Heterogeneous catalytic reactor design with optimum temperature profile II: application of non-uniform catalyst

A new methodology has been developed to design non-isothermal, non-adiabatic heterogeneous catalytic fixed bed and tubular reactors with optimal temperature profiles inside a reactor. Catalyst characteristics such as pellet diameter, shape and activity distributions inside a pellet are considered simultaneously for reactor design. Various types of non-uniform activity distributions inside a pellet are modelled and optimised for the maximisation of an objective such as yield or selectivity. Dirac-δ, layered and general non-uniform distribution profiles such as egg-shell, egg-yolk and middle peak distributions are applied for the reactor design. The research demonstrates that different catalyst distribution profiles can approach the optimum performance. Whilst it is known that the Dirac-δ profile (and its step-function equivalent) always gives the best performance for clean catalyst, other profiles can approach this performance and might offer advantages in catalyst manufacture and under degraded conditions. A profile-based synthesis approach is applied to generate various shapes of activity profiles for multiple sections along the reactor during the optimisation of non-uniform catalyst pellets. A case study with the ethylene oxidation process illustrates that the catalyst characteristics, such as activity distribution profiles inside a pellet, sizes and shapes can be manipulated to control the temperature through the reactor very effectively, leading to significant improvements in selectivity or yield. The non-uniform catalyst pellet is further applied to various reactor configurations such as inert mixing and side stream distributions. This work is the first to consider all of these effects simultaneously.     

Optimization of the active component distribution through the catalyst bed

A variational problem was formulated to determine the optimal axially non-uniform catalyst activity distribution along the fixed catalyst bed. It was observed that the mass transport limitations or non-isothermal temperature profile are necessary conditions for potential optimization of the catalyst distribution along the bed length. Under isothermal conditions with linear dependence of the reaction rate on concentration at a constant mass transfer coefficient, the uniform distribution is optimal. Analytical solution for the first-order reaction and numerical solutions for power-law kinetics were found.     

Hydrogenolysis of 1,4-dimethylcyclohexane on silica supported iridium catalyst: influence of time on stream on activity and selectivity

Opening cyclic or polycyclic alkanes by carbon–carbon bond cleavage can be a good route for the valorization of certain alkanes. A key point of any proposed catalytic system would be the catalyst poisoning. In this paper, we determine experimentally the influence of the time on stream on the distribution of products when 1,4-dimethylcyclohexane is introduced together with hydrogen over a Ir/SiO₂ catalyst. A simplistic model of selective catalyst poisoning is proposed to explain the influence of the time on stream on the activity and product selectivity.     

Optimum activity distribution in a catalyst pellet for a complex reaction

An analysis of the influence of the non-uniform distribution of the active species in a catalyst pellet on the effectiveness factor and point yield for the van de Vusse reaction network is carried out. The effect of different parameters on the point yield for a single pellet and on the integral yield for a fixed bed reactor is studied, and the optimum activity distribution is determined.     

镍基芳烃加氢催化剂硫中毒的失活动力学

将某一时刻催化剂表面上活性中心数目与初始时刻活性中心数目之比定义为催化剂表观活性因子,并认为此活性因子包含在主催化反应速率系数的指前因子中。考虑活性中心在催化剂表面分布的不均匀性,在现有级数型失活动力学模型基础上,经推导得到表观活性因子的具体表达式及其极限式,该式在一般情况下为不可分。最后,结合之前报道的具有较高普适性的反应转化率方程式,得到催化反应转化率与运转时间的关系方程,并以Ni-B/SiO₂催化剂在噻吩存在下芳烃加氢反应为例加以验证。结果表明,所提出的失活动力学表达式可以较好地描述镍基催化剂硫中毒的行为,方程参量具有物理意义,方程可以较精确地拟合实验数据。     

Nickel-catalyzed hydrogenation: A study of the poisoning effect of halogen-containing compounds

The poisoning effect of various halogen compounds on a nickel-based hydrogenation catalyst has been investigated. While alkyl monochlorides did not affect the catalyst activity, alkyl monobromide and-iodide had a strong poisonous effect. Vicinal dichlorides, 1,1-dichlorides, and HC1 also poisoned the catalyst. It is shown that with chlorine containing compounds, the poisoning mechanism involves a fission of the carbon-chlorine bond with the formation of HC1, while bromine and iodine compounds adsorb to the catalyst surface as such in a simple equilibrium reaction. A mackerel oil previously shown to exhibit three kinetically distinguishable poisoning effects has been further examined, and it is shown that its content of halogens can probably account for the observed poisoning pattern.     

Influence of catalyst structure on direct ethoxylation of fatty methyl esters over Al-Mg composite oxide catalyst

During direct ethoxylation of fatty methyl ester over Al-Mg composite oxide catalyst, the activity was nearly proportional to the total number of active Al acidic sites on the catalyst surface per unit weight of catalyst. Lower active Al acidic site densities resulted in a narrower ethylene oxide (EO) adduct distribution of obtained ethoxylate. We developed a new catalyst with a large surface area on which many acidic sites are distributed uniformly by partially poisoning the Al acid sites of high-Al content Al-Mg composite oxide with alkali. This catalyst was used for direct ethoxylation of fatty methyl esters to obtain ethoxylated fatty methyl esters with narrow EO adduct distribution efficiently.     

CO铜基变换催化剂氰化氢中毒机理热力学研究

为了掌握CO变换催化剂在高炉煤气气氛下的中毒机理,文中采用热力学非均相反应体系中的Gibbs自由能最小原理,从理论上分析了CO变换催化剂在高炉煤气气氛下,HCN中毒可能发生的化学反应及产物。对各个反应的吉布斯自由能及化学反应平衡常数进行了对比分析,结果表明:氰化氢(HCN)对于铁基高温变换催化剂在其活性温度下的中毒作用很小,而对于铜基低温变换催化剂在100—300℃的活性温度下,主要的中毒产物为CuCN,CuO,Cu2O,Cu(OH)2,C,且高炉煤气自身的主要气体混合也是使催化剂中毒的原因之一。此外,氧气的存在会加快催化剂的中毒反应。其中低温变换催化剂的活性物质Cu微晶与CO和O2反应的吉布斯自由能小于0,且绝对值最大,热力学平衡常数最大。这个反应是竞争能力最强的主导反应。     

Pt-Co supported on single-walled carbon nanotubes as an anode catalyst for direct methanol fuel cells

Catalyst of Pt-Co supported on single-walled carbon nanotubes (SWCNTs) is prepared using mixed reducing agents. The SWCNTs were pretreated in a microwave oven to enable surface modification. Pt-Co nanoparticles with narrow particle size distribution around 5.4 nm were uniformly deposited onto the SWCNTs. Under same Pt loading mass and experimental conditions, the SWCNTs-Pt-Co catalyst shows higher electrocatalytic activity and improved resistance to CO poisoning than the SWCNTs-Pt catalyst.     

ANALYSIS OF THE PERFORMANCE OF NON-UNIFORMLY ACTIVE CATALYST PELLETS FOR A NON-ISOTHERMAL SERIES REACTION

An analysis of the influence of the non-uniform distribution of the active species in a catalyst pellet on the effectiveness factor and selectivity for the case of a series non-isothermal reaction is carried out. The case in which the desired reaction has a lower activation energy than the undesired reaction is considered. The effect of different reaction parameters on the selectivity for a single pellet is studied, and the optimum activity distribution is determined for high values of Thiele modulus. It is shown that temperature gradients within the pellet have a detrimental effect on local and overall selectivity and that these gradients are a strong function of the distribution of catalytically active material within the pellet.     

CATALYST DEACTIVATION BY PORE PLUGGING AND ACTIVE SITE POISONING MECHANISMS II. PARALLEL POISONING IN BIDISPERSE STRUCTURED CATALYST

A catalyst deactivation model is formulated which includes the combined effects of pore plugging and active site poisoning in bidisperse structured catalyst particles. Intraparticle mass transfer is described by an equation which accounts for the configurational nature of diffusion in the micropores, and poison deposition is assumed to be in accord with a parallel poisoning mechanism. The model is used to explore the effects of macroporosity and micropore size on initial activity and activity maintenance, and model predictions appear to be consistent with observations from a recent coal liquefaction catalyst development program.     

An optimal catalyst activation policy for poisoning problems

Pontryagins minimum principle is used to calculate the optimum distribution of active material throughout a single pellet that is uniformly experiencing activity decay. The definition of optimality is based upon balancing catalyst costs against the net return from reactant conversion. The optimal policy is full activation to a fractional depth with an inert core, the depth depending upon the Thiele parameter, poisoning time constant, operating time, and an economic parameter. An isothermal, first order, irreversible reaction is considered.Auxiliary calculations of the effectiveness factor are given for the reaction rate in the nonisothermal pellet with the catalyst distribution found to be optimal in the isothermal case.The results of the pellet calculations are utilized in numerically calculating a uniform activation policy that is optimal for a homogeneously poisoned bed. The definition of optimality is on the same basis as that for the single pellet and the results depend upon the same physical parameters in addition to the time constant for convection in the tube relative to that for diffusion in the pellets. Four regimes of behavior arise depending upon catalyst costs: (i) the reactor cannot be operated economically, (ii) only partial activation policies are economical, a single one being optimal, (iii) both partial and full activation schemes are economical, with one of the former being optimal, and (iv) full activation is optimal but the reactor can be operated economically with partially activated pellets.The results for both the single pellet and the packed tube are viewed as the homogeneous poisoning limit of many practical problems and are believed to reflect the major characteristics of more complicated poisoning mechanisms.     

Sulphur poisoning of nickel-based hot gas cleaning catalysts in synthetic gasification gas II. Chemisorption of hydrogen sulphide

The effect of different components of gasification gas on sulphur poisoning of nickel catalysts were studied. In addition, the sulphur distribution and content of nickel catalyst beds were analysed to account the poisoning effect of sulphur on the activity of catalysts to decompose tar, ammonia and methane. The desorption behaviour of chemisorbed sulphur from the bed materials was monitored by temperature programmed hydrogenation (TPH). It was established that bulk nickel sulphide was active in decomposing ammonia in high-temperature gasification gas-cleaning conditions. The decomposing activity of methane was not affected by bulk nickel sulphide formation, but that of toluene was decreased. The activity of the catalyst regained rapidly when H₂S was removed from the gas. However, the conversion of ammonia was not regained at as high a level as before sulphur addition, most probably due to irreversible sulphur adsorption on the catalyst. The temperature increase could also be used to regenerate the catalyst performance especially in respect to methane and toluene. Sulphur adsorbed on nickel catalysts in different chemical states depends on the process conditions applied. At >900°C the sulphur adsorbed on the catalyst formed an irreversible monolayer on the catalyst surfaces, while at <900°C the adsorbed sulphur, probably composed of polysulphides (multilayer sulphur), was desorbed from the catalyst in sulphur-free hydrogen containing atmosphere. However, a monolayer of sulphur still remained on the catalyst after desorption. The enhanced effect of high total pressure on sulphur-poisoning of nickel catalysts could be accounted for the increased amount of sulphur, probably as a mode of polysulphides, adsorbed on the catalyst.     

烟气脱硝催化剂中毒机制与再生技术

随着烟气脱硝系统在火电厂的应用,对选择性催化还原催化剂的中毒机理和再生工艺的研究受到广泛关注。本文系统综述了脱硝催化剂的物理及化学中毒机制、再生方法及工艺。在中毒机制方面,将不同中毒机制归为三类:颗粒物或生成盐沉积在催化剂表面,堵塞催化剂通道和孔道;毒物与活性中心作用使表面的酸性性能和氧化还原性能降低;催化剂结构破坏和发生不可逆相变。在催化剂的再生方面,本文详细介绍了失活催化剂的再生工艺流程和再生液的选择,比较了不同再生技术的针对性和优缺点,最后介绍了电厂高钙项目的再生工业示范,其再生催化剂的相对活性恢复到原来的0.96,SO₂氧化率为1.0%,且各项指标达到了新鲜催化剂的水平。本文对延长催化剂使用寿命和制定废弃催化剂再生工艺具有重要指导意义。     

Revisiting the concepts of competition between reaction and diffusion in poisoned catalysts

The competition between diffusion and first‐order irreversible reaction in poisoned catalysts is revisited. Two cases are considered for isothermal slab catalysts: uniform and shell‐progressive (or pore‐mouth) poisoning. Analytical concentration profiles are derived, and the implications on catalyst performance are evaluated in different regimes (chemical‐ or diffusion‐controlled) for different levels of poisoning. It was found that depending on the poisoning mechanism, the activity decay can be more or less pronounced. Being of particular concern is the pore‐mouth poisoning at high Thiele modulus, conditions under which catalyst performance is drastically affected. The reagent concentration profiles allowed the explanation of the phenomena occurring at the particle scale, in particular the effectiveness factors, observed reaction rates, and poisoning factors' dependence on the Thiele modulus and fraction of the poisoned catalyst; it was found that such relationships are dependent on the mechanism of deactivation. © 2012 Canadian Society for Chemical Engineering     

工业V2O5-WO3/TiO2催化剂的碱土中毒研究

以某燃煤电厂应用的V₂O₅-WO₃/TiO₂蜂窝型SCR脱硝催化剂为研究对象,在实验室条件下模拟该催化剂Ca、Mg及二者复合中毒,研究了不同浓度的Ca和Mg对催化剂活性的影响,并用XRD、H₂-TPR及NH₃吸附FT-IR对催化剂进行表征。结果表明,Ca和Mg对工业SCR脱硝催化剂均具有毒性作用,其中Mg毒性相对较强,复合中毒在一定程度上弱化单组分对催化剂活性的影响;催化剂组分分散度和结晶度不受Ca和Mg的影响,但CaO和MgO的掺入抑制V₂O₅的还原能力;CaO和MgO对催化剂表面的酸性位均存在影响,尤其是B酸位,二者复合中毒对酸性位的影响小于单一组分。     

Sulphur poisoning of nickel-based hot gas cleaning catalysts in synthetic gasification gas: I. Effect of different process parameters

The effect of different process parameters on sulphur poisoning of nickel catalysts in tar (toluene), ammonia and methane decomposition was studied. Tests were carried out in a fixed-bed tube reactor at 800–1000°C at 5 and under 20 bar total pressure using a synthetic gasification gas mixture. In the same conditions, sulphur affected less the toluene and methane decomposing activity than the ammonia decomposing activity. Ammonia conversion was affected by the catalyst type but not by the nickel content of catalyst. When temperature was increased the effect of sulphur poisoning was decreased. At 20 bar pressure the poisoning effect of sulphur was stronger than at 5 bar pressure. To prevent sulphur poisoning of nickel catalysts in the tar and ammonia decomposition process at high pressure (20–30 bar), the catalyst process should operate at > 900°C. In addition, by decreasing the space velocity of the process the sulphur poisoning effect could be compensated in the test conditions.     

催化剂中微量砷含量的测定及其对活性的影响

通过实验确定了一种测定钴钼系CO耐硫变换催化剂中微量砷含量的方法——原子吸收分光光度法。实验中发现混合酸体系对测定结果影响较大,10%盐酸与2%硫酸是一种较理想的组合;不同砷含量对催化剂活性影响的考察结果表明,当催化剂中砷质量分数大于0.2600%后,活性开始明显下降;某些工业应用后的催化剂中砷含量检测结果表明,原料气砷杂质是引起催化剂中毒的主要原因之一。     

The influence of non-uniform catalytic activity on the performance of a single spherical pellet

This work investigates the influence of a non-uniform distribution of the intrinsic catalytic activity of a single spherical pellet on its effectiveness factor, selectivity and deactivation. In most cases the performance of non-uniformly active catalysts is superior to that of uniformly active catalysts which have the same volume averaged activity. The specific deactivation mechanisms and rate as well as the diffusional resistance have an important influence on the most desired activity profile. Concentrating the catalytic activity in the exterior shell most often yields the maximal selectivity but it also yields minimal resistance to deactivation when compared with catalysts with the same volume averaged activity and different activity profiles. For this reason, when it is important to maintain the catalytic activity for long periods it may be desirable not to use catalysts in which the activity is concentrated in the exterior shell.It is also shown that the maximal temperature rise during regeneration of a catalyst, in which coke is deposited non-uniformly, may largely exceed that obtained for a uniformly coked pellet.     

ZA—5型氨合成催化剂的研究：催化性能及其动力学

对ＺＡ－５型氨合成催化剂的活性及动力学进行了研究。实验结果证明,ＺＡ－５型氨合成催化剂的主要技术指标已大大超过ＩＣＩ７４－１型氨合成催化剂,具有活性温度低、催化活性高、耐热和抗毒性能好及机械强度高等显著特点。