DISTILLATION COLUMN DESIGNStripping ColumnCALCULATING NUMBER OF STAGES.The Underwoods and Gililands method is adopted in order to calculate the number of stages.Mr Ethanol = 46.069 g/molMr Water = 18.015 g/mol

Temperature of feed = 92.60 C (VLE data for ethanol-water mixture). Light Key - LK - Ethanol (More Volatile Component)Heavy Key - HK - Water (Less Volatile Component)

FEEDMole fraction of ethanol in liquid, = 0.067885Mole fraction of water in liquid, = (1 - 0.067885) = 0.93212Mole fraction of ethanol in vapor phase, = 0.267580Mole fraction of water in vapor, = (1 - 0.267580) = 0.732420

= 5.0164

DISTILLATE (55 wt. % ethanol)By interpolation,Mole fraction of ethanol in distillate, = 0.32338Mole fraction of water in distillate, = (1 - 0.32338) = 0.67661

BOTTOMS (0.02 wt. % ethanol)By Interpolation,Mole fraction of ethanol in bottoms, = 0.00008085Mole fraction of water in bottoms, = (1 - 0.00008085) = 0.99992

The minimum stages, Nmin requires at total reflux is estimated using the Fenske equation;

Similarly,

= 5.358 5

Minimum reflux at infinite, Rmin stages; = 0.067885 = 0.32338 = 0.00008085

= 0.2794

Reflux ratio is 1.2 to 1.5 times the minimum reflux ratio from reference. Taking average = 1.35

= 0.2794 1.35= 0.3772

Using Gillilands empirical relation between reflux ratio and number of stages (Table 11.42, Coulson and Richardson);

= 0.07

At 0.07, Replacing = 5,

Number of plates in stripping column = 14

= 0.65 from Table 11.1 (SINNOT, 2005)

Taking reboiler to be equivalent to one stage,

FEED POINT LOCATIONThe empirical equation given by Kirkbride (1944) in Chemical Engineering Design by Coulson and Richardson (Vol. 6) is used to find the feed point location.

Where, Nr = Number of plates below feed Ns = Number of plates above feed

From material balance,Mass flow rate of distillate = 2.894 kg/sMass flow rate of bottom product = 13.050 kg/s

Therefore,

(excluding the reboiler)

Taking overall column efficiency to be 65%, Ns = 20This means that the feed enters the column at the top and there are no stages above the feed.

APPROXIMATE COLUMN SIZING Height of columnSafety factor above and below the column: 0.4 mPlate spacing: 0.5 m

Diameter of columnAccording to SINNOT (2005), the maximum allowable superficial vapor velocity can be estimated using Souders and Brown equation.

Where

From mass balance,StreamMass Flow Rate (kg/s)

Feed, F15.945

Distillate, D2.894

Bottom product, B13.050

The stripping column operates at a pressure of 1 atm. From steam tables, at P = 101.325 kPA,T = 99.97 C

Reflux ratio, R = 0.3772

Since feed is entering at the bubble point, g = 1

Top ProductAssuming that the vapor behave as an ideal gas, Where,P = Pressure of vapor = R = Gas constant = 0.1805 kJ/kgKT = Temperature of vapor = (90.08 + 273.15) = 363.23 K = Specific volume (m3/kg)

Density of ethanol vapor,

Calculation of parachor, C2H5OH,

GroupContributionNumber

C4.89.6

H-O11.311.3

H-C17.185.5

O20.020.0

126.4

Molecular Weight of ethanol = 46

Bottom Product

Table 1: List of parameters used for calculationsPARAMETERTOPBOTTOM

Temperature, T (K)363.23373.14

Liquid Rate, L (kg/s)1.091817.036

Vapor Rate, V (kg/s)3.9863.986

Liquid Density, (kg/m3)789958.77

Vapor Density, (kg/m3)1.50.5976

Liquid Flow Rate, q (m3/s)

Vapor Flow Rate, Q (m3/s)2.5792

Surface Tension, (N/m)0.004805

Viscosity, (Ns/m2)

Molecular Mass, Mr (kg/kmol)18.0172

Note: Design methods adopted in this chapter have been mostly borrowed from Coulson and Richardsons Chemical Engineering Design, Volume 6 combined with Backhurst and Harkers Process Plant Design (1973). Also, the equations and correlations used are described in more details in Chapter 2.

1) Column Diameter

Flow parameter, FLV

Using figure 11.27 (SINNOT, 2005), for plate spacing = 0.5 m,

Correction for surface tension

Vapor velocity based on net area, (V)

The column is designed for 80% flooding at maximum flow rate (WINKLE, 1967)

Maximum volumetric flow rateTop Q = 2.5792 m3/sBase Q = 6.6702 m3/s Net area required (An)

Column cross-sectional area (Ac)As trial, downcomer area is taken as 12% of tower area.Ad = 0.12 AcAn = 0.88 Ac

Column diameter, (Dc)

Since same diameter above and below the feed, the column diameter 1.80 m2) Selection of Liquid Flow PatternThe maximum volumetric liquid rate is at the base, L = 0.01777 m3/s. From figure 11.28 (Coulson and Richardson, Vol 6), single pass cross-flow plate is selected.3) Provisional Plate DesignColumn Diameter, Dc = 1.80 mColumn Area, Ac = /4 Dc2 = /4 (1.80)2 = 2.5447 m2Downcomer Area, Ad = 0.12 2.5447 = 0.3054 m2 (At 12%)Net Area, An = Ac - Ad = 2.5447 - 0.3054 = 2.2396 m2Active Area, Aa = Ac - 2Ad = 2.5447 - 2(0.3054) = 1.9340 m2Hole Area, Ah = 0.10 Aa (First Trial) = 0.10 1.9340 = 0.1934 m2 Weir LengthFrom figure 11.31 (SINNOT, 2005)

Taking,Weir height = 50 mmHole diameter = 5 mmPlate thickness = 5mm

4) Check WeepingMaximum liquid rate = 17.036 kg/sMinimum liquid rate at 70% turn-down = 0.70 17.036 = 11.9254 kg/s

At minimum rate,

From Figure 11.30 (SINNOT, 2005), From the value obtained, it can be observed that the operating rate will be well above the weep point, which means no weeping will occur.5) Plate Pressure Drop

Dry plate pressure drop

Using Figure 11.34(SINNOT, 2005):

Orifice coefficient, Co = 0.84

Residual Head, (hr)

Note: An assumption of 100mm was made in order to calculate the base pressure. A change is physical properties will have little effect on the plate design. 112.5 mm per plate is considered to be acceptable (SINNOT, 2005).The pressure drop can also be expressed in

6) Downcomer Liquid Backup Downcomer pressure lossHeight of bottom edge of apron above plate, hap = hw - 10

As this value is less than Ad = 0.3054 m2, Aap is used in the equations that follow.

Since 0.2214 0.275, the chosen plate spacing of 0.5 m is acceptable. Residence timeSatisfactory since tr 3s Check entrainment

From Figure 11.29 (SINNOT, 2005),

It can be observed that the per cent flooding is also well below the design figure of 80, which means that the column diameter could be reduced. However, this would lead to an increase in pressure drop. The column diameter is maintained.7) Perforated AreaNote: 50 mm unperforated strip round plate edge is allowed and 50 mm wide calming zones

Angle subtended by the edge of the plate = 180 - 98.93 = 81.07

Total area of perforation, (Ap)

Hole pitch

Hence, triangular pitch pattern is used.From Figure 11.33 (SINNOT, 2005),

Since lp is greater than twice dh, it is acceptable.8) Number of holes

9) Hydraulic gradientThe hydraulic gradient for sieve trays is normally small but it should be checked.

From Figure 6-9 (Backhurst and Harker, 1973), at Fva = 1.63, Aeration factor, Qp = 0.62

Froth height on tray (hf)

Hydraulic radius of aerated mass (Rh)

Velocity of aerated mass, (Uam)

Where, = 0.22 has been obtained from Figure 6-9 (Backhurst and Harker, 1973), at Fva = 1.6

Reynolds Modules (Reh)From Figure 6-10 (Backhurst and Harker, 1973), sieve tray friction factor, f = 0.0205 Liquid gradient, Note: Lf = Lwg = 9.81

The value obtained clearly shows that the liquid gradient is considerably small and is thus, insignificant.10) Height of ColumnWhere,Hc = Height of ColumnNactual = Actual number of plates = 20lt = Tray spacing = 0.5 mH = Liquid hold-up (1.5 m) and vapor disengagement (1.0 m)

Plate thickness = 5 mmTotal thickness of trays = 20 0.005 = 0.1 m

EFFICIENCYThe AlChE method is adopted in order to make a rough estimate of the plate efficiency. This estimation is vital in order to obtain the actual number of stages of a given separation. Since maximum flow rates are considered in the section, conditions at the base of the column are considered.Liquid flow rate, L = 17.036 kg/sVapor flow rate, V = 3.986 kg/sThe point efficiency is first estimated for a stage where the concentration of ethanol is 0.02 wt. %.At x = 0.0002, y = 0.02 (From equilibrium plot)Assuming 60% tray efficiency, y = (0.020.6) = 0.012 Liquid propertiesBottom temperature = 100 C (Assumption)

Estimating density of liquid:From steam tables, at 100 C, specific volume of saturated liquid = 0.001043 m3/kgDensity of water at 100 C = 1/0.001043 = 958.77 kg/m3Density of ethanol at 100 C 789 kg/m3Mass of liquid = 17.036 kg/s

Estimating viscosity of liquid:Note: Viscosities are rarely additives. The viscosity for organic mixtures can be given by the modified Souders equation.

Where I = Souders index. From Table 8.1 (SINNOT, 2005), the Souderss index for each component can be estimated.For Ethanol (C2H5OH), contributions from Table 8.1:2 carbon atoms2 50.2=100.4

6 hydrogen atoms6 2.7=16.2

OH1 57.2=57.2

Total I1173.8

For Water (H2O), contributions from Table 8.1:2 hydrogen atoms2 2.7=5.4

1 oxygen atom1 29.7=29.7

Total I235.1

Surface tension of liquid: of ethanol = 22.39 mN/m (Perry, 1987) of water = 72.75 mN/mAssuming surface tensions can be additive,

Vapor properties

Estimating density of vapor:From steam tables, at 100 C, specific volume of saturated vapor = 1.6941 m3/kgDensity of water at 100 C = 1/1.6941 = 0.5903 kg/m3Density of ethanol at 100 C 1.5 kg/m3Mass of liquid = 3.986 kg/s

Estimating viscosity of vapor:Assuming viscosities of vapor can be additives,

The equation developed by Fuller at al. (1966) is used to estimate the vapor diffusivity, Dv.

From Table 8.5 (SINNOT, 2005), the special atomic diffusion volume for each component can be calculated. Diffusion volumes; EthanolElementNo. of

C16.502=33

H1.986=11.88

O5.481=5.48

50.36

Diffusion volumes; WaterElementNo. of

H1.982=3.96

O5.481=5.48

9.44

The equation developed by Wilke and Chang (1955) is used to estimate the liquid phase diffusivity, DL.

for water = 2.6 (SINNOT, 2005)Viscosity of water = 0.32 mNs/m2 (Perry, 1987)From Table 8.6 (SINNOT, 2005), the molar volume of ethanol;AtomVolumeNo. of

C0.01482=0.0296

H0.00376=0.0222

O0.00741=0.0074

Vm=0.0592 m3/kmol

Vapor and liquid load

Vapor velocity based on active area, (ua)

Column vapor factor, (Fv)

Volumetric liquid flow rate (q) across plate over average width of the plate (Lp)

The liquid hold-up on the sieve plates

Liquid contact time,(s)

Where, Dw is the Downcomer width

.Dw = (Radius of circle -h)H is calculated by considering the triangle and thus applying the tangent of the angle:h = 0.5842 mThus, Dw = (1.8/2) - 0.5842 = 0.3158 mFlow path, ZL =Dc - Dw = 1.4842 m

Number of liquid-phase transfer units

Estimating Eddy diffusivity for sieve trays

Peclet number for a tray

Estimating point efficiency

From previous calculations, R = 0.3772

From Figure 11.15 (SINNOT, 2005), Emv = 0.65 Point Efficiency = 65%Estimating Murphree Efficiency

From Figure 11.16 (SINNOT, 2005),

Murphree Efficiency = 84.5%Estimating overall column efficiency

Overall Column Efficiency = 56.9%