Oxidation of spent caustic

This process involves sulfidic spent caustic more than phenolic spent caustic and comprises the following steps  

---

Spent caustic neutralization is applied to both phenolic and sulfidic waste streams, but oxidation of spent caustics is limited to sulfide waste streams, since phenols inhibit the oxidation of sulfides in spent caustics. 

The pretreatment unit operations which may be necessary for various types of treatment facilities are shown in Table 12. The most prevalent in-plant treatment methods are sour-water stripping, neutralization and oxidation of spent caustics, ballast water treatment, and slop-oil recovery. These measures substantially reduce the waste loadings and to a significant degree are required to protect subsequent treatment. In addition to these in-plant control methods, refineries use gravity oil separators to recover free oil from process effluents. 

The precipitated sulfur is colloidal and gives a yellowish color to the deposits. The reaction can also occur without air oxidation of spent caustic if the caustic already contains a significant fraction of thiosulfates. This is often the true for SC caustic but is undetected by analysis. 

Oxidation of spent caustic by O2 in a reactor with a thermostated jacket keeping the temperature at TO C or 110°C under a pressure of 9 to 10 bar. 

JX Nippon Oil Energy Corp. Wet air oxidation (WAO), spent caustic Sent caustic Oxidation of sodium sulfide (Na S) component in the caustic scrubber effluent of olefin plants with air using wet air oxidation 13 NA... 

As a final product of spent caustic oxidation by air, or by oxygen in most industrial i facilities which can not provide the residence time, the pressure or the temperature ... 

Nonregenerative processes use caustic soda scrubbing sometimes in combination with air oxidation. They generate three types of spent caustic ... 

Acid consumption in the assumption of a final neutralization step is not insignificant, since it is defined by the initial free caustic plus the released caustic (and initial Na2C03). Accordingly, in comparison with the direct acidification of 1 m lT of spent caustic with 6% free NaOH, 5 gd" of and 12 g-H of S-RSH, the respective H2SO4 consumptions are still 81 kg after oxidation and 106 kg for direct neutralhation. This is notwithstanding the overconsumption in order to lower the pH from 7 to 4 in the second case. 

In addition to the sulfur compounds listed above, hydrogen sulfide has been found in many crude petroleums. Elemental sulfur has been definitely found in several crude petroleums by API Research Project 48 (23). Although Birch and Norris (5) isolated several disulfides from the spent caustic used in treating gasoline from Iranian petroleum, these compounds may have resulted from the oxidation of the thiols and their presence in the original petroleum is regarded as doubtful. Other types of sulfur compounds, such as thiophenes and aromatic thiols, have been identified in cracked petroleum products, but the presence of such compounds in naturally occurring petroleums has not yet been established. 

Hydrogen Sulfide. Extraction of the petroleum fraction with an aqueous solution of NaOH (5 to 15 per cent concentration) is usually used to remove H2S, frequently prior to other sulfur-removal operations. This will remove thiophenols as well. A line mixer (Chap. 9) followed by a settler in a one-stage contact, with recirculation of the caustic solution until its ability to remove H2S is spent, is usually the practice. The fouled extract, containing NaHS and Na2S, cannot conveniently be regenerated by heating, since the acidity of H2S increases with increased temperature and the caustic thus retains the H2S more firmly. Oxidation by aeration is an unsatisfactory alternative, and usually the fouled solution is discarded. 

In general, the in-plant control methods employed by the industry (sour-water stripping, spent-caustic neutralization and oxidation, slop oil recovery, etc.) will determine the final effluent characteristics and the level of pretreatment required for discharge to the municipal collection system. The following specific in-plant practices are frequently employed ... 

In spent caustic soda, the COD mainly comes from sulfides, mercaptans and phenols, with hydrocarbons (HC) as secondary coexisting components. After an air or O2 oxidation treatment, most of the residual COD is due to the thiosulfates formed or to slightly oxidized phenols. 

Under atmospheric pressure and temperature, reactions (1) and (2) are limited to the SzOs stage and are fairly slow. Reaction (5) oxidizing 8203 into SO 4 is even slower and is mainly developed in reactors or oxidizers at high temperatures (70 to 90°C) and pressures (5 to 10 bar) which make it more expensive. The presence of catalysts (Fe, Fe, Mn, Ni ", Co " ) accelerates reactions (1) and (2) considerably. These catalysts are already present in spent caustic (oxides) or added into process condensates, especially if the condensates have undergone physicochemical treatment. 

Workable for S concentrations of 30 to 300 mg-l" but involving instantaneous flows of about 20 to 200 m h h air oxidation does not serve the same purposes as when spent caustic is oxidized. 

After oxidation of sulfides into thiosulfetes, a fraction of the spent caustic could, however, be recycled for alkaline scrubbing of certain cuts. 

Theoretically, the first process requires reneutralising the acid effluent after it has been stripped and is more expensive in reagents. Meanwhile the second process is supposed to allow some recycling of the oxidized spent caustic which is still highly alkaline. 

Liquid oxygen conveniently available near the reactor makes it more advantageous to use as an oxidizing agent than air and widens the scope of oxidation under pressure. Spent caustic oxidation units have been set up on the following basis ... 

This type of reactor has developed more recertcly. In Europe, the first examples of implementation were designed for tannery and kraft paper pulp water, i,e. less concentrated in S than spent caustic. Operation at atmospheric pressure and at the only temperature allowed by the exothermic nature of the reactions naturally defines longer reaction times. However, the oxidation tanks built for this type of process are less sensitive to the maintenance and corrosion problems which may affect reactors working at high temperature and pressure. 

As for pressurized reactors, no provision has to be made for adding a catalyst since the spent caustic almost always contains enough metallic oxides of iron, nickel or cobalt to achieve expected oxidation kinetics. 

Sipma J, Svitelskaya A, van der Mark B, Hulstoff Pol LW, Lettinga G, Buisman CJN, Janssen AJH (2004) Potentials of biological oxidation processes for the treatment of spent sulfidic caustics containing thiols. Water Res 38 4331-4340... 

The niter and fresh caustic soda, required to maintain the fluidity of the salt bath in the reactor chamber, are added gradually. When the color of the saturated salts turns from a dark gray to white, the impurity metals are at their highest state of oxidation, and the lead content of the spent salts is very low. In a modification, the arsenic and tin are selectively removed as sodium arsenate and sodium stannate, followed by the removal of antimony as sodium antimonate. 

Spent acids are used where feasible to neutralize alkaline waste streams, or are neutralized with purchased lime, or caustic, and are then routed to the normal effluent treatment system for further cleanup before discharge [61]. Waste solids from refinery operations include such materials as spent clay from decolorization of lubricating oils or waxes, sand used as a catalyst support, filter aid, or filter base, and exhausted Claus catalysts (primarily ferric oxide on alumina). These are disposed of by landfilling by the majority of Canadian refineries. However, some use landfarming for disposal of these materials. One refinery recovers spent Claus catalysts for regeneration into new catalyst. 

---