ARTICLE pubs.acs.org/IECR
Limestone Particle Attrition in High-Velocity Air Jets Gang Xiao,*,† John R. Grace,‡ and C. Jim Lim‡ †
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, China ‡ Department of Chemical & Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, Canada V6T 1Z3 ABSTRACT: Experiments were carried out with limestone particles of several narrow size intervals (125180, 250300, 355425, 500600, 600710, 710850, and 8501037 μm) for times ranging from 0.5 to 144 h in a high-velocity jet apparatus to provide a more comprehensive understanding of jet attrition. The theory of cumulative damage for fatigue is applied to explain the particle attrition mechanisms and to build an attrition model. Fines generation processes differed for limestone particles of different initial sizes, especially in the initial stage, because of the effects of rough surfaces and cumulative damage needed for attrition. In the model, the fines generation rates in the initial stage was well fitted by an exponential function with an index inversely proportional to the particle volume until stable stages were reached, whereas the rate of fines generation during the stable stage appeared to be constant for narrowly sized limestone particles.
’ INTRODUCTION High-velocity air jets are commonly used to investigate attrition of particles. Forsythe and Hertwig1 reported the attrition characteristics of fluid cracking catalyst (FCC) particles for a single orifice which provided a high-velocity air vertical jet. This apparatus was later adopted as a standard method for testing attrition (ISO 5937:1980),2,3 proposed in 1980, but withdrawn in 2002. Extending Forsythe and Hertwig’s concept, Gwyn4 proposed a three-orifice distributor apparatus with high-velocity air jets for FCC attrition testing in 1969. This was the origin of the ASTM D 5757-00 method,3,5 a widely used standard test method for characterizing attrition and abrasion of catalyst powders. This standard method delivers an air jet index (AJI), a dimensionless value numerically equal to the percent attrition loss over the first 5 h of operation, in order to give a “relative estimate” of the attrition resistance of powdered catalysts. However, the AJI provides very limited information and is rarely used for attrition modeling because the first 5 h of attrition do not represent the entire attrition process, especially for rough and fragile materials. Limestone is a widely used sorbent for SO2 capture and is receiving widespread attention for CO2 capture in fluidized beds,69 and it is also sometimes used as a catalyst, e.g., for biomass tar cracking.10 Scala et al.11 studied attrition of calcinated and sulfated limestone in a downward impact apparatus and in a fluidized bed and reported microstructural changes by in situ abrasion. In this paper, a three-orifice distributor apparatus was used to investigate the attrition of limestone of several narrow size intervals (125180, 250300, 355425, 500600, 600710, 710850, and 8501037 μm) for times from 0.5 to 144 h to provide a more comprehensive understanding of attrition.
Figure 1. Schematic of air jet apparatus.
of 0.397 ( 0.003 mm diameter, equidistant from each other, each centered 10.008 ( 0.254 mm from the axis of the column. The distributor plate is attached to the bottom of the attrition tube, a 710-mm-long stainless steel cylindrical column of 35 mm inside diameter. The settling chamber is connected to the top of the attrition tube. It consists of a lower diverging cone 230 mm high, a 300 mm long cylinder of 110 mm inside diameter, and an upper cone 100 mm high, converging to an inside diameter of 30 mm. A port is available at the top of the upper cone for adding particles.
’ EXPERIMENTAL EQUIPMENT AND METHODOLOGY The air jet attrition apparatus is shown in Figure 1. It employs a three-orifice distributor plate, an attrition tube (cylindrical column), a settling chamber of larger diameter, and a fines collector. The distributor plate contains three upward-facing holes r 2011 American Chemical Society
Received: August 5, 2011 Accepted: December 9, 2011 Revised: December 4, 2011 Published: December 09, 2011 556
dx.doi.org/10.1021/ie201698r | Ind. Eng. Chem. Res. 2012, 51, 556–560
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Details of High-Velocity Air Jet Attrition Tests Forsythe and Hertwig1 jet(s)
1 jet, diameter 0.397 mm
Gwyn4
ASTM 5757-00 method5
3 jets, diameter
this paper
3 jets, diameter 0.381 mm
3 jets, diameter 0.397 mm
stainless steel, 710 mm high,
stainless steel, 710 mm high,
0.397 mm attrition tube
glass, 1524 mm high, 25.4 mm i.d.
brass, 700 mm high, 38 mm i.d.
pressure
3.844.05 bar (wind box)
35 mm i.d.
6.90 bar (air supply)
35 mm i.d.
2.00 bar (air supply); 1.301.80 bar (back pressure)
∼2.48 bar (air supply); ∼1.38 bar (freeboard); 1.01 bar (after ceramic filter)
gas flow
air, 7.08 L/min
air, 7.08 L/min
air (relative humidity 3040%), 10.0 L/min
air, 10.0 L/min
testing time
1h
123 h
5h
0.5144 h
temperature
room
room
20 °C
room (20 ( 5 °C)
particles
FCC, 50 g,
