Caustic washing

Esterifica.tlon. The process flow sheet (Fig. 4) outlines the process and equipment of the esterification step in the manufacture of the lower acryflc esters (methyl, ethyl, or butyl). For typical art, see References 69—74. The part of the flow sheet containing the dotted lines is appropriate only for butyl acrylate, since the lower alcohols, methanol and ethanol, are removed in the wash column. Since the butanol is not removed by a water or dilute caustic wash, it is removed in the a2eotrope column as the butyl acrylate a2eotrope this material is recycled to the reactor. 

Approximately 50—55% of the product from a coal-tar refinery is pitch and another 30% is creosote. The remaining 15—20% is the chemical oil, about half of which is naphthalene. Creosote is used as a feedstock for production of carbon black and as a wood preservative. Because of modifications to modem coking processes, tar acids such as phenol and cresyUc acids are contained in coal tar in lower quantity than in the past. To achieve economies of scale, these tar acids are removed from cmde coal tar with a caustic wash and sent to a central processing plant where materials from a number of refiners are combined for recovery. 

This secondary reaction starts at about 180°C, but the mass must be heated to 350—400°C to bring the reaction to completion and produce a nitrate-free product. The off-gases are extremely corrosive and poisonous, and considerable attention and expense is required for equipment maintenance and caustic-wash absorption towers. Treatment of the alkaline wash Hquor for removal of mercury is required both for economic reasons and to comply with governmental regulations pertaining to mercury ia plant effluents. 

A mixing valve in the form of a conventional globe valve is simple and economical. A typical service iavolves caustic washing of gas oil and water—oil mixing upstream of a desalter. The valve is normally specified to handle a pressure drop ia the range of 20—350 kPa (0.2—3.5 atm). 

The solvent is 28 CC-olefins recycled from the fractionation section. Effluent from the reactors includes product a-olefins, unreacted ethylene, aluminum alkyls of the same carbon number distribution as the product olefins, and polymer. The effluent is flashed to remove ethylene, filtered to remove polyethylene, and treated to reduce the aluminum alkyls in the stream. In the original plant operation, these aluminum alkyls were not removed, resulting in the formation of paraffins (- 1.4%) when the reactor effluent was treated with caustic to kill the catalyst. In the new plant, however, it is likely that these aluminum alkyls are transalkylated with ethylene by adding a catalyst such as 60 ppm of a nickel compound, eg, nickel octanoate (6). The new plant contains a caustic wash section and the product olefins still contain some paraffins ( 0.5%). After treatment with caustic, cmde olefins are sent to a water wash to remove sodium and aluminum salts. 

Alkali Treatment. Caustic washing is the treatment of materials, usually products from petroleum refining, with solutions of caustic soda. The process consists of mixing a water solution of lye (sodium hydroxide or caustic soda) with a petroleum fraction. The treatment is carried out as soon as possible after the petroleum fraction is distilled, since contact with air forms free sulfur, which is corrosive and difficult to remove. The lye reacts either with any hydrogen sulfide present to form sodium sulfide, which is soluble in water, or with mercaptans, foUowed by oxidation, to form the less nocuous disulfides. 

In both cases, the carbonate ion concentration increases and eventually equiUbrates in the system, releasing carbon dioxide in the stripping column and thereby reducing product purity. Hence, a small caustic wash tower is employed to remove any carbon dioxide that is Hberated in the stripper. 

The gases from the reactor are then cooled and subjected to a caustic wash to remove unreacted hydrogen chloride. This is then followed by a methanol wash to remove water introduced during the caustic wash. A final purification to remove aldehydes and ethylidene dichloride, formed during side reactions, is then carried out by low-temperature fractionation. The resulting pure vinyl chloride is then stored under nitrogen in a stainless steel tank. 

In the feed preparation section, those materials are removed from the reactor feed which would either poison the catalyst or which would give rise to compounds detrimental to product quality. Hydrogen sulfide is removed in the DBA tower, and mercaptans are taken out in the caustic wash. The water wash removes traces of caustic and DBA, both of which are serious catalyst poisons. Also, the water wash is used to control the water content of the reactor feed (which has to be kept at a predetermined level to keep the polymerization catalyst properly hydrated) and remove NH3, which would poison the catalyst. Diolefins and oxygen should also be kept out of poly feed for good operation. 

After reaction at 200 - 250 F and 350 psig the reactor effluent is stripped to remove recycle HCl. The stripper bottoms is cooled and caustic washed to remove remaining traces of HCl. The product can then be sent to the alkylation plant for fractionation or a tower provided in the isomerization plant for fractionation of isobutane and recycle of unconverted normal butane back to isomerization. 

Chemical Reactivity - Reactivity with Water No reaction Reactivity with Common Materials No reaction Stability During Transport Normally unstable but will be detonate Neutralizing Agents for Acids and Caustics Wash with water, rinse with sodium bicarbonate solution Polymerization May occur in contact with acids, iron salts, or at elevated temperatures and release high energy rapidly may cause explosion under confinement Inhibitor of Polymerization Monomethyl ether of hydroquinone 180-200 ppm phenothiazine (for tech, grades) 1000 ppm hydroquinone (0.1 %) methylene blue (0.5... 

Mercaptans are slightly acidic organic sulfur compounds. They can be removed from light ends mixtures by caustic washing in a Merox extraction... 

When both HjS and mercaptans are to be removed in the light ends treating plant, the amine scrubbing tower precedes the caustic washing step. If the order were reversed, the large quantity of HjS present would "fix" the caustic. Also, the caustic will act to remove any final traces of HjS when it follows the scrubbing tower. 

In a typical gas oil design, the lighter products overhead from the quench tower/primary fractionator are compressed to 210 psi, and cooled to about 100°F. Some Q plus material is recovered from the compressor knockout drums. The gases are ethanolamine and caustic washed to remove acid gases sulfur compounds and carbon dioxide, and then desiccant dried to remove last traces of water. This is to prevent ice and hydrate formation in the low temperamre section downstream. 

The acid extract phase is separated, diluted with water, and heated to regenerate isobutylene. The isobutylene is then caustic and water washed to remove traces of acid, distillation dried, and rerun. The unreacted C4 stream, containing normal butenes, is also caustic washed before further processing. 

Note that the methanation section is the last processing step in the HYGAS pilot plant, and it depends on the steady-state troublefree operation of the preceding steps (the gasification reactor, amine purification, and caustic wash sections for cleanup sulfur removal) before it can be brought on-line. 

Caustic washing. Here, caustic solution 5-15% strength is reacted with liquid gasoline or gaseous hydrocarbons. This interacts with H2S and lower molecular-weight mercaptans and they are thus removed. 

Caustic washing and thermal treatment of silver chloride precipitate are performed to achieve a 5X decontamination level for shipment to an offsite silver recovery operation. 

Several processes use the principle of an alkali (caustic) wash, which is a process in which distillate is treated with sodium hydroxide to remove acidic contaminants... 

Isomerization processes produce sour water and caustic wastewater. The ether manufacturing process utilizes a water wash to extract methanol or ethanol from the reactor effluent stream. After the alcohol is separated, this water is recycled back to the system and is not released. In those cases where chloride catalyst activation agents are added, a caustic wash is used to neutralize any entrained hydrogen chloride. This process generates a caustic wash water that must be treated before being released. This process also produces a calcium chloride neutralization sludge that must be disposed of off-site. 

The ethylene conversion to ethyl alcohol per pass through the reactor is only 4-6%, so most of the ethylene needs to be recycled. But first the reactor effluent is cooled and caustic washed to neutralize any vaporized H3PO4. As the effluent cools down, the ethyl alcohol liquifies, and the ethylene can easily be separated. The ethylene recycle stream is then scrubbed by sloshing it through water prior to recycle. 

Polymerization is a rather dirty process in terms of pounds of pollutants per barrel of charge, but because of the small polymerization capacity in most refineries, the total waste production from the process is small. Even though the process makes use of acid catalysts, the waste stream is alkaline because the acid catalyst in most subprocesses is recycled, and any remaining acid is removed by caustic washing. Most of the waste material comes from the pretreatment of feedstock, which removes sulfides, mercaptans, and ammonia from the feedstock in caustic and acid wastes. 

Processes such as caustic washing, sweetening, hydrodesulfurization, and water removal are common examples of processes used to improve the quality of finished fuels. These processes are described in the remaining portion of this chapter. 

Water Crude oil water washing, caustic washing condensate water and contamination during storage... 

Naphthenic acids occur primarily in distillate and some heavy fuel fractions. Topically, caustic treatment effectively removes these compounds. However, even after caustic treatment, alkali salts of heavier naphthenic acids may still remain oil soluble. In fuel, these compounds can act as very effective emulsifying agents. Fuel haze and particulate contamination can be due to these acid salts. Caustic solutions of various strengths can be used to wash fuel. Usually 10 to 20 vol% of a 5% to 10% caustic wash solution is effective for most applications. 

Removal of the objectionable odors due to the presence of H2S and mercaptans is the objective of the fuel sweetening process. Several methods can be utilized to remove these undesirable compounds including caustic washing, copper chloride sweetening, sulfuric acid treating, Merox processing, and hydrotreating. These methods will be discussed below. 

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