Abstract:The entropy production rates as obtained from the exergy analysis, entropy balance and the nonequilibrium thermodynamics approach are compared for two distillation columns. The first case is a depropanizer column involving a mixture of ethane, propane, n-butane and n-pentane. The other is a weighed sample of Mexican crude oil distilled with a pilot scale fractionating column. The composition, temperature and flow profiles, for a given duty and operating conditions in each column, are obtained with the Aspen Plus V8.4 software by using the RateFrac model with a rate-based nonequilibrium column. For the depropanizer column the highest entropy production rate is found in the central trays where most of the mass transfer occurs, while in the second column the highest values correspond to the first three stages (where the vapor mixture is in contact with the cold liquid reflux), and to the last three stages (where the highest temperatures take place). The importance of the explicit inclusion of thermal diffusion in these processes is evaluated. In the depropanizer column, the effect of the coupling between heat and mass transfer is found to be negligible, while for the fractionating column it becomes appreciable.

An exergy analysis to a Mexican flavor industry, which uses liquefied petroleum gas as a primary energy fuel in their process equipment was carried out.. The analysis used a proposed method that quantifies efficiency by means of exergetic indicators. To apply it to this case study equipment, the system or process was assumed to be a block that interacts with the surroundings in three ways: heat, work and mass transfer. The analyzed blocks were boilers, a thermal oxidizer, dryers, a distillation tower and extractors. Work and heat needs were covered by liquefied petroleum gas. The exergy indicators quantify the degradation of energy by determinining the difference between the actual operation efficiency of the block and the maximum operation, both of them obtained from second law point of view. These indicators were exergy loss, efficiency, effectiveness, performance and potential of improvement. Following the exergetic method application, it was found that the indicators of the effectiveness and performance in all blocks analyzed are near zero. This means that the process equipments are using a high exergy source to perform their function and also in large quantity. The results show that the oxidizer presented the major irreversibilities, and it is the equipment with the greatest potential for improvement and the key to reducing fuel consumption.