ABSTRACT
The controllability study is an integral part of chemical process design. In this work, the controllability of two special distillation techniques, extractive distillation and pressure swing distillation, designed for the separation of azeotropic mixtures is investigated with dynamic tools. The control design interface of Aspen Plus and Matlab are applied for the modeling and evaluation of the two systems. Dynamic controllability indices are determined and aggregated in a desirability function. The results are compared to obtain efficient help for process design activity. The pressure swing distillation shows significantly better controllability features than the extractive distillation. The reason can be the fact that in the case of the extractive distillation, a third compound, the extractive agent, is added to the system to carry out the separation, therefore making the system more complex. As far as the selection of manipulated variables is concerned, in the case of the extractive distillation, the reflux flows should be preferred to the reflux ratios but in the case of the pressure swing distillation, the reboiler heat loads are preferred to the reflux ratios since those are closer to the controlled compositions. Both separation systems show worse controllability features if the product purity requirement is approaching to the pure products, that is, close to 100%. Although the energy consumption of the pressure swing distillation is higher than that of the extractive distillation, it has the inherent feature that it can be automatically heat integrated due to a column operated at high pressure and, as a consequence, higher temperatures.
ABSTRACT
BACKGROUND: According to European trends, more children eat at school canteens than ever before, therefore food safety and quality have become increasingly significant in recent years. Nevertheless, there are large differences in food safety levels in different school canteens. We hypothesize that the microbial status of the served meal represents on the general hygiene of the kitchen. Our research examines whether mesophilic aerobic bacteria measured in served food are connected with the level of hygiene in the catering unit, and whether this indicator can be used as a criterion for assessing school kitchens. METHODS: Meal samples were collected from six school kitchens, and mesophilic aerobic bacterial count was measured. Samples were collected on five different days, so each kitchen was monitored five times. Two meals per visit were collected: a soup and a main course. RESULTS: Out of the 60 samples, 26 were good (CFU/g < 103), 24 were acceptable (CFU/g: 103–105), and in 10 samples, the microbial count was found to be above the limit (CFU/g > 105). Statistical calculations revealed that microbial contamination of served meals was influenced neither by the supplier nor by the type of meal (soup or main course). However, the level of hygiene in the serving kitchen significantly affects the microbial status of meals. CONCLUSIONS: Based on the results, a qualification system can be developed using the mesophilic aerobic bacterial count measurable in the served meal to assess hygiene. By regular determination of mesophilic aerobic bacterial count and the presence of Enterobacteriaceae, the food safety of a catering unit can be quantitatively evaluated.
