DEVELOPMENT OF A NOVEL SPOUTED BED APPARATUS

The main item of the investigated spouted bed apparatus is the gas distributor which consists of two adjust- able chopped cylinders (Mörl et al。 (2001), Piskova (2002))。 By rotating the cylinders, the free cross section area of the gas inlet can be varied (Fig。 1)。 Thus, the opening ratio of the gas distributor and consequently

the gas inlet velocity can be regulated directly during the operation。 This constitutes an advantage when the gas distributor gets

clogged with bed ma- terial。 By varying the gas inlet area, clog- ging and dead zones within the apparatus are eliminated without interrupting the proc- ess。 The fluidization gas is sucked into the apparatus horizontally through the two slits that extend along the whole apparatus depth (see Fig。 1) by a suck- ing ventilator。 After passing the slits, the gas flow is perted upwards mutually  by a center profile。  On top  of  the  centre pro-

file, the flows from both   sides   unite and

Fig。 1: Schematic of the spouted bed apparatus with slit-shaped adjustable gas inlet。

form a jet which passes the apparatus vertically from the bottom to the top。 Due to this region with high gas velocities, process gas is sucked into the jet flow from the upwards extending process chamber (solid arrows in Fig。 1)。 Thus, a special and clearly defined flow structure is formed in the apparatus, which is character- ized by an unequal velocity distribution over the cross section area。 In the central region above the centre profile, one or more nozzles can be integrated for supplementary coating or granulation (layering) of parti- cles。 These nozzles usually spray liquid into the bed in the upward direction (“bottomspray”)。 Particles lo- cated in the process chamber are entrained upwards by the core jet (dashed arrows in Fig。 1)。 In the upper process chamber the entrained particles are ejected to the sides and move back towards the gas entry zone。 Due to the slope of the inner profile, the particles are transported to the lower area of the gas jet, where they are entrained upwards。 Thus, a circulating particle motion is obtained, which is quite uniform and rather well defined。 Based on this flow structure, different regions within the apparatus cross section area can be distinguished。 The region with high gas velocities above the middle profile is referred to as the “jet zone”。 A flow structure similar to those experienced in pneumatic conveying can be encountered in that region。 The two zones adjacent to the jet zone are called the “back-flow zones”。 By the patented adjustable gas flow regulation (Mörl et al。 (2001)), the throughput of process gas as well as the direction of the entering gas can be influenced selectively。 Hence, flow conditions in the bottom region of the “jet zone” can be  regulated with respect to the gas velocity and the width of the jet as well as the force exposure on the particles。 Par- ticularly, during the development time of applications, this adjustability is an advantage。 Compared to con- ventional conical or conical-cylindrical spouted beds, the scale-up of the prismatic apparatus design can oc- cur just by increasing the apparatus depth or length, because the fluid dynamics of a pseudo-2D spouted bed behaves similar to three the dimensional case。

DETERMINING THE STABLE SPOUTING DOMAIN

Whether the pneumatic operation regime is stable or not depends on the distribution of both phases in the gas-solid spouted bed。 Visual observations and FFT analysis on bed pressure drop signals showed that the characteristics of instable operation are channel formation, bubbles and dead zones, whereas the stable op- eration regime exhibits a good mixing of both phases in the apparatus。 The ranges of stable and instable op- eration can be depicted as a geometric area of operating points which are characterized by different inlet gas velocities, Archimedes-numbers, bed masses and cross section areas of the gas inlet (Kojouharov (2004))。 To obtain the boundaries of the stable spouting range, the inlet gas velocity, the bed mass, the free gas entry area and the particle system were varied。 As stability criteria for characterization of the pneumatic operation ranges of fluidized beds and spouted beds, Mitev (1979) and Piskova (2002) proposed a dimensionless Re- G-Ar-diagram。 At a given gas inlet velocity and particle Archimedes number, a geometric parameter G is de- fined which describes the ratio between the cross section of the free gas inlet area Agas。inlet and the apparatus cross section area at the bed surface under fixed bed conditions: