SO2 removal

Most industrialized countries have problans with their SO2 emissions. Thus, for several decades the interest in research for controlling SO2 pollution has continued, use of ACs being one possible alternative for controlling these emissions [6,60,140-142]. 

The effect of the porous structure of ACs on SOj retention has been the subject of intense research. These studies give information about the retention mechanism (both adsorption and oxidation) and about the effect that porosity has on the retention of SO2. From our studies [140,141] it can be stated that a high volume of narrow micropores is also desirable to achieve high SO2 adsorption capacity. 

To analyze the SO2 adsorption capacity until saturation [140,141], our experimental system consists of IR and UV gas analyzers (Rosemount Binos 1001 SOj/ O2, Binos CO/CO2). A 1.2-cm-diameter differential reactor has been used, with 0.2 g of sample. The gas flow was 620 mL/min, with a concentration of2000 ppm SO2, S OjinNj. 

FIGURE 1.39 Breakthrough curves showing a delay time between CH4 and CO2. 

Separation of gases is a very important process in several industries (e.g., chemical, petrochemical, and related industries). Although cryogenics and absorption remain the most widely used processes, the last two decades have seen a tremendous growth in research activities and commercial applications of adsorption-based gas separation. Separation by adsorption is based on the selective accumnlation of one or more components of a gas mixture on the surface of a microporous solid. The separation is achieved by one of three mechanisms steric, kinetic, or equilibrium. Most processes operate by virtue of equilibrium (or competitive) adsorption of gases from binary or multicomponent mixtures [143]. 

Titania is the most common catalyst used in the chemical industry and oil refineries for the removal of SO2 through the Claus reaction [563,565,566] and the reduction of SO2 by CO [563,565]  

The main products of the adsorption of SO2 on Ti02 are SO3 and SO4 species, and no dissociation of the molecule is achieved [206,565,566]. 

The results show clearly that the Au/Ti02 system is much more chemically active than the Au/MgO (see Fig. 6.27). It appears that MgO plays a minor role in the chemical activity of the supported Au nanoparticles. The dissociation of SO2 on Au/MgO is very limited due to weak Au/MgO interactions [559]. In contrast, Au/Ti02 interactions are complex and simultaneously 

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Fig. 10. Effect of potassium on SO2 removal where (—) represents the NSPS limit, and (V) represents LMF4-T, ( ) LMF4-U, and ( ) LMF4-V.

Other gas-treating processes involving sulfolane are (/) hydrogen selenide removal from gasification of coal, shale, or tar sands (qv) (108) (2) olefin removal from alkanes (109) (J) nitrogen, helium, and argon removal from natural gas (110) (4) atmospheric CO2 removal in nuclear submarines (5) ammonia and H2S removal from waste streams (6) H2S, HCl, N2O, and CO2 removal from various streams (111—120) and (7) H2S and SO2 removal from... 

Three demonstrations of the LIMB technology have been carried out. The first was a privately funded project in the 75 MWt Boiler 405 at the No. 4 AC Station of Inland Steel Industries, Inc. (56). By injecting 70 wt % minus 200 mesh (74 -lm) limestone, approximately 40% SO2 removal was achieved at a Ca S ratio of 3. This rose to 50% removal when the Ca S ratio was increased to 4. The second LIMB demonstration was the backup desulfurization system installed by B W as part of the relocation, repowering, and reconfiguration of the PCS power plant (46). 

The General Electric in-duct scmbbing (IDS) process involves the atomization of a slaked lime slurry, using a rotary disk atomizer A test at the 12 MWe scale at the Muskingum River Station of Ohio Power, performed in a duct with a 4.3-m cross section, achieved 50% SO2 removal with good lime utilization... 

The Dravo hydrate addition at low temperature process involves a two-step injection of water and dry sorbent in a rectangular 19.8-m duct having a cross section of 2 m. In one step water is injected through atomization nozzles to cool the flue gas from 150°C to approximately a 15°C approach to adiabatic saturation. The other step involves the dry injection of hydrated lime, either downstream or upstream of the humidifica tion nozzles. Typical SO2 removals were 50—60% at a Ca S ratio of 2. 

Catalyst temperature, flush liquid, space velocity SO2 removal in 190 mm deep bed (%) Bed depth for 95% S02 removal (m) Pressure drop at 95% S02 removal (kPa)... 

Several uncertainties in this periodic process have not been resolved. Pressure drop is too high at SV = 10,000 h 1 when packed beds of carbon are used. Study of carbon-coated structured packing or of monoliths with activated carbon washcoats is needed to see if lower pressure drops at 95% SO2 removal can be achieved. Stack gas from coal or heavy oil combustion contains parts-per-million or -per-billion quantities of toxic elements and compounds. Their removal in the periodically operated trickle bed must be examined, as well as the effect of these elements on acid quality. So far, laboratory experiments have been done to just 80°C use of acid for flushing the carbon bed should permit operation at temperatures up to 150°C. Performance of periodic flow interruption at such temperatures needs to be determined. The heat exchange requirements for the RTI-Waterloo process shown in Fig. 26 depend on the temperature of S02 scrubbing. If operation at 150°C is possible, gas leaving the trickle bed can be passed directly to the deNO, step without reheating. 

Sulfur dioxide removal processes can be used to treat flue gas from industrial boilers, heaters, or other process gases where sulfur compounds are oxidized. These processes have generally been proven in utility applications. More recently, several industrial SO2 removal installations have been completed. 

Process Selection. The following guidelines are appropriate to the selection of an SO2 removal process. 

The presence of other sulfur recovery facilities that could be integrated with the SO2 removal protess should be considered. 

Process Installations. Because of past operating problems with utility SO2 removal processes, information on the type and number of industrial SO2 removal process installations is of interest here. As of April 1979, 36 industrial SO2 removal processes were in operation and an additional 21 installations were in some stage of planning, design, or construction. These 57 installations represent a total of around 8.5 million SCFM capacity with 163 systems operating on 305 different boilers. Table IV summarizes the parameters of interest for the operating SO2 removal processes (23). 

A total of 81 tests were conducted, 44 without magnesia addition, and 37 with magnesia. The tests were designed to evaluate the effects of magnesia addition, slurry flow rate to the scrubber, scrubber inlet liquor pH, and total height of spheres on SO2 removal. Gas velocity has no significant effect on SO2 removal for the range tested. 

The following correlation for prediction of SO2 removal fits the experimental data ... 

Equation 10 explains 95 percent of the variation in the data for SO2 removal with a standard error of estimate of 3.2 percent SO2 removal. Values of SO2 removal predicted by Equation 10 are plotted against the corresponding measured values in Figure 7. 

The effect of the concentration of dissolved sulfite, the reactive base, on SO2 removal in Equation 10 can be represented as ... 

Figure 8 shows the effects of dissolved sulfite concentration and slurry flow rate on SO2 removal as predicted by Equation 10 for the TCA with 15 inches of spheres and a scrubber... 

Na](M) pS02/pH20 0.02, infinite stages Steam Requirement Moles H20/Mole SO2 90% SO2 removal,, 1.0 M Citrate Nonlinearity Factor Capacity Moles S02/liter... 

Silica from zeolite migrates less readily. In the magnesia-alumina system, spinel, as identified by X-ray diffraction, is inactive for SO2 removal. The effect of temperature on steam stability, oxidative adsorption and reductive desorption of SO2 are described. Five commercial catalyst types are ranked for SOx removal. 

Selection of Oxides. At Amoco, previous studies in the literature on SO2 removal from flue gas have been used to guide the selection of oxides for the UltraCat process but they have been of limited direct usefulness. This was true because of the peculiar requirements of the UltraCat process of high adsorption temperature, low regeneration temperature, and non-interference with the cracking reactions. The previous literature studies generally assumed that SO2 would be adsorbed at temperatures close to a stack gas temperature of 600 E, and desorb at either the same temperature or higher. The conditions of these studies was set. 

To show the alumina effect quantitatively, a series of catalysts was made in which the amount of alumina in the matrix was varied from 25 to 100% by adding alumina sol to a 25% alumina, silica-alumina slurry. These catalysts were formulated with REY molecular sieve. The results for SO2 removal are shown in Figure 3 where SO2 removal (corrected for unit factor) increases with increasing alumina. Our conclusion that alumina was important for SO2 adsorption also confirmed the results of Blanton and Flanders at Chevron (22). The non-linearity of the relationship implies an antagonistic effect between silica and alumina. The silica-alumina antagonism will be discussed relative to deactivation subsequently. 

Rare Earths and Alumina. A much easier and cheaper way of getting the SO2 removal enhancement from rare earths that was observed with the well-exchanged rare earth Y zeolite was to add rare earths, especially cerium, by direct impregnation to high alumina cracking catalyst (24). 

Other rare earths, including yttrium (29) and lanthanum (30) are active for SO2 removal as shown on Table III. 

These materials were made to contain 10 wt% oxides on gamma alumina. The percentage of SO2 removed after 50 minutes was measured, at 1250°F, for these additives at the 1 wt% level mixed with cracking catalysts. They were then ranked by the ratio of the % removed to that removed by cerium on alumina. 

Platinum. Other materials are effective promotors for the oxidative adsorption of SO2. Figure 6, for instance, demonstrates the effect of platinum which is the best promotor and the earliest one used for the UltraCat process (31). The figure, which compares SO2 removal curves for alumina alone and with 2 and 100 ppm Pt at 1200, 1300 and 1400 F, indicates that alumina promoted with platinum at both levels is more efficient for removing SO2 than pure alumina. The catalytic effect of platinum, not unexpectedly, becomes less pronounced as the temperature is increased as can be seen by inspecting the curves and also by comparing the percentage of SO2 removed after 100 minutes as shown on Table IV. 

Without platinum, alumina becomes more effective for removing SO2 as the temperature is increased. In this unpromoted case, the rate of oxidative adsorption of SO2 controls the amount of SO2 removed. Increasing the temperature increases that rate. 

In contrast, with platinum, SO2 removal, while always greater than the unpromoted case, tends to decrease with increasing temperature. The presence of platinum increases the rate of oxidative adsorption of SO2 to the point that the capacity of alumina becomes the limiting factor rather than the rate. The capacity, limited by thermodynamics, decreases with increasing temperature because of the stability of surface sulfate species... 

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