Stream sulfur dioxide feed

Many large smelting operations that produce veiy high concentrations of sulfur dioxide feed the gas stream directly into a sulfuric acid plant The design and operation of acid plants of this type ate not discussed in this text, as they are considered to represent a separation and chemical manufacturing operation, not a gas purification process. On the other hand, the removal of sulfiu dioxide from dilute smelter oif-gas streams and the recovery of unconverted sulfur dioxide finm the acid plant tail gas constitutes gas purification problems and are reviewed in this chapter. 

Other components in the feed gas may react with and degrade the amine solution. Many of these latter reactions can be reversed by appHcation of heat, as in a reclaimer. Some reaction products cannot be reclaimed, however. Thus to keep the concentration of these materials at an acceptable level, the solution must be purged and fresh amine added periodically. The principal sources of degradation products are the reactions with carbon dioxide, carbonyl sulfide, and carbon disulfide. In refineries, sour gas streams from vacuum distillation or from fluidized catalytic cracking (FCC) units can contain oxygen or sulfur dioxide which form heat-stable salts with the amine solution (see Fluidization Petroleum). 

A derivative of the Claus process is the Recycle Selectox process, developed by Parsons and Unocal and Hcensed through UOP. Once-Thm Selectox is suitable for very lean acid gas streams (1—5 mol % hydrogen sulfide), which cannot be effectively processed in a Claus unit. As shown in Figure 9, the process is similar to a standard Claus plant, except that the thermal combustor and waste heat boiler have been replaced with a catalytic reactor. The Selectox catalyst promotes the selective oxidation of hydrogen sulfide to sulfur dioxide, ie, hydrocarbons in the feed are not oxidized. These plants typically employ two Claus catalytic stages downstream of the Selectox reactor, to achieve an overall sulfur recovery of 90—95%. 

In two processes under development as of 1997, the sulfur dioxide stream reacts with reduciag gas over a proprietary catalyst to form elemental sulfur. Both processes have achieved a sulfur recovery of 96% ia a single reactor. Multiple reactor systems are expected to achieve 99+% recovery of the feed sulfur. The direct sulfur recovery process (DSRP), under development at Research Triangle Institute, operates at high temperature and pressure. A similar process being developed at Lawrence Berkeley Laboratory is expected to operate near atmospheric pressure. 

SuRe [Sulphur recovery] A version of the Claus process in which the capacity of the plant is increased by using air enriched in oxygen in the production of the sulfur dioxide. There are two versions SURE SSB [Side Stream Burner], and SURE DC [Double Combustion], In the first, a small portion of the feed stream containing hydrogen sulfide is burnt sub-stoichiometrically in a second burner in the second, the hydrogen sulfide is oxidized in two stages, with cooling and sulfur separation between them. Both of these... 

Sulfur dioxide is contained in the feed and effluent streams of a chemical reactor, but it is neither a reactant nor a product. The volumetric flow rates of both streams (L/min) are measured with rotameters, and the concentrations of SO2 in both streams (mol/L) are determined with a gas chromatograph. The molar flow rate of SO2 in the reactor effluent (determined as the product of volumetric flow rate and concentration) is 20% lower than the molar flow rate of SO2 in the feed. Think of as many possible explanations for the discrepancy as you can. 

The simplest gas-solid containment systems conceptually are the direct adsorption ones. These accomplish adsorption on solids such as activated carbon, or alkalized alumina at relatively low temperatures and ordinary pressures [46]. In a separate unit a more concentrated sulfur dioxide stream is produced when the saturated absorbent is regenerated by heating. This is a more economically attractive feed to an acid plant or for liquefaction or sulfur generation. 

The process flowsheet as presently developed is shown in Figure 17. The exothermic Bunsen reaction produces two aqueous solutions of sulfuric acid and hydriodic acid from material feeds of water, sulfur dioxide and iodine. The reaction favors presence of excess water and iodine to make it spontaneous and with iodine rich hydriodic acid (Hix) formed to facilitate subsequent phase separation. The excess of water and iodide, however, imposes heavy process stream loads upon subsequent reactions, particularly so in the HI reaction steps. Though not yet reflected in the present flowsheet, improved reaction conditions are being studied with the goal of significantly reducing excessive reactants in order to simplify overall process and production cost. 

The effluent streams from Claus plants contain unreacted hydrogen sulfide and sulfur dioxide and elemental sulfur present as vapor and mist (77). They commonly also contain carbonyl sulfide and carbon disulfide formed by reactions with hydrocarbons present in the feed gas (77). It is usually required that the tail gas be incinerated, even though not otherwise treated, to convert the hydrogen sulfide, carbonyl sulfide, and carbon disulfide to the less toxic and malodorous sulfur dioxide. 

Once the feed stream has been purified, the sulfur dioxide and oxygen dimensions must be defined. Depending upon its source, the sulfur dioxide may vary from a few tenths of 1% to 100% (dry basis), in combinations with oxygen from 0% up to the line shown on Figure 2. The dotted line represents the gas composition that results when 100% sulfur dioxide (dry basis) is diluted with air. The only gas compositions to which Allied Chemical sulfur dioxide reduction technology is not directly applicable are those in the shaded area at the lower left of this diagram. This lower boundary represents a practical limit which has been established by heat balance and thermodynamic considerations rather than by economic factors. 

Impurities such as sulfur dioxide can react with ion-conducting membranes and reduce performance, and the reactivity depends on the partial pressure of SO2 in oxygen-containing stream and certain reactant species present in the ITM. It has been observed [12] that each membrane composition has a critical threshold SO2 partial pressure, above which SO2 will react with the ITM materials to impact oxygen flux. This result helps to determine the required level of sulfur removal from oxygen-containing feeds. 

Fig. 16 shows the SO2 breakthrough curve from the fixed-bed packed with 0.5 g of one of the sol-gel derived y-AbOa/CuO sorbent prepared by the solution-sol mixing method (Sample 3 in Table 8). The feed flow rate is 8.7 ml/min. The temperature for the experiment is 400°C. As shown, the concentration of sulfur dioxide in the effluent stream is zero in the first 3 hours. After 3 hours, the concentration of SO2 in the effluent stream begins to increase, but it takes more... 

---