Spent caustic washings

One way to reduce the amount of magnesium chloride is to use a caustic injection after the desalter. The injection amount is guided by the amount of salt in the overhead condensate. Spent caustic washes can be used if they do not contain contaminants that cause increased corrosion or plugging. For example, a spent alkylation plant caustic would add sulfur dioxide, which can react with hydrogen sulfide to cause sulfur plugging. The following steps are required to avoid problems with a caustic injection system ... 

Sulfur compounds such as alkyl and aryl thiols are treated with concentrated alkaline solutions in a process known as sweetening and cresols and xylenols are recovered from spent caustic washes, producing sodium cresolates/ xylenolates. 

Production of cresols and xylenols from spent caustic washes of the petroleum refineries has been confined mostly to the Untied States since cresolates feedstocks have been inadequate in other countries. Besides as a result of use of UOP s Merox process of sweetening which does not use NaOH solution, or of hydrotreating process, less and less cresols... 

Aqueous Drawoffs from Hydrocarbon Vessels - Water or aqueous materials that are withdrawn continuously or intermittently from vessels where they directly contact hydrocarbons (e.g., process water from distillate drums, and spent wash water or spent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 ... 

Improved drying, sweetening, and hnishing procedures to minimize spent caustics and acids, water washes, and hlter solids requiring disposal. 

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. 

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. 

Reaction of water and acetylenic HC with formation of polymerizing acetaldehyde in the form of sticky pink products. These products are thought to contain bonded sulfur. A washing pretreatment for SC spent caustic by light or aromatic cuts can keep these products from forming. This problem is believed not to exist for FCC spent caustic which is very rich in phenols. This is because phenols, powerful antioxidants, are thought to slow down gum formation. 

Spent caustic is washed by hydrogenated gasolines, BTX or toluene which dissolve the preformed gums and polymers or carry off the heavy HC. Washing is performed at the storage tank outlet in a drum ... 

Spent caustic is washed by steam which removes the acetylenic fractions or partially eliminates heavy aromatic fractions. It is effective only as long as it is carried out before the general storage tank, which in Itself induces polymerization. This operation is a preliminary stripping process in an alkaline environment ... 

Possible washing with gasoline or BTX if the spent caustic comes from the SC. 

The dinitro product is then cooled and filtered (the spent acid hquor is recoverable), the cake is washed with water, and the resulting washwater is sent to the wastewater treatment plant. The dinitro compound is then dissolved in an appropriate solvent and added to the amination reactor with water and soda ash. An amine is then reacted with the dinitro compound. The crude product is passed through a filter press and decanter and finally vacuum distilled. The saltwater layer from the decanter is discharged for treatment. The solvent fraction can be recycled to the reactor, and vacuum exhausts are caustic scrubbed. Still bottoms are generally incinerated. 

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. 

Prior to feeding the alkylate to distillation, the aluminum chloride and the hydrochloric acid are removed and recovered. The recovery of the spent alkylation catalyst involves two or three washing steps. The alkylate is first washed Chydrolyzed) with water to remove the dissolved AICI3 and then with dilute caustic in a second step to get rid of very small amounts of residual acid. A third water wash may be used as a final clean-up if necessary. 

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