Static mixers (SM) have become standard equipment in the process industries. They are widely used in applications that involve chemical reactions, heat transfer, blending of fluids, or a combination of these. Compared to mechanically agitated vessels, SMs consume less energy, require less maintenance, and can provide mixing with shorter residence time. Assessment of the performance of SM provides a means to categorize and rank the available devices and new designs, which in turn facilitates selection for specific applications. Applying the second law efficiency (SLE) principle, we derived and proposed a novel mixing parameter, the M number, which is a dimensionless ratio of mixing level to energy loss. The parameter is compared to an industry-standard method of mixing evaluation that relies on the coefficient of variation (CoV) change across the mixer. Both CoV and the M number are used to evaluate mixing performance from computational fluid dynamics (CFD) results for a static mixer for various inlet conditions. Unlike the CoV-based parameters considered, the M number offers the advantages of accounting for energy loss and the natural mixing effects of the system. In addition, an empirical relationship is obtained that relates the M number to the Reynolds number (Re). Potential applications for the M number are discussed and its limitations are noted. Work in progress includes investigation with other SM.
Static, or motionless, mixers are widely used in applications that involve chemical reactions, heat transfer, blending of fluids, or a combination of these. Within those applications, mixing can affect various parameters such as heat or mass transfer rates, process operating time, cost, safety, and product quality. Therefore, it is crucial to assess the performance of static mixers. In general, their performance is evaluated based on their ability to carry out mixing while minimizing energy loss. To accomplish this, a novel mixing parameter, the M number, is proposed and evaluated. The M number is a unitless parameter that describes the effects of the mixer using entropy change and pressure drop. The parameter is compared to another method of mixing evaluation that relies on Covariance (CoV) change across the mixer. Computational Fluid Dynamics (CFD) is executed using both methods to evaluate two static mixers and compare the results of each method. Potential applications for the M number are discussed and its limitations are noted.
Static mixers (SMs) are widely used in applications that involve chemical reactions, heat transfer, blending of fluids, or some combination of these. Accurately evaluating the performance of SMs is critical for the appropriate selection of these devices. Traditional approaches involve parameters that separately evaluate the mixing level and the energy spent in mixing (i.e., pressure drop) but do not account for both simultaneously. The M-number has been recently proposed as a means to simultaneously evaluate the quality of mixing as well as the pressure drop across the static mixer (SM). This work demonstrates an implementation of the M-number by evaluating the performance of a KOFLO static mixer (KSM) for mixing water vapor and air via Computational Fluid Dynamics (CFD), for Schmidt number of approximately unity. Pipe and mixer sizes as well as inlet profile distribution were varied. A single-element KSM was used based on the impetus of our study: Selective Catalytic Reactor (SCR) applications, which requires minimization of the pressure drop. The results are shown to be reasonably comparable with respect to previous works, containing experimental data.
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