Spent caustic solutions

Disposal of the spent caustic solution can be a troublesome environmental problem. Depending on the plant location, acid gases are either sent to a fired heater or treated in a Claus unit for conversion of hydrogen sulfide to elemental sulfur. 

Aqueous Drawoffs from Hydrocarbon Vessels - Water or aqueous materials that are withdrawn continuously or intermittently from vessels where they directly contact hydrocarbons (e.g., process water from distillate drums, and spent wash water or spent caustic solution from settlers) must be disposed of in such a way that entraimnent or inadvertent withdrawal of hydrocarbon will not create a hazard. Disposal is therefore a function of hydrocarbon category, as follows ... 

Spent caustic solutions from petroleum refining. Petrochemical refineries use caustics to remove acidic compounds such as mercaptans from liquid petroleum streams to reduce produced odor and corrosivity as well as to meet product sulfur specifications. Spent liquid treating caustics from petroleum refineries are excluded from the definition of solid waste if they are used as a feedstock in the manufacture of napthenic and cresylic acid products. U.S. EPA believes that spent caustic, when used in this manner, is a valuable commercial feedstock in the production of these particular products, and is therefore eligible for exclusion. 

Caustic soda scrubber Spent caustic solution High pH, alkalinity, and color. Extremely low average flow rate... 

Thermophilic (55 C) digestion of the caustic-treated feed at 0.4 lb VS/ft -day loading and a 6-day detention time showed the same methane yield of about 4 SCF/lb VS added as observed during mesophilic digestion of the pretreated feed at a 0.1 lb VS/ft -day loading and a 12-day detention time (Run 5T/24, Table II). However, the thermophilic gas production rate was 4-5 times the mesophilic rate. Also, the caustic requirement for feed pretreatment was lower for the thermophilic run owing to the recycling of the spent caustic solution. 

Caustic treatijnent of undiluted feed for 24 hr at 100 C with 3 wt % NaOH and recycling of spent caustic solution. Fresh caustic is added at the rate of about 3 meq per gram of VS. Spent caustic recycle flow rate is 75 vol % of the feed slurry flow rate. 

Solid-Bed Caustic Treatment. SoHd-bed caustic units utilizing methanol [67-56-1] injection into the LPG feed stream can be used for carbonyl sulfide removal. The methanol—caustic solution must be drained periodically from the beds and discarded. When the soHd bed is exhausted, the spent caustic must be discarded and replaced. The LPG from the treater has a low enough water content to meet the propane specification. 

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 simplest way of removing hydrogen sulfide is to scrub the hydrocarbon mixture with an aqueous solution of a strong base, such as sodium hydroxide. The hydroxide reacts with the HjS, which is a weak acid, and the hydrogen sulfide removal is quantitative even in a single contact stage. Spent caustic from this operation cannot be regenerated economically. 

Aqueous plant effluent and drawoff streams such as steam condensate, sour water, or spent caustic soda solution may require disposal to a disengaging drum. 

Tannin solutizer process. In this process, alkyl phenols or isobutyric acid is used in combination with tannic acid and caustic solution. The spent tannin solution is again regenerated by air blown into the solution. 

Thus a spent scrubbing solution is obtained that contains equimolar quantities of NaOCl and NaCl. The chlorine-containing vent streams may contain also and variously C02, HC1 and other components naturally occurring in atmospheric streams. Clearly the C02 and HC1 have the potential to reduce the capacity of the caustic stream for chlorine removal. 

According to a USEPA survey, many of the more than 150 separate processes used in petroleum refineries generate large quantities of hazardous wastes. Typical wastes generated from refinery processes include bottom sediments and water from crude storage tanks, spent amines, spent acids and caustics, spent clays, spent glycol, catalyst fines, spent Streford solution and sulfur. 

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. 

Overhead from the chlorinator is scrubbed to remove excess chlorine in two vessels in succession which employ water and 5% caustic solution, respectively. The water from the first scrubber is used in the chlorine solution tower. The caustic is recirculated in order to provide adequate wetting of the packing in the caustic scrubber fresh material is charged in at the same rate as spent material is purged. Following the second scrubber, propylene dichloride is recovered from the gas by chilling it. The spent gas is recycled to the chlorinator in the required amount, and the excess is flared. 

The reaction mixture is pumped away from the reactor with an alkymer transfer pump, through a steam heater and an orifice mixer into the alkymer wash and surge tank. Dilute caustic solution is recirculated from the a.w.s. tank through the orifice mixer. Makeup of caustic is from a dilute caustic storage tank. Spent caustic is intermittently drained off to the sewer. The a.w.s. tank has an internal weir. The caustic solution settles and is removed at the left of the weir the alkymer overflows the weir and is stored in the right-hand portion of the tank until amount sufficient for charging the still has accumulated. 

The Neutralization Module accepts the ton container contents from the TCC module and destroys the agent batchwise through hydrolysis with water followed by caustic addition. The Neutralization Module consists of three units, each located inside a Containment Level A toxic cubicle. There are two HD Reactors and one TCC Effluent Tank in each of the three neutralization units. In each neutralization unit, drained agent held in the Agent Holding Tank is processed in batch neutralization reactions. The rinse and spray water from the TCC Module and spent decontamination solution are stored in the TCC Effluent Tank and process in separate batch reactions. In the neutralization reaction HD reacts with water to yield the principal hydrolysis products of thiodiglycol and hydrochloric acid. After the hydrolysis is complete and sample analysis results confirm the destruction of HD, 18 percent sodium hydroxide is added to the reactor to raise pH in order to increase the hydrolysate biodegradability. The hydrolysate is then pumped to the Hydrolysate Tank in the VOC Treatment Module. 

In the process to produce alumina (Fig. 1), bauxite is crushed and wet ground to 100-mesh, dissolved under pressure and heated in digesters with concentrated spent caustic soda solution from a previous cycle and sufficient lime and soda ash. Sodium aluminate is formed, and the dissolved silica is precipitated as sodium aluminum silicate. The undissolved residue (red mud) is separated from the alumina solution by filtration and washing and sent to recovery. Thickeners and Kelly or drum filters are used. The filtered solution of sodium aluminate is hydrolyzed to precipitate aluminum hydroxide by cooling. The precipitate is filtered from the liquor, washed, and heated to 980°C in a rotary kiln to calcine the aluminum hydroxide. 

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. 

The electrolyzers producing sodium hydroxide are fed with saturated brine (300 to 310 NaCl per litre), which is prepared in most works by dissolving common salt. In mercury electrolyzers, the salt is dissolved iu the spent brine which as a rule is set free from the dissolved chlorine, and which contains after leaving the electrolyzer 260 to 280 g NaCl per litre. With the diaphragm electrolyzers, which yield a caustic solution containing 120 g NaOH and 200 g NaCl per litre, the so called return solution is added to the brine prepared by dissolving the salt. This return solution is obtained in a later stage of the process iu the course of evaporation of the caustic solutions. 

Decontamination solutions are dilute aqueous solutions of caustic or sodium hypochlorite. These solutions are used to wash (decontaminate) work areas where agent has spilled. They are also used to decontaminate a worker in PPE prior to removing the suit for disposal. Spent decontamination solution (SDS) usually contains very small amounts of the chemical agent breakdown products resulting from hydrolysis of the agent present on the surface being decontaminated. SDS is collected and stored on-site for later disposal either off-site or on-site by incineration. 

Spent decontamination solution (SDS) consists of caustic or bleach-based aqueous solutions that have been used in the decontamination of personal protective clothing or the operations areas. SDS may also result from rinsing drained TCs or munition cavities. These solutions are captured and stored for treatment and disposal, either on-site or off-site. SDS typically contains less than 1 percent levels of sodium chloride and organic decomposition products from agent hydrolysis. 

The excess caustic solution is drained off for reuse. Additional amounts are removed by forcing the sheets through a press. The sheets are still in a swollen state and retain from 2.7 to 3.0 parts of the alkali solution. The spent steeping solution squeezed out of the pulp is processed for recovering the caustic from the organic materials. 

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