A.C. Dimian

Fatty acid esterification by reactive distillation. Part 2: kinetics-based design for sulphated zirconia catalysts

Abstract In a previous study on the basis of reaction equilibrium, it was demonstrated that the synthesis of fatty acids esters of heavy alcohols by catalytic reactive distillation (RD) is basically feasible. Presently, the analysis is expanded to the kinetics-based design applied to the synthesis of 2-ethylhexyl dodecanoate. Experimental data for a super-acidic sulphated zirconia catalyst are employed. It is shown that this catalyst is highly selective even for the high alcohol/acid ratios as prevail in a RD column. To avoid catalyst deactivation, liquid–liquid segregation into an organic and an aqueous phase must be prevented, and operating temperatures beyond 373 K must be chosen. A kinetic model based on liquid activities is proposed, with parameters determined from both kinetic and equilibrium measurements. A typical column design has 13 reactive stages with a catalyst loading of 170 kg m −3 , temperature profile of 400– 440 K , and a space velocity of 0.022 kmol kg cat −1 h −1 for 99.9% purity product ester. A modified Damkohler number is proposed.

Interaction Between Design and Control of a Heat-Integrated Distillation System with Prefractionator

This paper analyses the relationship between the design and control of a heat-integrated distillation set-up consisting of a prefractionator and a side stream main column. The separation of a pentane-hexane-heptane mixture with moderate purity requirements is considered. Both forward and reverse heat-integration schemes are investigated. Different designs are possible, depending on the light/heavy split in the prefractionator and the heat-integration scheme. They are similar with respect to energy consumption, but very different with respect to dynamic behaviour. The differences are studied using frequency-dependent controllability analysis, as well as carrying out closed-loop simulation, in the presence of large feed composition disturbances. Thus, it is found that the forward heat-integration is much easier to control, using only temperature measurements. The low-cost design with a small prefractionator has the best closed-loop performance. The reverse heat-integration can be controlled only if a composition analyser of the side stream is available; a sharp light/heavy split in the prefractionator gives better disturbance rejection. The superior dynamic behaviour of the forward heat-integration scheme was confirmed by the study of a high purity separation of the benzene-toluene-xylene mixture.

Design of reactive distillation process for fatty acid esterification

Publisher Summary Esters of fatty acids are produced in batch reactors. This chapter presents an innovative continuous process based on reactive distillation that can be used as a multipurpose configuration allowing the synthesis of a variety of fatty esters. The approach consists of chemical and phase equilibrium analysis to identify the feasible design region and computer simulation to generate process alternatives. Results are presented for the esterification of lauric acid with methanol and 2-ethylhexanol, the lightest and the heaviest in the C 1 –C 8 alcohol series. Two process alternatives can be accommodated in the same hardware, but with different operation procedures: one with alcohol reflux and other with acid reflux. The first is feasible for heavy alcohols, forming heterogeneous azeotrope with water. The second is suited for both light and heavy alcohols and may be seen as a generic esterification process of fatty acids. Kinetic measurements are tested by simulation method. A laboratory setup is currently builtup to prove the feasibility of this new process.

Effect of recycle interactions on dynamics and control of complex plants

Abstract Plantwide inventories of main components and impurities in a complex plant are sharply affected by interactions between recycles which, in general, are not desirable. However, this paper demonstrates how the interactions between some recycle loops may be exploited to create feasible plantwide control structures that are impossible to achieve simultaneously with stand-alone units. Thus, flowsheet architecture, equipment design and control system design must be interrelated. Simultaneous design is necessary for items involved in plantwide control structures. The paper presents a simulation based methodology for evaluating the effect of recycle interactions on dynamics and plantwide control of complex plants. Steady-state and dynamic simulations are combined with controllability analysis, both in steady state and in dynamic mode. A case study handling the removal of impurities in a plant with nested loops illustrates the approach. The controllability of two flowsheet alternatives is evaluated. The steady-state analysis is confirmed at low frequencies. Possible difficulties may occur at higher frequencies, where the period of disturbances and the time constants of the distillation columns are of the same order of magnitude. The relative direction of disturbances plays a significant role. Closed loop simulation validates the main trends of the controllability analysis, showing in the same time the difficulty in managing a perfect multivariable control of the material balance.

Systems analysis in handling impurities in complex plants

This paper presents an application of systems analysis in handling impurities in a complex plant for operation diagnosis or for revamping. Tracing impurities necessitates careful calibration of the plant material balance and the study of the recycle loops dynamics. Thus may be predicted operation difficulties originating mainly from interactions between recycles rather than from insufficient equipment performance. Steady-state flowsheeting combined with controllability tools enables to detect recycles interactions, to evaluate plantwide control properties of flowsheet alternatives and to suggest design improvements. Then, dynamic simulation on a reduced flowsheet may be used to asses robust flowsheet alternatives. A case study of a VCM plant illustrates the methodology.

Dynamics and control of a reactive distillation process for fatty acid esterification

Abstract The paper investigates the dynamics and control of a reactive distillation (RD) process for fatty acids esterification using sulphated zirconia as catalyst, demonstrating the controllability of the process. The process consists of a RD column, a decanter and a vacuum evaporator. The flow rate of fresh acid is used as throughput manipulator. The alcohol recycle is fixed, and the fresh alcohol is added through level control. The setpoint of the recycle flow controller is adjusted, to avoid excessive changes of the reflux flow rate. Control of RD column reboiler temperature and evaporator temperature and pressure ensures the product quality.

Interaction between Design and Control of Heat-Integrated PFR

This article analyzes the interaction between design and control of a heat-integrated plugflow reactor. The space of the design parameters, feed-effluent heat-exchanger (FEHE) efficiency and steam-generator duty, is divided by bifurcation varieties into regions with different controllability properties. Close to the cusp variety, the system is sensitive to disturbances and control is as difficult. The same conclusion is valid for designs with large FEHE. Disturbances can be rejected, but fast control can not be achieved. The range of design parameters for which the system cannot be stabilized is computed.

Modeling and design of a biochemical process for NOx removal

Abstract Modeling and design of an industrial scale biochemical process for removal of NOx from flue gases (BioDeNOx) are presented. The process is based on the absorption of NOx in Fe(II)EDTA solution followed by the reduction to nitrogen in a biochemical reactor. Rate-based models of the absorption and reaction units are developed, taking into account the kinetics of chemical and biochemical reactions and the rate of gasliquid mass transfer. We demonstrate that the process is feasible at industrial scale. A spray column is more efficient as absorber than a bubble column since it minimizes the deactivation of Fe(II)EDTA by concurrent oxidation. In achieving high yield in NOx reduction, the regeneration and recycle of the Fe(II)EDTA complex is determinant.

Stable plantwide control of recycle systems

The interaction between design and control of chemical plants involving material recycles is addressed. Typical flowsheet and control structures are identified based on reaction stoichiometry and the physical properties of reactants and products. For the case of floating recycle, state multiplicity occurs associated with the instability of the operating points on the low-conversion branch. Fixing the reactor-inlet flow rate of each reactant is an effective means to control the mass balance, which makes the reactor behaviour similar with a stand-alone operation. When two or more reactants are recycled together, this would require composition measurements. In this case, designs achieving almost complete conversion of one reactant are suggested.

Multiplicity and stability of CSTR-separator-recycle systems

Publisher Summary This chapter discusses the nonlinear behavior of several continuous stirred-tank reactor (CSTR)-separator-recycle systems. The plant Damkohler number (Da) is introduced as a parameter of the dimensionless balance equations. A feasible operating point exist if Da > Da cr , where the critical value, Da cr corresponds to a singular point of the model equations. For one-reactant nth-order isothermal reaction, the feasible operating point is unique and stable. In other cases (second-order isothermal reaction involving two reactants; first-order adiabatic reaction), multiple steady states exist; the range of achievable conversion being limited by the instability of the low-conversion state. For the chemical reactor, a dynamic model can be derived based on unsteady mass and energy balance. The model contains a few nonlinear differential equations, being amenable to analytic or numerical investigation. The chapter considers a first-order reaction, taking place in an adiabatic CSTR.