.6 cm) of liquid per tray for vacuum an 2-5 in (5.1-12.7 cm) for pressure operations.

Design CalculationsKnowing the design considerations and criteria, a design calculation is done

to analyze the performance of the sieve tray column which includes:

Tray hydraulics (i.e. pressure drop, flow regime, froth density and entrainment and weeping factors)

Point efficiency Murphree tray efficiency Column efficiency

Tray HydraulicsPressure Drop Calculation in Vapor Phase

The pressure drop in the vapor phase across a sieve tray is calculated using:

∆ P=∆ Pdry+ ρ1gh1

Where dry pressure drop is given by:

∆ Pdry=12ρG( ug ,hCD

)2

And g is acceleration due to gravity, h1 is the liquid height or hold-up in m, ug,h is the vapor velocity in the hole in m/s, CD is the drag coefficient, and ρG, ρ1 are the densities of the vapor and liquid, respectively. The ρ1gh1 represents the static head due to the liquid hold-up on the tray. Hence, the liquid height, h1 must be predicted from correlations that depend on the weir geometry. One such equation that predict the liquid height is given by:

h1=0.6Hw0.5 p0.25(FPb )

0.25

25mm<Hw<100mm

Where Hw is the weir height in m, p is the pitch of the holes in the sieve plate in m, FP is the flow parameter which can be computed using:


FP=( u1ug )√( ρ1ρg )And b is the weir length per unit bubbling area in m. Also, CD is a function of the flow conditions near a hole and is dependent on the liquid present on the tray. It is correlated by:

CD=0.7 [1−0.14( gh1 ρ1ug ,h2 ρg )

2/ 3]Pressure Drop Calculations in the Liquid Phase

The liquid is transported down through the downcomer. The capacity of the downcomer should be should be sufficient to handle the liquid load without becoming the rate limiting factor, i.e. without the liquid backing up the downcomer to a significant extent. The extent of liquid back-up (hdc) can be estimated from:

hdc=ht+hda+hL

Where ht is the pressure difference between points a and b in the vapor phase that is necessary to keep the vapor flowing upwards, hL refers to the effective clear liquid height on the tray deck that must be overcome by the liquid in the downcomer, and had refers to the pressure loss due to the liquid flow under the downcomer apron. Note that ht is necessary to keep the upward flow of vapor, but acts as pressure differential that works against the natural liquid flow in the downcomer. If this pressure differential is large, the liquid will back up more in the downcomer. These points out the coupling between the pressure loss in the vapor phase through the tray deck area and the liquid flow in the downcomer. An optimal design must balance these two factors carefully. hL and ht can be estimated from the correlation:

hda=165.2U da2

Where hda is in mL of liquid and Uda is the velocity under the downcomer apron in m/s.

Froth Height and Density Calculations

The froth density (or the two-phase density) has been measured using gamma ray techniques. The average liquid volume fraction on a sieve tray, defined as ε-=h1/hb is correlated by:


1

ε1−¿−1=c1[ ug

(gh1 )0.5 (ρGρL )

0.5 ]n

¿

Where, hb is the froth or bed height in m and ug is the vapor velocity on bubbling area in m/s. The constant c1 and n depend on the type of flow regimes. In the spray regime, c1 is 265 and n is 1.7, while in mixed/emulsion regime, they are 40 and 0.8, respectively. This requires one to estimate the flow regime to be expressed under a given set of operating conditions.

Point Efficiency Calculation

The point efficiency (EOG) is related to the overall number of transfer units by:

EOG=1−e−NOG

The overall transfer units (NOG) can be correlated to data free of weeping and entrainment only for froth and spray regimes:

NOG=

11 1μ0.1φ0.14 ( ρLF s

2

σ 2 ) (DG tG )0.5

λ 1114 (DG ρG

DL ρL )0.5

(MG LM LG )+1

Here λ=mG/L is the separation factor, F s=us√ρG is superficial F-factor in kg0.5/m0.5S, tG = hf/us is the vapor phase contact time in s, and hf is the froth height in m. note that this correlation combines the geometrical parameters such as φ, the fractional perforated area, Ab the bubbling area, the system properties such as densities (ρL, ρG), diffusivities (DL, DG), viscosity (µ), interfacial tension (σ) in N/m, molecular weights (ML, MG) and operating conditions such as flowrates (L,G). This correlation appears to predict the point efficiencies to within 5% of experimental data over a wide range of pressures.

Murphree Tray Efficiency Calculation

The point efficiency is based on a detailed examination of mass transfer at vapor/liquid interface. The ideal equilibrium tray assumption used in the McCabe-Thiele method asserts that the flow condition on a tray is homogenous everywhere. If that were true, the point efficiency would be the same everywhere in the tray. But there is strong evidence that the flow is not homogenous, the degree of inhomogeneity is very evident in large-diameter columns. Information about flow


profile must be integrated with point efficiency to predict Murphree tray efficiency. This will apply to longhitudinal mixing only and is given by:

EMVEOG

= 1−e−(η+Pe)

(η+Pe) {1+ [(η+Pe)/n ]}+ e−n−1η {1+[η/(η+Pe)] }

Where:

η=Pe2 [(1+ 4 λ EOGPe )

1 /2

−1]And Pe is the Peclet number, defines as Pe=Z12/DEt1. Here Z1 s the length of the liquid travel, or the distance between the two weirs and t1 is the liquid residence time. The effective diffusivity is given by:

√DE=0.0124+0.017uG+0.0025 L+0.0150W

Where DE is in ft2/s, uG is superficial velocity in ft3/s divided by active bubbling area in ft2. As the Peclet number becomes large, this model predicts efficiency enhancement much large than unity. In large diameter columns (large Z1) the Peclet number can tend to take large value which would suggest significant efficiency enhancements.

Effect of Entrainment on Murphree Tray Efficiency

The effect of entrainment on the Murphree tray efficiency is estimated from:

EMV entrain=EMV [ 1

1+EMV ψ1−ψ ]

Where:

ψ= eL+e

=absolute entrainmenttotal liquid flowrate

Where e is the entrained liquid in mol/hr. liquid entrainment in spray regime is:

ψ=1.0 x10−8( hbH s)3

( ug ,hu1 ) for0.3< hbH s<0.9

Where Hs is tray spacing in m, hb is bed height, ug,h is vapor velocity in the hole in m/s and u1 is the horizontal liquid velocity.


Column EfficiencyWeeping Point Determination

When the vapor velocity is too small, the liquid on a tray deck can flow down through a holes on the sieve plate, instead of the downcomer, which is preferred path for the liquid. If weeping is significant, then it results in mixing of liquid streams between neighboring trays, thus degrading the performance of the column. The need to avoid weeping places a limit on the minimum vapor velocity which is computed by:

Mixed/free bubbling regime

C Fw=F √gh1[1−0.15 FPbh1 ]Emulsion flow regime

CF❑w=0.45F √gh1

Where C Fw=ug√ρG /¿¿ is the capacity factor at the weep point in m/s, and F is the fractional hole area per unit bubbling area. Note that weeping will seldom occur in spray regime as vapor velocities are sufficiently large under design conditions.

Extensions to Multicomponent Systems

The methods already discussed is developed largely using experimental data for binary systems. The question of whether they can be applied to multicomponent systems can be examined as follows:

Tray hydraulics factors such as pressure drops, flow regimes, froth densities, etc, depend only on the fluid mechanics of the two-phase mixture on sieve trays; hence one can expect the correlations to be useful for multicomponent mixtures as long as mixture properties for densities, viscosities, interfacial tensions, etc. are used.

The point efficiency (and hence the Murphree tray efficiency) depends on the mass transfer resistance on each component species in each phase. Since the diffusivity and the equilibrium ratios (or the slope of the equilibrium curve,m) could vary for each species, the point effciency will be different for each species. Thus, complicating the design calculations.

Issues Relating to Scale-up of Efficiency Data


Since the point efficiency data and correlations are based on local conditions, they should, in principle, remain valid on all scales. They are then integrated with flow conditions to predict the overall tray efficiency. However, correlations such as Murphree efficiency becomes variant in some scales thereby a detailed computational fluid dynamics can provide a detailed flow information to be used in design calculations.

References:

Chuang, K.T. and K. Nandakumar (2000). Tray Columns: Design. Canada: Academic Press, p 1135-1145

Fair, J. R. and J. L. Humphrey (1983). Liquid-Liquid Extraction Process. Texas: Fifth Industrial Energy Technology Conference, ESL-IE-83-04-130, vol 2.

King, C.J. (1980). Separation Processes. New York: McGraw Hill Book CoTreybal, R.E. (1980). Mass-Transfer Operations. New York: McGraw Hill Book

Co., 3rd ed.


sieve tray extractor

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