Advanced Sulfur Control Concepts in Hot Gas Desulfurization Technology. Quarterly Report, April 1--June 30, 1996 PDF Download
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Pages : 47
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Experimental effort during the past quarter was restricted to the fixed-bed reactor. Effort during April was devoted to the sulfidation and regeneration of cerium oxide. Sulfidation tests were plagued by over-sulfidation, i.e., the quantity of H2S removed from the gas phase exceeded the stoichiometric amount associated with the conversion of CeO2 to Ce2O2S. This was initially attributed to the formation of Ce2S3 which was found to be thermodynamically possible in the highly reducing feed gas. However, the addition of steam to the feed gas to prevent Ce2S3 formation did not eliminate the over-sulfidation problem. Later tests indicated that the apparent over-sulfidation was due to reaction between H2S and the walls of the reaction vessel. Apparently the alonizing treatment to passivate the reactor walls was either ineffective at the reaction conditions or had deteriorated with use to the point that protection was no longer viable. Limited Ce2O2S regeneration results, although very qualitative, were quite favorable. In one regeneration test in an O2-N2 atmosphere, no SO2 or H2S were detected by the chromatograph in the regeneration product. Significant amounts of total sulfur were detected, and the test had to be terminated prematurely when elemental sulfur caused the product line leading to the chromatograph to plug. Experimental tests during May and June examined the regeneration of FeS as a function of temperature, gas feed composition, and gas flow rate. Complete regeneration was achieved with as much as 75% of the sulfur liberated in elemental form. Low regeneration temperature and large ratios of H2O to O2 in the feed gas promote the formation of elemental sulfur. A number of changes in the reactor system were made during the quarter, including improvements to the sulfur condenser and filters on the reactor product line leading to the gas chromatograph.
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Experimental effort during the past quarter was restricted to the fixed-bed reactor. Effort during April was devoted to the sulfidation and regeneration of cerium oxide. Sulfidation tests were plagued by over-sulfidation, i.e., the quantity of H2S removed from the gas phase exceeded the stoichiometric amount associated with the conversion of CeO2 to Ce2O2S. This was initially attributed to the formation of Ce2S3 which was found to be thermodynamically possible in the highly reducing feed gas. However, the addition of steam to the feed gas to prevent Ce2S3 formation did not eliminate the over-sulfidation problem. Later tests indicated that the apparent over-sulfidation was due to reaction between H2S and the walls of the reaction vessel. Apparently the alonizing treatment to passivate the reactor walls was either ineffective at the reaction conditions or had deteriorated with use to the point that protection was no longer viable. Limited Ce2O2S regeneration results, although very qualitative, were quite favorable. In one regeneration test in an O2-N2 atmosphere, no SO2 or H2S were detected by the chromatograph in the regeneration product. Significant amounts of total sulfur were detected, and the test had to be terminated prematurely when elemental sulfur caused the product line leading to the chromatograph to plug. Experimental tests during May and June examined the regeneration of FeS as a function of temperature, gas feed composition, and gas flow rate. Complete regeneration was achieved with as much as 75% of the sulfur liberated in elemental form. Low regeneration temperature and large ratios of H2O to O2 in the feed gas promote the formation of elemental sulfur. A number of changes in the reactor system were made during the quarter, including improvements to the sulfur condenser and filters on the reactor product line leading to the gas chromatograph.
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Languages : en
Pages : 28
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Twenty-five reduction/sulfidation tests plus one sulfidation/regeneration test were completed during the quarter. The reduction/sulfidation tests examined the behavior of six cerium oxide sorbents from different sources with reaction variables of temperature, pressure, gas composition and flow rate. Most significantly, steam was added to the sulfidation feed gas for the first time. Tests using pre-reduced sorbents and tests in which reduction and sulfidation occurred simultaneously were performed. Prebreakthrough H2S concentrations less than 10 ppmv were obtained over a range of reaction conditions with prebreakthrough concentrations as low as 1 ppmv achieved at the most favorable conditions. The general response to reaction variables was as expected except when feed rate was varied. In some of these cases the FPD breakthrough time did not correspond to expectation. The single regeneration run was conducted at 600 C and 2 atm using 12% SO2 in N2 at a feet rate of 400 sccm. This was the first regeneration test at other than 1 atm pressure; favorable results were obtained. The only experimental objective remaining is additional high pressure regeneration testing.
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Pages : 43
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Three areas of research were pursued during the past quarter. Experimental CeO2 sulfidation and regeneration tests examined the effect of SO2 concentration and gas flow rate on the production of elemental sulfur during regeneration. The maximum number of cycles using a single sorbent charge was increased to 13, and initial tests using a second source of CeO2 (from Molycorp, Inc.) were carried out. In the process analysis effort, a third case study based on single-stage desulfurization using CeO2 sorbent was added. Capital and operating costs for this option were estimated under base case conditions. The sensitivity of the annual levelized cost of all three cases to variations in sorbent durability, sorbent unit cost, O2 and N2 unit cost, and capital cost was examined. As the sorbent cost was reduced, based on smaller sorbent replacement rate and/or smaller sorbent unit cost, the annual levelized cost of all three processes decreased, and the cerium process became more attractive. For example, at a sorbent replacement rate of 0.1% of the sorbent circulation rate, both cerium processes should be less costly than the single-stage zinc sorbent process. As the sorbent replacement rate approaches zero (infinite sorbent lifetime), income from the sulfur by-product and export steam produced by the cerium processes exceeds the other process costs and a profit of $2 to 2.5 million appears possible. In contrast, the annual levelized cost of the zinc-based process at zero sorbent replacement rate is about $5 million.
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Pages : 47
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The primary objective is to determine the feasibility of an alternate concept for the regeneration of high temperature desulfurization sorbents in which elemental sulfur, instead of SO2 is produced. Iron and cerium-based sorbents were chosen on the basis of thermodynamic analysis to determine the feasibility of elemental sulfur production. Experimental effort on the regeneration of FeS using the partial oxidation concept was completed during the quarter, and attention returned to the sulfidation of CeO2 and regeneration of Ce2O22S. Progress was made in the process simulation effort involving two-step desulfurization using CeO2 to remove the bulk of the H2S followed by a zinc-titanate polishing step. The simulation effort includes regeneration of Ce2O2S using two concepts - reaction with SO2 reaction with H2O. Elemental sulfur is formed directly in the reaction with SO2 while H2S is the product of the regeneration reaction with steam. Steam regeneration is followed by a Claus process to convert the H2S to elemental sulfur. The last test involving partial oxidation regeneration of FeS was completed in early July. Experimental problems were encountered throughout this phase of the program, primarily associated with erratic readings from the total sulfur analyzer. The problems are attributed to variable flow rates through the capillary restrictor, and, in some cases, to steam concentrations which exceeded the capacity of the membrane dryer. Nevertheless, sufficient data was collected to confirm that large fractions of the sulfur in FeS could be liberated in elemental form. Low regeneration temperature ((approximately)600°C), large steam-to-oxygen ratios, and low space velocities were found to favor elemental sulfur production.
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Pages : 59
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Good progress was made on both the experimental and process modelling fronts during the past quarter. All experimental tests used the fixed-bed laboratory reactor to study the sulfidation of CeO2 with H2S and the regeneration of Ce2O2S using SO2. A number of experimental problems were solved (or at least alleviated) during the quarter including malfunctioning mass flow controllers, excessive bed pressure drop, and elimination of the H2S plateau during early stages of sulfidation tests. Most CeO2 sulfidation tests were carried out a 800°C and 5 atm using a sulfidation gas containing 1% H2S, 10 % H2, balance N2. At these conditions sulfidation of CeO2 was rapid and complete. Sulfur material balance closure was satisfactory, and, except for the unexpected H2S plateau during the prebreakthrough period, the sulfidation results were as expected. Near the end of the quarter, the cause of the H2S plateau was tentatively identified as being due to reaction between H2 and elemental sulfur deposited downstream of the sorbent in the bottom of the reactor and in tubing leading to the gas chromatograph. The sulfur deposits occurred during regeneration tests, and chemically cleaning the lines between regeneration and sulfidation coupled with reducing the temperature of the transfer line during sulfidation greatly reduced the H2S plateau. A brief examination of the effect of sulfidation temperature between 700 and 850°C showed relatively little temperature effect, although the slope of the active portion of the breakthrough curve was somewhat smaller at 700°C, which is consistent with a smaller reaction rate at this temperature.
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Pages : 27
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The primary objective of this research project is the direct production of elemental sulfur during the regeneration of known high temperature desulfurization sorbents. The contract was awarded to LSU on April 12, 1994, and this quarterly report covers accomplishments during the first 2 1/2 months of the project. Effort during the initial 2 1/2 month period has been limited to Tasks 1 and 2, and involves a search of the literature to identify concepts for producing elemental sulfur during regeneration of known metal oxide sorbents and a thermodynamic evaluation of these concepts. While searching and evaluating the literature is a continuing process, concentrated effort on that phase is now complete and a detailed summary is included in this report. Three possible concepts for the direct production of elemental sulfur were identified in the LSU proposal, and the literature search has not uncovered any additional concepts. Thus, the three concepts being investigated involve: (1) regeneration with SO2, (2) regeneration with mixtures Of 02 and H2O, and (3) regeneration with H2O. While concept (3) directly produces H2S instead of elemental sulfur, the concept is included because the possibility exists for converting H2S to elemental sulfur using the Claus process. Each of the concepts will ultimately be compared to the Direct Sulfur Recovery Process (DSRP) under development by RTI. DSRP involves initial sorbent regeneration to SO2, and the inclusion of additional processing steps to reduce the SO2 to elemental sulfur.
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