Abstract
Waste minimization in reactor design is an effective approach for pollution control, when compared to the traditional practice of the end-of-pipe treatment. Reactor degree of backmixing and operating conditions are important factors that determine the performance of chemical process, including environmental impact. For the purpose of waste minimization, two modeling methods were used for simulating the performance of the acrylonitrile production reactor, based on the ammoxidation of propylene. The effect of residence time, temperature, degree of backmixing on the steady-state propylene conversion, and production of waste were determined. The tanks-in-series model and the axial dispersion model were used to account for the degree of backmixing. The two main by-products in the acrylonitrile process are acetonitrile and hydrogen cyanide, which are both highly toxic waste. Extensive reactor backmixing reduces propylene conversion, especially at high temperature and residence time. Minimum acetonitrile production is favored by low residence time, high to moderate temperature, and no backmixing. Minimum hydrogen cyanide production is favored by low residence time, low temperature, and no backmixing. At 450 °C, the percentage of increase in the selectivity of acrylonitrile, with respect to hydrogen cyanide at plug-flow reactor conditions, as compared to a continuous stirred tank reactor, is 87.1, 74.3, 50.9, 30.4, and 12.4% at a residence time of 1, 2, 4, 6, and 8 s, respectively. The reactor degree of backmixing and operating conditions are important factors that affect the environmental friendliness of the acrylonitrile production process.
Highlights
In the past, the approach to chemical reactor design had been to achieve high yield at low cost
The dimensionless concentration of ACN increased from 0.0148 at a low Pe number (CSTR conditions) to 0.0202 at a high Pe (PFR conditions), i.e, a 3.65% increase
The tanks-in-series model (TISM) is recommended to account for the effect of backmixing, because it is is recommended to account the effect of backmixing, because it is13 of 16 simple and computational advantages over for simple and has computational advantages over axial dispersion model (ADM)
Summary
The approach to chemical reactor design had been to achieve high yield at low cost. Recent trend is to include environmental factor in the design in addition to minimize the cost of the process. It was found that it is possible to decrease the production of waste in a chemical process via modifications of the design conditions. In addition to process modification (to reduce waste production), recycling is another strategy to minimize waste and increase the efficiency of the process [2]. Sepiacci et al [3] applied a waste reduction (WAR) algorithm to determine plant design’s environmental impact, in regard to chemical processing. The aim of WAR is to reduce environmental impact during the design stage of chemical processing. The process produces zeolite, ammonia, and hydrogen with no other wastes [4]
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