Anton A. Kiss

Energy efficient control of a BTX dividing-wall column

Abstract Dividing-wall column (DWC) is considered nowadays the new champion in distillation, as it can bring substantial reduction in the capital invested as well as savings in the operating costs. This work presents the simulation results of energy efficient control and dynamics of a dividing-wall column (DWC). In order to allow a fair comparison of the results with previously published references, the case-study considered here is the industrially relevant ternary separation of the mixture benzene–toluene–xylene (BTX) in a DWC. Rigorous simulations were carried out in Aspen Plus and Aspen Dynamics. Several conventional control structures based on PID control loops ( DB / LSV , DV / LSB , LB / DSV , LV / DSB ) were used as a control basis. These control structures were enhanced by adding an extra loop controlling the heavy component composition in the top of the prefractionator, by using the liquid split as an additional manipulated variable, thus implicitly achieving minimization of energy requirements. The results of the dynamic simulations show relatively short settling times and low overshooting especially for the DB / LSV and LB / DSV control structures. Moreover, the energy efficient control proposed in this work allows the operation of DWC with minimum energy requirements or maximum purity of products.

Innovative dimethyl ether synthesis in a reactive dividing-wall column

Abstract Dimethyl ether (DME) is of great industrial interest due to its use as clean fuel for diesel engines or in combustion cells, as a precursor to other organic compounds, as well as a green aerosol propellant that can effectively replace chloro-fluoro-carbons. Conventionally, high purity DME is synthesized by dehydration of methanol produced from syngas, in a process involving a catalytic fixed-bed reactor and a direct sequence of two distillation columns. The key problem of this classic process is the high investment costs for several units that require a large overall plant footprint, as well as the associated high energy requirements. To solve these problems, we propose in this work an innovative DME process based on a reactive dividing-wall column (R-DWC) that effectively integrates in one shell a reactive distillation (RD) unit with the DWC technology. The double integrated system allows the production of high-purity DME in only one unit, with minimal footprint and significantly lower costs. This study also makes a fair comparison between the reported conventional DME process and the optimally designed process alternatives based on RD and R-DWC, respectively. All processes are optimized in terms of minimal energy requirements, using the state of the art sequential quadratic programming (SQP) method implemented in AspenTech Aspen Plus. The results clearly demonstrate that the R-DWC process has superior performances as compared to the conventional or RD process: significant energy savings of 12–58%, up to 60% reduced CO2 emissions, as well as up to 30% lower capital investment costs.

Innovative process for fatty acid esters by dual reactive distillation

Catalytic reactive distillation offers new opportunities for manufacturing fatty acid esters, involved both in biodiesel and specialty chemicals. A key problem is the effective water removal in view of protecting the solid catalyst and avoiding costly recovery of the alcohol excess. This work proposes a novel approach based on dual esterification of fatty acid with light and heavy alcohols, namely methanol and 2-ethylhexanol. These two complementary reactants have an equivalent reactive function but synergistic thermodynamic features. The setup behaves rather as reactive absorption combined with reactive azeotropic distillation with heavy alcohol as co-reactant and water-separation agent. Another element of originality is the control of the inventory of alcohols by fixing the reflux of heavy alcohol and the light alcohol column inflow. This strategy allows achieving both stoichiometric reactant feed rate and large flexibility in ester production. The distillation column for recovering light alcohol from water is not longer necessary. The result is a compact, efficient and easy-to-control multi-product reactive setup.

REACTIVE DIVIDING-WALL COLUMNS—HOW TO GET MORE WITH LESS RESOURCES?

This work presents a novel integrated reactive-separation design based on a dividing-wall column (DWC) applied to an industrial case study within AkzoNobel Chemicals. To the best of our knowledge this is one of the first reported industrial applications of a reactive DWC. Due to changing market conditions, one of the by-products in a plant became more economically attractive than the main product. However, the design of the existing plant does not allow an increase of the by-product production rate at the cost of the main product. To solve this problem we developed a novel integrated design that combines reaction and separation into a feasible reactive DWC that allows 35% savings in capital and 15% savings in energy costs. This article describes the novel reactive DWC design, presents the rigorous simulation results, and makes a comparison with the base case alternative.

Design and control of recycle systems by non-linear analysis

Abstract By placing together the pieces of the design and control puzzle we developed a novel methodology that allows the screening and selection of feasible integrated designs at an early stage. The structure reactor–separator–recycle (R–S–R) system is a central design objective. The key piece of equipment remains the chemical reactor that should be large enough to ensure feasible and flexible operation. Multiple states are possible solely due to the effect of material recycles, but thermal effects may bring more complicated non-linearities. The so-called snowball effect is merely a matter of design than control. The behaviour depends on the reactor size and plantwide control structure. Larger reactors behave better than smaller ones. A clear distinction is made between self-regulation and regulation-by-feedback control structures. Usually, using the recycles to change the production rate is better than fixing the fresh feeds. By means of the phenol hydrogenation case study, we show that indeed the best control strategies are based on manipulating both reactants recycles.

Integrated reactive absorption process for synthesis of fatty esters

Reactive separations using green catalysts offer great opportunities for manufacturing fatty esters, involved in specialty chemicals and biodiesel production. Integrating reaction and separation into one unit provides key benefits such as: simplified operation, no waste, reduced capital investment and low operating costs. This work presents a novel heat-integrated reactive absorption process that eliminates all conventional catalyst related operations, efficiently uses the raw materials and equipment, and considerably reduces the energy requirements for biodiesel production--85% lower as compared to the base case. Rigorous simulations based on experimental results were carried out using Aspen Plus and Dynamics. Despite the high degree of integration, the process is well controllable using an efficient control structure proposed in this work. The main results are provided for a plant producing 10 ktpy fatty acid methyl esters from methanol and waste vegetable oil with high free fatty acids content, using sulfated zirconia as solid acid catalyst.

State multiplicity in CSTR-separator-recycle polymerisation systems

Abstract This article continues earlier work (Comput. Chem. Eng. 24 (2000) 209) concerning the design and control of isothermal reactor–separator–recycle systems. The multiplicity behaviour of six reaction systems of increasing complexity, from one-reactant, first-order reaction to chain-growth polymerisation, is investigated. Below a critical value of the plant Damkohler number, Da Dacr. For one-reaction systems, one stable steady state is born at a transcritical bifurcation. For consecutive-reaction systems, including polymerisation, a fold bifurcation can lead to two feasible steady states. Moreover, the transcritical bifurcation is destroyed when two reactants are involved. If the gel-effect is included, a maximum of four steady states are possible. When multiple steady states exist, the achievable conversion is constrained by the instability of the low-conversion branch. This has practical importance for polymerisation systems when the radicals’ quasi-steady state assumption is not valid or the gel effect is significant.

State multiplicity in PFR–separator–recycle polymerization systems

Abstract This article explores the non-linear behaviour of isothermal and non-isothermal plug-flow reactor (PFR)–separator–recycle systems, with reference to radical polymerization. The steady-state behaviour of six reaction systems of increasing complexity, from one-reactant first-order reaction to chain-growth polymerization, is investigated. In PFR–separator–recycle systems feasible steady states exist only if the reactor volume exceeds a critical value. For one-reaction systems, one stable steady state is born at a transcritical bifurcation. In case of consecutive-reaction systems, including polymerization, a fold bifurcation can lead to two feasible steady states. The transcritical bifurcation is destroyed when two reactants are involved. In addition, the thermal effects also introduce state multiplicity. When multiple steady states exist, the instability of the low-conversion branch sets a lower limit on the conversion achievable at a stable operating point. A low-density polyethylene process is presented as a real plant example. The results obtained in this study are similar to CSTR–separator–recycle systems. This suggests that the behaviour is dictated by the chemical reaction and flowsheet structure, rather than by the reactor type.

Advanced control of a reactive distillation column

Abstract The paper presents a detailed analysis of the dynamic behavior of a reactive distillation column. A control relevant dynamic model is derived using firstprinciples modeling and it is used to study the dynamic behavior of the process at high and low purity operating regimes. The results are used to analyze the performance of linear and nonlinear model predictive control in comparison to coupled PID control.

MODELING AND SIMULATION OF A PERVAPORATION PROCESS FOR FATTY ESTER SYNTHESIS

This study presents a generalized concept for modeling ester production in hetero/homogeneous catalysis by using a pervaporation membrane reactor (PVMR) that enhances the conversion by continuous removal of water byproduct. The system is rigorously described by the Maxwell-Stefan approach combined with the solution-diffusion model. The proper description of the membrane mass transport takes into account the subtle interplay between adsorption and diffusion, as well as the coupling between the diffusing species. This relatively simple yet robust modeling approach is deployed in MathWorks MATLAB, where rigorous dynamic simulations of the PVMR were performed. The esterification reaction of oleic acid with ethanol is considered as a relevant case study. The comparison between various catalytic systems—conveniently possible due to the flexible modeling approach—is supported by an appropriate sensitivity analysis study of the key design and operating parameters (e.g., permeability coefficient, membrane area-to-volume ratio, operating temperature and pressure). The simulation results were also successfully validated against reported experimental data, thus proving the effectiveness of this modeling approach.