Spent caustic neutralization

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 ... 

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. 

Chemical Reactivity - Reactivity with Water Reacts vigorously with water with the release of flammable hydrogen gas Reactivity with Common Materials No reactions Stability During Transport Stable at temperatures below 225 C Neutralizing Agents for Acids and Caustics Neutralize only when accidental reaction with water is complete. Do not neutralize the flammable solid with aqueous solutions. Spent reaction solution may be neutralized with dilute solutions of acetic acid. Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

The major discharges from sulfuric acid alkylation are the spent caustics from the neutralization of hydrocarbon streams leaving the alkylation reactor. These wastewaters contain dissolved and suspended solids, sulfides, oils, and other contaminants. Water drawn off from the overhead accumulators contains varying amounts of oil, sulfides, and other contaminants, but is not a major source of waste. Most refineries process the waste sulfuric acid stream from the reactor to recover clean acids, use it to neutralize other waste streams, or sell it. 

The most common waste stream from drying and sweetening operations is spent caustic. The spent caustic is characterized as phenolic or sulfidic, depending on which is present in the largest concentration this in turn is mainly determined by the product stream being treated. Phenolic spent caustics contain phenol, cresols, xylenols, sulfur compounds, and neutral oils. Sulfidic spent caustics are rich in sulfides, but do not contain any phenols. These spent caustics have very high BOD and COD. The phenolic caustic streams are usually sold for the recovery of phenolic materials. 

Spent caustics may therefore contain sulfides, mercaptides, sulfates, sulfonates, phenolates, naphthenates, and other similar organic and inorganic compounds [38]. Spent caustics can also be classified as phenolic and sulfidic [37]. Sulfidic spent caustics are rich in sulfides, contain no phenols, and can be oxidized with air. Phenolic spent caustics are rich in phenols and must be neutralized with acid to release and remove the phenols. 

Spent caustics usually originate as batch dumps, and the batches may be combined and equalized before being treated and discharged to the refinery sewer. Spent caustics can also be neutralized with flue gas to form carbonates. Sulfides, mercaptides, phenolates, and other basic salts are converted by the flue gas (reaction time 16-24 hours) stripping. Phenols can be removed, then used as a fuel or sold. H2S and mercaptans are usually stripped and burned in a heater. Some sulfur is recovered from stripper gases. The treated solution contains mixtures of carbonates, sulfates, sulfites, thiosulfates, and some phenolic compounds. 

The acid gases, usually CO2 and H2S, are removed by scrubbing with diethanolamine or monoethanolamine solutions and possibly an additional caustic treatment. Older processes used caustic solutions of 5-15 wt% NaOH followed by a water wash. Of course, the spent caustic creates a disposal problem it must be neutralized with acid and then properly disposed of according to prevailing pollution and hazard waste standards. Different column configurations have been proposed, but usually large scrubber towers with well over 30 valve-type trays are used. 

D-5) Concentrated HCI receiver (D-6) Concentrated HCI pump (D-7) Concentrated HCI storage (D-8) Acid recycle pump (E) Crude product pump (F-1) Caustic make-up tank (F-la) Caustic make-up agitator (F-2) Caustic transfer,pump (F-3) Caustic storage tank (F-4) Caustic feeder (F-6) Neutralizer (F-6) Spent caustic separator (G-1) Flash still feed pump... 

In order to reduce the caustic consumption even more, without ruining the overall selectivity, a low-temperature preneutralization step is introduced, in which acids are neutralized with spent caustic lye originating from the decomposition section. Effectively, the aqueous caustic stream and the organic oxidate stream are flowing in countercurrent mode. Fresh NaOH and preneutralized oxidate are supplied to the decomposition section that requires a high free... 

Mesophilic (35 C) and thermophilic (55°C) digestion of feed subjected to mild alkaline (sodium hydroxide) pretreatment with recycling of spent caustic for fresh feed treatment and neutralization of treated feed with digester gas to minimize acid neutralizer requirement. 

Oily process wastes and oil-free wastes are collected separately in some refineries so that the oily wastes can be treated for oil removal before mixing with other waste streams. The spent caustics and spent acids are generally collected and sold or disposed by other means. Few refineries neutralize these wastes for discharge, to the wastewater collection system. 

The major wastewater from the alkylation process is the spent caustic from the neutralization of the hydrocarbon stream from the reactor. Even though the spent acids are recovered as salable by-product, leaks and spills of acid catalysts could reach the sewer lines. The major pollutants from the solvent refining processes include solvents such as phenols, glycols, and amines. Other processes for the manufacture of waxes and asphalt, and for finishing and blending of gasoline, produce relatively low volumes of dilute wastewater. 

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 pretreatment unit operations generally consist of equalization, neutralization, and oil separation. In addition, phenol recovery (to reduce the phenol concentration) and spill protection for spent acids and spent caustics may be required in some cases. 

Outside of die treatments discussed in this chapter, the ways of breaking down or disposing of spent caustic are very different. It can be sold to an outside buyer, though this is done less and less. It can be recycled downstream from the desalter, diluted with different types of water or neutralized with acid effluents. 

Formed after the spent caustic oxidation treatment. Direct acidification can then neutralize these alkaline effluents and gives rise to S2O disproportionation ... 

Accordingly 40 to 180 kg of pure H2SO4 is required to neutralize a cubic meter of spent caustic. The added sulfuric acid, 10 or 18 N, will therefore dilute the spent caustic by 80-44 liters to 360-200 liters of water. 

Although this risk exists, it is not common. It has occurred during neutralization of very concentrated spent caustic by 20 N acid. 

Changing concentrations during neutralization of spent caustic... 

It should be pointed out that when the technology of the neutralization reactor imposes recycling part of the acidified and degassed spent caustic, the actual flow through the stripper can be one and a half to three times the useful throughput of the facility. 

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. 

Spent caustic can therefore be sent to aerobic biological purification after having undergone desulfurization by stripping or by air oxidation and after having been neutralized. 

The steam cracker spent caustic (1 to 1.5 fh ) is reused to neutralize acids after pressurized air oxidation. It finally comes back with the Aubette effluent. 

Due to these disadvantages, research on the transesterification reaction using heterogeneous catalysts for biodiesel production has increased over the past decade (Lee and Wilson, 2014). Fig. 6.9 summarizes the classification of catalysts. Zhang et al. (2003) argued there is a considerable incentive for the substimtion of liquid bases by solid bases for the following reasons (1) energy intensive product/catalyst separation, (2) corrosiveness, and (3) the costs associated with the disposal of spent or neutralized caustics. 

The first vessel in the blowdown system is therefore an acid-hydrocarbon separator. This drum is provided with a pump to transfer disengaged acid to the spent acid tank. Disengaged liquid hydrocarbon is preferably pumped back to the process, or to slop storage or a regular non-condensible lowdown drum. The vented vapor stream from the acid-hydrocarbon separator is bubbled through a layer of caustic soda solution in a neutralizing drum and is then routed to the flare header. To avoid corrosion in the special acid blowdown system, no releases which may contain water or alkaline solutions are routed into it. 

Some refineries neutralize the caustic with spent sulfuric acid from other refining processes, and charge it to the sour water stripper. This removes the H2S. The bottoms from the sour water stripper go to the desalter, where the phenolics can be extracted by the crude oil. 

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