Effect of heat losses on monolithic reactors for VOC abatement

Abstract

In this contribution a theoretical study of the thermal effects due to heat losses on monolithic reactors for the catalytic combustion of VOC is carried out. To describe the performance of the reactor under steady-state non-isothermal non-adiabatic conditions a heterogeneous 1D Multiple Channel Model is considered, where the monolith is represented by several zones in the form of square concentric rings, each one represented by a square section single channel. Internal and external mass transfer diffusional limitations are taken into account as well as external heat transfer resistance. Heat transfer by conduction through the solid (cordierite) is considered along the cross-section coordinate of the monolith, i.e., from the central zones to peripheral zones at lower temperatures. Although an efficient thermal insulation is selected, the channels of the periphery can operate several degrees colder that the channels of the central zones, these lower temperatures are associated to non-complete VOC conversions, which are emitted into the atmosphere. Studies of parametric sensitivity with respect of inlet temperature reveals that heat losses become more relevant when the reactor is operating near the Emission Limit Value. In relation to design considerations, as the reactor scale decreases and the external area/volume ratio increases, the effect of heat losses on VOC emissions is magnified. The inlet temperature set point should be high enough to prevent high VOC emissions due to lower inlet VOC concentration in a heat loss scenario. This results in higher energetic requirements to ensure VOC complete conversion. To face the reactor performance issues related with heat losses to the environment, optimized designs were proposed aiming to increase the residence times in peripheral zones, which effectively enhance the reactor performance.