Friday 18 March 2011

QUENCH TOWER


Introduction to Quench Towers

 The cracked gas first passes through a quench fitting which is only a mixing valve, where gas cools from 380°C to 185°C by direct contact with heavy refractory fuel oil. This cold mixture then enters the primary fractionator, the oil quench tower which is considered to be the trickiest system, as serious fouling and total plugging of the system is often experienced The risk of water condensation and foaming at the top are avoided by keeping the temperature at the top of the primary fractionator above the condensation temperature of water.
The gaseous stream exiting at the top of the primary fractionator is at 111°C, consisting of cracked gas along with the steam and evaporated reflux, then enters the water quench tower where the bulk of the process steam is condensed due to the circulating quench water, which when cooled is returned to the column at two different temperature levels depending on the plant feedstock capacity and local conditions. Here most of the gasoline fraction and the evaporated naphtha reflux is condensed and is passed along with the quench water to an oil water separator.

Method for Estimation of Number of Plates

An ingenious method is presented by Piccioti for the designing of the quench towers employing non evaporative liquid stream for quenching. He developed appropriate equations to simulate the quench tower. Piccioti presented a simple correlation between enthalpy of the cracked gas and the temperature given as
                                               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 feed stocks. For naphtha feed stocks the value is, a = 0.138.
           In this technique the quench tower is essentially considered as heat exchanger with several cooling sections. Word section is intended as a part of quench tower between two successive injections.
 For a particular section “s” the overall heat balance can be written as
                                             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)
 With the help of the above equation the intermediate temperature of the cracked gas between the sections can be found out. After finding the intermediate temperatures, the number of theoretical plates in the section is found out by 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)

1 comment:

  1. is it this type of tower is kind of cooling medium?..

    ReplyDelete