PTQ Q1 2025 Issue

PG : Pressure gauge MFC : Mass ow controller pH : pH meter TI : Temperature indicator P : Pump V : Vessel P SV : Pressure safety valve NV : Non-return valve V : Valve

C100

1500mm random packing

V200

V100

V20

P200

Oil phase

V300

P100

CO cylinder

Aq. phase to waste treatment

Figure 1 Schematic for treating spent caustic with CO 2

homogeneous and heterogeneous catalysts have been rec- ommended. However, the process works to its best under acidic conditions, and neutralising caustic still becomes a prerequisite. 4 Treating oxidation-resistant compounds In our quest for developing sustainable solutions to envi- ronmental issues of petroleum refining, in the current work, as a sustainable approach for treating spent caustic, the use of carbon dioxide (CO 2) is proposed if the major compo- nents of the spent caustic are oxidation-resistant phenolic or cresylic compounds. CO 2 is widely available as a waste gaseous stream in manufacturing industries and petroleum refining. CO2 reacts with the excess alkali of the spent caustic effluent in an acid-base reaction, forming sodium carbonate (Na 2 CO 3) and reducing the pH below 9. Being more acidic than phe- nolic compounds, CO 2 reacts further with the alkali metal salts of phenols and/or thiols. Thiophenol (pKa~6.7) salts may not react completely, while naphthenate salts will not react at all with CO 2 . The phenol(s) and thiols, generated in the neutralisation reaction, separate as an oil phase from the spent caustic. Surprisingly, no neutralisation of alkaline streams with CO 2 is practised in refineries, although the use of CO2 for the pH adjustment of spent caustic before WAO has been suggested.5 The pH adjustment was limited to 10, which was not enough to separate phenolic compounds from the spent caustic. For recovery of phenols, the pH must be brought at least one unit below the pKas of cresols/phenols. The treatment of Merox unit spent caustic with CO 2 gave a 65% COD reduction when the pH decreased below 9.0, while the sec - ond stage of solvent extraction gave an overall 90+% COD reduction. The process is controlled by the kinetics of mass transfer of CO 2 into aqueous solution. Figure 1 shows a schematic of a laboratory setup consist- ing of a stirred vessel equipped with a six-blade Rushton

turbine agitator running at 1,500 rpm and a flow meter to measure the amount of CO 2 sparged just below the impel- ler. The exit gaseous stream from the stirred vessel was connected to a packed column with a provision for spraying an alkali solution (1% w/v) to chemically absorb any acidic gases that might be liberated in the reaction or stripped off from the solution by CO 2 . Additionally, residual CO 2 in the exit gas reacts with alkali in this column. For comparison, the spent caustic waste was also neutralised using mineral acids with continuous pH monitoring, initially to a neutral pH and then to a pH below 3. Results Since it was not possible to identify each chemical compo- nent in the effluent, the COD was used as a monitoring var - iable. The pH of the spent caustic effluent from Refinery #1 was 12.9 with a COD of 40,600 ± 200 mg/L, while values were 12.4 and 44,500 ± 200 for the effluent from Refinery #2. During the first phase of pH reduction to 10, when CO2 reacts with excess alkali in reaction (R1), no COD reduction was seen:

2NaOH +CO2  Na 2 CO 3+H2O

(R1)

Thereafter, CO 2 reacts with sodium phenates, forming Na 2 CO 3 + sodium hydrogencarbonate (NaHCO3) , releasing phenol(s) as a separate oil phase and reducing the pH of the effluent below 9. Thiols can also react similarly:

2Ph-ONa + 2CO2  2PhOH + Na2 CO 3

(R2a)

Na2CO₃+H2O+ CO2  2NaHCO3

(R2b)

After a first slow decrease in the COD, with the addi - tion of CO 2 , there was a sharp drop in the COD, indicating separation of the organics from the aqueous solution (see Figure 2 ). The final CO2-treated effluent from the stirred vessel was an emulsion of the oil phase with the aqueous

PTQ Q1 2025