(292e) Effect Of Detector Response On Dynamic Column Breakthrough Experiments
Dynamic Column Breakthrough (DCB) experiments, routinely used to complement adsorption and diffusion studies at the particle scale, constitute an important step in the development and verification of a dynamic process model. A typical experimental set-up consists of: a mass flow controller to control the feed flow rate; pressure measuring and regulating devices; a mass flow meter to monitor the exit flow; valves to appropriately direct the flow; and (preferably) an on-line detector to analyze the exit flow composition. The tubing, fittings and accessories from the exit of the mass flow controller to the inlet of the adsorber and from the adsorber exit to the detector (including the detector volume) constitute the dead volumes. Our experience with laboratory-scale breakthrough experiments suggests that the tubing, fittings and accessories primarily contribute a delay to the response of the adsorber. However, the detector, in addition to a delay from its internal volume, may also have some degree of spread depending on the operating volumetric flow rate. A common practice is to measure a blank response under the same flow, pressure and temperature conditions as the actual experiment by simply bypassing the adsorption column with a tube (or a connector) of negligible volume and this blank response is then subtracted point by point from the combined response (i.e., including the adsorption column). The underlying assumption here is that blank and column responses are linearly additive, both in terms of mean residence time and spread.
In the proposed presentation, it will be shown that the delay and spread contributed by the dead volumes of a laboratory-scale breakthrough rig may be effectively represented as a combination of a plug flow reactor followed by several CSTRs acting in series. This model for representing the dead volumes will then be used to evaluate the conditions under which the linear additivity assumption behind the aforementioned method of point by point correction is valid. It will be shown that at other conditions the mixing effect in the detector, in fact, sharpens the response received from the adsorption column and hence the point by point correction leads to over-compensation. A rigorous procedure to calculate the breakthrough response at the column outlet from the combined breakthrough and blank responses will be presented with detailed illustration.