Hydrodynamics and Mass Transfer of Modular Catalytic Structured Packing

Abstract

Process intensification aims at the replacement of large, expensive, energy intensive processes to arrive at substantially smaller, more efficient, less costly and environmental friendly processes. The integration of two or more unit operations into single devices is the way to do this, as encountered in Catalytic Distillation where heterogeneously catalysed chemical reaction is combined with thermal separation in one shell. It presents the most significant class of multifunctional reactors/separators. Therefore to fit the needs of catalytic distillation processes modular arranged packings are developed to allow flexibility with regard to catalyst load in the column. The modular catalytic structured packing, MCSP, is build-up of alternating catalyst containing pockets (reaction section), with corrugated sheets (distillation section). Successful implementation requires solutions regarding enormous uncertainties that exist with reliable process design and scale-up. For these purposes the knowledge of hydrodynamics (dynamic liquid holdup, pressure drop and capacity) and mass transfer (HETP) characteristics imposed by the internal configuration is vital and lacking in open literature. To overcome this, the geometry based Delft MCSP Model, a parallel channel model, is developed to predict the hydrodynamic and mass transfer performance of Modular Catalytic Structured Packings (MCSP). The model performance is validated with pilot plant hydraulic (air/water) and total reflux distillation experiments carried out without reaction, for the base case Katapak SP, MCSP. Two different types were used, namely the MCSP-11 and MCSP-12, where the difference is the number of corrugated sheets sandwiched between the catalyst filled pockets, respectively 1 and 2. Three parallel flow channels are identified in the latter, the catalyst filled pockets, closed channels directly next to the pockets and open crossing flow channels in the middle of the distillation section. In the MCSP-11 this reduces to two, the pockets and closed channels. Liquid Hold-upThe liquid hold-up is the basic flow parameter and the total hold-up in the MCSP packed bed is determined by two contributions, the so called internal and external hold-up. The internal operating hold-up is the amount of liquid flowing in the catalyst filled pockets and is bound between the static hold-up as lower end and the hold-up at the catalytic load point, i.e. complete saturation of the catalyst beds as upper end. The basic requirement is that the catalyst is fully employed which means to approximate plug flow through the catalyst bed and this is possible at liquid loads at or above the catalytic load point. Above the catalytic load point the excess liquid is retained in the distillation section of the packing and adds to the dynamic liquid hold-up in this section. Loading effects are incorporated and up to flooding the model agrees well with the experimental results. Pressure Drop The dense structure of the catalyst beds and pockets prevents vapour flow through this part of the MCSP. Therefore due to the low porosity these packings have a lower capacity and high pressure drops compared to conventional structured packing. As in corrugated sheet structured packing, the pressure drop in the open channels is determined by gas-gas interaction in the crossing flow channels, gas-liquid interaction at the interface along the channel and direction change related losses at the wall and at the transition between elements which are accompanied with entrance effects. In the closed channels is assumed that the flowing phases are not able to escape the channel within the packing height. In these channels gas-gas interaction is absent however due to the reaction section geometry halfway the element height (and at the top and bottom of the element) there is a flow diverging/converging zone causing variation in vapour velocity which is associated with a considerable energy losses. This friction is taken into account by a momentum based expression. In the model constant pressure over each cross section is assumed and to ensure this equality the vapour should be redistributed accordingly. Due to the geometrical differences between the flow channels encountered in parallel the effective vapour velocities are different and not known and therefore are iteratively calculated assuming a constant pressure drop and uniform liquid distribution. The pressure drop is predicted accurately well into the loading region. Mass Transfer Performance Vapour-liquid mass transfer takes place in the space filled with corrugated sheets, including the outer surface of the pockets, since here a vapour-liquid interface is available. In the Delft MCSP model the open channels behave as the flow channels in conventional structured packing. In the closed channels both liquid and vapour are forced to follow the channel to the end. Assuming a uniform liquid distribution all three walls of the triangular channel are wetted, i.e. maximum efficiency is expected, however in practice the liquid tends to flow in the form of rivulets along the lowest point of the channel. In this way a limited interface is available for contact with the ascending vapour causing a lower efficiency. In the model at the transition between packing layers it is assumed that the flowing phases are able to fully mix and this means that the concentration over the column cross section is uniform. Of each flow channel the change in concentration is calculated and mixed according to the flow contribution. Based on the average concentration the number of equilibrium stages per layer is determined and from this the HETP. Experimentally in the preloading region the MCSP generally shows a higher HETP than around loading. The difference in performance is mainly caused by maldistribution of liquid, namely bypassing of the liquid inside the pockets by the vapour and reduced lateral spreading due to these pockets which is especially seen in the MCSP-11. The MCSPs exhibit their lowest attainable HETP around hydraulic loading of the packed bed where the model predictions agree with the experimental results.The Delft MCSP model takes all macro-geometry related effects properly into account without any adjustable parameters. It proved to be able to account correctly for geometrical variations and is well capable to predict the performance of MCSPs. With the model tailor made MCSP can be arranged to fit the needs (catalyst load/separation requirements) of a particulate catalytic distillation process and implementation of the model within a rigorous modelling environment should overcome some of the uncertainties associated with process design and scale-up.