Ulrich Nieken

CATALYTIC COMBUSTION WITH PERIODIC FLOW REVERSAL

Operation of fixed-bed reactors with periodic flow reversal as proposed and demonstrated by Matros and co-workers is a novel mode of operation for weakly exothermic or equilibrium limited reactions. In the present paper catalytic combustion in monolith type catalysts is considered. The influence of the operating parameters upon the shape of the temperature profiles in the stationary state als well as the stability of the ignited state are studied by model calculations. The advantages of inert front- and end sections and of hot gas withdrawal from the centre of the reactor are discussed. Experimental results for the catalytic combustion of traces of methane and propene in air show general agreement with the simulations.

Modeling and simulation of crystallization processes using parsival

Abstract Mathematical models describing particle-size distributions in crystallization processes are of a challenging complexity. Depending on the considered physical phase system and the considered process, mathematical models of crystallization processes include phenomena such as primary nucleation, crystal growth, fines dissolution as well as agglomeration and/or breakage (attrition) of crystals. From a numerical point of view, agglomeration and breakage of crystals add particular (quadratic) complexity, since all particle sizes are connected by integral terms. A new powerful algorithm for the treatment of all these structures is presented in this paper. The method — called Galerkin h–p method — is based on a generalized finite-element scheme with self-adaptive grid- and order construction and is connected to a time discretization of Rothes type. The algorithm can be applied to all combinations of the phenomena discussed above and needs no additional information on the form of the particle-size distribution. The numerical algorithm and the user-interface was implemented using object-oriented concepts leading to the simulation package P arsival (P ar ticle S i ze e val uation). The paper presents some basic features of P arsival and the numerical algorithm as well as one illustrating example.

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.

Control of the ignited steady state in autothermal fixed-bed reactors for catalytic combustion

Abstract Since autothermal reactors are always operated in the ignited steady state, control measures must be taken to prevent the reactor from extinction during times of lean feed as well as from overheating during times of rich feed. Respective control measures are discussed for autothermal reactors with integrated heat exchange for the case of catalytic combustion of traces of organics in exhaust air. They are shown to differ from control measures for conventional autothermal reactors with external heat exchange. Two types of autothermal reactors with integrated heat exchange are the fixed-bed reactor with periodic flow reversal and the countercurrent fixed-bed reactor. Control measures which proved successful in simulations were verified experimentally on a reverse flow laboratory reactor. They consist of adding supporting fuel or hot gas in times of lean feed and of using central heat exchange, cold gas injection or hot sidestream withdrawal in times of rich feed. Since none of these measures was able to prevent the reactor from overheating, a new concept using a structured fixed bed and sidestream withdrawal was developed and tested. It allows for an efficient heat recovery during times of lean feed as well as for the prevention of excess temperature and the utilization of almost all of the heat of reaction during times of rich feed.

Periodic operation of the Deacon process

The problems arising in two step Deacon process are corrosion, space time yield, chlorine purity, catalyst deactivation and overheating of the catalyst. Avoiding adsorption of hydrogen chloride on the catalyst carrier and limiting the maximum temperature, periodic operation with sufficient space time yield and chlorine purity is possible. To overcome corrosion problems a two-step adiabatic fixed-bed process was developed. Dynamic simulations showed that periodic flow reversal causes overheating, but with cocurrent operation it is possible to keep temperatures in the required range. The dynamics of the process are dominated by a travelling temperature and reaction front. A simple analytical model can be derived to calculate the most interesting features directly.

On the dynamic simulation of mass crystallization with fines removal

Abstract In mass crystallization the size distribution of the crystals produced is determined by the rate of growth and nucleation. Both rates are functions of the supersaturation present in the system. Usually a low nucleation rate is preferred for producing large crystals, but this requires low production rates. Fines removal is an alternative means of achieving the desired coarse product. It is applied in the industrial production of, for example, potassium chloride or ammonium sulfate. In this contribution the effect of fines removal, as well as the role of nucleation kinetics, has been investigated by dynamic simulation. The model used for simulation consists of the solute mass balance which is coupled by kinetics to the population balance of the crystals. The calculations show that true quantitative results cannot be obtained with simple models. Nevertheless, it is possible to investigate the dynamic behaviour of crystal size distributions. It is also shown how the intensity of fines removal and the presumed nucleation kinetics affect the stability of crystallizers.

Regelung eines instationär betriebenen Festbettreaktors mit Fuzzy-Kontrollregeln

In losungsmittelverarbeitenden Betrieben (Lackierstrasen, Mobelindustrie u.a.m.) fallen oft Abluftstrome mit niedriger Beladung an fluchtigen organischen Schadstoffen (VOC) an. Eine Ruckgewinnung und Ruckfuhrung der organischen Komponenten ist oft unwirtschaftlich oder technisch unmoglich. In solchen Fallen bietet sich als Entsorgungsmoglichkeit die vollstandige Oxidation der organischen Verunreinigungen zu CO2 und Wasserdampf an. Da die Temperatur des Abluftstromes in der Regel weit unterhalb der Zersetzungstemperatur der Schadstoffe liegt, mus er erst auf die erforderliche Reaktionstemperatur aufgeheizt werden. Um den Energieeinsatz dafur niedrig zu halten, werden ublicherweise Katalysatoren eingesetzt, welche die Zersetzungstemperatur herabsetzen, und vor dem Reaktor wird ein Warmeubertrager geschaltet, in dem das heise, gereinigte Abgas das Rohgas aufheizt.