Scrubbing liquor

The electrostatic effect can be incorporated into wet scrubbing by charging the particulates and/or the scrubbing-liquor droplets. Electrostatic scrubbers may be capable of achieving the same efficiency for fine-particulate removal as is achieved by high-energy scrubbers, but at substantially lower power input. The major drawbacks are increased maintenance of electrical equipment and higher capital cost. 

Autopurification A wet-scrubbing process for removing hydrodrogen sulfide from coke-oven gas. The scrubbing liquor was an ammoniacal suspension of ferric ammonium ferro-cyanide. The process was developed by ICI, Billingham, UK, in the 1930s and 40s, but was abandoned in 1947. 

Flakt-Hydro A flue-gas desulfurization process which uses seawater as the scrubbing liquor. The pH of the effluent is raised with calcium hydroxide before it is discharged to sea. Developed by Norsk Viftefabrikk. Now owned by ABB Flakt Industri. As of 1996, 16 plants had been installed worldwide. 

Generally, although not exclusively, a scrubber with a recycle loop of the caustic scrubbing liquor is used cases of once-through scrubbing liquor operation do exist. The scrubber may be operated in batch, semi-batch or continuous mode with respect to the liquid. Process hazards exist in batch and continuous mode, the most significant of which is over-chlorination. Batch-wise operations leads to periodic high loads on the hypochlorite destruction unit. In order to even out these loads, and improve the process safety, a study of alternative treatment options has been undertaken. 

The process concept is shown in Fig. 26.7 where the recycle loop of the caustic scrubbing liquor passes through a fixed-bed reactor and then through the normal cooler. The blow-down of spent caustic and make-up with fresh caustic can be carried on in the same fashion as without the in-loop hypochlorite decomposition. Consideration of the optimum locations for removal and addition may, however, be slightly different. 

Consider a chlorine scrubbing unit operated semi-batch-wise. That is, a system similar to that shown in Fig. 26.7 is operated batch-wise but, periodically, a portion of the contents of the sump are sent to treatment and the sump refilled with fresh caustic. This mode of operation allows continuous treatment of the chlorine-laden vent stream while allowing batch operation without the need for a dual scrubbing-liquor reservoir. 

In wet scrubbing of SOp from boiler flue gas by limestone slurry, the concentration of dissolved species in the scrubbing liquor that can react with incoming SOp gas is very low, about one to two m-mole/1. This is far below the SOp make-per-pass in the scrubber, typically about 10 m-mole of SOp absorbed per liter of liquor for one pass through the scrubber. Therefore, the SOp absorption rate is largely dependent upon the slow rate of limestone dissolution into the liquor passing through the scrubber. 

Addition of magnesia to the scrubbing liquor increases the concentration of two dissolved sulfite species, SO3 and MgS0 , with CaS0 remaining constant. This increase in dissolved sulfite concentration makes the SOp absorption rate more dependent on the very fast liquid phase reactions of the basic sulfite species with the strong dibasic acid SOp(aq) ... 

S02 removal is strongly correlated with the concentration of dissolved basic sulfite species in the scrubbing liquor. The enhancement effect of sulfite on the number of mass transfer units in a scrubber is expressed by a simple exponential relationship. 

The degree of gypsum saturation of a 1imestone/magnesia scrubbing liquor can be calculated from either dissolved calcium or dissolved S02 (sulfite plus bisulfite) analyses. 

After extraction, the organic solvent, loaded with phosphoric acid, is scrubbed with a phosphoric acid solution in equilibrium with the organic solvent. In this way, coextracted contamination is removed. The scrub liquor is recycled to extraction. 

To effectively remove carbonyl sulfide from a gas stream, special alkaline scrubbing liquors are used. These contain sodium aluminate or sodium plumbite, or they are made of alkalies with a hydrolysis catalyst based on Zn, Fe, Ni, or Cu. Diethanolamine, diglycolamine, or other alkanolamines (qv) mixed with water remove carbonyl sulfide from sour, ie, acid-gas-containing, gas streams (25,26) (see Carbon dioxide). 

The technical and economic aspects of wet flue gas simultaneous desulfurization and denitrification systems are presented so that their practicality for utilization by utility industry can be assessed. The emphasis is on the kinetics of the systems based on the employment of ferrous chelates to promote the solubility of NO and the reactivity of NO with SO2 in scrubbing liquors. Analytical techniques are developed for characterizing reaction intermediates and products. Alternative approaches and novel ideas that could develop into a more efficient and cost-effective scrubber system employing metal chelate additives are discussed. 

The development of more efficient ferrous chelates that can increase the binding rate and equilibrium constant with NO, and also the reaction rate of ferrous nitrosyl chelates with sulfite/bisulfite ion, would allow the employment of smaller absorbers, reducing tanks, and L/G (flow rate ratio of scrubbing liquors to flue gas) to achieve the same scrubbing efficiencies. The determination of optimum scrubbing conditions and chemistry such that the formation of undesirable products can be depressed or eliminated would allow the reduction of cost in the area of scrubbing liquor regeneration. This paper addresses the kinetics and thermodynamics of important reactions in-... 

Pure SO2 is injected to achieve the desired concentration in the flue gas. Regeneration of spent scrubbing liquor is performed in the four-tank-in-series reactor train with a total residence time of 80 minutes. Limestone is fed to the first reactor as 45% slurry. 

Ultraviolet spectrophotometry (DuPont 400 SO2 analyzer) was used to monitor the gas phase SO2 concentrations and SO2 removal efficiencies. The pH of scrubbing liquor in each reactor was measured hourly during pilot testing. Solid dewatering properties were characterized by hold tank slurry settling rate and filter cake insoluble solids concentration. Detailed descriptions of the test facilities and analytical procedures were reported earlier(S). 

For the study of magnesium ion (Mg2+) effects on the system performance of the limestone DA process, Epsom salt (MgS04 5H20) was added to adjust the Mg2+ concentration in the scrubbing liquor. The base case system performance was established without the addition of epsom salt the steady-state concentration of Mg2+ was 355 Dpm. In subsequent tests, all operating conditions -- except for Mg2 ... 

Chloride ions (Cl ) accumulating in the scrubbing liquor will be balanced by cations such as sodium (Na+) and Mg2. Since previous data indicated that high Mg2+ concentrations are not desirable, it is expected that Na+ will be the cation associated with the Cl in limestone DA scrubbing solutions. 

I, it was operated at several different conditions to explore the scrubbing efficiency, the effect of chloride buildup in the scrubbing liquor, and to obtain a material balance. Tables IA and IB summarize the data obtained in the 13 runs made in the pilot plant. 

II. The filtrate was recycled to the system, mixed with additional sodium hydroxide, and pumped back into the column as scrubbing liquor. Chloride built up as the scrubbing reaction occurred. 

Precipitation occurs when solubility limits of the components in solution are exceeded. This is the case when chloride concentration in the scrubbing liquor reaches values above 6.6 percent. These salts redissolve in the liquor, indicating that they are most likely sodium salts. 

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