  We consider reaction-transport processes in open reactors in which systems of
first order reactions involving a number of gas species and solid catalysts can
occur at localized active regions. Reaction products flow out of the reactor
into vacuum conditions and are collected at an exit boundary. The output
composition problem (OCP) is to determine the composition (molar fractions) of
the collected gas after the reactor is fully emptied. We provide a solution to
this problem in the form of a boundary-value problem for a system of
time-independent partial differential equations. We then consider network-like
reactors, which can be approximated by a network consisting of a collection of
nodes and 1-dimensional branches, with reactions taking place at nodes. For
these, it is possible to solve the OCP in a simple and effective way, giving
explicit formulas for the output composition as a function of the reaction
coefficients and parameters associated with the geometric configuration of the
system. Several examples are given to illustrate the method.
