Scrubbing reagent

The economics seem to be better for systems where dry powdered fresh hme plus ground recycled hme is injected along with a relatively coarse spray which impinges on and dries out from the reagent, as described by Stouffer et al. [Hs EC Res., 28(1) 20 (1989)]. Witnum et al. [9th Ann. Pitt. Coal Prep. Util. Euv. Control Contractors Conf. (1993)] describes an advanced version of that system that has been further optimized to the point that it is competitive with wet hme-stone scrubbing for >90 percent flue gas desirffurization. 

Other than the wear problems, actual in-plant maintenance usually involves removal of wood, pieces of blasting wire, and other trash from the ports. When a reagentized feed is used, layers of oily reagents can build up on the spiral surface and sometimes require scrubbing for removal. With feeds containing oily reagents that attack rubber, ra-sion-resistant alloy spiral sections are used. 

The combined action of chemical (or electrochemical) treatment and scrubbing is often more effective than either method alone. It is frequently advantageous to alternate short periods of immersion with scrubbing to remove any corrosion product that has become loosened by the action of the chemical reagent. 

Projected economics were also highly promising [41] capital and operating costs would be a fraction of those required by standard methods, e.g. scrubbing. Furthermore, no chemical reagents would be required and no waste stream produced. However, the high melting points of the alkali-metal sulfates (T > 512 °C) offered severe limitations to application, especially for use in power plants, where the flue gas typically is unavailable for treatment at temperatures below 400 °C. 

Before 1970, most of the data for nitrogen oxides were obtained by continuous measurements with a colorimetric analyzer that was similar in principle to the colorimetric oxidant analyzer shown in Figure 6-8. The scrubbing agent is a mixture of -(l-naphthyl)ethylenediamine, sulfanilic acid, and acetic acid in aqueous solution. The color is produced when both nitrogen dioxide and nitrites react with this reagent to form an azo dye. The color is not affected by nitric oxide in the air sample. 

Clean thoroughly and, if necessary, dry all apparatus before use. (If a formula calls for any dry or anhydrous chemicals, solvents, reagents, etc., then even the humidity in the air should be kept out of the reaction.) Scrub stubborn residue with acetone and a bristle brush, then use soapy water, and rinse thoroughly with water. 

The scrubbing liquid is controlled to a neutral pH with reagent addition to drive SO2 absorption. Caustic soda (NaOH) is typically used as the alkaline reagent. However, other alkalis, such as soda ash, magnesium hydroxide, and lime have also been utilized with excellent results in terms of performance and reliability. For FCCU applications, however, where a 5-7 year continuous operation is required, the use of lime as a reagent is not recommended. Multiple levels of spray nozzles provide sufficient stages of gas/liquid contact to remove both particulate and SO2. An illustration of the spray tower and the spray nozzles is provided in Figure 16.6. 

Historically, most wet scrubbing systems on FCCUs have utilized caustic (NaOH) as the reagent. Caustic is readily available in refineries, is easy fo handle, and has no solid reaction by-product. These systems have proven to be very effective and reliable, with continuous operation in excess of 5 years while handling all upset conditions that can occur. 

A three-channel approach was developed by Lee et al. (1993) to distinguish H202 from hydroxymethyl hydroperoxide and total peroxides. In this approach, one channel is used to scrub the air sample into a POPHA solution to obtain total peroxides. In a second channel, the air sample is scrubbed into Fenton reagent solution at a pH of 3. This converts the H202 into OH radicals ... 

Other reagents can be employed in a wet scrubbing process, but application is typically limited by the higher costs. 

Oxides of nitrogen, NO, can also form. These are generally at low levels and too low an oxidation state to consider water scrubbing. A basic reagent picks up the N02, but not the lower oxidation states the principal oxide is usually NO, not N02. Generally, control of NO, is achieved by control of the combustion process to minimize NO ie, avoidance of high temperatures in combination with high oxidant concentrations, and if abatement is required, various approaches specific to NO have been employed. Examples are NH3 injection and catalytic abatement (43). 

Choice of ideal scrub and strip reagents and complexants, resulting in a product of the highest purity and yield, an easy post-treatment of the stripped product, and the least amount of secondary wastes. 

Produced HBr is scrubbed by the use of 10 N sodium hydroxide solution to form sodium bromide. For all the runs, the initial inventory is 1.25 L of sodium hydroxide solution. The NaOH solution was sampled at regular intervals and sent to Argonne s Analytical Chemistry Laboratory for analysis. Volume aliquots of solution samples were diluted with reagent water and analysed by ion chromatography to determine bromide. Separate aliquots were diluted with acid addition and analysed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) to determine calcium. During the data analysis, adjustments are made for the addition of condensed unreacted steam to the neutralisation solution. 

Tributyl Phosphate. The reagent tributylphosphate (Eastman Kodak) was purified by scrubbing with sodium carbonate solution (5% v/v) and then with distilled water and by distillation in vacuo. 

Eight of the Bahco scrubbers are located on oil-fired boilers in pulp mills. The boilers range in size from 50,000 to 375,000 Ibs/hr of steam and produce 10,000-145,000 ACFM of oflF-gas at about 340°-360°F. Each of these units operates with caustic soda as the scrubbing reagent. The units operate at a controlled pH of 6.5 which maximizes the production of sodium bisulfite and limits unwanted sulfate oxidation. The soluble bisulfite product is utilized in the pulping operation. Particulate and mist concentrations are held to about 0.02 grain/SCFW and SO levels are reduced by 97-99% from about 1500 to about 30 ppm. These units have operated continuously without any operational upsets. 

---