We aim to construct and operate a prototype for continuous conversion of mixed plastic polymers feedstock to naphtha-range chemicals by SCW-hydrocracking (SCWH). Feedstocks of focus are polypropylene (PP), polyethylene in low- and high-density forms (LDPE, HDPE) and their mixtures. They are frequently used for packaging of foods, drinks among many commodity products. PP, PE, and derivatives are specifically resistant to recycling by the current technologies.
Key objectives. Toward construction of the prototype, the main objectives are:
O1. Construction of a continuous process prototype
O2. Comprehensive experimental programme for SCWH of pure and mixed plastic polymers.
O3. Investigations of the reactions’ mechanisms and kinetics, required for process design.
O4. Simulation of the process energy requirements and assessment of possible energy integration scenarios.
Experimental programme:
Polymers dissolution. In the mixing vessel we aim to attain sufficient temperature (T) and pressure (P) to form a homogeneous polymer-water mixture. Guided by literature1 a P,T value of 220 bar, 350 °C respectively, is sufficient to dissolve polymers in hot compressed water with minimal degradation. Initial solubility tests at T range of 250 – 360 °C will be carried.
Supercritical water hydrocracking (SCWH) of feedstock. The sought products are oils of C6–C18 chain-length. Reactor temperature (T), residence time (t) and water/polymer ratio (Rw/p) play main roles in the feedstock conversion and distribution of products. T and t influence the proportion of each species in the product stream while water quantity (Rw/p) governs the thermophysical properties of the process fluids as well as the input heat required to attain the reaction temperature. Products will be characterised by analytical techniques namely:
• Gases: GC-TCD
• Oils: GC-FID, GC-MS and TGA
• Solids: TGA, FTIR
Reaction pathways and kinetics investigations:
A proposed pathway for polymer decomposition in SCW, in a batch reactor will be the starting point whereby the experimental results for selected compound groups will be utilised to ‘map out’ reaction pathways that better describe the system under investigation. While the overall scheme is similar, we envisage differences instigated by (1) the reactor configuration, (2) feedstock compositions and (3) the interactive effects encountered in mixed-plastic feedstock.
The reaction pathways will be based on the Lumped Reaction Network kinetic models derived from a series of Pseudo first order reactions.
Energy Assessment:
Energy integration scenarios will be investigated via computer simulation packages namely ASPEN HYSIS® and MATLAB combined with experimental data at various process variables. Material and energy balances will be conducted, and the heat of reactions will be calculated using the stoichiometric equations and bond energies.
The successful candidate will be trained on experimental techniques and analytical skills. Applicants must have minimum of B.Eng. or M.Eng. in Chemical Engineering - Grade 2:1. High numeracy, well-developed laboratory skills, and high drive to work in new/advanced processes are highly preferred.
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