.. . . . . , ,~ :.. ~. ELSEVIER Powder Technoh,gy 97 119t)81 tiff I 17 Flow instability in non-fluidized standpipe flow J.-Y. Zhang ~'*, V. Rudolph b "'In.~titute Of Coal Chemi.~lry, Chinc.~' ,.It a~h'm.~ ot S~'ivnt'e~, l'c,iym~n 03~;001. ('ho~a ~" Ih'l~,rtment tqChemical i'.',gmeerin.e, l.;l~iver.sirl qq Qtwen.~hzmL Bri.~b, me. (2h1. 4072. :tuxtralia R¢ceixed 12 April I t)t~7; r¢ceixed m re,, e, cd Iorm 22 OcloK, r 1~4t~7:accepted 25 No~,cmbcr 1997 Abstract Solids flow through an t)rilice at the btmom of a standpipe, again~,t a counterflo~,, of gas. becomes unstable ax the gas flow is increamd. In the unstable condition, the solids flow in the standpipe is jerky, reflecting the oxcillationx bctv, een fltm and no Iio~, at the orifice, and remmble.~ .~lip-stick flow. PracticM operation of these systcm~,requires a straightforward quantitati~ e demarcation for distinguishing when flow belmviour changes from stable to unstable. This paper presents an analysis of flow stability m standpipes with gas countertlow, support~Mby small-scale experiments. For smaller orifices, solids Ilow instability occurs when a hemispherical bubble cap just .,,pans the orifice, corresponding to a certain critical negative pressure gradient. For large orilice.~, stable .,,olid.,,flow is abruptly cut off when the ctmnterflo~,ing ga.', rate is high enough, anti tit) ul).~table .~olid.,, th)~v region t~.-curs. ,~'~ 1998 Elxe~'ier Science S.A. All rights re.,,erved. Kt'wtord~." Slre~,.,, free ~,urlace,¢ Orilice.,,; Standpipe.,,: t:1o~, in,,labilit~ I. Introduction Standpipes are a necessary part of all circulating lluidized bed processes, including lluid catalytic crackers, circulating Iluidized bed combustors and caiciners (see e.g. Ref. I I I). Processes incorporating standpi.~s are often disrupted by sudden unexpected instabilities in their operation. Various explanations of instability, surveyed by Jones and Leung 121, demonstrate that analysis is complicated by the fact that a system may be judged stable from one analytical viewpoint, but unstable from another, so that no general rule is available to predict the flow character. Three flow regimes are normally distinguished in standpipe flow, namely, non-fluidized fl,.,w, Iluidized flow and streaming flow. l.ogicaily, problems of flow can be characterized by aspects of ( 1 ) quality -- what type of flow pattern occurs in the standpipe? -- and (2) quantity -- what solids flow rate is discharged from the standpipe'? In response to both of the above qucslions, at 2east two common types of standpipe flow instabil;t:t might be distinguished, namely, flow pattern insta- bility and flow t'ate instability 13 ]. Flow pattern instability, which arises from the co-existence in the standpipe of different regimes, or from difli~rent voi- dage conditions within the same flow regime, or the formation * Corresponding author. Tel.: + 61 7 3365 3708: Fax: ~-(,I 7 3365 4lt~ 0032-5910/98/$19.00 ~;~ 1998 Elsevier Science S.A All rights remP.'ed. PilS0032-5910{ 97 )03395-(I and travel of bubbles, is discussed from a variety of view- points in RelX. 12.4-1 I I. Flow rate instability occurs when the standpipe discharge starts to exhibit an unstable, periodic pulsing behaviour reflecting the oscillations between flow and no flow. It exists often in countercurrent non-fluidized flow of gas and solids, and arises from intermittent bubble fomlation at constrictions within the standpipe. This kind of instabili,y has been inves- tigated in Ret:s. 13,12-17 I- So far. understandin°, of flow in the unstable situation is largely descriptive and a general quantitative model tbr inves- tigative analysis or unit design is not available. This paper explores the reason why flow rate instabihty occurs and pro- vides the critical design features fc,r stable countercurrent non-fluidized standpipe flow. Normally, the valve at the base of the standpipe is used to regulate the solids flow rate and serves as a restriction to allow tbrmation of a dense bed of solids alx~ve this valve. Since solids flow behaviour in the standpipe is controlled by the barn valve, the following theoretical analysis and exper- intental demonstration will focus on this region when solids flow changes from stable to unstable flo~,. The critical stand- pipe pressure gradient, which is important for practical pur- poses, is then derived. Smooth operation occurs if this pressure gradient is not exceeded: higher pressure gradients result in unstable solids flow or even complete stoppage.