Biofiltration of periodically fluctuating concentrations of an ␣-pinene-laden waste gas was investigated to treat both high-frequency and low-frequency fluctuations. The effects of periodic concentration fluctuations on biofilter performance were measured. Controlled variables of periodic operation included cycle period and amplitude. The cycle period ranged from 10 min to 6 days, with the inlet ␣-pinene concentration fluctuating between 0 and 100 parts per million volume. At high-frequency concentration cycling (i.e., on the order of minutes), both cyclic and constant concentration biofilters maintained similar long-term performance with an average removal efficiency of 77% at an averaged loading rate of 29 g ␣-pinene/m 3 bed/hr. A first approximation suggests kinetics that are time-independent, indicating that steady-state data can be used to predict transient behavior at this time scale. Cyclic biofilter operation with a cycle period of 24 hr (with equal on/off time) was achievable for biofilters without a significant loss in performance. At longer time scales, cyclic biofilter performance decreased at the restart of the ON cycle. The recovery time to previous levels of performance increased with increasing cycle period; the recovery time was less than 1 hr for a cycle period of 24 hr and between 6 and 8 hr for a cycle period of 6 days. INTRODUCTIONIn treatment processes such as biofiltration, the feeds to the biofilter are byproducts of other processes and, therefore, one would generally not have very good control over it as a process input. Because changes in inlet concentration of pollutants in a waste gas may affect the operation and performance of a biofilter treating such streams, it is important to consider these types of concentration fluctuations. Fluctuations in air emissions or waste gases can be the result of seasonal changes in operation, daily variations in operating conditions, hourly fluctuations, or variations every minute caused by process conditions and operation. In the forest products industry, air emissions from some pulping operations (e.g., brownstock washers) and other manufacturing processes (e.g., press vent operation in the manufacture of particleboard or other laminated wood products) are cyclic in nature and, as such, the pollutant concentrations vary with time. 1,2 This is because of the cyclic nature of the process or unit operation producing the emission. It is difficult, if not impossible, to keep constant the amounts of pollutants in the off gases, and it is therefore impractical to control the concentration of contaminants in the air emission.Fluctuating concentrations caused by process operations (e.g., a press vent or brownstock washer operation) may have a significant impact on biofiltration because the residence time, , in biofilters (i.e., on the order of seconds to several minutes) is often on the same time scale as the concentration fluctuations. In the literature, one finds a broad range of residence times, ranging from seconds 3 to several minutes. 4,5 This...
A new approach based on catalytic distillation (CD) technology was proposed to remove water from ethanol. Isobutylene was introduced to react with water in the CD column. The commercial software simulation tool Aspen Plus was used to investigate the effects of key design factors such as operating pressure and temperature, reactant ratios, reflux and distillate to feed ratios, number of separation and reaction stages, and feed and reaction zone location. It was found that the CD technology offers potential advantages of reduced energy consumption and reduced capital cost over traditional approaches for the removal of water from ethanol.
This paper describes the development and simulation of an unsteady state biofilter model used to predict dynamic behaviour of cyclically-operated biofilters and compares it with experimental results obtained from three, parallel, bench-scale biofilters treating both periodically fluctuating concentrations and constant concentrations of an α-pinene-laden gas stream. The dynamic model, using kinetic parameters estimated from the constant concentration biofilter, was able to predict the performance of cyclic biofilters operating at short cycle periods (ie, in the order of minutes and hours). Steady state kinetic data from a constant concentration biofilter can be used to predict unsteady state biofilter operation. At a 24 h cycle period, the dynamic model compared well with experimental results. For long cycle periods (ie, hours and days), removal efficiency decreased after periods of non-loading: the longer the period of non-loading, the poorer the biofilter's performance at the re-commencement of pollutant loading. At longer time scales the model did not effectively predict transient behaviour, as adsorption and changes in kinetic parameters were not accounted for. Modelling results showed that similar biofiltration performance for the cyclic and constant concentration biofiltration of α-pinene is expected for biofilters operating solely in the first order kinetics regime. Poorer performance for cyclic biofilters following Monod kinetics spanning the entire kinetics range is expected as the cycle amplitude increases. The most important parameters affecting the performance of a cyclically-operated biofilter with short cycle periods are: amplitude of cyclic fluctuations, C g,max /C g , relative value of the half-saturation constant in the Monod expression, K s , and effective diffusivity of α-pinene in the biofilm, D e .
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