1150
Ind. Eng. Chem. Res. 2004, 43, 1150-1162
Process Intensification in Rotating Packed Beds (HIGEE): An Appraisal D. P. Rao,* A. Bhowal,† and P. S. Goswami Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208016, India
Rotating packed beds have received considerable attention as a means of process intensification for gas-liquid mass transfer over the past 2 decades. In this work, we take a critical view of the developments in understanding the transport processes in rotating packed beds. The intensification achieved so far falls short of the goal of 2-3 orders of magnitude volume reduction compared to that obtained conventional columns. The directions toward achieving this goal are outlined. 1. Introduction It has been over 2 decades since the first patent on rotating packed beds was filed by Ramshaw and Mallison,1 Imperial Chemical Industries, U.K., for carrying out distillation, absorption, and stripping under a centrifugal force field. The claims are that the height equivalent of a theoretical plate (HETP) of rotating packed beds can be as low as 1-2 cm and that a volume reduction by 2-3 orders of magnitude compared to that of conventional packed columns can be attained.2 Such a possible miniaturization of equipment stimulated interest in rotating packed bed units. The commercial acceptance has been slow. There have been reports of the commercial use of rotating packed beds by Glitsch, Inc.;3 HIGRAVITEC Center, Beijing University;4 and recently by The Dow Chemical Company, Midland, MI.5 We refer to such units hereafter as RPBs (rotating packed beds). It is also known as HIGEE, an acronym for high gravity. Ramshaw2 reviewed the earlier attempts to utilize centrifugal force field to affect separation and highlighted possible applications in a number of allied areas. Fowler3 described the experience gained on commissioning industrial-scale units by Glitsch Inc., Dallas, TX. Kelleher and Fair6 presented a brief review of some of the developments up to 1996. Progress has been made in understanding some of the basic aspects of transport processes in RPBs. Some intriguing aspects of RPBs have been reported. A comprehensive appraisal of the possible intensification that can be achieved with RPBs is not available. The objectives of this work are to present such an appraisal, highlight the unresolved problems, and point out the directions for future activities. Although RPBs have been used for adsorption, reactive precipitation, heat transfer, etc., the focus of this appraisal is limited to the use of an RPB as a gas-liquid mass-transfer device. In some of the studies of RPBs, rotors fitted with concentric perforated shells have been used, which are akin to trays in a distillation column. These too are beyond the scope of this work. 2. Description of an RPB Figure 1 shows sketches of two RPB units, one having a rotor with a vertical axis and the other having a rotor * To whom correspondence to addressed. E-mail: dprao@ iitk.ac.in. † Present address: Dept. of Chemical Engineering, Jadavpur University, Kolkata, India.
Figure 1. Sketch of rotating packed bed unit: (a) horizontal-axis RPB, (b) vertical-axis RPB. Legend: 1, liquid feed inlet; 2, liquid outlet; 3, vapor inlet; 4, vapor outlet; 5, packing; 6, motor.
with a horizontal axis. The rotor is an annular, cylindrical packed bed. It is housed in a casing and is driven by a motor. The flows of vapor (or gas) and liquid are in countercurrent directions in the rotor. The angular velocities employed are in the range of 500-2000
10.1021/ie030630k CCC: $27.50 © 2004 American Chemical Society Published on Web 01/27/2004
Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004 1151
revolutions per minute (rpm). Such values are in the range of those used for centrifugal pumps and fans, in contrast to the angular velocities of 10 000 rpm or more used in industrial centrifuges. The centrifugal field is 100-1000g, where g is Earth’s gravity. A variety of packings have been used: glass spheres, plastic beads, rigid reticulated wire-mesh, metal foam, metal wiremesh, and disks. The porosities and surface areas of these dry packing materials are in the ranges of 0.40.95 and 750-5000 m2/m3. The vapor is introduced into the casing. It flows through the rotor and emerges from the eye and through the outlet pipe as shown in Figure 1. The liquid is fed through a stationary distributor placed at the eye of the rotor as jets, broken jets, or spray of droplets onto the inner periphery of the packing. The liquid attaches onto the packing at the inner periphery and flows over it under the influence of the centrifugal field. It flows over the packing as thin films, rivulets, flying droplets, or films covering the pores of the packing depending on the type of packing, the gas and liquid flow rates, and the angular velocity. The mass transfer takes place in the packed section. However, a considerable gas-liquid interfacial area is available outside the rotor, especially with small beds, as the liquid emerges from the packing as a shower of droplets, which collects on the casing wall, flows down the wall as a film, and collects at the bottom of the casing. 3. Columns versus RPBs Broadly speaking, the desired extent of separation determines the height of a conventional packed column, whereas it is the radial width of the rotor (ro - ri) that determines the height for an RPB. The allowable gas and liquid throughputs dictate the required flow area of the bed at the inner periphery of the rotor. The diameter of a conventional column is determined by allowable throughputs (based on flooding), and these values are inherently governed by Earth’s gravity. On the other hand, in an RPB, these throughputs are governed by the centrifugal acceleration. The gas and liquid velocities vary along the radius. The centrifugal acceleration is lowest at the inner radius. Therefore, the inner radius and height determine the allowable flow rates. The outer radius is constrained by the mechanical stresses of the rotor and the pressure drop across the rotor. The design involves finding the appropriate inner radius, outer radius, packing height, and angular velocity. 3.1. Process Intensification. To achieve a reduction in volume of 2-3 orders of magnitude in an RPB as compared to a conventional column, the allowable throughputs have to be at least an order of magnitude higher, and furthermore, the mass-transfer rate has to be higher by a similar magnitude. To understand the possible reasons for the process intensification, we examine the following aspects of an RPB: (i) throughputs, (ii) gas flow, (iii) liquid flow, (iv) pressure drop, (v) flooding, (vi) liquid-side mass-transfer coefficient, (vii) gas-side mass-transfer coefficient, and (viii) power requirement 4. Throughputs and Other Parameters To interpret the results of different investigators, it is helpful to compare the range of liquid and gas velocities and other parameters that were employed in each case. Table 1 presents a comprehensive list of the
operating and equipment parameters employed by the various investigators. It lists studies readily accessible in the open literature. Some of the parameters given in Table 1 were not explicitly reported. They were inferred or computed from the reported data. To facilitate the comparison of data reported by different investigators, we have converted them all to a uniform basis and SI units. Most of the studies employed air-water systems, and the rest used systems that have properties not far from those of air-water systems. Figure 2 shows the ranges of velocities employed by the different investigators. It is preferable to base the gas and liquid velocities on the inner radius. In the literature, in some cases, these values are based on the arithmetic average of inner and outer radius. We recomputed these velocities on the basis of area at the inner periphery. The range is shown as a triangular area, spanning the minimum and maximum velocities of gas and liquid velocities. The actual range covered is less than that shown in Figure 2. It is worth mentioning that the objective of most of these studies was not to ascertain the allowable throughputs. Figure 2 also shows the range of velocities used for a conventional packed column with Raschig rings. In most cases, the velocities of gas and liquid employed in RPBs are only a few times higher than those employed in conventional columns. Guo et al.17 and Sandilya et al.23 employed gas velocities an order of magnitude higher than those of conventional columns. The former used cross-flow, and the latter employed a pair of rotating disks. A closer scrutiny of the studies in which higher liquid velocities were employed indicates that severe liquid ejection could have occurred at the inner periphery, an phenomenon akin to flooding (see section 6.2). Trent and Tirtowidjojo5 reported studies performed on a commercial RPB unit. The velocity range in these studies was about 5 times higher than that used in conventional columns and much higher than the range employed in laboratory units. 5. Gas Flow Knowledge of the nature of gas flow through an RPB is required to understand the possible process intensification, gas pressure drop, mass-transfer rate, and power requirement. Only recently has the gas flow received attention. At present, the flow description is limited to volume-averaged gas velocities. We examine here the gas flow through various regions of the unit under different operating conditions. 5.1. Stationary Rotor. Consider the flow of gas through an RPB with an unirrigated, stationary rotor. If the clearance between the rotor and the casing is adequate, then the gas will enter the rotor through its outer periphery with a uniform velocity. The gas flows into the eye of rotor, takes a 90° bend, and leaves through the outlet. The flow is convergent because of the variation in the flow area within the rotor. It is also “one-dimensional”, as in a conventional packed bed. The flow is similar to that in conventional packed beds except for the variation in flow area. The pressure drop across the rotor (i.e., packed section) can be estimated from the Ergun correlation 3 150(1 - ) dP dpFg ) + 1.75 2 dr (1 - )G Rep
(1)
GB, 4 WG (Cu)
12.7
-
3
12.7, 12.7, 12.7 8.3
-
FM
-
GB
WM
FM, SM, WM CF (Al)
Tung and Mah7
Keyvani and Gardner8
Flower3
Munjal et al.9,10
Kumar and Rao11 Singh et al.12
PG
MS
RP (PVC)
WG
RWM
FD
WM, GB, FP
Liu et al.15
Kelleher and Fair6
Burns and Ramshaw16
Guo et al.17
Zheng et al.4
Mukherjee18
Trent et al.19
Zheng et al.20 FB
GB
Basic and Dudukovic14
Lockett13
8.2
16
30
7
7, 9
22.9, 30.5, 38.1 17.8
15.5
7, 9
-
22.8
9
8.5
2.25, 7.5, 14
8.1, 10, 15
3
15, 15, 15
40.6
15.5
0.92
0.9, 0.95 0.38, 0.6
0.95
0.4, 0.433
4.8, 4.8, 4.8
25
1.8
25, 70
20
10
15
2
2.5
500, 500
>0.9, >0.92
0.94, 0.94, 0.94
0.92, 0.5, 0.5
V
V
H
H
H
V
V
V
V
V
V
V
-
H
V
V
0-1000 V
6602800, 1500, 1250
1333
-
-
0.7
1500
524, 1027 2500
3300, 1134
1770
25002067
4000
3300, 1134
2500
656, 1476, 2952
20005000 3300, 1650
packing area rotor (m-1) axisb
0.95
0.533, 0.389 0.92
0.4, 0.433
12.7, 0.9512.7, 0.934 12.7 5.8 -
2.5
2.5
1.5- >0.9 2.5
4.4
1.2, 2.5 4
ro h (cm) (cm) porosity
15, 30 30, 50
3.2
3.5
8.75
4.5
3, 5
3, 5
-
packinga
Ramshaw2
reference
ri (cm)
variety of packing supports at inner radius perforated sheet at outer radius
-
no details 10-mm metal 17 001/m -
perforated plate
none
support plate
none
perforated sheets and wire mesh
-
-
-
01300
7001500, 750
3001500
2 pipes gas inlet 0with spary and oulet 1090 holes pipes
tested several spray nozzles
-
5 nozzles
4001200
-
-
3101600
240800
02200 4001200
feed and outlet pipes -
610, 1140 1001000
9001550
-
5001300
-
casinggas pipe -
1000
ac (g)
0-15
0-60, 0-195, 0-365
20-180 at inner radius
0-67
0-3.3, 0-1.0, 0-0.58
0-3.0
-
0.801.7
0.20.3
-
0.00.0068 0.0010.0026
0.0040.167
0.0020.01
0.0220.027 0.00.062
00.012, -, -
0.01
0.030.06, 0.06
0.014
-
-
-
0.53.5
-
-
-
1.0-5.0
(kga) 50-120 (kla) 0.18-0.9 -
-
(kga) 2.9-12 -
-
0.0560.4
1.1722.9 -
(kla) 0.1-0.12 (kla) 0.14-1.08
-
0.51.14
-
(kla) 1.0-2.5
0.330.66
-
volumetric massmasstransfer transfer coefficient coefficient (103 m/s) (1/s)
0.0047- 0.80.045 1.0
-
0.00.036
0.0040.006
-
V lc (m/s)
not 0.01 available
0.0180.18 0.1350.63
0.02.9
15.0
0.130.22 0.473.26
02.07
-
0.01.70
-
Vgc (m/s)
80-370, 0.03185 0.18, -
3.2-79
6-55
15-140
0-240
2-87
5-60
40-200
12-43
26-80
800
13200
1250- 70-134 1750
-
-
casinggas pipe
-
-
ω (rpm)
perforated tube perforated pipe
2 spary bars
-
4 slits
16 holes in a pipe
-
12 tubes
-
-
none
-
liquid pressure distributor tapings
-
end plates
Table 1. Operating and Equipment Parameters Employed by Various Investigators
-
HTU 1.8-4.2
-
HTU 2.5-4
-
-
-
-
HTU 1e
ATU 0.02-0.06
-
-
HETP 1.5-2.5
HETP 2.0-5.5
HTU 1-2 -
HTU/ ATU/ HETPd (cm)
clarifies intriguing aspects of pressure drop
two units of 50 t/h and 300 t/h used for deoxygenation of water parallel cloth disks pilot scale
reported nature of liquid flow and maldistribution cross-flow
validity of film model, anisotropy of hold-up very radially short bed distillation at total reflux
flooding
early version of disclosure based on Ramshaw and Mallison data,1 assumed height residence time and power requirements measured status report from Giltsch, Inc., Dallas, TX reported interfacial area and solidliquid mass-transfer coefficient
remarks
1152 Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004
0.0020.008 0.67-2.37 across the rotor
6001600
42298
3.5-5.5
