Grigorios Kolios

Autothermal fixed-bed reactor concepts

The principles, properties and applications of autothermal fixed-bed reactor concepts are presented. First we focus on different reactor types for weakly exothermic reactions and discuss their basic behavior, their stability and nonlinear dynamic features. The second part is devoted to the autothermal coupling of endothermic and exothermic reactions. A systematic classification is proposed for the process alternatives developed so far and a simplified model is developed from which basic features of an optimal design can be deduced.

Efficient reactor concepts for coupling of endothermic and exothermic reactions

Multifunctional autothermal reactors are a novel concept in process integration and intensification. They can be implemented as a countercurrent or reverse-flow reactor. A promising field of application is the coupling of endothermic and exothermic reactions. Methane steam reforming coupled with methane combustion is considered as a particular example. Several novel reactor configurations with co- and countercurrent flow in the reaction zone will be discussed by numerical simulation and an example for experimental verification will be presented.

Fixed-bed reactors with periodic flow reversal: experimental results for catalytic combustion

The influence of design and operating parameters on the behavior of a fixed-bed reactor with periodic flow reversal has been studied in a laboratory set-up for the case of catalytic total oxidation. The results are in accordance with detailed model simulations published elsewhere. They show that the periodic operation is completely dominated by the regenerative heat exchange and that steady-state kinetics can be used. Like any other autothermal reactor, a fixed-bed reactor with periodic flow reversal has to be operated in the ignited steady state. It was shown that totally and partially ignited steady states may exist under the same operating conditions if several combustible components with different ignition temperature are present in the feed. Hot gas withdrawal from the middle of the packed bed proved to be a suitable method to utilize almost all of the heat of reaction at the highest temperature in the reactor and to prevent high temperature peaks at the respective exit valves. Together with an appropriate design of the fixed bed, composed of inert front and end sections with low effective axial conductivity and an active portion with large axial conductivity, hot gas withdrawal allows for an efficient control of the reactor under the conditions of both high and low feed concentrations.

Analysis of a novel reverse-flow reactor concept for autothermal methane steam reforming

Abstract One of the main shortcomings of existing multifunctional reactor concepts for the autothermal coupling of endothermic and exothermic reactions is inefficient heat integration leading to excessive maximum temperatures or poor reactor performance. For the asymmetric operation of a reverse-flow steam reforming reactor, conditions under which these shortcomings can be overcome are proposed. The asymmetric process is based on the formation of travelling reaction zones. The features of these transient phenomena are analysed by means of a simplified model. During the endothermic semicycle, the heat consumption forms a temperature wave with an expansive low-temperature and a compressive high-temperature part. During the exothermic semicycle a proper axial distribution of the heat supply is necessary in order to maintain a favourable temperature profile in the cyclic operation mode. The results obtained with the simplified model are verified by direct dynamic simulation.

Styrene synthesis in a reverse-flow reactor

Multifunctional reactor concepts for thermal coupling of endothermic synthesis reactions with an exothermic auxiliary reaction are presented. The processes are carried out in an autothermal operation mode incorporating regenerative heat exchange within the reactor through periodic flow reversal. Catalytic dehydrogenation of ethylbenzene to styrene is considered as a particular example, where catalytic combustion of the produced hydrogen serves as a heat source. Two alternative operation modes are discussed, the asymmetric operation mode, where the synthesis reaction and the combustion are separated in time and the symmetric operation mode with external combustion and hot gas injection, or with air injection and in situ hydrogen combustion. A lab-scale reactor with an integrated catalytic burner has been set-up and experiments with the symmetric operation mode have been performed. The symmetric operation mode with hot gas injection proves to be an attractive alternative to conventional processes for styrene synthesis: it allows for high conversions and high selectivities, combined with an efficient heat recovery.

A simplified procedure for the optimal design of autothermal reactors for endothermic high-temperature reactions

Abstract The occurrence of excessive maximum temperatures is presently the main obstacle in the design of countercurrent reactors for the autothermal coupling of endothermic and exothermic reactions. The reasons for the appearance of these high-temperature maxima are elucidated and discussed using a simplified reactor model. Based on it, measures are derived of how the maximum temperature can be reduced and the thermal efficiency of the integrated reactor can be improved. Simple formula and an efficient graphical procedure for a short-cut reactor design are provided. The results of the simplified design procedure have been verified through simulation with a more detailed reactor model.

On the efficient simulation and analysis of regenerative processes in cyclic operation

Abstract Examples and specific properties of cyclically operated regenerative processes are discussed. For the design of these processes their cyclic steady state is of primary interest. While direct dynamic simulation provides a basic understanding of the process dynamics it is generally not suitable for rigorous process design. This is due to the fact that it frequently takes a large number of cycles until the cyclic steady state is established. Therefore, two different approaches to the efficient determination of cyclic steady states are discussed and compared. The application of these methods for parameter studies and stability analysis of cyclic processes is demonstrated with the example of the reverse flow reactor.

Towards continuous processes for the synthesis of precursors of amorphous Si/B/N/C ceramics

The development of a process for the continuous synthesis of Cl3SiNMeBCl2 (DMTA) as a single-source precursor of highly durable Si/B/N/C ceramics is reported. The process is based on a series of gas phase reactions starting from silicon tetrachloride and methylamine. Cl3SiNHMe (TSMA), which is initially formed, is subsequently reacted with BCl3 to yield DMTA as a colorless, highly air-sensitive liquid. Amine hydrochloride byproducts are removed by filtration through ceramic filters. DMTA crystallizes in the monoclinic system, space groupP21/m, with a = 709.96(13) pm, b = 685.61(12) pm, c = 897.28(16) pm, and Z = 2. Polymerization of DMTA by aminolysis in n-hexane at −78 °C releases a polyborosilazane, SiBN2.4C2.5H8.8, which upon thermolysis releases amorphous SiBN2.3C1.7 ceramic in 56% yield. The latter resists thermal degradation by crystallization in Ar atmosphere up to at least 1700 °C.

Autothermal Reactor Concepts for Endothermic Fixed-Bed Reactions

Autothermal reactor concepts for the heat integrated coupling of endothermic and exothermic reactions are required for an efficient on-site production of hydrogen from alcohols or hydrocarbons for use in fuel cells. Existing experience with countercurrent reactors and reverse-flow reactors for weakly exothermic reactions can be utilized for their design. However, specific features of coupling endothermic and exothermic reactions must be taken into account. The respective considerations are presented and discussed for different modes of operation: the simultaneous coupling of both reactions; coupling with cocurrent recuperative heat exchange between the two process streams in the reaction zone; and asymmetric operation with counter-current recuperative or regenerative heat exchange. It is shown that asymmetric operation requires spatial distribution of the feed for the exothermic reaction in order to prevent excess temperatures and to ensure stable operation. Both simulation and experimental results are presented for methanol steam reforming, gasoline reforming and methane steam reforming.

Styrene synthesis over iron oxide catalysts: from single crystal model system to real catalysts

Surface science methods originating from analysis of noble metal catalysts are increasingly applied to metal oxides. These methods provide direct access to fundamental structural properties and phase equilibria governing the catalytic properties of metal oxide surfaces. However, no systematic way existed so far for transferring this knowledge to technical catalysts. The aim of this paper is to combine surface science with chemical engineering methods to bridge this gap. Styrene synthesis over pure and K-doped iron oxides is used as an example to develop and to explain the methodology. Single crystal films (SCF), grown epitaxially on a Pt-carrier are considered as ideal model surfaces. Comprehensive UHV analyses yield the structural properties of SCF as well as their interaction with relevant components of the reaction mixture. Their results are combined with conversion experiments to derive a mechanistic catalyst model along with quantitative information on the reaction rates. The activity of SCF as well as their phase transitions under reactive conditions can be described with a continuum model depending on the macroscopic properties of the system. This model forms the crucial link towards technical catalysts. It is shown that the behaviour of a powder catalyst can be described as a superposition of the above kinetic model and an appropriate porous model. In this paper we review the developed methodology and conclude with the evaluation of the concept.