Distillation Column Design

REPO REP ORT RT:: DISTILLATION COLUM UMN N DES ESIIGN

Abhimany Abhi anyu u, Ankur Abhinav, Ankit An kit Sahu, Sahu, Pranj ranjal al Goswami

REPORT: DISTILL STILLAT ATION COLUMN DESIG SIGN

Table of Contents Problem Statement .......................................................................................... 3 Executive Summary ......................................................................................... 3 Design Process Proc ess Overview .............................. .............................................................. .................................................. .................. 3 Process Description .......................................................................................... 4 Design of Distillation Column ............................................................................. 4 Thermodynamics ................................ ............................................................... ........................................................... ............................ 4 Equilibrium Data .............................. ............................................................. ........................................................... ............................ 4 Vapor Liquid Equilibrium Data ...................................................................... 5 Preliminary Calculations ................................................................................. 5 Calculation of Tower Diameter (Fair’s Method) ................................ .................................................. .................. 8 Heat Exchangers: Condenser and Re-Boiler .................................................... 10 Condenser: ............................... ............................................................... ............................................................... ............................... 10 Re-boiler: ................................................................................................ 11 Height of Column ........................................................................................ 13 Thickness of Column Material .............................. .............................................................. .......................................... .......... 13 Thickness of Tori-spherical Head ................................................................... 14 Weight of Tower To wer Material Materia l.............................. .............................................................. ................................................ ................ 14 Cost Data ................................................................................................... 14 Evaluation of Annualized Fixed Cost and Annual Operating Cost: .................... 15 Optimization ............................................................................................... 15 Hydraulic Design ......................................................................................... 16 Bubble Cap Trays: .................................................................................... 16 Nozzles ................................................................................................... 16 Design of Support S upport ................................ ................................................................ ..................................................... ..................... 18 Pressure Drop Calculations ........................................................................... 20

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REPORT: DISTILLATION COLUMN DESIGN

Problem Statement A process stream of a mixture of benzene and toluene having 50 %(w/w) composition of benzene has to be concentrated by Distillation. The distillate product stream has to contain 95% (w/w) Benzene. The bottom product stream should contain maximum 3% benzene in it. The operating pressure for the operation is 1.1 atm. Design the distillation column using data supplied and consulted reference handbooks.

Executive Summary This report consists of a detailed design of a binary distillation column to increase the concentration of benzene from 50% (w/w) in the feed to 95% (w/w) under given process conditions. The design procedure required determination of the optimum reflux ratio and the corresponding number of stages required for achieving the specified concentration of product. The optimum value for R/R min was found to be 1.1. Calculations corresponding to this ratio yielded the design of a distillation column which is 9.5 meters high, 2.7 meters wide with 19 Bubble cap trays. Each bubble-cap tray consists of 166 openings arranged in a triangular pitch.

Design Process Overview The optimization was carried out based on economic considerations of the design. The Annualized Fixed Cost and Operating cost for several reflux ratios were evaluated and plotted. It must be noted that the optimization parameters include the cost of column, trays, and heat exchange units only along with the utility costs. Following the determination of an optimum reflux ratio, the type of tray was selected after consideration of economic and operational parameters. Further, the theoretical number of stages required was estimated graphically using McCabe-Thiele method.

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The column height was then determined using the actual number of trays required and a fixed tray spacing. Next, the dimensions of the column heads were calculated followed by design calculations for tray and nozzles. Hydraulic design for the column was done after estimating the pressure drop. Check was made to avoid weeping, down-comer flooding and entrainment conditions for the optimum design. After the calculations, the support for column was designed using standard procedure for supports of tall vessels. The data used in the design procedure were noted from reliable sources acknowledged in the bibliography.

Process Description The feed containing 50% (w/w) mixture of benzene and toluene is available at ambient temperature. **Incomplete

Design of Distillation Column This section contains the detailed design process (mechanical and hydraulic) of binary distillation tower for separation of benzene and toluene.

Thermodynamics Equilibrium Data

x y

1 0.82 0.66 0.50 0.38 0.28 0.15 0.06 0.0 1 0.92 0.83 0.72 0.60 0.45 0.30 0.13 0.0

[Provided Data]

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Vapor Liquid Equilibrium Data

The vapor-liquid equilibrium data can be obtained using Antoine’s Equation given as

   –  Using the above equation, we obtain the T-x-y data.

T ( oC) X y

85 0.772 0.896

90 0.524 0.772

95 0.404 0.626

100 0.257 0.457

105 0.127 0.259

Preliminary Calculations Feed is saturated liquid mixture(saturation temperature = 92oC) F = 75 ton/hr

ρ = 871.28 kg/m3

F= 75000kg /hr => F = 86.1 m3/hr Molecular Weight of feed (MF): MF = xFMA + (1-xF)MB = 0.5(78) + 0.5(92) = 85 kg/kmol F = 882.35 kmol/hr

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F=D+W FxF = DxD + WxW Given: xD = 0.95, xW = 0.03, xF = 0.5 D = 450.77 kmol/hr W = 431.58 kmol/hr

Mavg|distillate = 0.95(78) + 0.05(92) = 78.7 kg/kmol Mavg|bottom =0.03(78)+ 0.97(92) = 91.58 kg/kmol

D = 35475.6 kg/hr = 450.77 kmol/hr W = 39524.1 kg/hr = 431.58 kmol/hr

Feed Stream Distillate Bottom Product

Molar Flow Rate

Mol. Weight

Mole Fraction

F = 882.35 kmol/hr D = 450.77 kmol/hr W = 431.58 kmol/hr

85 kg/kmol 78.7 kg/kmol 91.58 kg/kmol

xF = 0.50 xD = 0.95 xW = 0.03

Graphically, we obtain the minimum Reflux Ratio (R min )

     Rmin = 1.043

∴

Now, we consider R / R min = 1.1 ∴

R = 1.1473

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Enriching Section Molar flow rate of liquid = L L=R*D L = 1.1473 * 450.77 L = 517.17 kmol/hr

Molar flow rate of vapour, V V = (R+1)*D = (1.1473 + 1)*450.77 V = 967.94 kmol/hr Stripping Section Molar flow rate of liquid = L’ L’ = R D + F L’ = (1.1473 * 450.77) + 882.35 L = 1399.52 kmol/hr Molar flow rate of vapour, V V ‘= (R+1)*D V’ = V (∵ Feed is saturated liquid ) V’ = V = 967.94 kmol/hr

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Calculation of Tower Diameter (Fair’s Method)

ρbenzene = 876.5 kg/m3 ρbenzene = 866.9

kg/m3

               = 875.94 kg/m3

L1

                = 867.15 kg/m3

L1

Average liquid density,

L

= 871.54 kg/m3

Now, PMD =

PMB =





GB

RT (for vapor coming from tray 1)

GD



GD

= 11.55 kg/m3

RT (for vapor coming from bottom tray)



GD

= 11.55 kg/m3

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Calculation of Operative Velocity

Using the viscosity data of the components and the operating flow rates of the streams, the superficial velocity is obtained from the CSB vs FLV Graph (BK Dutta P/194) US,LV = 2.2 ft/s Operating Velocity = 0.7(2.2) = 1.54 ft/s (Assuming Op. Vel. = 70% of superficial velocity) ∴

Operating Velocity = 0.47 m/s

Estimating Tower Diameter:

Vapor Flow rate, V = 967.94 kmol/hr Mass flow rate of vapor (top) = V M avg|distillate = 76176.88 kg/hr Mass flow rate of vapor (bottom) = V M avg|bottom = 88643.945 kg/hr Average mass flow rate of vapor = 82410.4 kg/hr Avg. density of vapor = 10.84 kg/m 3 Volumetric flow rate of vapor = 7602.44 m3/hr

         = 4.49 m2 Assuming frictional downcomer area,f d = 0.2 Cross Sectional area of Tower, AT = Aa/(1-f d) AT = 5.62 m2 Diameter of Tower [D i] = 2.67 m 9


Heat Exchangers: Condenser and Re-Boiler Condenser: From T-x-y plot, Dew point of vapor entering condenser = 83oC Bubble point of distillate = 81oC

Vapor Dew Point = 83oC

Spent Cooling Water 45oC

Cooling Water 32oC

Condenser

Liquid Bubble Point = 81oC

LMTD = 43.27oC Enthalpy Balance: VHV = QC + VHL HV = xD[CPA(T-TD) + λA] + (1 - xD)[CPB(T-TB)+λB]

= Hv = 41789.82 kJ/kMol (Reference temperature = 0 oC)

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HL = xDCPA(T-To) + (1 - x D)CPB(T-To)

= Hv = 11004.5 kJ/kMol (Reference temperature = 0 oC) Condenser Heat Load = V(HV - HL) QC = 8277317.944 J/s

Ucondenser*A*(LMTD) = Qc U condenser = 851 W/m2K Acondenser = 224.79 m2 (area of condenser) mcw * Cpw

*

∆T = Qc

mcw = 152.11 kg/s Mass of cooling water req. in 1 year = 4490287.2 m3/year (Assuming plant operation to be 8200 hrs/year)

Re-boiler: Vapor Dew Point = 110oC

Condensed Water 160oC

Saturated Steam 160oC @ 5 atm(g) Re-Boiler

Bottom Liquid Bubble Point = 109oC

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LMTD = 50.50oC Enthalpy Balance: FHF + QB = QC + DHD + WHW HF = xFCPA(T-To) + (1 – xF)CPB(T-TB)

= HF = 13374.96 kJ/kMol (Reference temperature = 0 oC)

HW = xWCPA(T-To) + (1 – xW)CPB(T-To)

= HW = 16930.45 kJ/kMol (Reference temperature = 0 oC) HD = HL = 11004.5 kJ/kMol ….(from previous calculations)

Re-Boiler Heat Load = Q B QB = 30264290.72 kJ/hr ms*λ = QB λsteam

= 2758 kJ/kg

ms = 10973.27 kg/hr Mass of cooling water req. in 1 year = 89980849.86 m 3/year (Assuming plant operation to be 8200 hrs/year)

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Ure-boiler* A*(LMTD) = QB U re-boiler = 1845.45 W/m2K Are-boiler = 90.2 m2 (area of condenser)

Height of Column The number of stages required for desired concentration of product is estimated graphically using the McCabe Thiele Method for the above calculated parameters. No. of actual stages = 19 Selected tray spacing = 18’’ = 0.457 m (top spacing= 0.4m ; bottom spacing – 0.85m) Height of column = (19-1)*0.457 + 0.4 + 0.85 = 9.476 m

Thickness of Column Material (Reference – Process Equipment Design | B.C. Bhattacharya)

f = allowable stress = 98.1 x 10 6 N/m2 (IS:2825-1969) J = 0.85

    t = 1.7 mm + 2 mm(corrosion allowance) t = 3.7 mm Next standard thickness available = 5mm

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Thickness of Tori-spherical Head (Reference – Process Equipment Design | B.C. Bhattacharya)

f = allowable stress = 98.1 x 10 6 N/m2 (IS:2825-1969) J = 0.85

     t = 5.34 mm + 2 mm(corrosion allowance) t = 7.34 mm Next standard thickness available = 8 mm

Weight of Tower Material Volume of material = 0.5784 m3 Mass of Tower material = 4540.4 kg Mass of Tower material (including trays) = 1.3 * 4540.4 = 5902.6 kg

Cost Data Following are the costs for material and utilities. Particulars Cost of Steel Cost of Cooling Water Cost of Steam Cost of fabrication

Cost 9690 INR/MT 320 INR/1000m3 485 INR/MT 60% of Material Cost

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Evaluation of Annualized Fixed Cost and Annual Operating Cost:

Particulars

Cost

Cost of Cooling Water per Year (INR/Year) Cost of Steam per year (INR/Year) Cost of Tower Materail (INR) Cost of Condenser

Rs. 14,36,927 Rs. 4,36,42,840 Rs. 82,241 Rs. 20,07,949 Rs. 18,45,226

Cost of Boiler Total Fixed Cost(Without Piping/Insulation) Total Fixed Cost(INR) Annual Fixed Cost (INR/year) Annual Operating Cost (INR/year) Total Annual Cost (INR/Year)

Rs. 39,35,417 Rs. 53,12,813 Rs. 9,03,178 Rs. 4,50,79,768 Rs. 4.59,82,946

Optimization Following the calculations of one value of R/R min the annual cost was evaluated programmatically for all other values of R/Rmin R/Rmin 1.1 1.2 1.3 1.4 1.5 1.6 1.7

Total Annual Cost INR 4,66,46,436 INR 4,98,86,241 INR 5,20,47,051 INR 5,42,07,792 INR 5,63,69,152 INR 5,85,29,777 INR 6,06,90,341

The Optimum R/Rmin was found to be 1.1 15


Hydraulic Design Bubble Cap Trays: Bubble cap trays selected for the binary distillation column. Following is the calculation for Cap distribution Tower Diameter = 2.67 m Ref. B.D. Smith Selected: Carbon Steel 4 in. bubble caps Downflow area = 10% of Tower cross-section Liquid Distribution area = 5% of Tower cross-section End Wastage = 10% of Tower cross-section

Allocated bubble cap area = 100- (2x10 – 2x5 – 2x10) = 0.5 * 5.62 m2

Cap- distribution: Pitch = D + Clearance = 4 + 1.5 = 5.5 inch No. of Bubble (caps arranged in triangular pitch) = 166 Nozzles

Feed Nozzle : Feed rate = 86.1 m3/hr Velocity = 1.5 m/s Flow area required = 0.016 m2 16


Taking NPS = 6’’ SC 40 OD = 0.168 m ID = 0.161 m Area = 0.02 m2 Velocity = 1.2 m/s Top Nozzle: Vapor Flow rate = 7602.44 m3/hr Velocity = 25 m/s Flow area required = 0.0845 m2 Taking NPS = 14’’ SC 40 OD = 0.355 m ID = 0.346 m Area = 0.094 m2 Velocity = 22.5 m/s Bottom Nozzle: Bottom Product Flow rate = 46.6 m3/hr Velocity = 1 m/s Flow area required = 0.0127 m2 Taking NPS = 5’’ SC 40 OD = 0.141 m ID = 0.134 m Area = 0.094 m2 Velocity = 22.5 m/s

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Design of Support

Cylindrical skirt support is selected as the vessel support. Skirt height = 2 m Minimum weight of vessel with two heads and shell, Wmin = 44.62 kN Now, Wmax = Wshell + Winsulation + Wwater_during_test + Wattachments Wmax = 249.734 kN

Period of vibration at minimum dead weight, Tmin

                Tmin < 0.5 s ∴

K2 = 1

Period of vibration at maximum dead weight, Tmax

                Tmax < 0.5 s ∴

K2 = 1

Evaluation of Wind Load

PW = K1K2 p H D Hence, PW,min = 21.58 kN 18


PW,max = 22.786 kN

Minimum wind moment MW,min = PW,min

 

Maximum wind moment MW,max = PW,max

= 123.826 kJ

 

= 130.746 kJ

Now,

  

σZWM,min =

, where t is thickness of skirt.

σzwm(min) = 22.06/t kN/m2 σzwm(max) = 23.3/t kN/m2

Minimum and maximum dead load stresses: σzw(min) = Wmin/πdt = 5.29/t kN/m 2 σzw(max) = 29.63/t kN/m2

Maximum tensile stress σz(tensile) = σzwm(min) - σzw(min) = 16.77/t kN/m2 Substituting, σz(tensile) = fJ = 67200 kN/m2 Equating, 67200 = 16.77/t t= 0.249 mm

Maximum compression load σz(compression) = σzwm(max) - σzw(max) = 52.93/t kN/m2 σz(compressive) = 0.125E



= 9314456 t kN/m2

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Equating, 9314456 t =52.93/t t= 2.384 mm Following IS: 2825-1969 , minimum corroded skirt thickness is 7 mm. Providing 1 mm corrosion allowance, a standard 8 mm thick plate can be used for skirt.

Pressure Drop Calculations Pressure drop

Tray

–

               ρv = 10.84 kg/m3 ρl = 871.54 kg/m3 Q = vapor flow rate in fps = 74.5 ft3 /sec Ar = riser area = 5.53 ft2 ht = 3.39’’ Total pressure drop across Column = 64.5’’ of liq = 0.138 atm

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Distillation Column Design

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