(478c) On the Optimisation of Industrial Scale Oxidation Processes by Combining Fundamental Chemistry and Multi-Phase Hydrodynamics

Terephthalic acid (TPA) is an important industrial intermediate produced in large scale as a raw material for poly ethylene terephthalate (PET) and other polymers. TPA manufacturers face a challenging business environment and are constantly looking to improve the process efficiency without compromising the product quality specifications and the production targets. The situation is the same for other products manufactured by means of oxidation, such as isophthalic acid, phenol and cresol.

TPA is commercially produced through the oxidation of p-xylene. Though the selectivity of the oxidation reaction is generally high (>96%), the high proportion of raw material cost in the plant economics provides impetus to look for further opportunities to reduce the raw material consumption. In addition, the solvent (acetic acid) is also lost (by burning and other reactions) in the process and this needs to be reduced. Considering that these processes are already quite efficient due to empirical optimisation over the years, any improvement achieved is expected to be relatively marginal (in the order of 1%), but due to the scale of operation this is likely to yield a few million dollars in additional profit even for a medium sized TPA plant.

Improving a process that is already efficient may involve fine tuning of the process operating conditions, modifications to the plant reactor/equipment configurations and where feasible, identify new degrees of freedom to meet the objectives. Though one can tweak the operating conditions or modify the internal configurations based on experience, it is faster, lower risk and cheaper (esp. when changes to reactor configurations are involved) to carry out optimisation studies using model-based engineering tools. Considering that the improvement targets for these processes are modest, the models used for these type of studies must be sufficiently accurate and predictive in nature. To meet these requirements, a very detailed first principle model framework for multiphase oxidation reactors has been developed in gPROMS, which accounts for the major physical and chemical phenomena taking place within the reactor:

? Occurrence of vapour, liquid and solid phases

? Mass transfer between the vapour and liquid phases based on Maxwell-Stefan relationship

? Vapour liquid equilibrium at the phase interface

? Crystallisation of the product within the reactor

? Chemical reaction kinetics

? Hydrodynamics of the reactor

As the volume of industrial reactors is in the order of hundreds of cubic metres, assuming a well-mixed behaviour in the reactor is a gross simplification. The hydrodynamic behaviour in the reactor and its impact on the physical and chemical performance of the system is captured using a multi-zonal approach (Bezzo, 2004).

As the TPA process involves liquid recycle loops, it is essential to not only model the oxidation reactor but also the condenser systems, post-oxidation reactors and downstream crystallisers.

The abovementioned model framework has been successfully used to optimise the performance of several TPA and IPA plants around the world. This contribution describes a further improvement of this framework related to enhancing the reaction kinetic model.

Oxidation of hydrocarbons (e.g., p-xylene, m-xylene, cumene) using molecular oxygen is commonly termed as autoxidation and is based on the radical chain reaction mechanism. This mechanism has been proposed for several catalysed and non-catalysed oxidation reactions and is reasonably well understood. The oxidation of both p-xylene and m-xylene are homogenously catalysed reactions and the catalyst is often a mixture of cobalt, manganese and bromine salts (MC type catalyst). The primary role of the catalyst in these processes is to selectively decompose the intermediate peroxides formed during the reaction at low activation energies. Acetic acid is used as solvent in these processes. Like most gas ? liquid reaction systems, the performance of the oxidation reactors can be manipulated by changing the operating pressure, temperature and catalyst amount, but in addition, the catalyst composition and water concentration [Partenheimer, 1995] in the reactor are two added controls that can be manipulated to improve the process performance. Interestingly, water is known to be a catalyst inhibitor at high concentrations, while it favours the reaction at low concentration and hence the overall process optimisation is a non-trivial task.

The reaction kinetic models available in open literature (e.g. Cincotti et. al., 1999 for TPA) are simple lumped models which generally ignore the catalyst and water concentration effects discussed above and are hence not predictive over wider operating conditions. Cheng et. al.,(2006) proposed a reaction kinetic model based on a radical chain mechanism that takes in to account the role of catalyst, but ignores the effect of water concentration. Furthermore, this kinetic model involves too many model parameters and is not predictive.

We have carried out extensive reviews on autoxidation processes and based on this, have developed a detailed reaction kinetic model based on a fundamental radical chain reaction mechanism that captures all chemical phenomena described earlier. In addition to the above, the reaction kinetic model is also capable of accounting for colour imparting species as well.

Very often, industrial manufacturers buy a formulated Co/Mn/Br catalyst mixture and hence the degrees of freedom associated with the catalyst composition are not available to the plant operator. The kinetic model based on the radical chain mechanism accounts for the individual active species in the catalyst mixture and the optimal catalyst composition can be identified for the given system. This enables the manufacturer to purchase the right catalyst for the system from the market or otherwise formulate a catalyst mixture, which maximises the performance for their system. Thus, the reaction kinetic model based on fundamental radical chain mechanism opens new avenues to optimise the process performance.

In this paper, we describe how the radical chain mechanism based kinetic model is employed to improve the performance of an industrial scale oxidation reactor. The model prediction will be compared against a simple kinetic model that does not account for the catalyst effects.

References:

Qinbo Wang, Youwei Cheng, Lijun Wang, and Xi Li, ?Semicontinuous Studies on the Reaction Mechanism and Kinetics for the Liquid-Phase Oxidation of p-Xylene to Terephthalic Acid?, Ind. Eng. Chem. Res. 2007, 46, 8980-8992.

Partenheimer, ?Methodology and scope of metal/bromide autoxidation of hydrocarbons?, Catalysis Today 23 (1995) 69-158

Youwei Cheng, Xi Li, Lijun Wang, and Qinbo Wang, ?Optimum Ratio of Co/Mn in the Liquid-Phase Catalytic Oxidation of p-Xylene to Terephthalic Acid?, Ind. Eng. Chem. Res. 2006, 45, 4156-4162

Bezzo, F., Macchietto, S. and Pantelides, C.C. ?A general methodology for hybrid multizonal/CFD models - Part I. Theoretical framework? Comput Chem Eng, 28, 501-511, 2004. Cincotti, Roberto OrruÁ , Giacomo Cao ?Kinetics and related engineering aspects of catalytic liquid-phase oxidation of p-xylene to terephthalic acid? Catalysis Today 52 (1999) 331±347