Pattern formation in fixed-bed catalytic reactors

Sinh Han Trinh, Purdue University

Abstract

The analysis of catalytic reactors using heterogeneous models has been shown to display spatial pattern formation which cannot accommodated by pseudo-homogeneous models. When the presence of the catalyst phase in the reactor is accounted for along with the nonlinearity of chemical kinetics, it is possible for the reactor to display steady state patterns in isothermal stirred tank reactors. Virtually all the works on pattern formation in reactors has been restricted to stirred tanks. The present work focuses on the possibility of spatial patterns in packed bed catalytic reactors and its use to improve reactor performance. Several fundamental issues are addressed in this work. The analysis is based on indirect interaction models which assume lumped transport resistance in the catalyst phase. The models assume for the fluid phase (i) plug flow, and plug flow with axial dispersion. Such patterns involve discontinuous concentration profiles in the catalyst phase. It is shown that patterns, which arise due to multiplicity behavior of individual catalyst particles, can be created by providing for an initial pattern in the reactor. Start-up strategies for such initial patterns are established using multiple feeds. Asymptotic stability of patterns in which neighboring particles lie in distinct stable branches is established. In contrast to the stability of patterns in a stirred tank which may become unstable, even when the particles lie in distinct stable branches, the stability in the plug flow fixed bed reactor is dictated by the stability of the particle in isolation. Computations demonstrating the achievement of spatial patterns from suitable initial conditions are made. It is shown that reactor operation is concerned with a class of patterns which can provide for an acceptable range of product quality. Specific reaction systems are chosen to demonstrate the effectiveness of maintaining spatial patterns. Thus the reaction system ($A\sb{0}\to A\sb1, A\sb{0}\to A\sp2, A\sb1 + A\sb2\to A\sb3$) to produce $A\sb3$ is shown to benefit substantially by patterns which allows the simultaneous accumulation of $A\sb1$ and $A\sb2$ by shifting between steady states favoring one or the other.

Degree

Ph.D.

Advisors

Ramkrishna, Purdue University.

Subject Area

Chemical engineering

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