(97f) Study on Power Consumption of a Novel HIGEE —— Rotating Zigzag Beds

As one of the process intensification(PI) technologies, the HIGEE(high gravity) or the rotating packed bed(RPB) has been widely applied to distillation, absorption, making nano-material, micromixing of two liquid streams, and so on. The fundamental studies on the hydraulic characteristics of the rotating packed bed have been carried out in many literatures including the pressure drop, liquid holdup, flooding, the residence time distribution(RTD), visual studies of liquid flow. However, power requirement for rotating packed bed was seldom studied. Keyvani thought that the total power for rotating bed included: the power associated with liquid flow, the power consumed in overcoming the frictional windage drag of the rotor, and the power required for the bearing friction.

The rotating zigzag beds (RZB) is a new kind of HIGEE invented by Ji,J.B. in Zhejiang University of Technology in 2004. The concentric circular baffles fixed to the rotating disc coupled with those fixed to the stationary disc forms the rotor of RZB. The upper stationary disc is fixed to the rotor casing, while the lower rotating disc is fixed to a vertical rotating shaft driven by a motor. The gas-liquid intimately contact in the annular space between the rotating and stationary baffles and the mass-transfer occurs. It is very necessary to study the power requirement for RZB because the power requirement for RZB was much greater than that for the conventional RPB.

According to the analysis of Keyvani, the shaft power, NB, of RZB is

NB= NL + NW + Nbf (1)

where NL is the power requirement associated with the flow of liquid through the rotor, NW is the power consumed in overcoming the frictional windage drag of the rotor, Nbf is the power required for the bearing friction.

And NL = K1*NLR (2)

where NL is the power requirement of the liquid flow under the gas flow effect, and NLR is the power requirement of the liquid flow in the absence of gas flow.

And NLR = K2*NLD (3)

where NLR is the power requirement of the real and viscous liquid flow in the absence of gas flow, and NLD is the power requirement of the ideal liquid flow with the viscosity of zero in the absence of gas flow. Hence a mathematical model is presented to predict the NLD. It is assumed that (1)liquid film flow on the rotating disc is the two-dimensional steady ideal flow without friction, and (2) the liquid film spread uniformly over the whole surface of the rotating disc without any dry patch.

With the analysis of the liquid flow on the rotating disc, the theoretical correlation is presented, so the NLD as a function of liquid rate and rotational speed is obtained. The experiments were carried out with air-water system on a RZB. The casing of the RZB was of 320mm diameter and 800mm height. The rotor of the RZB with 600mm diameter and 80mm height is composed of six rotating baffles and six stationary baffles. Liquid rate, L, ranged from 0m3/h to 1.35m3/h; and gas rate, V, ranged from 0m3/h to 600m3/h. The rotational speed, n, ranged from 600rpm to 1400 rpm. The input power, Nin, of the motor was measured by an electric meter. The shaft power, NB, can be obtained from the input power, Nin, of the motor through the motor efficiency and the transmission efficiency.

The NLR is obtained using the experimental measurement, so K2 in eq.(3) is obtained as a function of ReL and Reù, where ReL is Reynolds number of the liquid film flow and Reù is the rotational Reynolds number.

The NL as a function of liquid rate, gas rate and rotational speed is also obtained using the experimental measurement, so K1 in eq.(2) is obtained as a function of ReL, Reù and ReG where ReG is Gas Reynolds number.

For the power requirement of NW + Nbf, the correlation is given using the experimental data.

The shaft power, NB, of RZB is calculated with the eq.(1)~(3). The power requirement for RZB was much greater than that for the conventional RPB, because the kinetic energy of the liquid from the liquid acceleration under the centrifugal force disappear when the liquid as droplets impact on the wall of the stationary baffle with high velocity. The shaft power of RZB will be reduced and energy consumption of RZB will be minimized with the structural optimization of RZB.