Costin Sorin Bildea

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.

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.

Nonlinear behavior of reactor—separator—recycle systems

The nonlinear behavior of reactor—separator—recycle systems is studied. The dimensionless mole-balance equations are parameterized by the plant Damkohler number (Da) and the separation specifications. Reactant accumulation does not occur if Da > Dacr. DaCr corresponds to a bifurcation point of the mole-balance equations and depends on separation performance. For one reactant recycle and high purity separation, Dacr = 1. When two reactants are recycled, multiple steady states are possible. In this case, Dacr depends also on the control structure. Close to Dacr, control structures must change the reaction volume or temperature in order to reject effectively the disturbances.

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.

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 tubular reactors in recycle systems

Abstract The article presents an approach to design tubular reactors in recycle systems, based on non-linear analysis. A pseudo-homogeneous plug-flow reactor model is used. It is assumed that the separation unit delivers product and recycle streams with fixed composition. The stand-alone reactor has a unique stable steady state. The coupled reactor–separation–recycle system shows four types of conversion versus plant Damkohler number bifurcation diagrams. A feasible steady state exists only if the reactor volume exceeds a critical value. For isothermal reactor, the steady state is unique and stable. For non-isothermal reactor, one or two steady states are possible. In the second situation the low-conversion state is unstable. In some parameter regions, the unique state is unstable. The design should ensure state unicity and stability, which are favoured by large heat-transfer capacity, low coolant temperature and high reactor-inlet temperature. A case study demonstrates that these phenomena can be easily found in real plants.

On the application of model reduction to plantwide control

Abstract The derivation and applicability of reduced-order models for selection and assessment of plantwide control structures is studied. The paper demonstrates the advantage of exploiting the intrinsic structure of a chemical plant, which mirrors the decentralization of the control problem. The recommended procedure is to apply model reduction to individual units, followed by coupling these reduced-order models. This procedure is flexible and accurate, and leads to major reduction of the simulation time. The importance of retaining the model nonlinearity is highlighted.

Optimal design of intensified processes for DME synthesis

Abstract Dimethyl ether (DME) is widely used as green aerosol propellant, precursor to other organic compounds, or as a clean fuel for diesel engines or in combustion cells. The classic method for producing DME is by dehydration of methanol in a catalytic gas-phase reactor, and purification in a direct sequence of two distillation columns. Reactive distillation (RD) is a much better alternative for DME synthesis, based on process intensification principles. This paper presents the optimal design of novel DME processes based on reactive distillation, and makes a fair comparison with the classic reactor-separation-recycle process (for a plant capacity of 100 ktpy DME). The new RD processes were optimized in terms of minimizing the total annual costs, leading to savings of 30% in CapEx and 6% in energy requirements for the RD process. The results indicate that a RD column is recommended for new DME plants, while a combination of gas-phase reactor and RD is suitable for revamping existing plants.