Hydrogenation of CO2 over Fe-K based catalysts in a fixed bed reactors at elevated pressure

Catalytic hydrogenation of CO₂to produce hydrocarbons was conducted in a fixed bed reactor (1.6 cm-IDx60 cm-High). Fe-K based catalysts (KRICAT-A, B) were used for more than 850 hours to maintain CO₂ conversion level up to 30 C-mol% in the fixed bed micro-reactor. Effects of operating variables on the CO₂ conversion, hydrocarbon yield and its selectivity were investigated. The CO₂ conversion and total hydrocarbon yield increased with increasing reaction temperature (250-325 °C), pressure (0.5-2.5 MPa) and H₂/CO₂ mol ratio (2-5); however, they decreased with increasing space velocity (1,000-4,000 ml/g_cathr) in the reactor. The selectivities of liquid products increased with increasing reaction pressure; however, they decreased with increasing temperature, space velocity and H₂/CO₂ ratio. From the results of an experimental study, optimum operating conditions for the maximum yield of olefinic liquid products were found as T=315 °C, P=1.5 MPa, SV=2,000 ml/g_cathr and H₂/CO₂ ratio=3 in the fixed bed reactor within these experimental conditions. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8-10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

Steam gasification of coal with salt mixture of potassium and nickel in a fluidized bed reactor

Australian coal loaded with a mixed catalyst of K₂SO₄+Ni(NO₃)₂ has been gasified with steam in a fluidized bed reactor of 0.1 m inside diameter at atmospheric pressure. The effects of gas velocity (2-5 U_g/U_mf), reaction temperature (750-900 °C), air/coal ratio (1.6-3.2), and steam/coal ratio (0.63-1.26) on gas compositions, gas yield and gas calorific value of the product gas and carbon conversion have been determined. The product gas quality and carbon conversion can be greatly improved by applying the catalyst; they can also be enhanced by increasing gas velocity and temperature. Up to 31% of the catalytic increment in gas calorific value could be obtained at higher temperatures. In the experimental runs with variation of steam/coal ratio, the catalytic increments were 16-38% in gas calorific value, 14-57% in carbon conversion, 5-46% in gas yield, and 7-44% in cold gas efficiency. With increasing fluidization gas velocity and reaction temperature, the unburned carbon fraction of cyclone fine for catalytic gasification decreased 4-18% and 13-16%, respectively, compared to that for non-catalytic gasification. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8–10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

Hydrogen production by catalytic decomposition of methane over carbon catalysts in a fluidized bed

A fluidized bed reactor made of quartz tube with an I.D. of 0.055 m and a height of 1.0 m was employed for the thermocatalytic decomposition of methane to produce CO₂ — free hydrogen. The fluidized bed was used for continuous withdrawal of the carbon products from the reactor. Two kinds of carbon catalysts — activated carbon and carbon black — were employed in order to compare their catalytic activities for the decomposition of methane in the fluidized bed. The thermocatalytic decomposition of methane was carried out in a temperature range of 800–925°C, using a methane gas velocity of 1.0–3.0 U _mf and an operating pressure of 1.0 atm. Distinctive difference was observed in the catalytic activities of two carbon catalysts. The activated carbon catalyst exhibited higher initial activity which decreased significantly with time. However, the carbon black catalyst exhibited somewhat lower initial activity compared to the activated carbon catalyst, but its activity quickly reached a quasi-steady state and was sustained over time. Surfaces of the carbon catalysts before and after the reaction were observed by SEM. The effect of various operating parameters such as the reaction temperature and the gas velocity on the reaction rate was investigated.     

Characteristics of slug flow in a fluidized bed of polyethylene particles

The slug flow behavior of polyethylene particles was examined in a fluidized bed of 7 cm ID and 50 cm in height. The employed polymer particles were high density polyethylene (HDPE) with the average particle size of 603 μm. The slugging flow of polyethylene particles was analyzed from the measured pressure drop signals by classical statistical methods such as absolute average deviation, probability density function, power spectrum, auto-cor-relation, and cross-correlation. The results show that in spite of high dielectric constant of polymer particles, the slugging phenomena such as incipient slugging velocity, slugging frequency and slugging rise velocity were very similar to the Geldart B type non-polymeric particles. It was observed that slug frequencies decreased with gas velocity and the limiting slug frequency was observed for the gas velocities in this study.     

Gas yields from coal devolatilization in a bench-scale fluidized bed reactor

Devolatilization behavior of Australian bituminous coal-gasification was determined in a 0.1 m diameter fluidized bed at 650-900 °C. To predict gas yields from devolatilization, several correlations reported for gas yields were evaluated with the present experimental data. The correlation of Goyal and Rehmat [1993] was found to be good one. Also, a correlation for the product gas yields has been proposed from devolatilization of bituminous coals as a linear function of temperature with constants from the experiment. The experimental yields of product gas show good agreement with the values calculated by the proposed correlation. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8-10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

Studies on the preparation of C/SiC composite as a catalyst support by CVI in a fluidized bed reactor

In this research, in order to improve the mechanical strength and oxidation resistance of a catalyst support, we studied the formation of SiC layer on the pore surfaces of activated carbons by permeating and depositing SiC from a reaction between hydrogen and dichlorodimethylsilane (DDS). The porous structure should be kept during deposition. A fluidized bed reactor and activated carbons of size of 20-40 mesh were used. By studying characteristics of deposits under various deposition conditions, we confirmed that the best conditions of manufacturing catalyst support are a lower pressure and a lower concentration. In this work, at the conditions of 5 torr of total pressure and 3% of DDS concentration, during the 10 hr processing time, deposition occurred on the pore walls before plugging pores. The results from the mathematical modeling were compared with the experimental results.     

Attribution of sewage sludge and sludge-based char in a fluidized bed reactor

Fluidized bed pyrolysis has been recognized as an innovative technology for sewage sludge treatment. The physical and attrition properties of sewage sludge are changed through the fluidized bed pyrolysis. The minimum fluidization velocities and attribution rate constants for sewage sludge and sludge based-char were obtained from pressure drop and attribution tests. As a result, sewage sludge with 20% moisture content and char were classified as Geldart B solids and the superficial gas velocity for bubbling fluidization was 0.2142-0.8755 m/s. In addition, attribution of the sewage sludge and char was more affected by particle size than by material type. The equations for the overall attrition rate constants are K _a × 10⁵ = 1.09U − 14.82 for sludge and ln k _a = 0.1(U−U _mf )− 13.63 for char, respectively.     

Catalytic carbon dioxide hydrogenation to methanol: A review of recent studies

Methanol demand is continuously increasing in the chemical and energy industries. It is commercially produced from synthesis gas (CO + CO₂ + H₂) using CuO/ZnO/Al₂O₃ catalysts. Today, much effort is being put on the development of technologies for its production from carbon dioxide (CO₂). In this way, the Greenhouse effect may be mitigated. Over the years, several useful works on CO₂ hydrogenation to methanol have been reported in the literature. In this article, we present a comprehensive overview of all the recent studies published during the past decade. Various aspects on this reaction system (such as thermodynamic considerations, innovations in catalysts, influences of reaction variables, overall catalyst performance, reaction mechanism and kinetics, and recent technological advances) are described in detail. The major challenges confronting methanol production from CO₂ are considered. By now, such a discussion is still missing, and we intend to close this gap in this paper.     

Heat transfer in a high temperature fluidized bed

The heat transfer characteristics between the bed and immersed tube in a high temperature fluidized bed (7.5 cm I.D.×70 cm H) were investigated with sand and iron ore particles. The heat transfer coefficients were measured at operating temperatures of 200–600°C and gas velocities of 1–10 U_mf. The bed emissivity measured by the radiation probe was found to be 0.8–0.9. The experimentally obtained radiative heat transfer coefficient was in the range of 30–80 W/m²K for the operating temperature of 400–800°C and the contribution of radiation to total heat transfer was about 13% and 18% for the operating temperatures of 400°C and 600°C, respectively.     

二氧化碳加氢合成甲醇催化剂研究进展

介绍了CO2加氢合成甲醇催化剂的研究进展,重点对负载型铜基催化剂的最新进展进行了综述。     

Functionalization of HDPE powder by CF4 plasma surface treatment in a fluidized bed reactor

The surface of HDPE polymer powder was fluorinated by CF₄ plasma in a fluidized bed reactor. Plasma is generated by an inductively coupled electrode at 13.56 MHz (rf) frequency, connected to an auto matching network and an rf power generator. In plasma surface fluorination, the CF₄ gas is diluted with He gas. The experimental variables are treatment time and rf power. The chemical property of the modified powder has been determined by using ESCA and FTIR. Plasma surface fluorination with the powder in a fluidized bed reactor results from the formation of CHF-CF₂, CHF-CHF and CF₂ groups. These fluorine functionalities and the fluorine atomic ratio on the surface increase with the treatment time and rf power. It has been found that the composite parameter is a good measure for determining the effect of total energy input on the plasma surface treatment of polymer powder in a fluidized bed reactor.     

Synthesis of multi-walled carbon nanotube in a gas-solid fluidized bed

Multi-walled carbon nano-tubes (MWCNTs) were produced by acetylene decomposition on Fe-catalyst in a fluidized bed reactor (0.056 m-IDx1.0 m-high) with a sintered metal distributor (40 μm pore size). The Fe-catalysts were tested in decomposition of the different ratios of acetylene, hydrogen and nitrogen at the temperature range of 823–973 K. The physical properties of the carbon nano-tubes were determined by HR-TEM, SEM and Raman spectroscopy. The multi-walled carbon nano-tubes produced from the fluidized bed reactor are sub-aggregates and entangled with each other. The synthesized MWCNTs have outer diameters of a few tens of nanometers at 823–973 K. It has been found that the synthesized CNT agglomerates are in good condition with less amorphous carbon with the reaction time of 30 to 60 minutes from the analyses of Raman Spectra, SEM and TEM, The ratio (I _D /I _G ) of amorphous carbon (I _D = 1,295 cm^-1) and crystalline carbon (I _G =1,590 cm^-1) range from 1.15 to 1.49.     

The basic study of methanol to gasoline in a pilot-scale fluidized bed reactor

A fluidized bed reactor, used for methanol to gasoline (MTG), was designed followed the theory of gas–solid two-phase flow, and the effects of some factors, such as temperature, space velocity and the regeneration process, on the performance of MTG catalyst were systematically examined. The results show that: heat and mass transfer can be effectively conducted in the fluidized bed reactor; with the reaction temperature was increased, the methanol conversion rate maintained at 100% and the yield of gasoline gradually increased, then reached its highest value of 25.22% at 410 °C, after that it began to decline; and the C₅ aromatics content increased with temperature and reached its maximum value of 49.86% at 430 °C. With the weight space velocity was increased, the yield of gasoline firstly increased and then decreased, while the C₅ aromatics content was decreased; In addition, the effect of inner-regenerated process for used catalyst is very good. Low temperature can help to generate lighter olefin polymer, the higher extent of hydrogen transfer and cracking of large molecules at middle temperature, the carbon deposition reaction and aromatization reaction of low carbon olefin occurred at higher temperature, all of these contributed the above mentioned rules. While the weight space velocity acts on the performance of catalyst mainly via influencing the contact time and the carbon deposition reaction.     

Attrition characteristics of alumina catalyst for fluidized bed incinerator

Attrition characteristics of alumina catalyst for catalytic incineration have been studied in a fluidized bed cold mode combustor (F10 cm, 160 cm height). The particle size and density of alumina catalyst were 1.4-1.7 mm and 1.13 g/cm3. As operating variables, excess gas velocity (U-U_mf) and bed weight (W_b) were selected. The experimental results show that attrition rate of alumina catalyst increased with excess gas velocity and bed weight due to intensive rubbing and collision caused by bubble coalescence. The size of the entrained particles collected in cyclone ranges over 0.5 to 100 mm, and the mean size for number base increases with an increase of excess gas velocity.     

Slugging bed height of polyethylene particles in a fluidized bed with an expanded section

Slugging experiments were performed in a fluidized bed of 7 cm ID and 50 cm in height to examine the maximum bed height with an expanded section. High-density polyethylene (HDPE) particles of 0.5 mm were employed as the bed materials. The slugging bed height was linearly increased with the gas velocity in the beds of uniform cross section as well as expanded section with different slope. From the results of this study, it was found that the existing correlation to predict the slugging bed height based on the heavier particles for the uniform cross section area was satisfactorily applied for the lighter particles of HDPE and for the expanded section, a slight modification was made for the particle of HDPE in the slugging bed. Presented at the Int’/Sym. on Int’l Symp. on Chem. Eng. (Cheju, Feb. 8–10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

The mass production of carbon nanotubes using a nano-agglomerate fluidized bed reactor: A multiscale space-time analysis

The scaled-up mass production of carbon nanotubes (CNTs) was reviewed by a multiscale analysis from the delicate catalyst control needed at the atomic level, CNT agglomerate formation at the mesoscopic scale, to the continuous mass production process on the macroscopic scale. A four level analysis that considered CNT assembly, agglomerate structure, reactor hydrodynamics and coupled processing was used. Atomic scale catalyst design concepts were used to modulate the CNT structure. On the reactor scale, the design consideration was on getting suitable CNT and catalyst agglomerates with good fluidization behavior and transport properties. A pilot plant with high yield (15 kg/h) and purity (> 99.9%) was demonstrated, which made a great stride for extensive applications of CNTs. Other nano-agglomerate structures can also be considered using the multiscale time and space analysis, which will benefit mass production and applications of nanomaterials in future.     

Steam gasification of an australian bituminous coal in a fluidized bed

To produce low calorific value gas, Australian coal has been gasified with air and steam in a fluidized bed reactor (0.1 m-I.Dx1.6 m-high) at atmospheric pressure. The effects of fluidizing gas velocity (2–5 U_f/U_mf), reaction temperature (750–900 °C), air/coal ratio (1.6-3.2), and steam/coal ratio (0.63–1.26) on gas composition, gas yield, gas calorific value of the product gas and carbon conversion have been determined. The calorific value and yield of the product gas, cold gas efficiency, and carbon conversion increase with increasing fluidization gas velocity and reaction temperature. With increasing air/coal ratio, carbon conversion, cold gas efficiency and yield of the product gas increase, but the calorific value of the product gas decreases. When steam/coal ratio is increased, cold gas efficiency, yield and calorific value of the product gas increase, but carbon conversion is little changed. Unburned carbon fraction of cyclone fine decreases with increasing fluidization gas velocity, reaction temperature and air/coal ratio, but is nearly constant with increasing steam/coal ratio. Overall carbon conversion decreases with increasing fluidization velocity and air/ coal ratio, but increases with increasing reaction temperature. The particle entrainment rate increases with increasing fluidization velocity, but decreases with increasing reaction temperature. This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

含内构件的循环流化床的动态压力特性

针对含内构件的循环流化床,以石英砂为物料,使用动态压力传感器测量了含内构件的流化床中气固两相流的动态压力,分析了床内的瞬时压力特性. 结果表明,在进出口总压降中,文丘里压降最大,占主床压降的60%以上. 表观气速和固体颗粒循环流率共同影响循环流化床内的压力特性. 压力瞬时波动功率谱分析表明,压力波动对应一个主频,表观气速越小、颗粒循环流率越大时,压力波动越大,且循环流化床底部压力波动比上部大. 加入内构件能有效引导气流,使流动更均匀.     

Production of hydrogen and carbon nanotubes from methane decomposition in a two-stage fluidized bed reactor

Methane decomposition over a Ni/Cu/Al₂O₃ catalyst is studied in a two-stage fluidized bed reactor. Low temperature is adopted in the lower stage and high temperature in the upper stage. This allows the fluidized catalysts to decompose methane with high activity in the high temperature condition; then the carbon produced will diffuse effectively to form carbon nanotubes (CNTs) in both low and high temperature regions. Thus the catalytic cycle of carbon production and carbon diffusion in micro scale can be tailored by a macroscopic method, which permits the catalyst to have high activity and high thermal stability even at 1123 K for hydrogen production for long times. Such controlled temperature condition also provides an increased thermal driving force for the nucleation of CNTs and hence favors the graphitization of CNTs, characterized by high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and XRD. Multistage operation with different temperatures in a fluidized bed reactor is an effective way to meet the both requirements of hydrogen production and preparation of CNTs with relatively perfect microstructures.     

Gas backmixing in the dense region of a circulating fluidized bed

The gas backmixing characteristics in a circulating fluidized bed (0.1 m-IDx5.3-m high) have been determined. The gas backmixing coefficient (D_ba) from the axial dispersion model in a low velocity fluidization region increases with increasing gas velocity. The effect of gas velocity onD _ba in the bubbling bed is more pronounced compared to that in the Circulating Fluidized Bed (CFB). In the dense region of a CFB, the two-phase model is proposed to calculate D_bc from the two-phase model and mass transfer coefficient (k) between the crowd phase and dispersed phase. The gas backmixing coefficient and the mass transfer coefficient between the two phases increase with increasing the ratio of average particle to gas velocities (U_p/U_g).