Douglas P. Harrison

Hydrogen from methane in a single-step process

Addition of a calcium-based CO2 acceptor to commercial steam methane reforming catalyst permits the production of 95+% H2 in a single-step process. The combined reforming, shift, and CO2 separation reactions have been studied using a laboratory-scale fixed-bed reactor as a function of temperature, steam-to-methane ratio, acceptor-to-catalyst ratio, feed gas flow rate, and methane content of the feed gas. The combined reactions were sufficiently rapid above 550°C that equilibrium was closely approached at all reaction conditions studied. The H2 content of the product gas was relatively independent of temperature with the primary impurity at lower temperatures being CH4. Higher temperatures resulted in increased CH4 conversion but decreased carbon oxide removal leaving CO and CO2 as the primary impurities. Potential advantages of the single-step process, in addition to reducing the number of processing steps, include improved energy efficiency, elimination of the need for shift catalysts, and reduction in the temperature of the primary reactor by 150–200°C. Key unanswered problems involving the single-step process include continuous separation of catalyst and acceptor and the durability of the acceptor for multiple cycle operation.

Simultaneous shift reaction and carbon dioxide separation for the direct production of hydrogen

Abstract Hydrogen production via the reversible water-gas shift reaction normally requires multiple catalytic reaction steps followed by CO 2 separation in order to produce the required H 2 purity. Removal of CO 2 as it is formed via the noncatalytic gas—solid reaction between CO 2 and CaO provides the opportunity to combine reaction and separation into a single processing vessel. Resultant process simplifications include elimination of the need for heat exchangers between catalyst beds as well as the absorption and stripping units required for CO 2 removal (or the pressure swing adsorption unit). Process savings may be realized by reducing the quantity of excess steam to the reactor, and the expensive, sulfur-sensitive shift catalyst is not needed in the 500–600°C range of interest. The combined shift and carbonation reactions were studied in a laboratory-scale fixed-bed reactor containing dolomite sorbent precursor. The effect of shift-carbonation temperature and pressure, synthesis gas composition, space velocity, and composition and properties of the sorbent were studied. The rapid rates of the combined reactions permit equilibrium CO conversion and CO 2 removal to be closely approached. Greater than 0.995 fractional removal of carbon oxides was achieved over a range of reaction conditions.

High-temperature desulfurization using zinc ferrite: Reduction and sulfidation kinetics

The reduction and subsequent sulfidation of single cylindrical pellets of ZnFe2O4 have been studied in a microbalance reactor. Reduced zinc ferrite in the form of ZnO plus Fe3O4 is capable of rapid and complete reaction with H2S in the temperature range of 500–700°C. In strongly reducing atmospheres at the highest temperature, further reduction of Fe3O4 to FeO occurs and produces a negative effect on sulfidation kinetics. Time-conversion results during sulfidation are consistent with the unreacted core model assuming the global kinetics are controlled by mass transfer and pore diffusion. Values of the mass transfer coefficient and effective diffusivity determined from the experimental data are considerably larger than values obtained from literature correlations. Possible reasons for these deviations are discussed.

High temperature gas desulfurization with elemental sulfur production

Preliminary results on the use of cerium oxide as a high-temperature desulfurization sorbent are presented. The primary advantage of cerium over current zinc-based sorbents is the potential to produce elemental sulfur during the regeneration phase of the process. Although CeO2 is less effective for H2S removal during sulfidation, the sulfided product, Ce2O2S, will react with SO2 to produce elemental sulfur directly. Rapid and complete regeneration is possible over the range of 500 to 700°C, and only elemental sulfur is formed. Elemental sulfur concentrations (considered as S2) as large as 20 mol% have been produced in the regeneration product. The sorbent has been subjected to ten sulfidation–regeneration cycles using a laboratory-scale fixed-bed reactor with negligible activity loss. Effectively complete conversion of CeO2 to Ce2O2S during sulfidation and subsequent regeneration to CeO2 was achieved in each cycle. A two-stage desulfurization process using CeO2 for bulk H2S removal followed by a zinc sorbent polishing step has been proposed to meet specifications of the integrated gasification combined cycle (IGCC) process. Economic comparison with a single-stage desulfurization process using zinc sorbent followed by elemental sulfur recovery using the direct sulfur recovery process (DSRP) shows that the two-stage cerium process may be less costly if the cerium sorbent is sufficiently durable.

The variable property grain model applied to the zinc oxide-hydrogen sulfide reaction

Abstract The variable property grain model, including the effect of grain diffusion resistance, has been applied to experimental results from the reaction of hydrogen sulfide with zinc oxide. Grain radius is assumed to vary under the combined effects of sintering and extent of reaction. Property variations are correlated by the specific surface area, an easily measured quantity. All model parameters with the exception of the grain diffusion coefficient have been evaluated by literature correlations or independent experimental measurements. Significant improvement in the match with experimental data, as compared to the constant property grain model, has been achieved.

Partitioning of chloromethanes between aqueous and surfactant micellar phases

Abstract The partition constants for three volatile organic chemicals (VOCs)—methylene chloride, chloroform and carbon tetrachloride—between aqueous and surfactant micellar phases were determined using an equilibrium partitioning method. The surfactants used were sodium dodecylbenzene sulfonate (DDBS), sodium dodecylsulfate (DDS) and hexadecyltrimethyl ammonium bromide (HTAB). The vapor concentration of the VOCs remained unchanged until the critical micellar concentration (CMC) of the surfactant was reached. Above the CMC the vapor concentration decreased linearly with increasing surfactant concentration in the aqueous phase. The partition constants (Km) were observed to increase with increasing ionic strength of the solution. The partition constants also increased with increasing hydrophobicity of the VOCs. A pseudo-two-phase model was used to determine the Km values and was found to fit the experimental data well. Cationic micelles of HTAB gave larger Km values than anionic micelles of DDS and DDBS surfactants. Km values also increased with increasing temperature. From a thermodynamic treatment of the equilibrium process the values of the partition constants for transfer of VOCs from an ideal vapor phase to the micellar phase were calculated. These values were found to fall in between the values for aliphatic hydrocarbons and alcohols.

High-temperature desulfurization using zinc ferrite: regeneration kinetics and multicycle testing

Commercial use of a metal oxide for the high-temperature desulfurization of coal-derived gas will require that the sorbent undergoes successive sulfidaton and regeneration cycling without suffering a major activity loss. In this paper, the regeneration of sulfided zinc ferrite (ZnS + 2FeS) has been studied in a single-pellet electrobalance reactor at atmospheric pressure over a temperature range of 550-850°C. The regeneration atmosphere consisted of O2, H2O, SO2 and N2 in varying proportions. Regeneration is complicated by the formation of zinc sulfate at lower temperatures and/or high O2 and SO2 concentrations. The zinc sulfate can sunsequently be removed by thermal decomposition in an inert atmosphere or by direct exposure to a reducing atmosphere. Both methods appear equally effective in preparing the sorbent for further sulfidation. At high temperature (850°C) complete regeneration is not possible because of severe structural property changes associated with sintering. Multicycle runs using favorable regeneration temperatures indicate that the sulfidation reactivity increases slightly during the second and sometimes third cycles, and gradually decreases in later cycles. A maximum of eight sulfidation-regeneration cycles were conducted.

Multicycle performance of a single-step process for H2 production

Abstract Combining the chemical reaction and product separation steps in a single processing vessel is currently of great interest. In this study, reaction and separation are combined by carrying out the water-gas shift reaction in the presence of a calcium-based CO2 acceptor. The continuous removal of CO2 from the gas phase alters the shift reaction equilibrium and permits almost complete CO conversion and CO2 removal. The reaction temperature is significantly higher than employed in the traditional shift process, and, as a consequence, no shift catalyst is required. Previously reported results showed that greater than 99% removal of total carbon oxides could be achieved over a range of experimental conditions with greater than 99.9% removal achieved at the most favorable conditions. Total carbon oxide content of the product gas in the latter case was approximately 30 ppmv (dry basis). However, for the process to be economical, it is necessary that the CO2 acceptor retain its activity and capacity throug...

The role of solids in CO2 capture: A mini review

Publisher Summary The problem of global warming caused by greenhouse gas emissions is gaining increasing importance. Greenhouse gas (GHG) emission reductions can occur as a result of increased energy efficiency, substitution of low- or non-carbon fuels, or by the capture and storage of CO2. Growth in demand will no doubt erase gains associated with increased efficiency. Non-carbon energy sources are more limited, more expensive, and/or not accepted by the public. Fossil fuels will continue to be the major source of energy for the next few decades and there is increasing agreement that capture and storage will be required if atmospheric concentrations of CO2 are to be stabilized. Solid-based CO2 capture processes, while not having received the level of attention given to gas absorption processes, possess many potential advantages including high operating temperature and avoidance of liquid wastes. This paper discusses recent developments in both the low and high temperature capture of CO2 from flue gases, sorption enhanced H2 production and chemical looping combustion. The latter processes include CO2 capture as an integral step in the overall process.

Surface area and pore development during lignite activation

Samples of five Louisiana lignites were steam activated for up to eight hours at atmospheric pressure over a temperature range of 750–900 °C. The products were characterized in terms of their structural properties — specific surface area, pore volume, and pore size distribution. Pore volume increased monotonically with increasing activation temperature and time. Specific surface area reached a maximum of approximately 460 m2/g at 850 °C and three hours activation; further activation reduced the surface area. The average pore radius of approximately 20 A was essentially independent of activation temperature or time. A separate pyrolysis step preceding activation slowed the rate of activation but did not change significantly the product structural properties when compared at equal burnoff values.