rise of flowrate when without gas, but keeps nearly constant when with gas injection. With the rise of swirl number, pressure drop increases, while PDR declines.
Proceedings of The Thirteenth (2003) International Offshore and Polar Engineering Conference Honolulu, Hawaii, USA, May 25 –30, 2003 Copyright © 2003 by The International Society of Offshore and Polar Engineers ISBN 1 –880653 -60 –5 (Set); ISSN 1098 –6189 (Set)
Pressure Characteristics of Hydrocyclones with Gas Injection Lixin Zhao Dept. of Environmental Engineering, Harbin Institute of Technology Harbin, Heilongjiang, P. R. China
A. Belaidi, M. Thew School of Engineering, Design & Technology, University of Bradford Bradford, West Yorkshire, United Kingdom
separation function just by taking advantage of a part of the pressure loss.
ABSTRACT Focus on a solid-liquid hydrocyclone, effects of both geometric parameters and operating parameters on pressure characteristics are studied. Results of pressure when with free gas injection are obtained, and at last the energy dissipation is analyzed. The results show that with gas injection, the pressure drop values of both overflow and underflow increase simultaneously. Pressure drop ratio (PDR) decreases with the rise of flowrate when without gas, but keeps nearly constant when with gas injection. With the rise of swirl number, pressure drop increases, while PDR declines. With the rise of gas-liquid ratio, pressure drop increases basically, while PDR remains nearly constant when with gas injection.
KEY WORDS:
Overflow outlet pi
po lo
Tangential
do
Vortex cavity
inlets
Cone segment Underflow outlet
Split flow
Hydrocyclone; experimental research; pressure;
separation; gas
du
Water
INTRODUCTION Application and research of solid-liquid hydrocyclones can be traced back to several decades ago (Bradley, 1965; Svarovsky, 1984), but have rarely discussed about the effect of the existence of gas on pressure characteristics and separation efficiency of hydrocyclones. The pressure characteristics and separation performance would be different with those of without gas injection.
pu Collector pot
(a) Main view twin inlets S =16 D
xi
Hydrocyclones perform the separation function by taking advantage of the density difference among different immiscible phases (Jiang, Zhao and Wang, 2002). Under the action of centrifugal force, the solid particles will be moved to near the wall of hydrocyclone, and then be separated. During this process, pressure energy will be lost, which is indicated by pressure characteristics of hydrocyclones.
twin inlets S =29
(b) View from top Fig. 1 Structural sketch of tested hydrocyclone
For a solid-liquid hydrocyclone, there are two outlets, one is underflow outlet, which is mainly for particles; the other is overflow outlet, mainly for water (as shown is Fig. 1). Most of the gas that mixed inside the hydrocyclone will be ejected with overflow together with water and a little part of fine particles (Jiang, Zhao, Li and Wang, 2000), and a strong vortex field exists (Zhao, Jiang and Wang, 2002), so it produces the pressure drop. Actually, hydrocyclones accomplish effective
During the process of experimental research, it was found that when with gas injection, the central gas core diameter of hydrocyclone increased (as shown in Fig. 2), which inevitably affected the pressure characteristics of the hydrocyclone. The objective of this research is to find out when with gas injection the
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effects of parameters on pressure drop and pressure drop ratio (PDR), i.e., the ratio of overflow pressure drop to underflow pressure drop; and at the same time, to analyze the ways of decreasing energy consumption. This research is also very important for guiding the design and operation of hydrocyclones.
shown in Table 1. Table 1. Main geometric parameters of testing hydrocyclone
Hydrocyclone diameter D, mm
Overflow tube diameter do, mm
Overflow tube length lo, mm
Underflow outlet diameter du, mm
Total cone angle θ, °
50
12
50
10
8
PRESSURE CHARACTERISTICS Pressure Drop Underflow pressure drop ∆piu and overflow pressure drop ∆pio are defined as follows: ∆piu = pi − pu ∆pio = pi − po (a) Without gas injection
(b) With gas injection
(2) (3)
where, pi is inlet pressure, pu underflow pressure, and po overflow pressure.
Fig. 2 View of the hydrocyclone’s flow field (near the overflow tube)
As mentioned before, the separation process of the mixed media inside a hydrocyclone obtains the working power by losing a part of pressure energy. Therefore, under the same working flowrate and the same separation effect, the lower the pressure drop the better.
TEST FACILITIES AND PROTOTYPE PARAMETERS This research was carried out in the hydrocyclonic separation laboratory of Bradford University by using the existing experimental rig of solid-liquid hydrocyclone. Gas was supplied from the compression house via a regulator-dryer and was fed into the liquid line. The pressure values can be displayed in digital on the control pad. The flowrate values of liquid and gas can be shown on the flow meters. There is a valve near the top of the particle collector pot used to discharge split flow, which is measured by a small flow sensor and then displayed with the mode of digital number.
For solid-liquid separation hydrocyclones, split ratio, a dimensionless unit, is often defined as the ratio of underflow flowrate to inlet flowrate. During this research, it is defined as the ratio of split flowrate Qu out of the collector pot to the inlet flowrate Qi of the hydrocyclone, i.e., Rf =
As shown in Fig. 1, the pressure of inlet is pi, overflow outlet po, underflow outlet pu. The measured pressure is the static value.
Qu × 100% Qi
(4)
A valve is located on the split flow tube to control the split ratio. Focus on an H/C12-1 hydrocyclone, the pressure characteristics experiments were carried out. Effects of flowrate, swirl number, and gasliquid ratio (GLR) on pressure drop and PDR are discussed in this paper.
Flowrate Qi. When S = 9 and Rf = 0 (it means no split flow out from the collector pot), the relationship curves between pressure drop and flowrate of this hydrocyclone is shown in Fig. 3 (with Pio represents ∆pio, and similar to Piu).
The hydrocyclone is made with transparent Plexiglas. It has two pairs of different size twin inlets in order to conveniently adjust the swirl number of the hydrocyclone. Swirl number is a dimensionless parameter mentioned by Belaidi and Thew (2000). It is defined as:
πDxi 2 Ai
Pio, GLR=3 Pio, GLR=0
0.30 Pressure Drop, M Pa
S=
0.35
(1)
where, xi is the mean offset distance at which inlet fluid is tangential; Ai is the sum of inlet area nominal to tangent where xi is measured (as shown in Fig. 1, taking the larger twin inlets for example).
0.25
Piu, GLR=3 Piu, GLR=0
0.20 0.15 0.10 0.05
The swirl number directly affects the strength of hydrocyclone’s inside flow field. Under the same other conditions, the larger the swirl number, the stronger the swirling flow field of the hydrocyclone. The swirl numbers of the smaller size and the larger size twin inlets are 29 and 16 respectively. When using both two pairs of twin inlets, the swirl number is 9. The other main geometric parameters of the test prototype are
0.00 0
10
20
30
40
50
60
Q i , l/min
Fig. 3 Flowrate Qi (l/min) vs. Pressure Drop (MPa), when S = 9, Rf = 0
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Two of the curves are of the hydrocyclone without gas injection, which means the gas-liquid ratio (by volume) GLR = 0. The other two are of with gas injection when GLR = 3. With the rise of flowrate, both overflow pressure drop and underflow pressure drop increase gradually no matter whether gas exists or not. After regression, the following equations were got (where ∆p is pressure drop by the unit of MPa; Qi flowrate by the unit of l/min):
drop declines. Besides, the pressure-drop-increasing gradient with S when with gas injection is little larger than that of without gas, which shows again that the existence of gas increases the pressure drop inside hydrocyclones. Gas-Liquid Ratio GLR. When S = 9 and Rf = 0, Fig. 5 is obtained. It is shown that with the rise of GLR, pressure drop values increase basically. At lower flowrate, such as 20 l/min and 30 l/min, certain fluctuation exists. It is sure that the pressure drop values of the hydrocyclone with gas injection are larger than those without gas injection (GLR = 0).
when without gas (GLR = 0, the fitting quality is the best when r2 = 1): ∆pio = 1×10−5Qi2.46 (regression degree r2 = 0.99) ∆piu = 2×10−6Qi2.93 (regression degree r2 = 0.99)
(5) (6)
Qi=20l/min Qi=30l/min Qi=40l/min Qi=50l/min
0.40 ∆pio = 4×10−5Qi2.27 (regression degree r2 = 0.98) ∆piu = 2×10−5Qi2.39 (regression degree r2 = 0.98)
Overflow Pressure Drop, M Pa
when with gas (GLR = 3): (7) (8)
The injecting gas produces additional pressure drop inside hydrocyclones. With the rise of flowrate, the pressure drop values increase dramatically (the index values of Qi are all larger than 2). If the working flowrate of hydrocyclones is much larger than the design value, the pressure drop will increase evidently, which maybe results in the pressure supply shortage of its following facilities. On the other hand, it may also result in lower separation efficiency (Jiang and Zhao, 2002).
0.30 0.25 0.20 0.15 0.10 0.05 0.00 0.0
Fig. 3 shows that although the pressure-flowrate curves of hydrocyclone when with gas injection have the same trend with those without gas, the pressure drop values with gas injection are larger than those without gas.
1.5
3.0
4.5
6.0
7.5 GLR
Fig. 5 GLR vs. Overflow Pressure Drop (MPa), when S = 9 and Rf = 0 After further research, the mathematic relationship between GLR and pressure drop could be obtained so as to estimate the gas volume inside mixed media during actual application.
Swirl number S. From Fig. 4 (only three different swirl number values can be obtained by controlling the inlet valves), it is found that with the increase of S, overflow pressure drop values increase. The similar curve for underflow was also obtained.
Pressure Drop Ratio
Qi=40l/min, GLR=3
0.6 Overflow Pressure Drop, M Pa
0.35
Pressure drop ratio (PDR) is a dimensionless unit, which reflects the pressure drop distribution into two outlets of hydrocyclone.
Qi=40l/min, GLR=0 0.5
Qi=30l/min, GLR=3 Flowrate Qi. According to Eqs. 5~8 and the definition of PDR, we can get
Qi=30l/min, GLR=0
0.4 0.3
PDR = kQi−m(m >0, k >0)
0.2
where, k and m are experimental constant numbers.
0.1
Eq. 9 shows that PDR decreases with the rise of flowrate, which is shown in Fig. 6 (Rf = 0, S = 9).
0 0
5
10
15
20
25
(9)
When with gas injection, there is no apparent difference among different GLR values, especially at higher flowrate region. The changing trend is the same with that of no gas, but the changing amplitude is much less.
30 S
Fig. 4 Swirl number S vs. Overflow Pressure Drop (MPa), when Rf = 0
Swirl number S. Fig. 7 (Rf = 0) shows that PDR decreases with the rise of S, which means that with the rise of inlet flow velocity, underflow pressure drop increases much faster, especially when the flowrate is lower. It is also found that with the rise of S, the decreasing slope of PDR declines.
Eq. 1 shows that with the rise of swirl number, the inlet section plane area reduces, which makes the enlargement of inlet pressure drop, and then results in the rise of total pressure drop inside the hydrocyclone. But sometimes it is, to some extent, beneficial for the improvement of hydrocyclone’s separation efficiency, which will be verified in the future research.
Gas-liquid ratio GLR. Fig. 8 (Rf = 0, S = 9) shows the relationship between PDR and GLR.
It is also found that with the rise of S, the increasing slope of pressure
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It is shown that at lower flowrate, with the rise of GLR, PDR decreases; but at higher flowrate, the results are totally reversed, which results in the PDR values of different flowrate become similar at higher GLR.
Energy Dissipation Energy Dissipation (per time unit) ED is defined as following: ED = ∆PQ = ∆PioQo+∆PiuQu
3.0 GLR=0 GLR=1.5 GLR=3 GLR=4.5 GLR=6 GLR=7.5
2.6
PDR
2.2
(10)
Eq. 10 shows that ED is related to the two parts of pressure drop, i.e., both overflow pressure drop ∆Pio and underflow pressure drop ∆Piu. Therefore, based on the analysis that mentioned above, in order to reduce the energy dissipation of hydrocyclones, the most effective way is to reduce flowrate, swirl number or GLR, of course, under the premise of getting satisfactory separation efficiency.
1.8
DISCUSSIONS 1.4
In the applications of hydrocyclones, especially when used in oilfields, free gas often exists. Under this condition, the pressure characteristic of hydrocyclones is different with that of hydrocyclones operating without gas injection. The experimental research of the pressure characteristics of hydrocyclones with gas injection is very important for understanding the influence of main parameters, such as flowrate Qi , swirl number S, and GLR, on pressure drop and PDR, and guiding the design and operation of hydrocyclones.
1.0 20
30
40
50 Q i , l/min
Fig. 6 Flowrate Qi (l/min) vs. PDR, when S = 9, Rf = 0
Qi=40l/min, GLR=0
2
Because the pressure drop of hydrocyclones increases with the rise of flowrate, swirl number and GLR, in order to decrease the energy loss of hydrocyclones, reducing flowrate, swirl number, or GLR is the feasible way. Besides, PDR remains nearly constant with the rise of flowrate when with gas injection, which means when with gas injection, both underflow and overflow pressure drop are affected by flowrate at a same level. This is different with that of without gas injection. PDR decreases with the rise of swirl number S, and, at higher flowrate, i.e., near the design flowrate, the PDR values of hydrocyclones with gas injection are higher than those without gas injection.
Qi=40l/min, GLR=3 Qi=30l/min, GLR=0
1.8
Qi=30l/min, GLR=3 PDR
1.6 1.4 1.2
CONCLUSIONS 1) The existence of gas increases the pressure drop inside hydrocyclones, which would affect the application at many aspects, such as more power would be needed to operate the hydrocyclone; energy deficiency of its following device would be appeared.
1 0
5
10
15
20
25 S
30
Fig. 7 Swirl number S vs. PDR, when Rf = 0
2) PDR decreases with the rise of flowrate when without gas, but keeps nearly constant when with gas injection. It means when without gas injection, with the rise of flowrate underflow pressure drop increases faster than overflow pressure drop, i.e., flowrate affects underflow pressure drop more than overflow. But when with gas, the influence of flowrate on the pressure drop of both outlets is nearly the same.
Qi=20l/min
3.0
Qi=30l/min Qi=40l/min
2.5
3) With the rise of swirl number, pressure drop increases dramatically, while PDR declines, which means the inlet parameter (relating to the swirl number) affects the pressure drop to a great extent, and this kind of influence acts more on underflow pressure drop.
PDR
Qi=50l/min
2.0
4) With the rise of gas-liquid ratio, pressure drop increases basically. That is to say, the amount of free gas directly affects the pressure drop value. The more the free gas, the higher the pressure drop, under the same other conditions.
1.5
1.0 0.0
1.5
3.0
4.5
6.0
5) The way of reducing the unit energy dissipation of hydrocyclone is to reduce flowrate, swirl number or the gas-liquid ratio of the mixed media, among which the most feasible way is to reduce gas-liquid ratio at the entrance of hydrocyclones, i.e., to enhance the efficiency of the free gas knockout device to remove gas content of the mixed fluid as
7.5 GLR
Fig. 8 GLR vs. PDR, when S = 9, Rf = 0
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much as possible before entering hydrocyclone.
nology,” Publishing House of Harbin Institute of Technology, Harbin, pp 1~4 (in Chinese). Jiang, MH, Zhao, LX, et al. (2002). “Effects of Geometric and Operating Parameters on Pressure Drop and Oil-Water Separation Performance for Hydrocyclones,” Proc. of 12th International Offshore and Polar Engineering Conference, ISOPE, Kitakyushu, Japan, Vol I, pp 102~106. Jiang, MH, Zhao, LX (2002). “Test Research of Oily-Water High Efficient Treatment System,” China Chemical Engineering, Vol 30, No 1, pp 77~82 (in Chinese). Svarovsky, L (1984). “Hydrocyclones,” Holt, Rinehart and Winston, London, pp 1~4. Zhao, LX, Jiang, MH, et al. (2002). “Influence of Cone Angles on Both Tangential and Axial Velocity Distributions in Hydrocyclones,” Proc. of 12th International Offshore and Polar Engineering Conference, ISOPE, Kitakyushu, Japan, Vol I, pp 97~101.
ACKNOWLEDGEMENTS Lixin Zhao would like to acknowledge the sponsorship by China Scholarship Council for his research work at the University of Bradford, UK.
REFERENCES Belaidi, A, and Thew, M (2000). “Drop Size Effects on a De-Watering Hydrocyclone,” Proc. of Vortex Separation, York, England, pp 119~129. Bradley, D (1965). “The Hydrocyclone,” Pergamon Press, London, pp 1~8. Jiang, MH, Zhao, LX, et al. (2000). “Hydrocyclonic Separation Tech-
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