PTQ Q1 2025 Issue

effluent that was sent to a phase separator where it sep - arated into two phases: an oil phase of mostly phenolic compounds and another aqueous phase having residual organic pollutants and Na2 CO 3. Due to high viscosity and densities close to that of the aqueous solution, the oil droplets do not coalesce easily. The oil phase separated from the spent caustic of Refinery #1 after the treatment with CO2 had a density of 0.98 and viscosity of 25.9 mPa.s. It took more than 72 hours to get two clear, distinct phases on standing. The residual COD of the aqueous solution at pH <9 was 12,980 ± 1,340 mg/L. About 1.8% of oil by weight separated from the aqueous solutions, giving a 70% reduction in the COD. The sample from Refinery #2 also showed a 67% reduction in COD after treatment with CO2. Due to the acid-base type of reaction of CO2 with alkali, no free CO2 is expected to exist in the solution. The sol - ubility of CO2 in water is 2.03 x 10 -3 gmol/L at 30ºC. For an agitated stirred vessel, the mass transfer coefficient and hold-up were estimated as 0.36 1/s, and 3.0 ± 0.5%, respectively, from the following correlations: 6

10000 15000 20000 25000 30000 35000 40000 45000 50000

5000

0

0

50

100

150

200

250

CO (mg/L)

Figure 2 Reduction in COD of spent caustic waste by treatment with CO2 ( ο : Refinery #1,  : Refinery #2).

Treatment with activated carbon

Activated carbon

Residual CO (mg/L) 2,221±33 1,977±91 1,757±41 4,030

Adsorbed COD

Sr. no

(wt%)

(mg/L)/gm

0

-

1 2 3

0.5 1.0 1.5

362±7 205±9 152±3

(1)

Table 1

and

COD reduction A substantial decrease in the COD of the aqueous phase was seen after the toluene extraction. Figure 3 shows that more than 60% of residual COD was transferred to toluene in a single stage. The COD of the CO2-treated spent caustic reduced from 12,540 to less than 5,100 mg/L even with 10% (v/v) of toluene used for the extraction. With equal volume of toluene, the COD reduced further to 4,030 ± 100 mg/L. Table 1 shows another 50% reduction of COD after the adsorption on activated carbon activated the carbon at dif - ferent loadings in a single-stage operation, as a polishing stage. The spent caustic, treated with CO2 at Refinery #2 at 600 L scale, showed a higher value of COD (28,400 mg/L) and pH 9.42, indicating that the CO2 treatment was not complete. It was directly treated with activated carbon

(2)

The estimated mass transfer rate of CO2 into the aqueous caustic solution is 7.4 X 10 -4 gmol/L/s and is controlled by the hydrodynamic conditions and poor solubility of CO2. Not all the CO2 that is sparged into the reactor reacts. Therefore, to react in the once-through operation, suitable mass trans - fer equipment, such as a stirrer vessel with a gas-inducing impeller, will be necessary for the complete utilisation of CO2 . Since the oil phase mostly consists of compounds of phenolic/cresylic nature, toluene was added in the second step of treatment to dissolve the oil phase and even extract phenols from the aqueous phase. Paraffinic solvents do not dissolve phenolic compounds well. The toluene extract can be sent for recovery of phenols by physical methods or recycled back to the refinery for blending with a suitable stream.

7 8

70

68

6

66

4 5 3

64

62

2

60

0 1

58

0

0.5

1

1.5

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4500

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5500

V org aq /V

COD of aqueous phase (mg/L)

Figure 3 (left) % transfer of organics to toluene (right) Partition coefficient of organics towards toluene in terms of COD

54

PTQ Q1 2025

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