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2

Pressure and

Temperature Effects in

Fluid-Particle Systems

Ted M. Knowlton

1.0INTRODUCTION

Most investigations of fluidization parameters take place at ambient temperature and pressure. Yet, nearly all processes operate at elevated temperature, and many at elevated pressure. Therefore, it is necessary to know how increasing temperature and pressure affect the operation of fluidized systems. However, the operation of fluidized test facilities at temperature and pressure is much more difficult and costly than operating them at ambient conditions. It is not surprising then that information on how temperature and pressure affect the operation of fluidized beds is not as prevalent as would be desired. However, many researchers have undertaken the difficult task of building and operating units to obtain these badly needed data. The purpose of this chapter is to present what is known about operating fluidized beds at elevated temperature and pressure.

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112 Fluidization, Solids Handling, and Processing

From the 1940’s (when the significant research on fluidized beds began) until the early 1970’s, there was very little information in the literature dealing with the effects of temperature and pressure on the operation of fluidized beds. This was primarily because (i) high-pressure and/or high-temperature research rigs were (and still are) costly to build and to operate, and (ii) during this period, fluidization research was primarily directed toward improving fluidized-bed catalytic cracking, which was conducted at low pressures.

The early 1970’s saw the development of many new coal-based, synthetic-fuel, fluidized-bed processes which operated at high pressures. The scientists and engineers charged with designing these processes realized that there was a severe lack of information on how pressure (and also temperature) affected the operation of fluidized beds. Therefore, several studies to determine the effect of pressure on the operation of fluidized beds were commissioned. During the same period, other researchers in Japan, Europe, and the U.S. were also starting to conduct research to determine the effects that temperature and pressure have on fluidized systems.

As a result, most of the information on how temperature and pressure affect fluidized beds has been obtained during the last twenty-five years. Much of this research has been conducted in a piecemeal fashion by several researchers. However, enough information is available to allow the construction of an overall picture of how these two parameters affect fluidized beds, and fluid-particle systems in general.

Temperature and pressure affect the operation of fluid-particle systems because they affect gas density and gas viscosity. It is the variation in these two parameters that determine the effects of temperature and pressure on fluid-particle systems. Increasing system temperature causes gas density to decrease and gas viscosity to increase. Therefore, it is not possible to determine only the effect of gas viscosity on a system by changing system temperature because gas density is also changed and the resulting information is confused. Very few research facilities have the capability to change system pressure to maintain gas density constant while the temperature is being changed to vary gas viscosity.

Changing system pressure essentially only changes gas density in a system because gas viscosity is an extremely weak function of system

Pressure and Temperature Effects 113

pressure. For example, increasing the pressure of nitrogen gas from 1 to 70 bar increases its viscosity by less than 10%.

Most fluidized-bed processes operate within the temperature and pressure ranges of ambient to 1100°C and ambient to 70 bar, respectively. Over this temperature range, gas viscosity increases by a factor of about 3 to 4, depending upon the type of gas. If the pressure of the system remains constant while temperature is changed, the gas density decreases over this temperature range by a factor of 1373/293 = 4.7. If system pressure is increased without changing temperature, the gas density is increased by the same factor as the pressure ratio—which would be approximately 70:1 for a change in pressure from ambient to 70 bar.

The effects of temperature and pressure on fluidized-bed systems cannot be considered independently of particle size. Whether temperature and pressure have an effect (and indeed, even the direction of that effect) on a system, depends strongly on particle size. In addition, the type of interaction between gas and solids, i.e., whether the interaction is due to momentum or drag, determines if gas viscosity has an effect upon the system. As will be shown, gas viscosity is not important in systems in which momentum is important, but is important in systems dominated by drag.

This chapter will look at the effects of pressure and temperature on various types of fluidized beds: bubbling, turbulent, and circulating. More studies at elevated temperature and pressure have been conducted in bubbling fluidized beds than higher-velocity beds. Therefore, how temperature and pressure affect bubbling fluidized beds dominate this chapter. However, recent work has been conducted on how temperature and pressure affect the transitions between the various fluidization regimes, as well as limited work in the circulating fluidized bed regime.

The chapter begins by describing how temperature and pressure affect parameters important for bubbling fluidized beds, and then discusses their effect on regime transitions, circulating fluidized beds, and cyclones.

1.1Minimum Fluidization Velocity

One of the basic parameters to be determined when designing bubbling fluidized-bed systems is the minimum fluidization velocity, Umf. The effect of temperature and pressure on Umf has been investigated by many researchers (Botterill and Desai, 1972; Botterill and Teoman, 1980;

114 Fluidization, Solids Handling, and Processing

Botterill et al., 1982; Geldart and Abrahamsen, 1981; Geldart and Kapoor, 1976; Knowlton, 1977; Mii et al., 1973; Richardson, 1971; Rowe, 1984; Sakuraya et al., 1976; Sobreiro and Monteiro, 1982; Varadi and Grace, 1978; Yoshida et al., 1976). They have found that the effects of temperature and pressure on Umf are strongly dependent upon particle size. As shown in Fig. 1 from Rowe et al. (1984), increasing system pressure causes Umf to decrease for particle sizes greater than about 100 microns. Materials of this size are essentially Geldart Group B and D powders (Geldart, 1973). For particles smaller than 100 microns (Geldart Group A powders), pressure does not affect Umf .

	1.2
			Particle Density: 1250 kg/m			3
			Gas: Nitrogen
	1		Ambient Temperature
m/s
Velocity,	0.8
Fluidization	0.6
Minimum	0.4
				5000 microns
	0.2	500 microns
					1000 microns
	100 microns
	0
	0	20	40	60	80	100

Pressure, bar

Figure 1. The effect of pressure on minimum fluidization velocity. (Rowe.)