PTQ Q1 2025 Issue

The cost of WAO depends on the operating conditions, which are related to the COD of the effluent. More severe operating conditions are needed to treat effluent with higher values of COD. Higher-strength waste streams are frequently diluted to bring COD below 50,000 for better efficiency of the WAO process. The carbonation can eas - ily manage even a larger COD-containing spent caustic stream. The stronger the organic content of spent caustic, whether phenolic or cresylic acids, the more economical it is, as more recovery of phenols in the form of organic material is possible. If required, the waste stream from the carbonation operation, with or without a solvent extraction step, can be fed to the WAO unit to oxidise residual organic and inorganic sulphides in slightly alkaline conditions to sulphates, which can be run at milder conditions of 100ºC. Since the pH is not brought below 8.5, there was no evo- lution of malodorous sulphide gases, which is common if the pH of spent caustic is brought down by the addition of strong mineral acids. The carbonation process, however, does not touch inorganic sulphides, thiophenols and naph- thenates in the solution. Deep acidification of the spent caustic releases phenols (at pH <9), thiophenols (at pH <6.0), and naphthenic acids at (pH <4). However, the acid- gas absorption system was overloaded with the release of copious amounts of H 2 S and mercaptans gases, generating a secondary waste. The mineral acid addition rate must, therefore, be adjusted appropriately to ensure a managea- ble rate of gas release in the neutralisation reactor, a factor often not discussed during deep acidification. Ionic liquid treatment The main limitation of the carbonation process is the slower rate of CO 2 transfer into the aqueous solution. If the CO 2 available is a pure gas, a gas-inducing impeller can be used in the stirred vessel to use it completely in the reactor. Operating costs can be significantly reduced if CO2 is avail- able at the site as an effluent stream from any manufactur - ing facility or even when mixed with other gases. Sabri et al 7 reported an extremely high removal of COD (99.8%) by treatment of spent caustic with ionic liquids (ILs). At high pH values, all organics are in ionic form, preventing their extraction into organic solvent. So, direct solvent extraction without pH adjustment at pH of 12.9 is unlikely to recover any of the phenolic/cresylic compounds into an organic solvent. The acidification enables the con - version of ionic species to their non-ionic form. At pH below 9, all the phenolic compounds must be in molecular form to allow their complete extraction. Considering that COD-contributing organic compounds are strongly ionic at alkaline pH conditions, ion exchange with ILs cannot be ruled out. However, the work did not disclose the physicochemical processes involved nor the regenera- tion of the IL phase. Considering the prohibitively excessive cost of ILs, their application in the recovery of phenolics from spent caustic is not practical unless efficient regener - ation and recycling methodologies are developed to reduce the IL cost per unit of treated spent caustic to economically workable levels.

1% w/v 2% w/v 3 % w/v

0.95

0.9

0.85

0.8

0.75

0.7

Time (min)

without the intermediate solvent extraction step. Figure 4 shows the kinetics of the adsorption for three different loadings of activated carbon. At 1% w/v loading, the acti- vated carbon was able to pick up almost 8,560 COD/gm of carbon. The exponential decay of COD with time shows that the process is of first order and controlled by the exter - nal mass transfer of organics to the solid adsorbent surface. One of the major concerns in the treatment of spent caus- tic waste is the nature of compounds, mostly phenolic and thiophenols, which could be refractory to the usual biologi- cal treatment. The UV spectra of the spent caustic from two refineries show strong absorbance peaks at 275 nm and 240 nm corresponding to phenols/cresols and thiophenols, respectively. After the treatment with CO 2 and toluene, the effluent shows a substantial reduction in absorbance in the UV region. Thus, the process definitely removes the phenolic com - pounds, and the residual solution becomes amenable to biological treatment before the water can be released to the environment. After the activated carbon treatment, there was negligible absorbance at these wavelengths, showing complete removal of biologically refractory compounds. Comparative results It will be worthwhile to compare the results of carbonation and solvent extraction with the well-established WAO for treating the spent caustic effluent. The carbonation works at ambient conditions of temperature and pressure. It thus would not require large capital investments and operating costs, while WAO needs temperatures of 200ºC and 27.5 bar to 260ºC and 86 bar pressures and thus high-pressure equipment. WAO destroys the organics, adding them to the environmental load as gaseous CO 2 . In contrast, the car- bonation process uses CO 2 and recovers the phenolics and cresylics as a separate oil phase. This may become more economical when recovering these high-value compounds from spent caustics with significant amounts of compounds. A typical WAO, in industrial conditions, gives about an 80% reduction in COD in about 45-120 minutes, while the first stage of the proposed carbonation process alone brings down the COD by 65-70%. In the two-stage pro - cess, the COD reduction was 90+%. Figure 4 Kinetics of adsorption of COD on activated carbon at different loadings

PTQ Q1 2025