This work presents an approach for
the model-based design of a
load-flexible fixed-bed reactor for performing the partial water–gas
shift (WGS) reaction in the gas cleanup section of a coal-to-methanol
plant. Three optimization problems with various combinations of objective
functions (H2/CO ratio, pressure drop, and operating costs)
are formulated and solved using the nondominated sorting genetic algorithm
(NSGA-II) to obtain the Pareto-optimal fronts, and the selected solutions
are subjected to dynamic stability analysis. The optimum values of
decision variables such as the weight of the catalyst, catalyst particle
size, ratio of the reactor length and diameter, feed gas temperature,
and steam to CO ratio are estimated and are found to be dependent
on the choice of the objectives and tolerable deviation from their
required values. The dependence of the H2/CO ratio on the
inlet gas temperature varies with the choice and combination of the
objectives. In contrast, higher values of the steam to CO ratio (S/C) are associated with a lower deviation
from the desired H2/CO ratio for all three cases. However,
optimum values of S/C are found
to be below 1:1 because of its effect on the pressure drop and operating
costs. The solutions obtained in the equilibrium-limited regime are
found to be more stable in the presence of fluctuations in the feed
flow rates but show intermediate stability with fluctuations in the
inlet gas temperature. Four designs are recommended as the feasible
designs, and the criteria of their application are discussed. Even
though the work is related to a WGS reactor, the methodology used
can be applied for the design of any other reactor system operating
under variable feed conditions.
The
injection of nitrogen monoxide (NO) into an internal combustion
engine has been shown in the literature to reduce the net generation
of nitrogen oxides (NOx). In this work, engine simulations
are performed using two different microkinetic models to carry out
an analysis of reaction pathways responsible for the net decrease
in NOx formation when NO is injected into a hydrogen–fuelled
internal combustion engine. The net NOx formed when 5000
ppm NO is injected at the engine intake is found to be 60% lower than
the case without any NOx injection. The major reactions
contributing to the formation and conversion of NO are obtained for
various phases of combustion, and the corresponding reaction pathway
diagrams are constructed. During the early phase of combustion with
an increasing in-cylinder temperature and pressure, the Zeldovich
reactions primarily contribute toward NO formation (60–82%
contribution). During this phase, NO is primarily a product for various
reactions, and the net rate of production of NO decreases with the
external injection of NO. Further, any nitrogen dioxide (NO2) present in the intake mixture gets decomposed to NO owing to the
high temperature. Thus, the in-cylinder NOx concentrations
at the end of the power stroke are governed by the rate of consumption
and formation of NO, irrespective of the NOx speciation
at the intake. During the later phase of combustion with a decreasing
in-cylinder temperature and pressure, the NOx concentration
decreases, and the reactions constituting the Zeldovich mechanism
dominate NO consumption, with the reactions involving fuel-NOx interactions playing a secondary role (<14% contribution).
A comparison of the net rate of consumption of NO, the rate of formation
of N2, the major reactions involved in NO reduction, and
their relative contribution shows conclusively that the mechanism
of NOx reduction is not impacted by the addition of NO.
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