Exhaust gases, scrubbing

The effluents from the exhaust gas scrubbing unit are constituted by acidic drippings from the ESP (-1.8 to 2.0 kg/t of LAS) and Na2S03/water solution (9-10% dry matter content). 

The exhaust gas is very acidic (pH < 1 using wet pH paper), and therefore should be scrubbed by bubbling through a solution of potassium hydroxide. 

Black Liquor Soap Acidulation. Only two-thirds of a typical black liquor soap consists of the sodium salts of fatty acids and resin acids (rosin). These acids are layered in a liquid crystal fashion. In between these layers is black liquor at the concentration of the soap skimmer, with various impurities, such as sodium carbonate, sodium sulfide, sodium sulfate, sodium hydroxide, sodium lignate, and calcium salts. This makes up the remaining one-third of the soap. Crude tall oil is generated by acidifying the black liquor soap with 30% sulfuric acid to a pH of 3. This is usually done in a vessel at 95°C with 20—30 minutes of vigorous agitation. Caution should be taken to scrub the hydrogen sulfide from the exhaust gas. 

CVD processing can be accompanied by volatile hot-reaction by-products such as HQ or HF, which, along with unused precursor gases, must be removed from the exhaust gas stream. This is done by scrubbing the chemicals from the gas using water to dissolve soluble products or by burning the precursor gases to form oxides. 

The gas then consists mainly of TiCl4, and is cooled below 0°C, causing most of the TiCl4 to condense (f). The small amounts of TiCl4 and Cl2 remaining in the exhaust gas (C02, CO, and N2) are removed by scrubbing with alkali (t). 

The 1500-ml. sample of 100 p.p.m. of potassium cyanide as CN (cyanide ions) aqueous solution, with potassium hydroxide added to raise the pH, was introduced into the reactor (bottle type or absorption column). The desired catalytic solution was added by means of a pipet. The ozonized oxygen was passed into the reactor and the exhaust gas was measured by a wet-test meter, after the excess ozone had been scrubbed out by a potassium iodide solution. At the same time a separate sample of exit gas was analyzed by using potassium iodide solution and measuring the gas with a gas meter. The gas stream to the reactor was led into another gas absorption bottle with potassium iodide solution at definite intervals, so that samples of the residual cyanide solution could be withdrawn. 

Environmental concerns of lime kiln operation chiefly relate to exhaust gas dust control and are usually solved by water scrubbing. The slaked lime (calcium hydroxide) produced by the scrubber can be employed as a base in the operations of a chemical complex, or sold. Electrostatic precipitation of precooled gases is also used, sometimes in series with water scrubbing. The dissociated carbon dioxide discharged is not ordinarily regarded as a pollutant. The amount discharged from this source is far less than the carbon dioxide produced by fossil fuel combustion. Lime kilns associated with Solvay ammonia-soda plants may be able to recycle a part of the dissociated carbon dioxide, particularly if the concentrations are raised by the use of oxygen-enriched combustion air. Recently, interest has been shown... 

The problem of sulfur dioxide pollution is made more complicated by the energy crisis. As petroleum supplies dwindle and the price increases, our dependence on coal will probably grow. As supplies of low-sulfur coal are used up, high-sulfur coal will be utilized. One way to use high-sulfur coal without further harming the air quality is to remove the sulfur dioxide from the exhaust gas by means of a system called a scrubber before it is emitted from the power plant stack. A common method of scrubbing is to blow powdered limestone (CaC03) into the combustion chamber, where it is decomposed to lime and carbon dioxide ... 

Nutrient Rerxn/eryFrom Gas Streams- A multistage scrubbing system captures fumes from the reactors and any dust that escapes the cyclones. The nutrient content of the scrubber liquor increases as it moves countercurrently throi Qh the reactor gas scrubbing system to the granulator exhaust gas scrubber. Nutrient salts in scrubber liquor are recycled to the reactors. There are no liquid effluents from the plant. 

Recently, a new exhaust gas treatment based on the chemical scrubbing of the unreacted SO3 (by reaction with freshly added LAB) has been developed and retrofitted in several industrial plants located in areas where the acidic dripping disposal indeed has a high cost and poor feasibility. 

Moreover, the new system entails an increase in the overall production yield ( 1%) obtained by recovering the partially sulfonated materials generated by contacting the exhaust gas with the sprayed amount of LAB. The impact of this recovered stream on the quality of the final LAS-sulfonic acid is limited to a minor increase in the unsulfonated matter content and color. The overall gas scrubbing efficiency has proven to be the same ensured by the standard ESP-based gas treatment units. 

The exhaust gas leaving the scrubbing system should normally have a residual SO2 concentration of max. 5 mg/m during plant start-up, when the conversion of SO2 SO3 will rise from 50% to 98%, the SO2 concentration in the exhaust gas may be for a short period somewhat higher (10-15 mg S02/m ). 

Scrubbing of residual JSO. from exhaust gas (see figure 44 and table 19)... 

Exhaust Gas Treatment The gas effluent must be treated to comply with environmental regulations. The exhaust gas from the reactor contains traces of organic matter and unreacted SO3 and SO2 gases. The first two impurities are removed by the electrostatic precipitator. The remaining SO2 gas is removed by the reaction with caustic soda solution flowing countercurrently inside the scrubbing column. The concentration of the residual SO2 in the exhaust gas vented into atmosphere is kept at 5 ppm maximum, well below the allowable standards. 

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