Caustic Scrubber

Caustic (NaOH) scrubbing systems are used in Olefins plants to remove CO₂and 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 CO_{2 &} 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 = 45^oC = 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-Q_2.C₂]/Q₁.C₁ = [2.G (Y₁-Y₂)]/Q₁.C₁…………….(2)

LET X = Conversion of NaOH = [Q₁.C₁-Q₂.C₂]/Q_1.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