Bisulfite gaseous

The calcium bisulfite acid used in the manufacture of sulfite cellulose is the product of reaction between gaseous sulfur dioxide, liquid water, and limestone. The reaction is normally carried out in trickle-bed reactors by the so-called Jenssen tower operation (E3). The use of gas-liquid fluidized beds has been suggested for this purpose (V7). The process is an example of a noncatalytic process involving three phases. 

Sodium sulfite scrubbing can be configured as a regenerative process if the regeneration is performed thermally in an evaporator system. Thermal regeneration removes gaseous sulfur dioxide with the sodium bisulfite being converted back to sodium sulfite. The sulfur dioxide is then reduced to elemental sulfur or converted to sulfuric acid. 

Solutions containing ammonium sulfate, with or without the addition of ammonium hydroxide, have been widely used. The ammonium system can operate effectively only within a pH range of 4.0 to 7.0. As the pH value increases above 7.0, progressively more gaseous ammonia is liberated and this reacts in the gaseous phase with water vapor and SO2 to produce a dense aerosol (white plume) which is difficult for scrubbers to remove. In an ammonia system, in order to regenerate the scrubbing solution, the ammonium bisulfite and sulfite mixture is heated... 

Alternatively, the ammonium bisulfite/sulfite mixture can be treated with calcium hydroxide. Gaseous ammonia is evolved and trapped in water, which is then recirculated to the scrubber. 

The bisulfite leaving the scrubber is then reduced with gaseous hydrogen sulfide, which precipitates elemental sulfur by a modified Qaus reaction ... 

Sulfiu dioxide, in the presence of sxilfuric add, reduces the predpitated manganese dioxide to the soluble sulfate the removal of this large quantity of flocculent material greatly facilitates separation of the heptoic acid. The addition of sulfim dioxide requires about two hours, and an excess is to be avoided. Sodium bisulfite may be used if gaseous sulfm dioxide is not available. 

Figure 2. Typical gas volume data for the absorption of gaseous Oe into a bisulfite solution. Sodium acetate-acetic acid buffer at 0.5 M initial [HSOf] = 0.012 M [0 ] = 1.23 X 10 3 M. Insert O, 0 absorption into H20 , initial 02 absorption into HSOi solution.

The mechanism of the Claus reaction is complicated, and the sequence of the surface reaction steps is not fioUy elucidated (361). The primary step is certainly the strong adsorption of SO2 on acid—base pairs with formation of sulfite and bisulfite species (354—356), which later react with gaseous or weakly adsorbed H2S. The strength of the SO2 adsorption depends on the surface basicity, which is influenced by the amount of sodium present (225) however, it must be taken into account that the reactivity of the formed sulfites is lower on more basic surface sites. Thus, the acid—base properties of the catalyst play a key role in this reaction. According to Clark et al. (362), sulfate and thiosulfate species apparently form in addition to the sulfites. It was proposed that sulfate and thiosulfate react to form [HS404] ions, which then react with H2S to form the S3 sulfur polymorph, which is subsequently converted into cychc Se and Sg molecules. 

Fruits for dehydration are often treated with gaseous SO2 from burning sulfur as used in the manufacture of dried apricots, peaches, bananas, raisins, and sultanas. Alternatively, apple slices are generally dipped in solutions of the additive (prepared by dissolving sodium bisulfite or SO2 in water) and may receive an extra treatment with gaseous SO2 during drying. 

Applications. Sulfur dioxide is used in its gaseous or liquid form, or in the form of one of its salts mainly on dried fruits, molasses, syrups, concentrates, fruit juices and wines. Sulfur dioxide, sulfites, bisulfites and pyro-sulfites are more or less equivalent permitted as active ingredients in food worldwide. Many countries do not allow the use of sulfite for meat, fish or processed meat and fish products anymore. 

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