Friday 18 March 2011

WATER QUENCH TOWER DESIGN


Flowrate of cracked gas (ethylene) in to the tower = G = 11870 Kg/ hr
Enthalpy per Kg of the cracked gas at 111°C = hs+1      = 558.24Kj/Kg
Quench liquid flowrate irrigated to the tower= Ls        = 66760Kg/hr
Specific Heat Capacity of water                    = Cs         = 1 Kcal/kg
Temperature of quench water before section = ts-1        = 53°C
Temperature of water after the section           = ts          = 73°C
                                 Now using the relation
                                             G (hs+1-hs) = LsCs (ts-ts-1)
   Where
     G   = Hourly ethylene flow rate (Kg/hr)
    hs+1 = Cracked gas enthalpy before entering the section (Kcal/Kg of ethylene)
    hs    =   Cracked gas enthalpy after the section ( Kcal/Kg of ethylene)
    Ls  = Quench liquid flow rate (Kg/hr)
    Cs  = Heat capacity of quench liquid (Kj/Kg°C)
    ts     =  Temperature of the quench liquid before entering the section (°C)
    ts-1 =  Temperature of the quench liquid after section (°C)
  
          11870*(558.24 - hs) = 67760 (73 - 53)
                    hs = 444.22 Kj/kg
Now we know the enthalpy at which the gas is leaving the section, and so we can find the temperature at which the gas is leaving the section by using relation
                                              h = aT2
Where                
h = Cracked feed gas enthalpy, (Kcal/Kg of ethylene)
                                         T = Temperature of the cracked gas (°C)
a = A constant, the value of which is defined for different feedstocks.For   naphtha feed stocks the value is, a = 0.138.
444.22 = 0.138*T2
T2= 444.22/0.138
T2 =3218.98
Ts = 57°C

Calculations for Number of Ideal Stages

The number of ideal stages for this section ie the lower section can be calculated by using the relation,
                        
                         ln{(Ts+1-ts)/(Ts-ts-1)}
 
Ns=_____________________________________
              ln{(Ts+1-Ts)/ts-ts-1)} + ln {(Ts+Ts+1)/2(TsTs+1)0.5
Where
 Ts+1­= Temperature of the cracked gas before entering the section (°C)
Ts   = Temperature of the cracked gas after the section (°C)
 ts    = Temperature of the quench liquid before entering the section (°C)
 ts-1= Temperature of the quench liquid after section (°C)
Now putting the values and solving the equation we get the Number of plates theoretically required,
                         ln{(111-73/57-53)
 Ns=   ___________________________________________
              ln{(111-57)/73-53)} + ln{(111+57)/2(111*57)}               
                                                                        
                                                     Ns = 3.029
 Now taking the overall thermal coefficient of the plate as 0.45(As                             reported by Piccioti and in the other literature as well), we find the actual Number of Plates by using relation
                                                      (Ns) Eff =Ns/Es
 So the actual Number of Plates for the lower section is
                                                       (Ns) eff =3.029/0.45
                                                       (Ns) Eff = 6.73 = 7                                                                                                      

Plate Calculations for Second Section
          The same above described procedure is repeated for finding the number of plates in the upper section of the tower, we find the theoretical number of plates for the upper section of the quench tower and these came to be
                                                     Ns2 = 3.6
Actual number of plates for the upper section were found to be =8 plates
                                  Total number of plates required = 8+7 = 15 plates

Tower Design Calculations

Estimate vapor and liquid flow rates
Vapor and liquid flowrates are estimated by applying material and energy balances around water quench tower.
Calculate vapor load (VLoad)
                                                      The vapor load is given by the relation,
                                                       VLoad = Q {ρv/­(­ρLv)}0.5
Where                                           Q   = Volumetric flow of vapors (ft3/sec)
                                                     ρ= Density of vapors (lb/ft3)
                                                     ρL = Density of liquid (lb/ft3)                

 Substituting the values  Vapour Load= 5.2 ft3/sec
Estimation of the tower Diameter
From the figure the tower diameter comes out to be 4.25ft when a single pass tray is used with a tray spacing of 2 ft.
Calculations of active area(Aa)
                                                       The active area is calculated by,
                                                       Aa,min = {VLoad + q*L/13000]C*F
Where                                        
                                                       q = liquid flowrate (gal/min)
                                                       L = 9Dt/N……………….. (1)
                                                       Dt = Estimated tower diameter (ft)
                                                       N = Number of tray passes
                                 C = 0.44 (as for hydrocarbons S=1 So C*S = C = C*)
                                                       F = Flood factor =0.8
                                                       From equation (1) L = 38.25 inches.
Putting the values the value comes to be
                                                        Aa,min  = 14.7 ft2
Calculations of downcomer area (Ad)
                                                        The downcomer area Ad, min = q/U*F
Where
                                  U = Ideal downcomer design velocity (gal/min ft2)
                                                  U = U*S
 The ideal downcomer design velocity U* is found as the smallest value of the following equations
                                                        U* = 41[ρLv] 0.5
                                                         U* = 7.5[TsLv)] 0.5
Substituting the values the smallest downcomer velocity is
                                                        U* ≈ 250 gal/ min ft2
So the actual downcomer value is
                                                       U = U**S
Where                                           S = 1 (for hydrocarbons)
                                                       U = 250 gal/ min ft2
                                                       The downcomer area Ad, min = 2.55 ft2
Since the down comer area is greater than 11% of the active area hence it is all right.
Calculations of tower area (AT)
 The tower area is obtained as equations
                                                        AT = Ad, min +  Aa, min
                                                        The calculated tower area is 17.25 ft2
Calculations of tower diameter
 The diameter of the tower is given by the relation
                                                          Dt = (4*AT/π) 0.5
                                                               = (4*17.25/3.1416)0.5
                                                               = 4.7 ft
The tower diameter comes out to be 4.7 ft.
Calculations of valve units                 
        The number of valve units is taken to be 14/ft2 of the active area.
            So the number of valve units are = 14*14.7
                                                                 = 206
Weir Sizing
                                                            The ratio Ad/AT   = 2.55/17.25
                                                                                       = 0.14
So from the graph weir length/tower dia                       = 0.78
So weir length is                0.78*4.7                              = 3.66ft
Pressure drop calculations
 The dry pressure drop is given by
                                                            Δ PDry = K2V2hvL)
Where                             Vh = Velocity of gas through valves =
  K2 = A constant = 1.05 (for lighter hydrocarbon)                                                              Δ PDry = 3.9 in water/tray
 The wet tray Pressure drop is given by
                                                    Δ PT = Δ PDry + 0.4[q/Lw]0.67+ 0.4 hw
  Where Lw = Weir length,   hw = Weir height,   q = Liquid flowrate
Putting the values and getting the answer, the total tray pressure drop comes to be 6.6 inches water

Design of the Upper Section

The design of the upper section of water quench tower is parallel to the lower section design
 Estimate vapour and liquid flow rates
                        Mass flow rate of cracked gas (ethylene) = 26114 lb/hr
                        Volumetric flowrate of cracked gas         = 75.6 ft3/sec
                        Mass flowrate of water in                         = 106480 lb/hr
                        Volumetric flowrate of water (q)              = 212.4 gal/min
Calculate vapour load
                        VLoad = 2.96 ft3/sec
Estimation of tower diameter
From the graph the tower diameter comes out to be 3.25 ft and a single pass tray is used with a tray spacing of 2 ft.
Calculation of active area
                                          Aa, min =6.6 ft2

Calculations of downcomer area
  The ideal downcomer design velocity comes to be U = 250 gal/min ft2
  The downcomer area               Ad, min =q/U*F = 1.062 ft2
The down comer area is greater than the 11% of the active area hence it is all right.
Calculations of tower area
                                            AT = 8.724 ft2
Calculation of tower Diameter
The diameter of the tower is given by the relation
                                                          Dt = (4*AT/π) 0.5
                                                               = 3.3 ft
Calculations of number of valve units
      The numbers of valve units are taken to be 14/ft2 of the active surface
                                              So the number of valve units = 16*6.6 = 105
Weir sizing
                                          Weir Length/Tower dia   = 0.72
                                          So that weir length = 0.72*3.3 = 2.37 ft
                                         Also down comer width/tower dia =0.15
                                         So the down comer width = 0.15*3.3 = 0.695 ft
Pressure drop calculations
                                                Δ PDry = 5.3 in water/tray
                                                Δ PT    = 7.6 in water/plate

15 comments:

  1. THIS IS AWESOME !!!! IS THERE ANY WAY YOU CAN GIVE ME THE REFERENCES FOR ALL OF THE EQUATIONS ?!?!

    ReplyDelete
  2. or just the textbook or paper you found them in please !

    ReplyDelete
    Replies
    1. Hi, is it possible if you could provide the name of the references you used to perform the quench water tower design?
      Thanks!

      Delete
  3. This comment has been removed by the author.

    ReplyDelete
  4. I also need the reference books or links please

    ReplyDelete
  5. can you add the reference sir? thanks.

    ReplyDelete
  6. i need references of this design

    ReplyDelete
  7. Hello, I'm a chemical engineering student currently designing an ethylene plant, it will help me to understand your calculations if you could share the textbook/reference you got these equations from. Hope you reply soon! Thanks

    ReplyDelete
  8. Can I get the reference please?

    ReplyDelete
  9. HOW TO FIND THE TOWER HEIGHT

    ReplyDelete
  10. Can you please provide references?

    ReplyDelete
  11. Can you provide references please?

    ReplyDelete
  12. Encyclopedia of Chemical Processing and Design: Volume 11 by John J. McKetta Jr

    ReplyDelete
  13. Can any one please tell that if I have a gas flow rate, how can I measure the liquid flow rate for designing

    ReplyDelete