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Distillation Tower Design As computer technology advances, the fundamental aspects of plant design are becoming a lost art. … N.P. Lieberman, Refinery Manager, GHR Energy Inc., La The following steps are taken to design and optimize a distillation tower: R.A. Hawrelak, 22 Jan 02, CBE 497

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(a) Select a Process Sequence Consider a five component feed as shown below. Arrange in order of descending vapor pressure. C23 C320 C437 C535 C65 Total100lb moles/hr

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Process Sequence Cont’d Make a split between C3 and C4 Show this as C2, C3 / C4, C5, C6 This called a depropanizer. C3 is identified as the light key. C4 is identified as the heavy key.

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Establish Key Component Specs C3, light key composition in bottoms shall be 1.0 mole %. (2.0% sales spec) C4, heavy key composition in the overheads shall be 1.5 mole %. (3.0% sales spec).

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Set Up Mass Balance for Tower

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Mass Balance Equations

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Mass Balance Solution

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Obtain Antoine Constants Need Antoine constants for Vapor Pressure Vap Press, VP = 10^(A + B / (t°C + C)) psia

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Feed Conditions Temperature of feed = 225 deg F = deg C. Pressure of feed = psia

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Determine Bubble Point of Feed

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Determine Dew Point of Feed

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Assess Feed Condition Feed Bubble Point = deg C Feed Temp = deg C Feed Dew Point = deg C Feed temp between Bubble Pt. and Dew Pt. Feed must be in a two-phase V / L state. Special care will have to be taken for feed distributor design on feed tray.

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Determine V / L for Feed

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Solve For Ø, The Underwood Parameter Example In article by J.M. Ledanois, Hydrocarbon Processing, April, 1981, P- 231 Trial and error solution with as many solutions as there are components. Solution is a Newton convergence method. Not all cases converge.

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Solve For Ø, The Underwood Parameter, Cont’d

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Calc Minimum Reflux Ratio by Underwood See Perry VI, Chem Eng HB, Page Solution For Minimum Reflux Ratio By Solving For ∑ [Alpha*xDi / (Alpha - Ø)] = L/D min. + 1 Ø, The Underwood Parameter, was determined above.

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Calc Minimum Reflux Ratio by Underwood, Cont’d

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Determine Minimum No. Trays by Fenske - Underwood Assume top and bottom pressure equal feed pressure of psia for now. Assume overhead distillate is removed as a vapor from the condenser.

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Determine Minimum No. Trays by Fenske – Underwood, Cont’d

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Determine geometric Average Alpha between top and bottom of the tower. Geometric Avg = (Alpha Top*Alpha Btm)^0.5 Avg Alpha = ((2.51)(1.99))^0.5 = 2.23

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Determine Minimum No. Trays by Fenske – Underwood, Cont’d Min. Trays = LN((C3 lkD / C4 hkD)* (C4 hkB / C3 lkB)) / LN(Alpha Avg) Minimum No. Trays, Sm = 10.11

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Determine Trays versus Reflux Ratio by Gilliland Method Use Chang equation to represent Gilliland. Huan Yang Chang, HC Proc, Oct 1981, P- 146 A partial condenser and a reboiler represent two theoretical trays. No. trays = S – 2. Assume the economic reflux ratio is 1.2 times the minimum reflux ratio, Plot the results.

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Determine Trays versus Reflux Ratio by Gilliland Method, con’d

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Plot of Trays Versus Reflux Ratio

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Determine Feed Tray Location

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Determine Reflux Flow & Comp’n

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Calculate Overhead Vapor Flow from Top Tray = 19

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Calc Vapor Composition from Top Tray 19

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Show Molar Balance Around Top Tray 19

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Calc Dew Pt of Vapor Fr T19 and Liquid Comp’n Fr T19

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Vapor Comp’n From Tray 18

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Calc Dew Point of Vapor V18

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Design Data For Top of Tower

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Input Shortcut Tower Dia. (FWG)

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Shortcut Method by Dr Prakash

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Check Tower Mole Balance

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Calc Bubble Point of Bottoms

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Vapor Rate To Tray 1

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Final Vapor & Liquid Data to Tr 1

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Tower Diameter For Bottom Tray 1

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Shortcut Method by Dr Prakash For Bottom of Tower

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Tray Efficiency

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Tray Efficiency cont’d

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Actual No. of Trays & Feed Tray Location

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Tower Dimensions

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Vessel Specs

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Cost of Towers Database v1.1

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Cost Estimate for Tower with Trays

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Shortcut Method for Packed Towers

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Ekert Packing Factors

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Approximate HETP From Tray Tower design, TS = 18 inches. For Approximated Packed Tower Design assume one HETP = one Tray Spacing. HETP = 18 inches. Determine Tower Dimensions as for a Trayed Tower. Allow 6 ft. for feed tray and top tray for liquid distributer. No packing height should exceed 20 ft. If packing height exceeds 20 ft., must redistribute liquid which adds another 6 ft.

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FRI Packed Tower V1.2

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FRI Packed Tower Results For 2 ft. Diameter Tower

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FRI Packed Tower Results For 1.5 ft. Diameter Tower

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FRI Detailed Method for Designing a Packed Tower Select a Packing Factor from 18 selected packing types. FRI have determined the design factors which are too numerous to list here. FRI Packed Tower V1.2 will use this packing data and the other data in the shortcut method to design % Flood and estimate the HETP.

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FRI Detailed Method For PT

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FRI HETP Values for 2 ft. Diam.

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Packed Tower Cost Estimate

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Summary

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Word of Caution – Trayed Towers Towers with trays are huge mixing devices. Any slight restriction will cause flooding. Three controlling factors: (1) % Flood by Liquid and Vapor Load (2) % Spray Height by number of holes. (3) % Downcomer flood. Trays must be level and well supported.

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Word of Caution – Trayed Towers Vendors will often quote towers with many holes to reduce diameter and obtain the bid. Later on detailed design, they find they must reduce holes for specified diameter. This increases spray height beyond acceptable level and entrainment will be too high. Buyers must be aware of all design details.

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Word of Caution – Packed Towers Packed towers are low pressure drop systems. Flows don’t always go where they should. HETPs offered by vendors are optimistic. Vendors claim a wide range of operation. In actual practice there is a narrow range.

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Word of caution – Packed Towers cont’d Uniform liquid distribution is difficult. If packing ht. Exceeds 20 ft.. Liquid must be redistributed. This adds cost. Vapor is easily misdirected to walls. Vapor distributors are often required.

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Good Luck On Your Distillation Tower Design Presented to CBE Jan 02 R.A. Hawrelak

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