Alexandre C. Dimian

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.

Fatty acid esterification by reactive distillation. Part 1: equilibrium-based design

Abstract Esters of fatty acids are currently produced in batch processes. In this study we present an innovative process based on reactive distillation (RD). The synthesis of a several fatty esters is possible in the same RD set-up, making possible the development of a continuous multipurpose process. Conceptual design is presented as a systematic methodology based on thermodynamic analysis combined with computer simulation. This approach sets also targets for limited experimental work. The methodology is illustrated by the esterification of lauric acid with 2-ethylhexanol and methanol, the heaviest and lightest alcohol in the C 1 –C 8 series. The first part presents the design of a RD set-up based on chemical equilibrium. Both reactions can be accommodated in the same hardware, but with different operation procedures. The alternative with alcohol reflux is suitable for heavy alcohols that form heterogeneous azeotropes with water. Another alternative with acid reflux can accommodate both light and heavy alcohols, and may be seen as a generic process for fatty acid esterification.

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.

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.

Sulfated zirconia as a robust superacid catalyst for multiproduct fatty acid esterification

Sulfated zirconia catalysts obtained by employing chlorosulfuric acid show significantly higher activity in the esterification of fatty acids with different alcohols compared with catalysts made using sulfuric acid. The superior performance results from higher sulfur content, larger pores and stronger acid sites. These catalysts are robust and do not leach out sulfonic groups. Catalyst performance depends strongly on the sulfation reagent and the calcination conditions of the intermediate zirconium hydroxide. A series of kinetic experiments was carried out with lauric acid and various alcohols (methanol, 2-ethylhexanol, propanols and butanols). The new catalysts are ca. five times faster when using primary alcohols independent of the alcohol chain length. When using secondary and tertiary alcohols the reaction rate drops considerably. This is explained by a linear free energy relationship of substituent reactivity. The kinetic investigation shows that chlorosulfated zirconia is suitable as a multiproduct catalyst for manufacturing fatty esters, by employing a catalytic reactive distillation process.

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.

Renewable raw materials: chance and challenge for computer-aided process engineering

In the context of the climate change and with the perspective of rapid exhaustion of fossil hydrocarbon resources, the use of renewable raw materials becomes vital for the future of Chemical Process Industries. The first oil crisis from 1974 kicked-off the advent of process simulation. Today the emergence of bio-fuels, boosted by a serious petroleum and environmental crisis, is an exiting challenge for developing new design methods and simulation tools, as well as a chance for CAPE rejuvenation.

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.

An effective modular process for biodiesel manufacturing using heterogeneous catalysis

We present an innovative reaction set-up and process for biodiesel manufacturing by heterogeneous catalysis. This process has two key advantages over the state-of-the-art process: it enables a variable reaction time and easy catalyst switching/replacement. The process principle presented here is generic for liquid-phase reactions requiring long residence times, where conventional fixed-bed column reactors offer little flexibility. This is especially important when one switches between feedstocks or when the catalyst activity declines over time. Biodiesel manufacturing is a highly relevant example, because the reactor performance depends on the feedstock nature and composition. The concept is demonstrated in a scaled-down continuous laboratory reactor, keeping a similar reaction time and comparable heat and mass transfer to a large-scale process by optimising the reactor dimensions, fluid velocity and catalyst pellet size. We then provide the design of the large-scale process, which consists of serpentine-type plug flow reactors assembled as vertical tubes filled with catalysts. The reactor productivity can increase significantly by reducing the catalyst pellet size. A switching system allows connecting/bypassing the tubes and easy catalyst replacement. The reactor can be employed in a two-stage reaction technology, or in a one-stage reaction combined with membrane separation. Production capacity can be scaled-up simply by adding parallel modules. The versatility and ease of application make this catalytic process concept suitable for low-cost mobile biodiesel production plants.