Article pubs.acs.org/IECR
A New Crossflow Rotating Bed, Part 1: Distillation Performance Guang Q. Wang, Cheng F. Guo, Zhi C. Xu, Yu M. Li, and Jian B. Ji* Zhejiang Province Key Laboratory of Biofuel, College of Chemical Engineering & Materials Science, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, People’s Republic of China
ABSTRACT: A new kind of high-gravity devicethe crossflow concentric-baffle rotating bed (CRB)has been developed, the rotor of which contains a set of perforated concentric baffles. In this type of rotating bed, the gas flows in a zigzag pattern toward the center of the bed while the liquid traveled radially from the inside to the outside of the rotor. Between the adjacent concentric rings, the gas phase could be fully kept in contact with the dispersed liquid phase in crossflow. The rotor structures and the twophase flow arrangements determined its potential features, such as the low shaft power and the little backmixing, which, together with stagewise contacting, were favorable to multistage separation processes widely used in chemical process industries. In a pilot CRB, the pressure drop, shaft power, and mass-transfer performance were investigated under different operation conditions. The experimental results showed that the pressure drop, the shaft power, and the efficiency in each theoretical stage fell within the range of 100−600 Pa, 100−250 W, and 10%−15%, respectively. Compared to the rotating zigzag bed (RZB), the CRB generated a lower pressure drop and needed less shaft power for every contacting stage, which can be attributable to its different structures. However, the stage efficiency of the CRB was, at most, one-third as much as that of the RZB. This rather poor mass-transfer performance of the CRB shadowed, to a large degree, its advantages in the pressure drop and shaft power reductions. Therefore, it was necessary to optimize the structures of the CRB and enhance its mass-transfer performance.
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BACKGROUND High-gravity technology (abbreviated to HIGEE), which appeared in the 1970s, was used to improve mass-transfer processes.1 In this technology, gravity was replaced by the high centrifugal force generated in the rotating doughnut-shaped rigid bed. In the rotating bed, the liquid phase generally was accelerated to several hundred times the force of gravity and easily sprayed into the gas phase. The micromixing effect and mass-transfer rate could be increased by 1 or 2 orders of magnitude. This was its most attractive feature, in comparison with its conventional counterparts. In the literature, there exists several types of rotating beds2 with different internals, such as packing (e.g., disk, helical, and split packing). What was noteworthy was the rotating zigzag bed (RZB), which is characterized by a uniquely structured rotor3 and is used commercially in industrial processes.4 In the aspect of the RZB structure, the core component of the RZB was a nonpacking rotor, which was comprised of a rotating disk and stationary disk, on which two sets of concentric circular baffles were installed. In accordance with the flow pattern in the RZB rotor, the continuous gas phase could flow along a zigzag path and the dispersed liquid phase could be (re)distributed and (re)collected. The striking merits of RZB were the higher masstransfer efficiency, the capability of middle feed, and the easy achievement of multirotor configuration in one casing. However, in the RZB, there existed disadvantages, such as the large shaft power and possible liquid entrainment (backmixing), which, to some extent, limited RZB’s application. It also suggested that the RZB could be further improved within its framework. That just was the starting point of this study. With the above considerations, a new type of rotating bed, called a crossflow concentric-baffle rotating bed (CRB), has © 2014 American Chemical Society
been proposed, based on the structural modifications of the RZB. In the first paper in this series, the structures and characteristics of CRB are illustrated in detail. Its hydrodynamics and shaft power, as well as the mass-transfer performance, have also been experimentally investigated. In addition, the CRB was also compared to its prototype, RZB, to determine the validity of its structure modifications. The results could provide further insight into the mechanisms of the RZB and help to optimize the structures of the CRB. The structure optimization of the CRB will be presented in the second paper in this series.5
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CONSTRUCTION AND PRINCIPLES OF CRB The crossflow concentric-baffle rotating bed (CRB) could be viewed as the modified version of the RZB. The schematic diagram of CRB is presented in Figure 1, with the RZB also shown for comparison. It could be seen from Figure 1 that both CRB and RZB had the same structure framework: a combined rotational and stationary rotor. The stationary part of the CRB rotor was a stationary disk with a set of concentric grooves on its lower side. The rotational part was a rotating disk upon which a set of concentric baffle (rotational baffle) was installed on the upper side. The upper parts of the rotational baffles were extended into the grooves on the stationary disk. Each baffle was divided into three zones in the axial direction: gas-hole zone, liquid-hole zone, and nonhole zone (see Figure 2). These baffles were assembled together with the alternate baffles in the vertically opposite direction. The liquid distributor rotating Received: Revised: Accepted: Published: 4030
September 29, 2013 February 10, 2014 February 18, 2014 February 18, 2014 dx.doi.org/10.1021/ie4032296 | Ind. Eng. Chem. Res. 2014, 53, 4030−4037
Industrial & Engineering Chemistry Research
Article
Figure 1. Schematic diagram of a crossflow concentric-baffle rotating bed (CRB) and rotating zigzag bed (RZB). 1−shaft seal; 2−casing; 3− rotational disc; 4−rotational baffle; 5−gas inlet; 6−stationary disc; 7−liquid distributor; 7′−stationary baffle; 8−gas outlet; 9−liquid inlet; 10−liquid outlet; 11−shaft.
Although the CRB was developed from the RZB, it had obvious differences from the RZB. First, the upper parts of rotational baffles of the CRB were extended into the stationary disk. It is very likely that liquid entrainment exists in the clearance between the rotational baffles and the stationary disk in the RZB. For the CRB, this clearance was replaced by the gas-hole zones on baffles, which can effectively prevent the liquid entrainment. Second, the CRB was without stationary baffles and it could be structurally considered as RZB with the stationary baffles also “rotated”. The larger shaft power of the RZB was due to the fact that the liquid droplets dispersed by rotational baffles impinged on stationary baffles. This impingement action would cause a more kinetic energy loss and a larger shaft power. For the CRB, the “rotated” stationary baffles can greatly reduce the kinetic energy loss produced by impingement. To “rotate” the stationary baffles, the lower parts of stationary baffles must be extended into and fixed with the rotational disk. Third, from the perspective of mass transfer, the gas−liquid contact in the CRB could be considered purely crossflow, as indicated in Figure 3. This contacting manner was Figure 2. Simplified schematic of a concentric baffle.
with the shaft was installed at the center of the rotor to obtain a spatial uniform initial distribution. The gas-hole zones on all baffles and the annular space between the adjacent baffles formed the zigzag flow channels for the gas phase, and the liquid-hole zones on all baffles provided the radial flow paths for the liquid phase. To prevent the gas phase from bypassing the baffles, a labyrinth seal was employed between the rotational baffles and the stationary disk. The gas was introduced by the gas inlet into the casing and flowed inward successively through the gas-hole zones on each baffle driven by pressure difference. The liquid flowed into the distributor and traveled radially outward through liquid-hole zones on each baffle, in the form of fine droplets, because of centrifugal force. The liquid left from the rotor as droplets, which were collected on the casing wall and exited through the liquid outlet. Thus, in the rotor, the continuous gas phase uniformly mingled with the dispersed liquid phase in a crossflow manner. Each annular space between the adjacent (rotational) baffles could be defined as a contacting stage (see gray zone in Figure 1a). This stagewise-contacting mechanism is particularly advantageous to the multistage separation processes in chemical process industries.
Figure 3. Flow path and contacting process of gas and liquid in the CRB.
similar to the first step of mass-transfer process3 in the RZB, where there still existed a second step: gas−liquid counterflow contacting. Although the absence of counterflow contacting made the CRB at a disadvantage in mass-transfer performance, the lesser liquid entrainment would perhaps offset this disadvantage. In addition, the CRB made full use of the total volume of the rotor, because the mass transfer took place in all 4031
dx.doi.org/10.1021/ie4032296 | Ind. Eng. Chem. Res. 2014, 53, 4030−4037
Industrial & Engineering Chemistry Research
Article
Figure 4. Schematic diagram of the experimental setup. 1−U-tube differential manometer; 2−condener; 3−rotameter; 4−crossflow rotating bed; 5− falling film reboiler; 6−circulating pump; 7−motor; 8−frequency modulator; 9−sampling.
respectively. In this device, the rotor was driven by a motor and could be operated from 400 rpm to 1200 rpm tuned by a frequency modulator, which provided a centrifugal force of 30g−275g, based on the arithmetic mean diameter. The hydrodynamics, mass-transfer performance, and shaft power were experimentally studied by the separation of an ethanol− water mixture in the distillation process. A simplified schematic diagram of the experimental system, which mainly consisted of a CRB, a reboiler, a condenser, and a motor, is presented in Figure 4. As shown, the system was configured for total reflux operation. A predetermined amount and composition of feed was initially charged to the reboiler. In operation, the vapor from the reboiler was introduced tangentially into the casing, flowed radially inward in a zigzag way through the rotor due to pressure gradient, and left the rotor through the gas outlet located on the casing. The outlet vapor then entered the condenser and was condensed to liquid as reflux. The reflux liquid flowed into the rotor from the liquid inlet after measuring the flow rate and sampling. The liquid traveled radially outward through the rotor via centrifugal force. The liquid exiting from the outer periphery of the rotor was splashed onto the casing, collected at the bottom, and returned to the reboiler.
spaces between the adjacent (rotational) baffles. However, in the RZB, mass-transfer zones were just a part of spaces between the adjacent rotational baffles. That is to say, for the rotor with the same size, the mass-transfer zones of CRB were larger than those of RZB. Based on these considerations, it was still possible that the mass-transfer capacity of the CRB was as high as that of the RZB.
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EXPERIMENTAL SETUP AND PROCEDURES In the experimental investigations, a pilot CRB installed in a casing 300 mm in diameter was employed. The axial height of the rotor was 80 mm, and the inner and outer diameters were 100 and 242 mm, respectively. The concentric baffles with an axial height of 72 mm were arranged at the unequal radial intervals, but the annular areas between the adjacent baffles were equal. In the rotor, there were a total of 10 baffles (equivalent to 9 contacting stages), among which the innermost and outermost ones had diameters of 120 and 234.8 mm, respectively. The axial heights of the gas-hole and liquid-hole zones on the baffles were 16 and 31 mm. The holes for both the gas phase and the liquid phase were arranged in equilaterally triangular pattern with a pitch of 2.7 mm, and the diameters of the gas and liquid holes were 2 and 0.8 mm, 4032
dx.doi.org/10.1021/ie4032296 | Ind. Eng. Chem. Res. 2014, 53, 4030−4037
Industrial & Engineering Chemistry Research
Article
Figure 5. Dependence of stage efficiency on (a) F-factor and (b) rotational speed.
The pressure drop across the CRB was measured using a Utube manometer installed between the inlet pipe and the outlet pipe. The shaft power of the motor was determined by a threephase Watt-Hour meter with the aid of a stopwatch. The top and bottom samples were taken from the inlet and outlet liquid streams, and then their composition was analyzed via gas chromatography to evaluate the mass-transfer performance. The volumetric flow rate of reflux was measured by rotameter. Besides, the temperature of reflux liquid was also measured by a thermometer to calibrate the rotameter. It was obvious that the vapor flow area was variable as it moved radially toward the rotor center, because the total areas of gas holes on each baffle were different. Therefore, a cross section must be selected as the basis of velocity calculation. Since it was the annular spaces between the adjacent baffles that the mass transfer took place, the vapor velocities were calculated based on the annular area between the adjacent baffles. The vapor load factor F was calculated as follows:
⎛Q ⎞ F = ⎜ V ⎟ρV 0.5 ⎝ A ⎠
Es =
(1)
Q R ρR ρV
(3)
where ns is the number of contacting stages (defined previously) and NT was the number of theoretical stages achieved, which could be estimated using the McCabe−Thiele graphic method, according to the concentration of top and bottom samples. In order to compare the hydrodynamic and power consumption performance of different HIGEEs accurately, the comparison must be made under the same amount of separation. Each contacting stage or each baffle did not mean the same degree of separation, because of the difference in separation efficiency. Therefore, the pressure drop and power consumption in this study were evaluated based on each theoretical stage. The experimental procedures included the measurements of the pressure drop, shaft power, and composition of liquid samples under the different reflux liquid flow rates and rotational speeds. The steady state would occur when the composition of consecutive samples varied by
