Adsorption Dynamics Of Structured Fixed Bed And Its Applications

Separation and purification technologies play a significant role in areas of chemicalengineering, pharmaceuticals, food, environment and buildings. As the development of scienceand technologies, the associated environmental and health problems are worsening. Volatileorganic compounds (VOCs), which are sorts of the most typical industrial pollutants, presentsignificant hazards to both human health and ecological environment, which arouses publicconcerns. The related purification technologies have become the focus of scientific researchersaround the world. At the meantime, the separation technologies for hydrocarbon separation ofLPG (a very important raw material in chemical industries) and desulfurization have alsoattracted increasing attention. Specifically, the method for refining green refrigerant R290(propane of high purity) from LPG by purification is a front subject with both theoreticalsignificance and engineering value. Traditional fixed bed adsorbers have some drawbacks, forexample, low mass-transfer rate, high mass-transfer resistance and low bed utilization.Structured fixed bed adsorbers, which gain a massive variety of researches in recent years, areable to solve the above problems. Although a large amount of articles have been studied on theapplications of structured fixed beds, there are scarce literature about the adsorption dynamicsbased on structured fixed beds using non-empirical mathematical models. In this paper, theapplications of fixed bed adsorption for VOCs purification and hydrocarbon separation werereviewed. The adsorption dynamics of sulfur contents and hydrocarbons in fixed bed werestudied. The adsorption dynamics of toluene in structured fixed bed was studied and amathematical model was built for predicting the adsorption dynamics of structured fixed bed.The adsorption dynamics and mass-transfer mechanisms in the structured fixed bed wereanalyzed and discussed.Firstly, adsorption dynamics of sulfur in LPG in the fixed bed was investigated. Under thesame experimental condition, by comparing the breakthrough curves of sulfur contents in fixedbeds loading with different adsorbents, the performance of desulfurizers were evaluated. Yoon-Nelson, Bed Depth Service Time (BDST) and Adam’s-Bohart models were used to fit thebreakthrough curve by linear regression analysis. The results show that when setting the corresponding time to outlet concentration2ppm as the breakthrough time, the breakthroughtime of13X zeolite reaches32min, which indicates that13X zeolite has the desirableperformance for LPG desulfurization. The results of linear regression of Yoon-Nelson andBDST model agree well with the experimental data.Secondly, multi-components adsorption dynamics of propane, n-butane and i-butane inactivated carbon fixed bed were investigated. The effects of gas flow rate, bed length andadsorption temperature on the breakthrough curves of multi-components were studied. Inaddition, the multi-components competitive adsorption mechanisms were discussed. The resultsshow that adsorption waves of three components advanced and early breakthroughs of threecomponents happen as the increase of flow rate. Also, the breakthrough curves become steeper.Both the breakthrough time and the total adsorption time are influenced by the bed length. Byincreasing the bed length, the breakthrough and saturation time increase, so as the time foradsorption process. As the increase of adsorption temperature, the breakthrough time shortens,which means that higher adsorption temperature is unfavorable for the adsorption of n-butaneand i-butane on activated carbon. The mechanisms of multi-components competitive adsorptionwere discussed.Finally, based on mass conversion and LDF equations, a structured fixed bed adsorptionmodel considers both pore and surface diffusion was built. The LUB theory was used tocalculate and analyze the LUB values of different bed structures. Adam’s-Bohart model wasapplied to the linear regression analysis. The results show that the calculated breakthroughcurves for structured fixed bed agree well with the experimental data. The mathematical modelcan be used to predict the adsorption dynamics of VOCs in structured fixed bed. Thebreakthrough curves for structured fixed beds are steeper. In addition, the slope of thebreakthrough curve increases as the increase of the ACF bed length ratio, indicating that themass transfer rate has been enhanced and mass transfer resistance has been reduced. ACF offerslower toluene adsorption capacity. The LUB values decrease as the increase of ACF loadingratio. The linearity for ln C/C0to t agrees well with the experiment data. The adsorptioncapacityN0decreases when adding more ACF. On the other hand, the mass transfer rate of adsorbentk ABis enhanced and the pressure drop is decreased.