Quench Tower Design [PDF]

Citation preview

Table of Contents 1.0 DESIGN OF A QUENCH TOWER..................................................................................4 1.1 Problem Statement........................................................................................................4 1.2 General Overview on Quench Towers..........................................................................4 1.2.1 Spray towers...........................................................................................................4 1.2.2 Venture scrubber.....................................................................................................5 1.2.3 Packed tower...........................................................................................................6 1.3 MATERIAL BALANCE...............................................................................................6 Chemical engineering design..............................................................................................8 1.4.1 The density of the gas mixture is calculated as......................................................8 1.4.2 The volumetric flowrate of the gas (QG) can be calculated by...............................9 1.4.3 The ratio of the liquid mass flow rate to the gas mass flow rate is given by..........9 1.4.5 Calculation of pressure drop at flooding..............................................................10 1.4.6 Superficial gas velocity calculation......................................................................10 1.4.7 The diameter of the column can be calculated from.............................................11 1.4.8 The wall factor can be important for columns with an inadequate ratio of effective particle diameter to inside column diameter, and is given by:.......................12 1.4.9 The effective particle diameter, dp, is given by.....................................................12 1.4.10 The Reynolds number of the gas can be calculated as.......................................13 1.4.11 Calculation of dry-gas-pressure drop..................................................................13



1



1.4.12 The liquid mass velocity can be calculated as....................................................14 1.4.13 The Froude number of the liquid can be calculated as:......................................14 1.4.14 Calculation of specific liquid holdup.................................................................15 1.4.15 Calculation of pressure drop when the bed is irrigated......................................16 1.4.16 Height Equivalent of Theoretical Plate (HETP).................................................16 1.4.17 Number of Transfer Units (NTU).......................................................................17 1.4.18 Height of Overall Gas Transfer Unit (HOG)......................................................18 1.4.19 COLUMN HEIGHT...........................................................................................18 1.5 MECHANICAL ENGINEERING CALCULATIONS...............................................20 1.5.1 Design Pressure....................................................................................................20 1.5.2 Design Temperature..............................................................................................20 1.5.3 Minimum Vessel Thickness..................................................................................20 1.5.4 Dead Weight of Vessel..........................................................................................21 1.5.5 Weight of Empty Vessel........................................................................................21 1.5.6 Wind Loading.......................................................................................................21 1.5.7 Analysis of Stress..................................................................................................22 1.5.8 Dead-Weight Stress...............................................................................................22 1.5.9 Total Longitudinal Stress......................................................................................23 1.5.10 Maximum Stress Intensity..................................................................................23 1.5.11 Vessel Support.....................................................................................................23 REFERENCES..................................................................................................................28 2



Tables Table 1.1 Inlet stream of quench tower……………………………………………………...6 Table 1.2 Outlet 1 of quench tower………………………………………………………….7 Table 1.3 Outlet 2 of quench tower………………………………………………………….8 Table 1.4 Summary of chemical engineering design of quench tower…………………….19 Table 1.5 Summary of mechanical engineering calculations………………………………27



3



1.0 DESIGN OF A QUENCH TOWER 1.1 Problem Statement To Design a quench tower to cool hot gases flowing at a rate of 5730 kg/h using water flowing at 18400 kg/h. 1.2 General Overview on Quench Towers Quenching of reactor products is sometimes needed for sudden cooling, for removing impurities and to avoid side reactions. Cooling by liquid quenching is essentially accomplished by introducing the hot gases into a liquid contacting device. When the liquid evaporates or gets heated up, the energy necessary to heat up the liquid is obtained at the expense of hot combustion gases, resulting in the reduction of gas temperature. The temperature of the gases discharged from the quencher is at the adiabatic saturation temperature of the gases if the operation is adiabatic and the gas leaves the quencher with little or no water vapours. There are 3 types of quenchers   



Spray towers Venturi scrubbers Packed towers



1.2.1 Spray towers Spray towers or spray chambers consists of empty cylindrical vessels made of steel or plastic and nozzles that spray liquid into the vessels. The inlet gas stream usually enters the bottom of the tower and moves up, while the liquid is sprayed downward from one or more levels. This flow of inlet gas and liquid in the opposite direction (counter current flow), exposes the gas to the liquid, thereby enhancing the heat transfer. 1



Many nozzles are placed across the tower at different heights to spray all of the gas as it moves up through the tower. The reason for using many nozzles is to maximize the heat transfer. The liquid droplet must be large enough not to be carried out of the scrubber by the scrubbed outlet gas. Advantages of spray tower 



The design is completely open. It is simple to construct. This feature eliminates



 



many of the scale buildup and plugging problems associated with other scrubbers. This is an inexpensive and control device primarily used for gas conditioning. Very little space is required and only that amount of water that is needed to maintain







the desired temperature of the gases at the discharge is used. Its installation and operating cost are generally considered to be less than that of other cooling methods.



1.2.2 Venture scrubber A venturi scrubber accelerates the gas stream to atomize the scrubbing liquid and to improve gas-liquid contact. In a venturi scrubber, a throat section is built into the duct that forces the gas stream to accelerate as the duct narrows and then expands. As the gas enters the venturi throat, both gas velocity and turbulence increases. Depending on the scrubber design, the scrubbing liquid is sprayed into the gas stream before the gas encounters the venturi throat, or in the throat, or upwards against the gas flow in the throat. Disadvantage of venturi scrubbers   



Contact area available for water and gas is less. Construction is complex. Large amount of water is required for cooling.



2



1.2.3 Packed tower Packed bed quenchers consist of a chamber containing layers of variously shaped packing materials, such as Raschid rings, interlock saddles, pall ring, berl saddles, that provide large surface area for liquid gas contact. The packing is held in place by wire mesh retainers and supported by a place near the bottom of the scrubber. Cooling liquid is evenly introduced above the packing and flows down through the bed. Quench towers can either be cooled by a water or oil medium which gives the name quench-water tower or quench-oil tower. Their function is to cool the superheated gas in order to eliminate any further reaction that may occur and to also decrease the temperature of the gas.



1.3 MATERIAL BALANCE Table 1.1 Inlet stream of quench tower COMPONEN



MASSS



MASS



T



FLOW,kg/hr



FRACTION



H2 + CH4



542



9%



Ethylene



1630



28%



Propylene



168



3%



Butadiene



39.6



1%



butenes



44.9



1%



C5+



109



2%



N2



83.1



1%



CO2



194



3%



Mixed



3



H2O



1780



31%



ethane



1060



18%



propane



70.9



1%



butane



9.11



0%



TOTAL



5730



100%



Table 1.2 Outlet 1 of quench tower COMPONEN



MASSS



MASS



T



FLOW,kg/hr



FRACTION



H2 + CH4



5.42E+02



14%



Ethylene



1.63E+03



41%



Propylene



1.68E+02



4%



Butadiene



3.96E+01



1%



butenes



4.49E+01



1%



C5+



1.09E+02



3%



N2



8.31E+01



2%



CO2



1.94E+02



5%



ethane



1.06E+03



27%



propane



7.09E+01



2%



butane



9.11E+00



0%



TOTAL



3.95E+03



100%



Mixed



Table 1.3 Outlet 2 of quench tower COMPONE



MASSS



MASS 4



NT



FLOW



PERCENTA



kg/hr



GE



WATER



18400



100%



Chemical engineering design Parameters to be calculated are:            



The superficial gas velocity The diameter of the column The dry-gas-pressure drop The liquid holdup in the column The actual pressure drop when the bed is irrigated The overall gas-phase transfer units The height of the gas-phase transfer unit The height of the liquid-phase transfer unit The overall height of a gas-phase transfer unit The packed height Residence time Data: The packing used is 50mm metal pall ring random packing Cp = is a packing constant, 0.763, a = specific surface area of packing, 112.6 m2/m3, ɛ = packing void fraction, 0.951, FP = packing factor, 27m2/m3, Ch = is a characteristic of the particular type and size of packing, 0.784



Mass flowrate of gas, G = 5370 kg/h = 1.5 kg/s 1.4.1 The density of the gas mixture is calculated as ρg =



P×M R ×T



ρg =



2.3 × 68.26 0.08206× 613



(Perry et al, 1997)



ρg = 3.73 kg/m3



5



1.4.2 The volumetric flowrate of the gas (QG) can be calculated by G ρg



QG =



Where, ρG = gas density = 3.73 kg/m3 G = gas mass flowrate = 1.5 kg/s



QG =



1.5 3.73



= 0.402 m3/s 1.4.3 The ratio of the liquid mass flow rate to the gas mass flow rate is given by L G



=



5.11 1.5



= 3.41



Where, L = liquid flowrate = 5.11 kg/s Flooding data for quench columns with countercurrent flow of gas and liquid can be correlated in terms of the flow parameter(X) given by



X=



ρG L ¿ G ( ρL



3.73 ¿ X = 3.41 ( 1000



0.5



0.5



= 0.208



Flooding curve in quench tower can be accurately described by the polynomial regression lnYflood = [3.50221+1.028lnX+ 0.11093(lnX)2] 6



(Leva,1954)



2



−[ 3.50221+1.028 lnx+ 0.11093( lnx) ] Yflood = e



Yflood = 0.115



Csflood = (



µL ¿ ¿ F p¿ Y flood ¿



)0.5



(Leva,1954)



Where, FP = packing factor, 27m2/m3 (Wiley and Jaime, 1987) µL = liquid viscosity, 0.001Pa-s (Sinnot, 2005)



Csflood = (



0.115 0.5 27 × 0.0010.1 ) = 0.092 m/s



1.4.4 Calculation of the superficial gas velocity at flooding The superficial gas velocity can be calculated as



VGF



ρG ¿0.5 ρL −ρG ¿ Csflood ¿ ¿



(Leva, 1954)



Where, VGF = the superficial gas velocity at flooding, m/s



VGF =



3.73 1000−3.73 ¿ ¿ ¿ 0.092 ¿



= 1.5 m/s



The superficial gas velocity at flooding is 1.5 m/s 7



1.4.5 Calculation of pressure drop at flooding The pressure drop at flooding is strongly dependent on the packing factor for both random and structured packing and it is given by the empirical expression: ∆Pflood = 93.9(FP)0.7 (Kister and Gill, 1991) Where ∆Pflood has units of Pa per meter of packed height ∆Pflood = 93.9(27)0.7 = 943.236 Pa/m of packing 1.4.6 Superficial gas velocity calculation For a given fluid flow rates and properties, and a given packing material, superficial gas velocity can be calculated from the expression given by: VG = VGF × f



(Wiley and Jaime, 1987)



Where, VG = superficial gas velocity, m/s f = a fraction of flooding and is usually 0.7 for quench towers (Wiley and Jaime,1987) VG = 1.5 × 0.7 = 1.05 m/s Hence the superficial gas velocity, VFG = 1.05 m/s 1.4.7 The diameter of the column can be calculated from 4 × QG D = ( f ×V gf × π )0.5



4 ×0.402 D = ( 0.7× 1.05 × π



(Kister, 1992)



)0.5



= 0.83 m



D = 0.83 m



8



Hence the diameter of the column is 0.83 m The area of the column can be calculated as: 2



A=



π ×D 4



A=



π ×(0.83)2 4



= 0.541 m2



1.4.8 The wall factor can be important for columns with an inadequate ratio of effective particle diameter to inside column diameter, and is given by: 1 =¿ KW



1+



2 1 ( 3 1−ɛ



dp ) D



(Leibson et al, 1956)



Where, ɛ = packing void fraction = 0.951 (Wiley and Jaime,1987) Kw = wall factor 1.4.9 The effective particle diameter, dp, is given by dp = 6(



1−ɛ a



) (Leibson et al, 1956)



Where, dp = the effective particle diameter, m a = specific surface area of packing, 112.6 m2/m3 (Wiley and Jaime,1987)



dp = 6(



1−0.951 112.6



1 =¿ KW



1+



) = 0.0026110



2 1 ( 3 1−0.951



)



0.0026110 0.83



9



1 KW



= 1.043



KW = 0.959 1.4.10 The Reynolds number of the gas can be calculated as ReG =



v G ×d p × ρG × K W (1−ɛ)(µG )



Where, µG = kinematic viscosity of the gas mixture, 3×10-5Pa.s



ReG =



1.05 × 0.0026110 ×3.73 ×0.9590 (1−0.951)(0.00003)



The Reynolds number of the gas ReG = 6671.24 The dry-packing resistance coefficient (a modified friction factor), is given by the empirical expression: 1.8 64 Ψ = Cp ( R eG + ( ReG )0.08 )



(Leibson et al, 1956)



Where, Ψ = the dry-packing resistance coefficient (a modified friction factor) Cp = is a (packing constants) characteristic of the particular type and size of packing = 0.763. (Wiley and Jaime,1987) 1.8 64 Ψ =0.763 ( 6671.24 + (6671.24)0.08 ) = 0.686



The dry-packing resistance coefficient = 0.686



10



1.4.11 Calculation of dry-gas-pressure drop The dry-gas-pressure drop can be calculated from the dimensionally consistent correlating equation given by: V ∆ Po Z



(¿¿ G)2 Ѱ × a× ρ × G = ( ɛ)3 ×2 × K w ¿



(Stichlmair et al, 1989)



Where, Z= packing height, m △PO = the dry-gas-pressure drop, Pa ∆ Po Z



2



=



0.686× 112.6 ×3.73 ×(1.05) (0.951)3 ×2 ×0.9900



Hence the dry-gas-pressure drop,



∆ Po Z = 177.39 Pa/m



1.4.12 The liquid mass velocity can be calculated as



Gx =



L 2 π ×(D) 4



(Seader and Henley, 1998)



Where, Gx = liquid mass velocity, kg/m2.s



Gx =



5.11 2 π ×(0.83) 4



= 9.444 kg/m2.s



1.4.12 The Reynolds number of the liquid can be calculated as:



11



ReL =



ReL =



Gx a× µ L



Gx a× µ L



(Seader and Henley,1998)



=



9.444 112.6 ×0.001



= 83.87



Hence the Reynolds number of the liquid ReL = 83.87 1.4.13 The Froude number of the liquid can be calculated as: FrL =



G x2 × a g



(Seader and Henley,1998 )



Where, FrL = Froude number of the liquid g = acceleration due to gravity, 9.81m/s2 (Wiley and Jaime,1987) 2



FrL =



G x × 112.6 2



100 × 9.81



FrL = 0.1028 For ReL ≥ 5, the ratio of specific areas is given by : R eL ¿ ¿



ah =0.85 Ch ׿ a



(Seader and Henley)



Where, Ch = is a (packing constant) characteristic of the particular type and size of packing = 0.784. (Wiley and Jaime,1987)



12



ah = hydraulic, or effective, specific area of packing, m2/m3 R eL ¿ ¿



ah =0.85 Ch ׿ a



Therefore, the ratio of specific areas is



ah =¿ 1.606 a



1.4.14 Calculation of specific liquid holdup The specific liquid holdup (i.e. volume of liquid holdup/volume of packed bed) in the preloading hL



=(



region can be calculated from the dimensionless expression: 12 F rL ReL ¿¿



1 3



(



ah a ¿¿



2 3



(Billetand Schultes,1995)



Where, hL, = specific liquid holdup, m3 holdup/m3 packed bed VL = superficial liquid velocity, m/s



hL



=(



12(1.028) 83.87 ¿¿



1 3



2



( 1.606 ¿ 3



= 0.724



h L = 0.724



Hence the liquid holdup in the column is = 0.724



13



1.4.15 Calculation of pressure drop when the bed is irrigated When the packed bed is irrigated, the liquid holdup causes the pressure drop to increase. The Correct pressure drop for liquid holdup is calculated with the equation



∆P ɛ =¿ ( ɛ−hL ) exp( ∆ PO



R eL 200 (Billet and Schultes) ¿ ¿1.5



Where, △P = Actual pressure drop when the bed is irrigated, Pa ∆P 0.951 =¿ ( ∆ PO 0.951−0.724 ) exp (



∆P ∆ PO



83.87 200 ¿¿ 1.5



= 5.5



= 5.5



The actual pressure drop when the bed is irrigated is therefore ∆P Z



= 645.24 Pa/m



1.4.16 Height Equivalent of Theoretical Plate (HETP) HETP is calculated as;



HETP =



A



−0.19



0.21



[ ] [ ] σ 20



μ 0.2



Where, A = Size of packing σ



= 50 mm



= surface tension of liquid = 69.8 mN/m 14



D = 0.83 m μ = Overall viscosity of feed stream = 0.0006 Pa s



−3 69.8 HETP = 50 ×10 20



−0.19



( ) (



0.0006 0.2



0.21



)



HETP ¿ 0.0116 m 1.4.17 Number of Transfer Units (NTU) Number of transfer units is given by;



NTU =



[



x −y 1 ln ( 1−β ) 2 1 + β 1−β x 1− y 1



]



Where, β



= L/HG



= 0.000381



L = Molar liquid flow rate



= 1022.22 kmol/h



G = Molar gas flow rate



= 78.74 kmol/h



H = Henry’s Law Constant



= 3410Pa/mol fraction



x2 = Solute contents in liquid inlet stream mole fraction = 0.00 x1 = Solute contents in liquid exit stream mole fraction = 0.00 y1 = Solute contents in gas at bottom mole fraction Substituting the above values into equation (5);



15



= 0.00



NTU = 4.3 NTU = 5 1.4.18 Height of Overall Gas Transfer Unit (HOG) Height of overall gas transfer unit is given as;



Hog =



1 −1 β HETP 1 ln β



( ( ))



(6)



Hog = 2.01 m 1.4.19 COLUMN HEIGHT Packing height is calculated as; Htotal = Hog x NTU Htotal = 2.01 x 5 Htotal = 10.05 m Giving 0.457 allowance for disengagement of vapors at top and at bottom for liquid, Htotal = 10.51 m Therefore, total height of tower = 10.51 m



16



Table 1.4 Summary of chemical engineering design of quench tower Parameter



value



The superficial gas velocity, m/s



1.05



The diameter of the column, m



0.83



The dry-gas-pressure drop, Pa/m



117.4



The liquid holdup in the column



0.724



The actual pressure drop when the bed is irrigated, Pa/m



645



Number of transfer units



5



The overall height of a gas-phase transfer unit, m



2.01



The packed column height, m



10.51



1.5 MECHANICAL ENGINEERING CALCULATIONS The material of construction is carbon steel. 1.5.1 Design Pressure Design pressure (Pi) is taken as 110% of operating pressure (Sinnott, 2005a) :



Pi =



110 100 × 233.047 kPa = 256.352 kPa 17



1.5.2 Design Temperature Highest Operating temperature = 340oC 1.5.3 Minimum Vessel Thickness



Pi D i , 2f i - Pi e=



(Sinnott, 2005b)



Where, Di is the internal diameter = 0.83 m = 830 mm e is the minimum thickness required f is the design stress of stainless steel at 340oC = 100 N/mm2 (Sinnott, 2005c) Pi is the internal design pressure of the shell = 256.352 kPa = 0.256 N/mm2



 0.256  830 (2  100)  0.256 e=



 2 mm



Allowing a corrosion allowance of 2 mm (Sinnott, 2005d), the minimum thickness required to withstand internal pressure is 4 mm. 1.5.4 Dead Weight of Vessel The major sources of dead weight for the unit are;  



The weight of the empty vessel (Wv) The weight of the material (Wm)



1.5.5 Weight of Empty Vessel



 C v  π  ρ m  g  D m  H v  0.8D m  t Wv



(Sinnott, 2005e) 18



Where, CV = factor to account for the weight of nozzles, man ways, internal supports etc, which can be taken as 1.08 for vessels with few fittings. Hv = height of the cylindrical section, 10.51 m g = gravitational acceleration = 9.81 m/s2 t = wall thickness of vessel = 4 mm = 0.004 m ρm = density of vessel material (carbon steel) = 8000 kg/m3



Dm = width of vessel = 0.83 + 2(0.004) m = 0.838 m Wv = 1.08 π × 8000 × 9.81 × 0.838 [10.521 + (0.8 × 0.838)] × 0.004 = 9988.98 N Thus, dead weight of vessel = 9988.98 N 1.5.6 Wind Loading Bending stresses result from the bending moments to which the vessel is subjected. Bending moments will be caused by the wind loads on tall self-supported vessels, dead weight and wind loads on piping and equipment which is attached to the vessel, but offset from the vessel centre line (Sinnott, 2005f).



Dynamic wind pressure (



Pw



) is 1280 N/m2 (Sinnott, 2005g).



W (loading per unit length)=Pw Deff =1280 ( 0.838 ) =1072.64 N /m



Bending moment at bottom tangent line, Mx = (1072.64/2) x 10.512 = 59241.96 Nm



19



1.5.7 Analysis of Stress 1.5.7.1 Longitudinal Stress Longitudinal stress due to pressure is given by Pi d i 4t



σl 



(Sinnott, 2005h)



Hence,



σl 



0.256 x 830  13.28 N/mm 2 4x4



1.5.7.1 Circumferential Stress Circumferential stress due to pressure is given by Pi d i 2t σ h  2  13.28 σh  2  i 



σh



=



26.56



N/mm2



1.5.8 Dead-Weight Stress Dead-weight stress of the vessel is given as



σw 



WT π D i  t  t (Sinnott, 2005i)



Where WT is the total weight which is supported by the vessel wall



20



σw 



9988.98  0.953 N / mm2 π 830  4  4



1.5.9 Total Longitudinal Stress σ Total axial or longitudinal stress ( z )



σ c ( compressive )=σ l + σ w =13.28+ 0.953=14.233 N /mm2



(Sinnott, 2005j)



1.5.10 Maximum Stress Intensity σ s ( tensile ) =σ h−σ z=26.56−14.233



¿ 11.33 N /mm2



The maximum allowable stress for the material of construction is 100 N/mm 2. Since this stress is higher than the maximum stress intensity at any point in the material, the design is not prone to failure under stress. 1.5.11 Vessel Support The support system designed for a separator and all tall vessels depends on the size, shape, and weight of the vessel; the design temperature and pressure; the vessel location and arrangement; and the internal and external fittings and attachments. A skirt support is used for vertical columns. Its thickness is designed to withstand the deadweight loads and bending moments imposed on it by the separator. (Sinnott, 2005k) 1.5.11.1 Skirt Support Thickness Data 21



Specified skirt angle = 90 °C (straight cylinder skirt)



Maximum allowable stress,



(σ max )



and Young’s modulus, (E) at ambient conditions are



165 N/mm2 and 11350 N/mm2 respectively. The maximum dead-weight load on the skirt will occur when the vessel is full of water, Approximate Maximum Dead-Weight = (Di2 x Hv x (π/4) x ρw x g) ρw = density of water g = acceleration due to gravity Approximate Maximum Dead-Weight = 0.832 x 10.51 x (π/4) x 1000 x 9.81 = 55785.05 N Dead-weight of vessel = 9988+ 55785.05 N = 65773.045 N Wind loading = 1072.64 N /m Bending moment at base of skirt = 59241.96 Nm Assuming a skirt thickness of 20 mm



σ bs=



4 Ms π ( Ds +t s ) t s



,



σ ws =



W π ( D s +t s ) t s



Where, σ bs = bending stress in the skirt 22



Ms = maximum bending stress at the base of skirt W = total weight of vessel Ds = inside diameter of the skirt ts = skirt thichness Resultant stresses in skirt are; σ s ( tensile ) =σ bs −σ ws



σ s ( compressive )=σ bs +σ ws



σ bs=



4 ×59241.96 Nm ×1 0 π ( 830+20 ) 830× 20



σ ws (test )=



55785.05 ×1 03 π ( 830+ 20 ) × 20



σ ws (operational)=



3



= 5.35 N/mm2



=1.044 N/mm2



9988 ×1 03 π ( 830+ 20 ) × 20



= 0.19 N/mm2



^ Maximum σ s (compressive)=¿ 5.35 + 1.044 = 6.394 N/mm2



^ Maximum σ s ( tensile ) =5.35−0.19=5.16 N/mm2



Criteria for design The skirt thickness should be such that under the worst combination of wind and deadweight loading the following design criteria are not exceeded: 23



σs (tensile) < fsJsinϴs (Sinnott, 2005l)



σ s ( compressive )