Real-time production scheduling in a multi-grade PET resin plant under demand uncertainty

We deal with the real-time production scheduling of a continuous-process multi-grade PET resin plant. The process is surcharged by sequence-dependent changeovers, sequential processing with production and space capacity, and mixed and flexible finite intermediate storage. The management called us to develop a time responsive production scheduling tool that copes with demand uncertainty, urgent orders and increased lead times. We adopt simulation as a methodology approach and create a tool based on tangible control rules and simple production engineering methods that make dynamic analysis tractable. Our goal is to maximize the aggregate fill rate, taking into consideration the number of required equipment transitions which cause undesirable variations in base resin properties. The model is tested under a real-world six-month demand instance and ten other hypothetical scenarios. The results are compared with those of the optimal solution derived from a preexisting Mixed Integer Linear Programming model that considers short-term demand as known. (C) 2012 Elsevier Ltd. All rights reserved

The development of mixed oxide adsorbents and catalysts through continuous flow processes.

The aim of this project was to produce solid adsorbents and catalysts in a continuous flow process. These materials are traditionally made in a batch process. The materials were made and the effect of the change in synthesis explored. When hydrotalcite was produced in a batch process it was found to yield material that was synthesised by a reaction governed by thermodynamic control. Contrary, solid formation in a continuous flow it was found to be governed by a kinetically controlled reaction. During the batch reaction the reactants had time and mobility to precipitate, dissolve and re-precipitate in order to arrive at the lowest energy configuration. This configuration resulted in aluminium from one meixnerite intermediate layer aligning with aluminium in the opposing layer (referred to as “in phase”). This allowed carbonate anions to join the layers together creating a crystalline structure by balancing its charge between the two aluminium atoms. In a continuous flow process the reactants were not afforded either the time or space to move. This resulted in aluminium being randomly located within the meixnerite intermediate. Due to the random placement of aluminium, the aluminium in opposing layers did not consistently align (referred to as “out of phase”). Carbonate anions bound to these out of phase aluminium atoms are unable to balance their charge by linking layers together. The anions compensated by taking up hydrogen and forming a bicarbonate anion. The remaining aluminium took up monovalent nitrate anions in the absence of any divalent anions. Hydroxide anions would have been more favourable but they were required in the formation of the lattice. It was also found that the kinetically controlled product could be converted to the thermodynamically controlled product by washing. Washing, however, degraded the lattice structure and washed away an amount of material. The hydrotalcites produced by continuous flow process were thermally decomposed to act as a carbon dioxide adsorbent. They decomposed much faster than their batch counterparts, suggesting the carbonate bridge provides thermal stability to the material. The decomposition also revealed that the magnesium/aluminium spinel recorded in other studies did not form for the materials produced in a continuous flow process. Instead the material decomposed to the separate metal oxides. This did not have an effect on the ability of the material to adsorb carbon dioxide and performed comparably to the batch process materials. The continuous flow process was used to create a Ni/Mg/Al hydrotalcite that could be used as a catalyst in ethanol steam reforming. The continuous flow process allowed three processes for introducing nickel into the system: the doping of the starting reagents, ion exchange during the washing process and the impregnation of the dry product. The doped samples produced material comparable to those produced in continuous flow without doping or washing. The ion exchange and the impregnation samples produced samples that are comparable to those produced in continuous flow with washing The doped samples exhibited a fourth region of weight loss in thermogravimetric analysis which is speculated to be the formation of nickel aluminide at high temperatures. The catalytic activity corroborated this idea with an increase in metallic behaviour. The doped sample was found to be the more active sample tested. In order to further test the effect of kinetic and thermodynamic control during the continuous flow process, an attempt to control the particle size of zirconium basic sulfate was made. It was found that the particle size could be altered in the batch process due the variable environment within the reactor. Inefficient mixing created concentration gradients and inefficient heating created temperature gradients. It was these gradients within the batch reactor that facilitated the thermodynamic product to be produced and the particle size to increase. In a continuous flow reactor these gradients did not exist. The advantage of the flow reactor was the ability to control the environment of the reactor in an even and uniform manner. The uniformity of the reactor environment eliminated any temperature or concentration gradients, keeping all the materials at similar chemical potentials, limiting particle growth