Computer-aided total flowsheet synthesis

Building on a previous approach, a systematic computer-based method for synthesizing a total flowsheet has been developed. This work uses a hierarchical concept, whereby separation tasks selected by the algorithm at the top level are converted to cost-estimated sharp separation sequences at the second level. The strategy at both levels is heuristic and evolutionary. Only the top-level system is described here, although the results include both phases. The synthesis of separation tasks is described by Lu and Motard, and extends the work of Nath and Motard.The flow rates and sizes of processing tasks are determined by a new goal-lumping strategy and a successive linear programming algorithm. Information from the lower level design of separations is fed back to the top-level decision procedure. The entire system is self-adaptive, but also includes opportunities for interaction by the user. Several test problems indicate that the present approach is superior to earlier work.     

An interactive process flowsheeting and simulation system based on relational data structures

An interactive graphical interface has been developed for a modular-sequential steady-state chemical process simulation program. The interface software, called AGPSS (A Graphical Process Simulation System), consists of FORTRAN-IV routines for creating new equipment unit schematics, for drawing a process flowsheet, for accepting process equipment and stream parameters, for performing automatic analysis for recycle nets, for supervising execution of the steady-state material and energy balancing simulator, and for displaying results. The program is designed to operate from refresh terminals functioning as remote time-sharing devices on the University of Michigan's Amdahl 470V/6 computing system. The Integrated Graphics (IG) software package is used for all interactive graphical operations.

All data, including both picture and process information, are organized in a relational data structure containing a total of thirteen relations. The general algorithms for addition, retrieval, and deletion of data in the relational structure require just six set-theoretic operators, which are encoded as FORTRAN utility routines. These data structures and the associated set operators assisted materially in simplifying the design, programming, and testing of the interactive graphical interface.     

Computer-aided multivariable control system design for processes with time delays

A computer-aided control system design package is described which handles processes with multiple time delays and allows subsequent design of interaction compensators. The package synthesizes control system designs for complex problems in a short time while also providing dynamic simulation for the purpose of evaluating controller performance. Example problems are presented along with a case study on sensitivity to errors in the delays.     

Applications of mixed-integer linear programming in process synthesis

It is shown that important problems arising in process synthesis can be tackled effectively using mixed-integer linear programming techniques. Two examples are considered: the synthesis of chemical complexes that maximize its present value for a given number of periods where prices and demands fluctuate; and the synthesis of stream generation systems that minimize the total annual cost. Numerical results are presented for both cases.     

Effectiveness factor and selectivity estimation for a parallel reaction system

The case of double parallel reaction scheme taking place in a porous catalytic pellet is analyzed. Effectiveness factor expressions for both reactions are derived after matching asymptotic solutions strictly valid for small and large values of the Thiele moduli.It is assumed that the kinetics of both reactions follow a general irreversible power law model, that isothermal conditions prevail and that external mApproximate results in terms of effectiveness factors compare fairly well with exact results obtained by numerical integration of the differential equa     

Studies on the synthesis of chemical reaction paths—I: Reaction characteristics in the (ΔG,T) space and a primitive synthesis procedure

The creation of new reaction paths to accomplish a certain chemical goal has received increasing attention in the recent past. The present series attempts to establish an analytic framework that systematizes the search for the production of a certain chemical product, knowing that there is a wide number of chemicals which could be either raw materials or by-products of the eventual chemical reaction. In this paper some invariant algebraic properties of the chemical reactions in the space of ΔG (free energy change), T (temperature) are developed. Furthermore, based on these properties, a search strategy is proposed for the creation and screening of alternative reaction paths, leading to a desired chemical product. The above analysis has been concerned with single-step and multiple-step reactions including open and closed sequences of reactions (clusters of reactions).     

The hysteresis of steady states in a gradientless liquid-phase reaction system

The dynamic behaviour of a reaction system in a continuous-flow stirred tank reactor (CSTR) is described by a model comprising the unsteady mass and heat balances and the kinetics of the reaction system concerned.This model predicts that under certain conditions a hysteresis effect will occur, which manifests itself when the steady-state temperature of the reactIn the present paper a number of experiments concerning the hysteresis effect will be reported. The measurements were carried out with a well-mixed andIn a wide range of conditions with regard to acidity and water content of the reaction system, the reaction rate is found to be first-order in the MO cIn view of the low boiling point of MO, the temperature of the reaction system had to be kept well below 300 K.Since the total change of the temperature of the reaction system was estimated to exceed 40 K, feed-stream temperatures of about 250 K were required toAs a consequence of these low feed-stream temperatures, methanol was added to the feed streams to prevent crystallizing. The methanol did not take partWith respect to the hysteresis of steady states very good agreement between experimental results and calculated values was obtained.     

Period doubling and complex dynamics in an isothermal chemical reaction system

Stability and bifurcation theory have been used to organize a computational study of the dynamics of a three-variable isothermal reaction system with quadratic kinetics. This system exhibits a cascade of period-doubling bifurcations over decreasing intervals of parameter changes. This simple example illustrates a wide variety of complicated dynamics and shows that the qualitative dynamic features of isothermal reaction systems may be extremely sensitive to parameter values.     

An improved algorithm for diagnosis of system failures in the chemical process

The detailed techniques for improving the efficiency of a fault-diagnosis algorithm can be based on the signed directed graph. This graph represents the structure of the system, and the pattern on the graph represents a state of the system. The usefulness of the improved algorithm is demonstrated using a model of a chemical process comprising a reactor, a heat exchanger and a distillation tower.     

Thermal radiation controlled ignition/extinction phenomena in a system of separated catalyst layers with exothermal chemical reaction

In analogy to fixed beds ignition/extinction phenomena were observed in an exothermic and adiabatic chemical tubular reactor containing ten separated layers of catalyst particles. Obviously the thermal feed back by radiative exchange caused the system to perform like a quasihomogeneous medium. The application of the two phase homogeneous model equations revealed that the predicted ignition temperatures, which agreed well with those measured in a packed bed, were on the average 10 K lower than those measured in the multilayer system. This result is attributed to the missing contact points which in packed beds are privileged areas for the progress of ignition.     

Mixing and fast chemical reaction—VI: Extension of the reaction zone

Work reported earlier[1, 3] as well as here indicates that mixing at the molecular scale (micromixing) takes place by molecular diffusion in fluid elements, which are simultaneously undergoing laminar deformations. Whereas simple shear was considered before, the present analysis describes diffusion and reaction in regions deformed by extensional flow. After outlining various simple types of extension, results in the literature[4,7] are quoted, which were obtained from observations on the diffusion of heat spots in turbulent air. These indicated stretching motions in two directions with a corresponding shrinkage in the third. Direct application of these findings to diffusion in the direction of shrinkage leads to predictions of th product distribution from competitive, consecutive reactions. Without fitting any parameters, these predictions agree quite well with the distributions of two dyestuffs produced by two diazo coupling reactions. Insufficient fluid mechanical information is available to enablea more accurate comparison; matters requiring further study are listed.     

A note on approximation techniques used for process optimization

The need for increasingly complex models for process simulation has led several researchers to develop more efficient steady-state simulation strategies. In a common approach, rigorous physical property calculations are replaced by simplified ones which approximate locally the behavior of the rigorous ones. Periodically the rigorous models are executed to check if the simplified ones are still accurate enough, and, if not, parameters for the simplified ones are adjusted so both the rigorous and simplified models again predict the same values locally. Convergence is assumed when no adjustments are needed for these parameters and the simplified model is converged.This approach works well for phase equilibrium and even general process simulation problems. Some researchers have extended this approach to optimization problems as well. In this note we discuss problems which arise when adapting this approach for process optimization problems. We also present three examples where the use of simplified models can lead to detection of false optima or convergence failures.     

Gas absorption with photochemical reaction

Gas-liquid reactions can be activated by high energy radiations (photons). When gas absorption is accompanied by pseudo-first order reaction further enhancement in the specific rate of absorption can be realised for situations where either the reactive species in the liquid phase is activated or the dissolved solute gas is activated.     

Single-input—single-output control system synthesis : Part 1: Structural analysis and the development of feedback control schemes

A systematic, noniterative procedure requiring minimal numeric information has extended structural analysis to the synthesis of regulatory control systems comprising single-input—single-output controllers.     

Simultaneous absorption with reversible instantaneous chemical reaction

The approach presented recently [1] for analyzing absorption and desorption mass transfer problems with instantaneous chemical reaction is extended to the case of simultaneous absorption of two gases, A and A′. The analysis is developed for arbitrary stoichiometry. The following simple case is discussed in detail:A + B₁?B₂A′ + B₁?B₃ where B₁ is the liquid phase reactant and B₂, B₃ the reaction products. The analysis takes into account the “shift” reaction, which for the simple case above is:A + B₃?A′ + B₂This reaction takes place in the region near the interface. The analysis differs from previous work which, with one exception, ignored the “shift” reaction and restricted attention to zero values of the concentrations of B₂ and B₃ in the bulk liquid.The analysis shows that the conditions where the physical driving forces (a_i-a_o) for absorption of both gases are large and positive does not imply that the chemical driving forces (α_i-α_o) are both positive. In fact, it is shown that cases arise where one component may desorb even though its physical driving force is positive.A simplified thermodynamic model useful for extrapolation of mixed CO₂ and H₂S equilibrium data in amine solutions to very low values of acid gas loading in solution is developed. Tower profiles for simultaneous absorption of CO₂ and H₂S in monoethanolmine solution are considered in light of the new analysis. The good kinetic selectivity measured for H₂S at the absorber lean end is due to the fact that carbon dioxide is not absorbed in the instantaneous reaction regime. At the absorber rich end, where a temperature bulge develops, CO₂ is absorbed in the instantaneous regime, causing H₂S to be desorbed even though its physical driving force favors absorption.     

Effect of char gasification reaction order on bounding solutions for char combustion

The rate of oxidation and surface temperature of a char particle during combustion is determined by the combined action of reactions of carbon with oxygen and CO₂ at the surface and carbon monoxide with O₂ in the gas phase. Limiting values for the rates and temperature were determined by Amundson et al. assuming that the CO₂ and CCO₂ reactions were first order with respect to the gas reactant concentration. Results are presented here for the cases in which the CO₂ reaction is zeroth order and the CCO₂ reaction is both first and zero order to bound the reaction orders found in practice. Allowance for zeroth order kinetics is found to have a profound effect on the shape of the bounding solutions for burning rates and temperatures, most notably by showing that diffusion-limited combustion can occur at much lower temperatur than would be predicted using first order kinetics.