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Ind. Eng. Chem. Res. 2009, 48, 134–141
Prediction of Solids Circulation Rate of Cork Particles in an Ambient-Pressure Pilot-Scale Circulating Fluidized Bed Yue Huang,† Richard Turton,*,† Parviz Famouri,‡ and Edward J. Boyle§ Chemical Engineering Department, Lane Department of Computer Science and Electrical Engineering, West Virginia UniVersity, Morgantown, West Virginia 26506, and National Energy Technology Laboratory, P.O. Box 880, Morgantown, West Virginia 26507
Circulating fluidized beds (CFB) are currently used in many industrial processes for noncatalytic and catalytic gas-solid reactions. The prediction of solids flow rate is important because its effective control is the key to smooth operation of a CFB system. This paper presents a method for solids flow metering from pressure drop measurements in the standpipe dense phase. A model based on the Ergun equation is developed to predict the solids flow rate and voidage in the dense phase of the standpipe. The profile of the solids flow rate under unsteady state is also presented. With the use of this method, the dynamic response time at different locations along the standpipe of a pilot-scale fluidized bed operating at ambient conditions with 812 µm cork particles is estimated successfully. Through the use of a pressure balance analysis, solids flow models for the standpipe, riser, and other sections of the flow loop are combined to give an integrated CFB model. 1. Introduction Circulating fluidized beds (CFB) are currently used in many industrial processes for noncatalytic and catalytic gas-solid reactions. The key to smooth operation of a CFB system is the effective control of the solids circulation rate to the riser. Solids circulation rate is one of the most important parameters in the operation of CFBs, since it affects mass and heat transfer characteristics, which in turn impact the efficiency of the processes. For example, the rate at which a catalyst can be circulated has significant effects on the operability of catalytic cracking units. A system that circulates badly is difficult to operate and may be run at lower catalyst-to-oil ratios than desired, having an adverse effect on yields and product selectivities.1 The primary operational parameters of the system are the pressure drop and the gas flow rate, whereas the solids flow rate is normally unknown and must be estimated. Some techniques have been used to determine this variable;2–4 however, the measurement of this parameter in industrial-scale CFB units operating at extreme process conditions is very difficult. A method based on the pressure measurement around the loop of a CFB was developed.5,6 In comparison with direct measurement techniques, this is an indirect method, but it is still a useful and practical approach for this application because it is online and nonintrusive. The objective of this study is to present a method for solids flow metering from pressure drop measurement in the dense phase of the standpipe and to combine this model with other models to give a comprehensive flow description of the circulating solids flow in the CFB. Crucial parameters in the standpipe are the pressure gradients under different solids flow rates. In this work, these were obtained first and a model was subsequently developed based on the Ergun equation. Following this, the profile of solids flow rate in the dense phase under unsteady-state conditions is developed. * To whom correspondence should be addressed. E-mail: Richard
[email protected]. Tel.: +1 (304) 293 2111, ext 2415. Fax: +1 (304) 293 4139. † Chemical Engineering Department, West Virginia University. ‡ Lane Department of Computer Science and Electrical Engineering, West Virginia University. § National Energy Technology Laboratory.
On the basis of these results and a model for the riser,7 an integrated model is developed to simulate the entire loop of a pilot-scale CFB under transient conditions for the transport of cork solids using ambient air. 2. Experimental Methods A cold flow circulating fluidized bed (CFCFB) has been built at the National Energy Technology Laboratory (NETL), in Morgantown, West Virginia. The experimental setup is shown in Figure 1. The riser has an inside diameter of 0.305 m and a height of 15.45 m, and the standpipe has an inside diameter of
Figure 1. Illustration of the cold flow circulating fluidized bed (CFCFB) facility at NETL.
10.1021/ie8001843 CCC: $40.75 2009 American Chemical Society Published on Web 07/29/2008
Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 135 Table 1. Bed Material (Cork) Properties cork characteristics Fs dp Umf εmf εp Ψ
kg/m3 µm m/s
189 812 0.07 0.49 0.45 0.84
0.254 m. The standpipe and riser are equipped with pressure transducers along their length. Solids are transported from the standpipe to the riser through a loop-seal that acts as a nonmechanical valve. The standpipe and loop-seal are equipped with pressure transducers and aeration ports. Solids leaving the top of the riser are collected and returned to the standpipe through a primary cyclone. An aeration port is located near the base of the standpipe at approximately 0.4 m above the loopseal. This aeration controls the circulation rate of solids and is referred to as the move-air. For the experiments performed in this work, the move-air is varied, which causes the circulating flow of solids to change. The bed material used in the current studies is cork, whose properties are listed in Table 1, and the gas phase is air at approximately ambient conditions. According to the particle size and density, this powder belongs to group A of the Geldart classification of powders. The standpipe was instrumented with nine pressure transducers, connected in series, to measure the incremental pressure drops along the standpipe. A helical-shaped spiral vane was installed 4.0 m above the inlet to the nonmechanical solids valve at the bottom of the standpipe, and the frequency of its revolution was recorded and used to estimate the solids flow rate. The frequency of the rotation of the spiral was calibrated for the bed material by draining solids from the bottom of the standpipe and weighing the solids over a period of time.8
Figure 2. Inconsistency between data for move-air and solids flows.
Figure 3. Linear relationship between pressure gradient and superficial velocity of solids for the current system.
3. Results and Discussion 3.1. Flow Pattern in the Standpipe. Standpipe flow refers to the downward flow of solids with the aid of gravitational force against a gas pressure gradient. The gas flow with respect to the downward-flowing solids is in the upward direction. Although, in standpipe flow, the actual direction of flow of gas relative to the wall can be either up or down, generally both gas and solid flow cocurrently in the downward direction. Two types of flow patterns are possible:9 fluidized bed flow (in which particles are in suspension) and moving bed flow (in which particles move en bloc at the voidage of a packed bed with little relative motion between particles). The flow type can be determined by the slip velocity as follows:10 Usl
