Thursday 10 March 2011

ACID GAS REMOVAL



Caustic Scrubber

Caustic (NaOH) scrubbing systems are used in Olefins plants to remove CO2 and H2S from the cracked gas. This removal is required prior to cryogenic fractionation to prevent CO2 solidification. Also H2S poisons the acetylene hydrogenation catalyst, removal of these gases is accomplished by countercurrent scrubbing of cracked gas with a dilute caustic solution. The caustic scrubbing systems positioned after the third stage of compression. Cracked gas is fed to bottom of the tower. The spent caustic, containg reaction products and some dissolved hydrocarbons is drained from the top. Since the reaction velocity of H2S is much faster than CO2 and NaOH, tower design is based on the rate of absorption of CO2 only. As an additional specification, CO2 content is assumed to equal to the total sour gas concentration. The sour gas concentration in cracked gas is 250 ppm and NaOH scrubbing system lowers it to 1 ppm.

Design considerations

Design of packed column involves the following Parameters
Ø     Capacity
Ø     Column height
Ø     Type and size of packing
Ø     Pressure drop
Ø     Flooding velocity
Ø     Column internals
Ø     Mechanical design

Objective

                                                To remove CO2 & H2S

Equipment Used

                                                          Caustic Scrubber

Absorption Media

                                                       NaOH

Column Operating Conditions

Inlet cracked gas flow rate          =      410 Kg-mol/hr
Inlet sour gas concentration        =                250 ppm
Outlet sour gas concentration     =      1 ppm
Column operating temperature   =        45oC   =   318k
Column operating pressure         =      1114.5 KPa = 11.145 bar
Inlet NaOH concentration         =        1.55 Kg-mol/m3

Material Balance

The concentration of scrubbing system is calculated from stoithometric relations considering that for conversion of each mole of CO2, two moles of NaOH are needed, where as one mole of H2O and Na2CO3 are formed. The overall material balance for NaOH can be written as
Q1.C1   =   Q2.C2 + 2G (Y1-Y2)………………………... (1)
The equation (1) can be rearranged as,
[Q1.C1-Q2.C2]/Q1.C1 = [2.G (Y1-Y2)]/Q1.C1…………….(2)
LET X = Conversion of NaOH = [Q1.C1-Q2.C2]/Q1.C1
So that EQ. (2) becomes
X   = [2.G (Y1-Y2)]/Q1.C1…………….. (3)
Now Y1 = [250Kg-mol CO2/106 Kg-mol of cracked gas]*450Kg-mole/hr
              = 0.112 Kg-mole/hr
Similarly Y2 = 0.0004 Kg-mol/hr
Substituting these values in equation (3), we get
0.9 = 2(0.1025-0.00041)/Q1*1.55
Q1 = 65.86m3/hr = 4083.32 Kg/hr
The spent caustic is drawn at the same flow rate as that of fresh caustic soda
i.e.,      Q2 = 408332 Kg/hr

Colums Diameter And Area

(LW/VW)[DG/DL] 1/2 = 0.028

K4 =   0.8

K4 (at flooding) = 5.5

VW =   K4DG (DL-DG) 1/2         = 3.8 kg/m3-sec
          13.1. FP (mL/DL) 0.1
A = column area required = Gas flow rate/Vw = 0.7m2
Diameter = [(4*A)/Pi] 1/2= 0.938m = 0.65m (app.)

D (after round off) = 0.70 m

Vw(at actual dia) = 3.56Kg/m2-ses                             
Packing type             Pall rings (p)
Packing size               25mm

Specifications of pall rings

Bed weight             =      80Kg/m3
Area/volume           =      206m2/m3
Packing factor (Fp) =       180m-1
Ratio of column dia. to packing size = 25*10-3m/0.7855m = 38.2% =38%
Percentage flooding at selected Dia = [0.7368]/0.7855]*38
     = 35.64%=36%    

Effective Area

Aw=a [I-exp {-1.45(sc/ sL) 0.75 (LW/amL) 0.1 (Lwa/rL2g)-0.05(LW/ rLsL a) 0.2       
                                      =150m2/m3

Mass Transfer Coefficient

KL (rL/mLg) 1/3             = 0.0051 (LW/awmL) 2/3 (mL/rLDL)-1/2(ADP) 0.4
                                       =       1.423*10-6 m/s

            KGRT/aDv            =       K5 (Vw/amv) 0.7 (mV/rvDv) 1/3(ADP)-2

                                                =       6.25* 10-5 Kmol/m2.s.bar

Film Transfer Unit Heights

HG                        =       Gm/KGawP   =       1.22m

           HL                                       =       Lm/KLawCt =       1.53m

Number Of Transfer Units

NOG   = ln (yb/ya) = ln (250) = 5.52 = 6 (after round off)
HOG   = HG    + (mGm/Lm) HL
                             =       1.4m

Packing Height

            Z     =       HOG*NOG        =       8.4m

Total Height   

Z + 25%Z     =      10.5m

Pressure Drop

           DPt   =      DPd+DPL

Fs       =      G/ [(rG) 0.5 .3600]   =   0.7961ft/sec (lb/ft2) I/2

Gf           =       986Fs*(Fdp/20)1/2   =   2354.86lb/hr-ft2

Lf             =      L (62.4/rL) (20/Fpd) 1/2(mL) 0.1       =    494.1 lb/hr-ft2
DPd    = C3Gf2*10C4Lf   = 23mm of water/m of packing
DPL      = 0.4(Lf/20,000)0.1[C3Gf210C4Lf] 4 = 0.737 mm of water/m of packing
Pt      = 23.737 mm of water / m of packing height                                        

Superficial Velocity

 Ut     =       Fs/ (rG) 1/2   =       0.88 ft/sec

Operating Velocity

V=0.62 ft/sec

Thickness Of Shell And Head


Material used                   =       Carbon steel
Tensile strength               =      550 N/mm
Design stress at 50oC       =       240 N/m

Shell

e        =     PiDi/2f-Pi      =       2.34mm
Corrosion allowance        =       1mm
So total thickness            =       3.33 mm

Head

e        =       PiDi/2F-0.2Pi        =       2.32 mm
Corrosion allowance                 =       1mm
So total thickness                      =       3.32mm

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