Separator
In recent years, the Institute for Chemicals and Fuels from Alternative Resources (ICFAR) has been developing and testing reactor technology for the conversion of biomass and heavy oil feedstocks to useful bio-oil, bio-char, syngas, and other valuable chemical products via pyrolysis. Among the various biomass conversion processes developed at ICFAR, a downer reactor was designed for the pyrolysis of biomass and heavy oil feedstocks to maximize the liquid yield. The downer configuration was selected over other reactor types for careful control of thermal cracking reactions and gas-solids contact times.
To help achieve maximum liquid yield and careful control of cracking and contact time, a novel gas-solids separator was developed and tested (Huard, 2009; Huard et al., 2010b) in a full-scale cold model downer. The gas-solids separator was designed to integrate aspects of the circulating fluidized bed (CFB) reaction column exit, of primary gas-solids separation, and of product vapor recovery using stripping gas into one effective
separation-plus-stripping device. In order to assess the characteristics and performance of the integrated gas-solids separator in a comprehensive manner, and to optimize the design of the separator, the cold model downer was modified and supplemented with new instrumentation.
A 6.9 cm diameter (D), 134 cm tall (L), transparent acrylic cold model downer apparatus used in Huard (2009) and Huard et al. (2011) was modified for the work described in this thesis. Figure 2.1 shows a process and instrumentation diagram for the cold model downer and associated equipment. Figure 2.2 illustrates to scale the geometry of the downer apparatus and gas-solids separator with some of the internals and instrumentation used for the majority of the studies in this thesis. The vertical position of the separator could be adjusted such that the cone rim was a maximum of 10.5 cm above the gas outlet pipe to a minimum of 1.8 cm below the gas outlet, as illustrated in Figure 2.3. The downer was not equipped with a recirculation loop and was therefore operated in a once- through mode. The gas and solids outlets were located 99 cm and 134 cm below the downer inlet, respectively. The gas outlet diameter was 0.95 cm. Solids exiting the downer were collected in a cylindrical tank of diameter 20 cm and height 22 cm. The main changes to the system from Huard (2009) were positive pressure air delivery from a compressor (versus vacuum pressure delivery by an axial fan blower installed in the downer exhaust line) and far greater process instrumentation and control including converging-diverging nozzle gas mass flowrate controllers, flowmeters, pressure transducers, and data acquisition.
Figure 2.2 – Illustration of (a) the cold model downer, (b) gas-solids separator, and (c) top view of sheds and tracer sparger
Figure 2.3 – Illustration of separation length: a) negative separation length with cone rim below height of gas outlet, b) zero separation length with cone rim at same height as gas
outlet, c) positive separation length with cone rim above height of gas outlet Compressed air at room temperature was supplied to the apparatus for fluidization. The mass flowrate of air was controlled using a bank of three converging-diverging nozzles of various sizes (two at 0.20 cm diameter, one at 0.31 cm diameter) upstream of the downer. The air mass flowrate could be controlled up to 10 g/s, which resulted in a maximum superficial gas velocity of 2 m/s. A 750 W Omega AHP-7561 inline electrical heater was installed just downstream of the converging-diverging nozzles to heat the incoming air stream for local solids concentration measurements (whose procedure is explained in further detail below). Silica sand (particle density = 2650 kg/m3, Sauter mean diameter = 180 μm, full particle size distribution shown in Figure 2.4) was delivered to the downer by pressurized gravity flow from a feed tank mounted directly above the downer up to a maximum flowrate of 100 g/s. This allowed operation of the cold model downer for roughly three minutes at the highest solids flowrate. The sand particle size distribution was measured using a Sympatec GmbH HELOS H2316 particle size analyzer. The gas and solids were mixed in a Y-shaped pipe fitting immediately upstream of the downer inlet. Solids escaping from the gas-solids separator were captured in a 1 μm mesh filter bag.
Figure 2.4 – Particle size distribution of sand used in the majority of experiments As shown in Figure 2.2, sheds were used to segregate the downer from the gas-solids separation zone, and were located 14 cm above the gas outlet. The sheds had a criss-cross pattern, consisting of two rows of three sheds in each row, as shown in Figure 2.2. The purpose of the sheds was to create gas jets entering the separation zone, thereby inducing strong mixing with the tracer injected immediately downstream, and to prevent gas recirculation back into the downer. In this way the sheds created an approximation to a true closed boundary condition essential to accurate RTD measurement (Levenspiel, 1999). Assuming that the gas mixing condition entering the separator was representative of most downers, an axial dispersion coefficient (Dax) of 0.2 m2/s can be assumed (Brust & Wirth, 2004). Over the range of superficial gas velocities resulting in fully turbulent gas flow in the downer (from a minimum of roughly Ug = 0.8 m/s), the dispersion number (Dax/UgLd) at the sheds had a minimum value of around 0.1, where Ld was the length of the downer. A dispersion number of 0.1 is characteristic of “intermediate” dispersion (Levenspiel, 1999), and is reasonable for gas flow. However, the assumption of Dax = 0.2 m2/s was quite conservative and so it is reasonable to assume that dispersion at the sheds was actually quite low.