Hw Hub Piepers

The effect of interparticle forces on the stability of gas-fluidized beds - I.Experimental evidence

The stability theory for homogeneous fluidized beds presented earlier is reviewed. This theory is based on the concept of the elasticity of the bed structure as a consequence of interparticle forces. It is shown that this theory explains the effect of gas viscosity and gravity. It is further shown that the elasticity modulus is increased by gas adsorption to the solid surface at elevated pressure and, thus, explains the effect of gas pressure on bed expansion. The theory is compared with experimental results obtained with fluidization of fresh cracking catalyst and polypropylene by different gases and at gas pressures up to 15 bar. It is further shown that the elasticity modulus can be used to correlate bed expansion and bubble size during heterogeneous fluidization. The stability theory of Foscolo and Gibilaro is criticized and rejected on the basis of serious mistakes made in their theoretical derivation.

Effects of pressure and type of gas on particle-particle interaction and the consequences for gas—solid fluidization behaviour

Abstract The fluidization behaviour of cracking catalyst has been studied up to pressures of 15 bar with different fluidization gases (Ar, N 2 , H 2 ). A number of parameters of both the homogeneous and heterogeneous fluidized bed has been examined experimentally. The experimental results reveal that the minimum fluidization velocity (U mf ) is independent of the pressure. The bubble point velocity (U bp ) and the maximum bed expansion (H bp ) at this velocity increase with increasing pressure. This also holds for the dense phase voidage (e d ) and the dense phase gas velocity (U d ) in the bubbling bed. The bubble size decreases drastically with increasing pressure. However, the above-mentioned parameters are also strongly dependent on the type of fluidization gas used. The cohesion constant of the powder was measured, using a tilting bed technique. The results reveal that the cohesion constant increases with increasing pressure. Analysis of the results of adsorption measurements of the different gases to the solid reveals for the adsorption as well as for the cohesion and for the beu expansion the same pressure dependence. It is believed that the gas adsorption influences the cohesion between the particles and hence the elasticity modulus introduced by Rietema and Mutsers [1,2]. The increasing elasticity modulus with increasing pressure also explains the increasing bed expansion with pressure.

Liquid-induced pulsing flow in trickle-bed reactors

This contribution describes the experiments on pulse induction by cycling the liquid feed in a column of 3.2 m height. Based on a square-wave cycled liquid feed, two feed strategies are developed that involve the artificial induction of natural pulses and a separation of the wetting efficiency in time. The feed strategies aim at increasing the mass transfer rate of the limiting reactant and to prevent flow maldistribution and hot spot formation. The feed strategies are categorized upon a relatively fast and slow cycling of the liquid feed. The potential consequences of the developed feed strategies on reactor performance are evaluated. Cycling the liquid feed results in the formation of continuity shock waves in the column. The shock waves decay by leaving liquid behind their tail. This decaying process limits the frequency of the cycled liquid feed to rather low values since at relatively high frequencies, total collapse of the shock waves occurs. By the induction of natural pulses inside the shock waves, the integral mass and heat transfer rates during the liquid flush will be improved. Shorter flushes can therefore be applied and the usual encountered periodic operation is optimized. This feed strategy is termed the slow mode of liquid-induced pulsing flow. The second feed strategy termed the fast mode of liquid-induced pulsing flow may be viewed as an extension of natural pulsing flow. Individual natural pulses are induced at an externally set pulse frequency less than 1 Hz. This feed strategy is the only fast mode of periodic operation possible since pulses are stable while shock waves decay. The characteristics of the induced pulses equal the pulse characteristics of natural pulsing flow at equivalent gas flow rates. A critical liquid holdup in between pulses is necessary for the induced pulses to remain stable.

The effects of interparticle forces on the stability of gas-fluidized beds—II. Theoretical derivation of bed elasticity on the basis of van der Waals forces between powder particles

In Part I of this series it was shown, on the basis of elasticity of the bed structure, that a fluidized powder bed can be stable. It was also suggested that the origin of the elasticity is to be found in the existence of interparticle forces. These interparticle forces are the subject of the present paper. The van der Waals forces between the two neighbouring particles are discussed, while the effect of particle deformation is calculated. Starting from the interparticle forces at the asperities, a model is derived that describes the demndence of the elasticitv modulus on the characteristics of the particle bed as, e.g., the bed porosity, the pa-kicle diameter and the coordination number.

Nature and characteristics of pulsing flow in trickle-bed reactors

Abstract Pulsing flow is well known for its advantages in terms of an increase in mass and heat transfer rates, complete catalyst wetting and a decrease in axial dispersion compared to trickle flow. The operation of a trickle-bed reactor in the pulsing flow regime is favorable in terms of a capacity increase and the elimination of hot spots. Extending the knowledge on the hydrodynamic nature and characteristics of pulsing flow stands at the basis of further exploitation of the effects of this flow regime on reactor performance. An analysis of the hydrodynamics of pulsing flow reveals that pulse properties as liquid holdup, velocity and duration, are invariant to the superficial liquid velocity at a constant gas flow rate. The pulse frequency, however, increases with increasing superficial liquid velocity. The relative contribution of the pulses and the parts of the bed in between pulses to an average measured property can thus be obtained. By applying this concept it is shown that the linear liquid velocity inside the pulses varies between 0.1 and 0.2 m s −1 . The linear liquid velocity in between pulses, however, is invariant to gas and liquid flow rates and packing properties and equal to about 0.05 m s −1 . This suggests that a linear liquid velocity of about 0.05 m s −1 is the maximum velocity possible in the bed to maintain the trickle flow regime. All liquid in excess is transported as pulses. The liquid holdup in the parts of the bed in between pulses equals the liquid holdup at the transition to pulsing flow at all gas flow rates. The same trend holds for the linear liquid velocity in between pulses. Pulsing flow then is a hybrid of two transition states. The pulses reside at the transition to bubble flow, while the parts of the bed in between pulses reside at the transition to trickle flow. The enhanced particle-liquid heat transfer coefficient inside the pulses is mainly the result of the high linear liquid velocity inside the pulses. Particle-liquid heat transfer rates in between pulses are constant due to the constant linear liquid velocity.

Particle-liquid heat transfer in trickle-bed reactors

Abstract One of the major disadvantages of trickle-bed reactors is their poor capability to eliminate the heat involved in reaction. Knowledge of particle–fluid heat transfer rates in trickle-bed reactors assesses the possibility to prevent the formation of hot spots. Safety problems, deactivation of the catalyst and less than optimal selectivities in catalytic reactions can be avoided. In this contribution, experimental results on particle–liquid heat transfer rates in trickle-bed reactors are presented. Local time-averaged particle–liquid heat transfer rates are determined with custom-made probes in the trickle and pulsing flow regime. In the trickle flow regime, the local heat transfer coefficient increases both with increasing liquid flow rate and, to a lesser extent, with increasing gas flow rate. The transition to pulsing flow results in a substantial increase in heat transfer rates. In both flow regimes, heat transfer rates are governed by the linear liquid velocity. No principal difference exists between the trickle and pulsing flow regime, although the hydrodynamic behavior is very different. Local heat transfer rates are strongly dependent on the local structure of the packed bed. Virtually instantaneous measurements of particle–liquid heat transfer rates are conducted using constant temperature anemometry. During pulsing flow, heat transfer rates inside the pulses are roughly 3 to 4 times higher with respect to heat transfer rates in between the pulses. Constant temperature anemometry is an accurate experimental method to determine instantaneous particle–liquid heat transfer rates in trickle-bed reactors.

Enlargement of the pulsing flow regime by periodic operation of a trickle-bed reactor.

Potential advantages of pulsing flow in trickle-bed reactors include capacity increase and elimination of hot spots through the enhanced mass and heat transfer rates. A disadvantage of naturally occurring pulsing flow is the necessity of relatively high gas and liquid flow rates, especially at elevated pressures, resulting in rather short contact times between the phases. To maintain the advantages but to avoid the drawbacks of pulsing flow, a study has been set up to expand the pulsing flow regime. This is achieved by periodic operation of a trickle-bed, e.g. by cycling the liquid, respectively, the gas feed. It is observed that, due to the periodic operation of a trickle bed, it is possible to shift the transition boundary from trickling to pulsing flow towards lower average gas and liquid flow rates. An additional effect of induced pulsing flow is the possibility to predetermine the pulse frequency, and therefore the time constant of the pulses.

The induction of pulses in trickle-bed reactors by cycling the liquid feed

The operation of a trickle-bed reactor in the pulsing flow regime is well known for its advantages in terms of an increase in mass and heat transfer rates. However, fairly high gas and liquid flow rates necessitate the operation in the pulsing flow regime, resulting in relatively short contact times between the phases. By means of the periodic operation of a trickle-bed reactor it is possible to obtain pulsing flow at average throughputs of liquid usually associated with trickle flow during steady-state operation. This feed strategy to force pulse initiation is termed liquid-induced pulsing flow. The advantages associated with pulsing flow may then be utilized to improve reactor performance in terms of an increase in capacity and the elimination of hot spots, while interfacial contact times are comparable to trickle flow. An additional advantage of liquid-induced pulsing flow is the possibility to tune the pulse frequency and therefore the time constant of the pulses. During the periodic operation of a trickle bed, continuity shock waves are initiated in the column due to the step-change in liquid flow rate. This results in the division of the column into a region of high liquid holdup and a region of low liquid holdup. At high enough gas flow rates, the inception of pulses takes place in the liquid-rich region. Analysis of the performed experiments indicates that besides gas and liquid flow rates, an additional criterion for pulse inception is the available length for disturbances to grow into pulses. For self-generated pulsing flow this results in the upward movement of the position of the point of pulse inception with increasing gas flow rate. With liquid-induced pulsing flow this means that higher gas flow rates are necessary to induce pulses as the length of the liquid-rich region decreases. For both self-generated and liquid-induced pulsing flow this relationship between the gas flow rate and the available length for pulse formation is identical.

Advantages of forced non-steady operated trickle-bed reactors

Trickle-bed reactors are usually operated in the steady state trickle flow regime. Uneven liquid distribution and the formation of hot spots are the most serious problems experienced during trickle flow operation. In this paper, we advocate the use of non-steady state operation of trickle-bed reactors. Based on a square-wave cycled liquid feed, several operation modes are developed that involve the artificial induction of natural pulses and control of the catalyst wetting efficiency over longer times. The operation modes aim at increasing the mass transfer rate of the limiting reactant and simultaneous prevention of flow maldistribution and hot spot formation. The operation modes are distinguished by a relatively fast and slow cycling of the liquid feed. The potential advantages of the developed feed strategies on reactor performance are evaluated.

SCALING AND PARTICLE SIZE OPTIMIZATION OF MASS TRANSFER IN GAS FLUIDIZED BEDS.

Abstract Conversion was measured in a gas fluidized bed, using the decomposition of ozone as a model reaction. The results were expressed in terms of the height of a mass transfer unit. Mass transfer data from our own experiments and a large number of literature data were analyzed. We found a Scaling parameter S, that could describe all experiments. This Scaling parameter can be helpful in scale up. For A type powders, the height of a mass transfer unit was shown to rise with increasing particle size, using S as reference.