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Effects of water on steam rectification in a packed column
chemical engineering research and design 8 9 ( 2 0 1 1 ) 2560–2565
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Effects of water on steam rectification in a packed column Qingli Qian b,∗ , Hongxing Wang c , Peng Bai a,∗ , Guoqing Yuan b a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Beijing National Laboratory for Molecular Sciences (BNLMS), Laboratory of new materials, Institute of chemistry, Chinese Academy of Sciences, Beijing 100190, China c College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, China b
a b s t r a c t The effects of water on steam rectification, i.e., multi-stage saturated steam distillation, were investigated in a packed column. N-octane–p-xylene and 1,3,5-trimethylbenzene–1,2,4-trimethylbenzene were used as test systems. Both binary systems are nearly ideal systems and insoluble in water, thus the effects of water in steam rectification can be clearly and definitely revealed. Such unpolar organic liquid is named as “oil”. The water/oil at column top can be separated and refluxed at different ratio. Compared with conventional rectification, there are some peculiar phenomena in steam rectification. Water greatly enhances the flooding vapor velocity of the rectification, in addition, water plays a predominant role in pressure drop of the packed bed near flooding point. It is clear that liquid water in the packed bed can promote mass transfer of steam rectification, especially for materials with higher viscosity. In a word, steam rectification can be operated at low temperature with good mass transfer. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Steam rectification; Packed bed; Mass transfer; Multiphase flow
1.
Introduction
Steam distillation has been widely used for a long time in industry, especially for the extraction and purification of natural products, such as essential oils, because most of them are thermally unstable and insoluble in water. As a carrier gas, steam can lower the distillation temperature. Because of the torrential characteristics, it can also reduce residence time of natural products in distillation column, preventing decomposition or polymerization. According to Gibbs phase law, the steam distillation can be sorted into two modes: superheated steam distillation and saturated steam distillation. Currently, the superheated steam distillation is extensively applied in industry. The saturated steam distillation can be operated below 100 ◦ C under atmospheric pressure, and is more desirable for separating thermally unstable substances. However, the saturated steam distillation is mainly used in single equilibrium stage distillation for crude extraction of essential oils from biomass (Kishore et al., 1997; Perineau et al., 1992; Reverchon and Seatore, 1992). To increase the yield of extraction, Chemat (2006) and Masango (2005) designed new
∗
processes that force the saturated steam through a packed bed of the raw biomass. Multi-stage saturated steam distillation was defined as “Steam rectification” by Qian et al. (2003). For the convenience of discussion, we call rectification of binary organic systems as conventional rectification. According to Gibbs phase rule, steam rectification can be operated at lower temperature than conventional rectification. Ottenbacher and Hasse (2007) investigated the steam rectification as a process for separating thermally unstable substances, and deemed that it is a promising alternative or addition to state of the art processes like vacuum distillation. Steam rectification, involving two liquid phases and a vapor phase, is three-phase distillation. Actually, three-phase distillation has been widely investigated by researchers (Harrison, 1990; Repke and Wozny, 2002; Schoenborn et al., 1941; Siegert et al., 2000). However, the effects of water on steam rectification have not been thoroughly investigated. In this paper, we investigate the steam rectification with two test systems: n-octane–p-xylene and 1,3,5Both binary trimethylbenzene–1,2,4-trimethylbenzene.
Corresponding authors. Tel.: +86 10 62560247; fax: +86 10 62559373. E-mail addresses: [email protected] (Q. Qian), [email protected] (P. Bai). Received 6 October 2010; Received in revised form 19 May 2011; Accepted 18 June 2011 0263-8762/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2011.06.004
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Table 1 – The properties of the chemicals in the test systems. Chemical n-Octane p-Xylene 1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene Water
Tbp [◦ C] 125.7 138.4 164.7 169.4 100.0
Density [g cm−3 (20 ◦ C)] 0.70 0.86 0.87 0.88 1.00
organic systems are nearly ideal systems and insoluble in water. Such unpolar organic liquid is named as “oil”. In order to simplify the operation and reduce the impact of other factors than water, the column was specially designed, and water/oil at column top can be separated and refluxed at different ratio. Therefore the effects of water on steam rectification can be more clearly and definitely evaluated. The difference between steam rectification and conventional rectification are also demonstrated by experiments.
2.
Material and methods
2.1.
Material
Two test systems, n-octane–p-xylene and 1,3,5trimethylbenzene–1,2,4-trimethylbenzene were tested in steam and conventional rectification. Both binary organic systems are nearly ideal system and insoluble in water. The chemicals in the test systems are slightly toxic, inexpensive, and easily available in large amounts. The properties of the chemicals are displayed in Table 1.
2.2.
Methods
2.2.1.
The rectification column and experimental procedure
The flow chart of steam rectification in our experiment is revealed in Fig. 1a. The steam rectification was carried out in a glass column, the diagram of which is shown in Fig. 1b. The name and function of the parts on the column are as follows: (1) heater, (2) zeolite, (3) column kettle, (4) bottom sampling tap, (5) condenser, (6) packed bed, (7) thermal barrier, (8) vapor velocity flowmeter (for calculation), (9) top sampling tap, (10) oil/water separator, (11) reflux regulator, (12) liquid velocity flowmeter (for calculation), (13) manometer, (14) voltage regthermometer. The ulator, (15) oil/water interface regulator, column was packed with Ф3 × 3-net ring of stainless steel (random packing area: 2275 m2 m−3 , mesh number of the steel net: 100). The inner diameter of the column is 0.04 m, and the effective packing height is 0.8 m. -Net ring, also known as Dixon ring, is a highly efficient packing of small particles made of metal mesh and with same diameter and height. Net ring is used mainly for laboratory and low volume, high purity product separation process. For better reflex distribution, a cone-shaped stainless net was tightly placed on the top of the packed bed. In order to obtain continuous and steady state, there was no feed during the rectification, and the product collected at column top was returned to column bottom. The velocities of vapor and liquid in the column were calculated according to the liquid volume determined by the flowmeters in specified time. There was no liquid reflux during vapor velocity measurement, and the calculation of vapor velocity (Uv ) is based on the ideal gas equation of state. F factor (Fv ) is the product of vapor velocity and square root of vapor density (v ) in
Hevap [kJ mol−1 ]
Viscosity [mPa s (20 ◦ C)]
34.10 35.98 39.04 39.25 40.68
0.546 0.644 1.154 0.894 1.002
Surface tension [mN m−1 (20 ◦ C)] 21.62 28.54 28.00 29.71 72.88
√ the column, i.e., F = U . In our experiment the variation of vapor density is minor for specified test system, thus Fv can be obtained by simply adjusting Uv with a constant (square root of average vapor density (kg m−3 )0.5 ). The constants can be calculated from the ideal gas equation of state and the partial pressure of each component. The relationship between Fv and Uv has been supplied in related figures. The pressure drop of the packed bed was obtained by U-type manometer, and the tinct n-heptane was used as measuring medium. The samples were collected via taps at the top and bottom of the packed bed, and analyzed with gas chromatography. For comparison, conventional rectification was also conducted in the packed column. The operation of conventional rectification is similar to that of steam rectification, but the oil/water separator is no longer needed. The relfux can be manipulated with automatically controlled reflux regulator, through which relfux and output proceed alternately, and reflux ratio is the ratio of time for reflux and output. In addition, water and oil can be regulated respectively after oil/water separator. The reflux ratio (R) of oil and water can be obtained according to Fig. 1a: Roil =
2.2.2.
LRo , Doil
Rw =
LRw . Dw
HETP calculation
In this paper, we use HETP to evaluate mass transfer of the rectification column. According to definition, HETP = Z/N, where Z is the height of the packed bed, and N is the theoretical plate number. Because the liquid organic phase has little solubility in water, we assume that water has slight impact on the relative volatility of the binary organic systems, therefore N in steam rectification can also be determined with the binary organic systems. To simplify the problem, we studied the rectification under total reflux, thus N can be obtained from Fenske equation: N=
log [(x1 /x2 )T (x2 /x1 )B ] log ˛m
where “T” indicates column top, “B” indicates column bottom, ˛m represents the average value of the relative volatility at the column top and bottom, x1 and x2 represents the concentration of the light and heavy component in each binary system respectively. The relative volatility (˛) at certain temperature can be calculated directly from the equation: ˛=
Ps1 Ps2
where Ps1 and Ps2 represents saturated vapor pressure of the light and heavy components in the binary system, respectively. Ps1 and Ps2 can be obtained from Antoine equation: log Ps = A −
B t+C
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Fig. 1 – (a) Flow chart of the steam rectification. (b) Diagram of the rectification column. where t represents temperature (◦ C), and the unit of Ps is mm Hg. A, B and C are parameters of each component, as is listed in Table 2.
3.
Results and discussion
3.1.
The hydrodynamics of steam rectification
The test system for hydrodynamics experiment was noctane–p-xylene. The factors affecting the pressure drop, which is predominant for hydrodynamics of a packed column, were investigated. When the column is operated at lower vapor velocity, the pressure drop of the packed bed changes linearly with vapor velocity, we call that operation scope as constant liquid holdup section. When vapor velocity increase to a certain point, the liquid stream from column top cannot flow down fluently and is partly loaded in the packed bed. As a consequence, the pressure drop increases more remarkable with the elevation of vapor velocity, and that point is usually named as loading point. If the vapor velocity is further enhanced to some point, the liquid stream cannot flow down and the pressure drop rise dramatically, we call such point as flooding point. The operation scope between loading and flooding point is denoted as loading section.
3.1.1.
total reflux, the steam rectification leaps from constant liquid holdup section directly to flooding point. Such phenomenon is similar to conventional rectification. As for pure water under total reflux, the flooding vapor velocity is much higher than that of steam rectification, and the pressure drop varies linearly with vapor velocity in a wide scope. We also measured the pressure drop of the dry packed bed with air, and the flow rate of the air was determined with a gas flowmeter. The pressure drop of the packed bed also changes linearly with gas (air) velocity. The pressure drop of water vapor is higher than that of air at the same vapor (gas) velocity, due to the existence of liquid water in the packed bed. This is a basic and common feature of the packed column.
3.1.2.
The effect of reflux ratio
The pressure drop increases with increasing reflux, as is revealed in Fig. 3. The vapor velocity is 0.06 m s−1 , locating in constant liquid holdup section. The reflux ratio of water and oil is equal. With increasing reflux, the liquid flow in the packed bed becomes more remarkable, which may block some passages for up-flowing vapor, causing higher resistance. To maintain the steady vapor velocity, more driving
The effect of vapor velocity
The impact of vapor velocity on pressure drop of the packed bed is shown in Fig. 2. The vapor velocity, based on whole cross area of the column, is superficial vapor velocity. When the steam rectification is operated under partial reflux (the ratio is 0.2 for both water and oil), there is an evident liquid loading section before the flooding point. When operated under
Table 2 – Parameters of the Antoine equation. Component
A
B
C
n-Octane p-Xylene 1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene
6.91868 6.99052 7.07436 7.04383
1351.990 1453.430 1569.622 1573.257
209.15 215.31 209.58 208.56
Fig. 2 – The impact of vapor velocity on pressure of the packed bed.
chemical engineering research and design 8 9 ( 2 0 1 1 ) 2560–2565
Fig. 3 – The impact of reflux ratio on pressure drop of the packed bed.
Fig. 5 – The pressure drop of steam rectification and conventional rectification (based on vapor velocity).
3.1.4.
Fig. 4 – The impact of oil/water ratio on pressure drop of the packed bed. force is needed. In rectification, the driving force for flowing vapor is the pressure drop, hence the pressure drop of the packed bed becomes higher.
3.1.3.
The comparison with conventional rectification
The comparison between steam rectification and conventional rectification, under total reflux, is shown in Fig. 5. The flooding vapor velocity of steam rectification is 2 times that of conventional rectification. In constant liquid holdup section, water has little impact on the pressure drop of steam rectification. The hydrodynamics of a packed column is closely related to rectification operation. When rectification is operated between loading and flooding points, both mass transfer and column capacity are desirable. The flooding vapor velocity is the maximum vapor velocity for design and operation of a rectification column. The oil flux represents the amount of organic vapor flowing through unit cross area of the column in unit time. The oil flux determines the practical capacity of a steam rectification column. Under total reflux, the oil flux of steam rectification at flooding point is lower than that of conventional rectification (Fig. 6). For a specified column, steam rectification can lower distillation temperature, but at the cost of capacity loss. For n-octane–p-xylene, the oil flux of steam rectification at flooding point is 75.9% that of conventional rectification.
The effect of water/oil ratio
If the pressure is fixed in steam rectification, water content in vapor phase is certain, therefore, regulating liquid water and oil in the packed bed is crucial for controlling steam rectification. When the vapor velocity is 0.10 m s−1 , close to the flooding point, the impact of liquid water on pressure drop is more remarkable than that of oil (Fig. 4). Although oil volume condensed at column top is 3 times that of water, and oil reflux is regulated in reverse ratio to water (Table 3), the pressure drop still enhances markedly with elevating water reflux. Thus water plays a predominant role near flooding point in affecting pressure drop of the packed bed. It is a unique feature of steam rectification. There is a strong possibility that water causes strong turbulence when propelled by speedy vapor, and blocks up the passages of flowing vapor.
Table 3 – The regulation of reflux ratio for oil and water (inverse ratio). Oil ∞ Water 0
5:1 1:5
4:1 1:4
3:1 1:3
2:1 1:2
1:1 1:1
1:2 2:1
1:3 3:1
1:4 4:1
1:5 5:1
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Fig. 6 – The pressure drop of steam rectification and conventional rectification (based on oil flux).
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Fig. 7 – The impact of vapor velocity on HETP of steam rectification.
3.2.
The mass transfer of steam rectification
3.2.1.
The effect of vapor velocity on HETP
With elevating vapor velocity, the dynamic liquid holdup increases and the liquid distribution becomes better, enhancing liquid–vapor mass transfer area. Moreover, liquid disturbance becomes more violent, thus the molecular kinetic energy increases and the resistance in liquid and vapor layers decreases. These factors are good for mass transfer in steam rectification. On the other hand, the liquid–vapor contact time decreases, which diminishes the chance of vapor–liquid mass transfer. In steam rectification, the HETP values of both test systems increases with elevating vapor velocity (Fig. 7), but they change at different rate. The difference in mass transfer behavior of the two test systems may originate from their different chemical properties. The viscosity of 1,3,5-trimethylbenzene–1,2,4-trimethylbenzene system is much higher than its counterpart, therefore liquid water flow plays a predominant role in improving the mass transfer at low vapor velocity. However, the content of water vapor in steam rectification of 1,3,5-trimethylbenzene–1,2,4trimethylbenzene system is also much higher than that of its counterpart, thus at high vapor velocity the resistance of water vapor on mass transfer becomes more evident.
3.2.2.
The effect of liquid water on HETP
The experimental results of 1,3,5-trimethylbenzene–1,2,4trimethylbenzene system show that the separation efficiency of steam rectification with total water reflux is much higher than that without water reflux (Fig. 8). It clearly indicates that liquid water in the packed bed is markedly beneficial to mass transfer, especially at lower vapor velocity. The unique phenomenon can be attributed to the different nature of water and oil: water is insoluble in oil, and the surface tension of water is much higher than oil. Our results agree well with those of other researchers. Hoffmann et al. (2005, 2006) and Repke et al. (2007) have reported that water may form droplets on the oil film or rivulets below the closed oil film, and no evident overflow of the two liquid phases takes place. Ottenbacher and Hasse (2007) also found that the two phases did either flow more or less independently or that the aqueous phase was covered by the organic phase. The existence of a second liquid phase during distillation in packed column does not inevitably lead to a reduction of the separation efficiency (Repke and Wozny, 2002; Siegert et al., 2000). Siegert (1999) concluded from their experiments that the mass transfer area of the organic
Fig. 8 – The impact of liquid water on the HETP of steam rectification (1,3,5-trimethylbenzene–1,2,4-trimethylbenzene).
phase remains either unchanged or even increases because of the formation of ripples.
3.2.3.
The comparison with conventional rectification
Steam rectification can effectively reduce the distillation temperature. For two test systems, the bottom temperature can be lowered from 130.7 and 166.4 ◦ C of conventional rectification to 90.4 and 92.6 ◦ C of steam rectification. Water turbulence can promote the flowing of oil films, and increase effective mass transfer area in steam rectification. However, the lower temperature of steam rectification is also a disadvantage to mass transfer. The two factors mutually affect the mass transfer behavior of steam rectification. For noctane–p-xylene, the HETP of steam rectification is close to that of conventional rectification (Fig. 9). Whereas for 1,3,5trimethylbenzene–1,2,4-trimethylbenzene the mass transfer of steam rectification is much better than that of conventional rectification (Fig. 10). The viscosity of 1,3,5-trimethylbenzene and 1,2,4-trimethylbenzene is greatly higher than that of noctane and p-xylene (Table 1), thus water turbulence plays a more important role in enhancing effective mass transfer area of steam rectification.
Fig. 9 – The HETP of steam rectification and conventional rectification (n-octane–p-xylene).
chemical engineering research and design 8 9 ( 2 0 1 1 ) 2560–2565
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References
Fig. 10 – The HETP of steam rectification and conventional rectification (1,3,5-trimethylbenzene–1,2,4-trimethylbenzene).
4.
Conclusion
Steam rectification, multi-stage saturated steam distillation, can effectively reduce distillation temperature. It is a promising method for separating natural substances. For n-octane–pxylene and 1,3,5-trimethylbenzene–1,2,4-trimethylbenzene, the bottom temperature can be lowered 40 ◦ C and 75 ◦ C, respectively, by steam rectification. The flooding vapor velocity of steam rectification is about 2 times that of conventional rectification. However, at flooding point, the oil flux in steam rectification is lower than in conventional rectification. At lower vapor velocity water has little impact on pressure drop of the packed bed, however, near flooding point water plays a predominant role. The HETP of the packed bed in steam rectification increases with the elevation of vapor velocity. From the HETP data, we can contrast the mass transfer rate of the two systems in conventional and steam rectification. For noctane–p-xylene, the mass transfer rate in steam rectification is close to that in conventional rectification. Whereas for 1,3,5trimethylbenzene–1,2,4-trimethylbenzene, the mass transfer rate in steam rectification is much higher than in conventional rectification. Liquid water in packed bed may enhance mass transfer, especially for materials with higher viscosity. In a word, steam rectification can separate substances insoluble in water, at low temperature and with good mass transfer.
Chemat, F., Lucchesi, M.E., Smadja, J., Favretto, L., Colnaghi, G., Visinoni, F., 2006. Microwave accelerated steam distillation of essential oil from lavender: a rapid, clean and environmentally friendly approach. Anal. Chim. Acta 555, 157–160. Harrison, M.E., 1990. Consider three-phase distillation in packed columns. Chem. Eng. Prog. 86 (11), 80–85. Hoffmann, A., Ausner, I., Repke, J.-U., Wozny, G., 2005. Fluid dynamics in multiphase distillation processes in packed towers. Comput. Chem. Eng. 29, 1433–1437. Hoffmann, A., Ausner, I., Repke, J.-U., Wozny, G., 2006. Detailed investigation of multiphase (vapor–liquid and vapor–liquid–liquid) flow behavior on inclined plates. Chem. Eng. Res. Des. 84 (A2), 147–154. Kishore, W.M., Keshavan, N., Pangarkar, V.G., Taskar, A., 1997. Recovery of dissolved essential oils from condensate waters of Basil and Mentha arvensis distillation. J. Chem. Technol. Biotechnol. 69, 362–366. Masango, P., 2005. Cleaner production of essential oils by steam distillation. J. Cleaner Prod. 13, 833–839. Ottenbacher, M., Hasse, H., 2007. Continuous three-phase distillation: a process for separating thermally unstable substances. Chem. Eng. Res. Des. 85 (A1), 144–148. Perineau, F., Ganou, L., Vilarem, G., 1992. Studying production of lovage essential oils in a hydrodistillation pilot unit equipped with a cohobation system. J. Chem. Technol. Biotechnol. 53, 165–171. Qian, Q., Wang, H., Bai, P., 2003. Steam rectification. Tianjin Chem. Ind. (in Chin.). 17 (1), 25–27. Reverchon, E., Seatore, F., 1992. Isolation of rosemary oil: comparison between hydrodistillation and supercritical CO2 extraction. Flavour. Fragr. J. 7, 227–230. Repke, J.-U., Wozny, G., 2002. Experimental investigation of three-phase distillation in a packed column. Chem. Eng. Technol. 25 (5), 513–519. Repke, J.-U., Ausner, I., Paschke, S., Hoffmann, A., Wozny, G., 2007. On the track to understanding three phases in one tower. Chem. Eng. Res. Des. 85 (A1), 50–58. Schoenborn, E.M., Koffolt, J.H., Withrow, J.R., 1941. Rectification in the presence of an insoluble component. Trans. Am. Inst. Chem. Eng. 37, 997–1021. Siegert, M., 1999. Dreiphasenrektifikation in Packungskolonnen. VDI-Verlag, Düsseldorf. Siegert, M., Stichlmair, J., Repke, J.-U., Wozny, G., 2000. Heterogeneous azeotropic distillation in packed columns: experimental results. Chem. Eng. Technol. 23 (12), 1047–1050.
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Effects of water on steam rectification in a packed column Qingli Qian b,∗ , Hongxing Wang c , Peng Bai a,∗ , Guoqing Yuan b a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Beijing National Laboratory for Molecular Sciences (BNLMS), Laboratory of new materials, Institute of chemistry, Chinese Academy of Sciences, Beijing 100190, China c College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, China b
a b s t r a c t The effects of water on steam rectification, i.e., multi-stage saturated steam distillation, were investigated in a packed column. N-octane–p-xylene and 1,3,5-trimethylbenzene–1,2,4-trimethylbenzene were used as test systems. Both binary systems are nearly ideal systems and insoluble in water, thus the effects of water in steam rectification can be clearly and definitely revealed. Such unpolar organic liquid is named as “oil”. The water/oil at column top can be separated and refluxed at different ratio. Compared with conventional rectification, there are some peculiar phenomena in steam rectification. Water greatly enhances the flooding vapor velocity of the rectification, in addition, water plays a predominant role in pressure drop of the packed bed near flooding point. It is clear that liquid water in the packed bed can promote mass transfer of steam rectification, especially for materials with higher viscosity. In a word, steam rectification can be operated at low temperature with good mass transfer. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Steam rectification; Packed bed; Mass transfer; Multiphase flow
1.
Introduction
Steam distillation has been widely used for a long time in industry, especially for the extraction and purification of natural products, such as essential oils, because most of them are thermally unstable and insoluble in water. As a carrier gas, steam can lower the distillation temperature. Because of the torrential characteristics, it can also reduce residence time of natural products in distillation column, preventing decomposition or polymerization. According to Gibbs phase law, the steam distillation can be sorted into two modes: superheated steam distillation and saturated steam distillation. Currently, the superheated steam distillation is extensively applied in industry. The saturated steam distillation can be operated below 100 ◦ C under atmospheric pressure, and is more desirable for separating thermally unstable substances. However, the saturated steam distillation is mainly used in single equilibrium stage distillation for crude extraction of essential oils from biomass (Kishore et al., 1997; Perineau et al., 1992; Reverchon and Seatore, 1992). To increase the yield of extraction, Chemat (2006) and Masango (2005) designed new
∗
processes that force the saturated steam through a packed bed of the raw biomass. Multi-stage saturated steam distillation was defined as “Steam rectification” by Qian et al. (2003). For the convenience of discussion, we call rectification of binary organic systems as conventional rectification. According to Gibbs phase rule, steam rectification can be operated at lower temperature than conventional rectification. Ottenbacher and Hasse (2007) investigated the steam rectification as a process for separating thermally unstable substances, and deemed that it is a promising alternative or addition to state of the art processes like vacuum distillation. Steam rectification, involving two liquid phases and a vapor phase, is three-phase distillation. Actually, three-phase distillation has been widely investigated by researchers (Harrison, 1990; Repke and Wozny, 2002; Schoenborn et al., 1941; Siegert et al., 2000). However, the effects of water on steam rectification have not been thoroughly investigated. In this paper, we investigate the steam rectification with two test systems: n-octane–p-xylene and 1,3,5Both binary trimethylbenzene–1,2,4-trimethylbenzene.
Corresponding authors. Tel.: +86 10 62560247; fax: +86 10 62559373. E-mail addresses: [email protected] (Q. Qian), [email protected] (P. Bai). Received 6 October 2010; Received in revised form 19 May 2011; Accepted 18 June 2011 0263-8762/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2011.06.004
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chemical engineering research and design 8 9 ( 2 0 1 1 ) 2560–2565
Table 1 – The properties of the chemicals in the test systems. Chemical n-Octane p-Xylene 1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene Water
Tbp [◦ C] 125.7 138.4 164.7 169.4 100.0
Density [g cm−3 (20 ◦ C)] 0.70 0.86 0.87 0.88 1.00
organic systems are nearly ideal systems and insoluble in water. Such unpolar organic liquid is named as “oil”. In order to simplify the operation and reduce the impact of other factors than water, the column was specially designed, and water/oil at column top can be separated and refluxed at different ratio. Therefore the effects of water on steam rectification can be more clearly and definitely evaluated. The difference between steam rectification and conventional rectification are also demonstrated by experiments.
2.
Material and methods
2.1.
Material
Two test systems, n-octane–p-xylene and 1,3,5trimethylbenzene–1,2,4-trimethylbenzene were tested in steam and conventional rectification. Both binary organic systems are nearly ideal system and insoluble in water. The chemicals in the test systems are slightly toxic, inexpensive, and easily available in large amounts. The properties of the chemicals are displayed in Table 1.
2.2.
Methods
2.2.1.
The rectification column and experimental procedure
The flow chart of steam rectification in our experiment is revealed in Fig. 1a. The steam rectification was carried out in a glass column, the diagram of which is shown in Fig. 1b. The name and function of the parts on the column are as follows: (1) heater, (2) zeolite, (3) column kettle, (4) bottom sampling tap, (5) condenser, (6) packed bed, (7) thermal barrier, (8) vapor velocity flowmeter (for calculation), (9) top sampling tap, (10) oil/water separator, (11) reflux regulator, (12) liquid velocity flowmeter (for calculation), (13) manometer, (14) voltage regthermometer. The ulator, (15) oil/water interface regulator, column was packed with Ф3 × 3-net ring of stainless steel (random packing area: 2275 m2 m−3 , mesh number of the steel net: 100). The inner diameter of the column is 0.04 m, and the effective packing height is 0.8 m. -Net ring, also known as Dixon ring, is a highly efficient packing of small particles made of metal mesh and with same diameter and height. Net ring is used mainly for laboratory and low volume, high purity product separation process. For better reflex distribution, a cone-shaped stainless net was tightly placed on the top of the packed bed. In order to obtain continuous and steady state, there was no feed during the rectification, and the product collected at column top was returned to column bottom. The velocities of vapor and liquid in the column were calculated according to the liquid volume determined by the flowmeters in specified time. There was no liquid reflux during vapor velocity measurement, and the calculation of vapor velocity (Uv ) is based on the ideal gas equation of state. F factor (Fv ) is the product of vapor velocity and square root of vapor density (v ) in
Hevap [kJ mol−1 ]
Viscosity [mPa s (20 ◦ C)]
34.10 35.98 39.04 39.25 40.68
0.546 0.644 1.154 0.894 1.002
Surface tension [mN m−1 (20 ◦ C)] 21.62 28.54 28.00 29.71 72.88
√ the column, i.e., F = U . In our experiment the variation of vapor density is minor for specified test system, thus Fv can be obtained by simply adjusting Uv with a constant (square root of average vapor density (kg m−3 )0.5 ). The constants can be calculated from the ideal gas equation of state and the partial pressure of each component. The relationship between Fv and Uv has been supplied in related figures. The pressure drop of the packed bed was obtained by U-type manometer, and the tinct n-heptane was used as measuring medium. The samples were collected via taps at the top and bottom of the packed bed, and analyzed with gas chromatography. For comparison, conventional rectification was also conducted in the packed column. The operation of conventional rectification is similar to that of steam rectification, but the oil/water separator is no longer needed. The relfux can be manipulated with automatically controlled reflux regulator, through which relfux and output proceed alternately, and reflux ratio is the ratio of time for reflux and output. In addition, water and oil can be regulated respectively after oil/water separator. The reflux ratio (R) of oil and water can be obtained according to Fig. 1a: Roil =
2.2.2.
LRo , Doil
Rw =
LRw . Dw
HETP calculation
In this paper, we use HETP to evaluate mass transfer of the rectification column. According to definition, HETP = Z/N, where Z is the height of the packed bed, and N is the theoretical plate number. Because the liquid organic phase has little solubility in water, we assume that water has slight impact on the relative volatility of the binary organic systems, therefore N in steam rectification can also be determined with the binary organic systems. To simplify the problem, we studied the rectification under total reflux, thus N can be obtained from Fenske equation: N=
log [(x1 /x2 )T (x2 /x1 )B ] log ˛m
where “T” indicates column top, “B” indicates column bottom, ˛m represents the average value of the relative volatility at the column top and bottom, x1 and x2 represents the concentration of the light and heavy component in each binary system respectively. The relative volatility (˛) at certain temperature can be calculated directly from the equation: ˛=
Ps1 Ps2
where Ps1 and Ps2 represents saturated vapor pressure of the light and heavy components in the binary system, respectively. Ps1 and Ps2 can be obtained from Antoine equation: log Ps = A −
B t+C
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Fig. 1 – (a) Flow chart of the steam rectification. (b) Diagram of the rectification column. where t represents temperature (◦ C), and the unit of Ps is mm Hg. A, B and C are parameters of each component, as is listed in Table 2.
3.
Results and discussion
3.1.
The hydrodynamics of steam rectification
The test system for hydrodynamics experiment was noctane–p-xylene. The factors affecting the pressure drop, which is predominant for hydrodynamics of a packed column, were investigated. When the column is operated at lower vapor velocity, the pressure drop of the packed bed changes linearly with vapor velocity, we call that operation scope as constant liquid holdup section. When vapor velocity increase to a certain point, the liquid stream from column top cannot flow down fluently and is partly loaded in the packed bed. As a consequence, the pressure drop increases more remarkable with the elevation of vapor velocity, and that point is usually named as loading point. If the vapor velocity is further enhanced to some point, the liquid stream cannot flow down and the pressure drop rise dramatically, we call such point as flooding point. The operation scope between loading and flooding point is denoted as loading section.
3.1.1.
total reflux, the steam rectification leaps from constant liquid holdup section directly to flooding point. Such phenomenon is similar to conventional rectification. As for pure water under total reflux, the flooding vapor velocity is much higher than that of steam rectification, and the pressure drop varies linearly with vapor velocity in a wide scope. We also measured the pressure drop of the dry packed bed with air, and the flow rate of the air was determined with a gas flowmeter. The pressure drop of the packed bed also changes linearly with gas (air) velocity. The pressure drop of water vapor is higher than that of air at the same vapor (gas) velocity, due to the existence of liquid water in the packed bed. This is a basic and common feature of the packed column.
3.1.2.
The effect of reflux ratio
The pressure drop increases with increasing reflux, as is revealed in Fig. 3. The vapor velocity is 0.06 m s−1 , locating in constant liquid holdup section. The reflux ratio of water and oil is equal. With increasing reflux, the liquid flow in the packed bed becomes more remarkable, which may block some passages for up-flowing vapor, causing higher resistance. To maintain the steady vapor velocity, more driving
The effect of vapor velocity
The impact of vapor velocity on pressure drop of the packed bed is shown in Fig. 2. The vapor velocity, based on whole cross area of the column, is superficial vapor velocity. When the steam rectification is operated under partial reflux (the ratio is 0.2 for both water and oil), there is an evident liquid loading section before the flooding point. When operated under
Table 2 – Parameters of the Antoine equation. Component
A
B
C
n-Octane p-Xylene 1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene
6.91868 6.99052 7.07436 7.04383
1351.990 1453.430 1569.622 1573.257
209.15 215.31 209.58 208.56
Fig. 2 – The impact of vapor velocity on pressure of the packed bed.
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Fig. 3 – The impact of reflux ratio on pressure drop of the packed bed.
Fig. 5 – The pressure drop of steam rectification and conventional rectification (based on vapor velocity).
3.1.4.
Fig. 4 – The impact of oil/water ratio on pressure drop of the packed bed. force is needed. In rectification, the driving force for flowing vapor is the pressure drop, hence the pressure drop of the packed bed becomes higher.
3.1.3.
The comparison with conventional rectification
The comparison between steam rectification and conventional rectification, under total reflux, is shown in Fig. 5. The flooding vapor velocity of steam rectification is 2 times that of conventional rectification. In constant liquid holdup section, water has little impact on the pressure drop of steam rectification. The hydrodynamics of a packed column is closely related to rectification operation. When rectification is operated between loading and flooding points, both mass transfer and column capacity are desirable. The flooding vapor velocity is the maximum vapor velocity for design and operation of a rectification column. The oil flux represents the amount of organic vapor flowing through unit cross area of the column in unit time. The oil flux determines the practical capacity of a steam rectification column. Under total reflux, the oil flux of steam rectification at flooding point is lower than that of conventional rectification (Fig. 6). For a specified column, steam rectification can lower distillation temperature, but at the cost of capacity loss. For n-octane–p-xylene, the oil flux of steam rectification at flooding point is 75.9% that of conventional rectification.
The effect of water/oil ratio
If the pressure is fixed in steam rectification, water content in vapor phase is certain, therefore, regulating liquid water and oil in the packed bed is crucial for controlling steam rectification. When the vapor velocity is 0.10 m s−1 , close to the flooding point, the impact of liquid water on pressure drop is more remarkable than that of oil (Fig. 4). Although oil volume condensed at column top is 3 times that of water, and oil reflux is regulated in reverse ratio to water (Table 3), the pressure drop still enhances markedly with elevating water reflux. Thus water plays a predominant role near flooding point in affecting pressure drop of the packed bed. It is a unique feature of steam rectification. There is a strong possibility that water causes strong turbulence when propelled by speedy vapor, and blocks up the passages of flowing vapor.
Table 3 – The regulation of reflux ratio for oil and water (inverse ratio). Oil ∞ Water 0
5:1 1:5
4:1 1:4
3:1 1:3
2:1 1:2
1:1 1:1
1:2 2:1
1:3 3:1
1:4 4:1
1:5 5:1
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Fig. 6 – The pressure drop of steam rectification and conventional rectification (based on oil flux).
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Fig. 7 – The impact of vapor velocity on HETP of steam rectification.
3.2.
The mass transfer of steam rectification
3.2.1.
The effect of vapor velocity on HETP
With elevating vapor velocity, the dynamic liquid holdup increases and the liquid distribution becomes better, enhancing liquid–vapor mass transfer area. Moreover, liquid disturbance becomes more violent, thus the molecular kinetic energy increases and the resistance in liquid and vapor layers decreases. These factors are good for mass transfer in steam rectification. On the other hand, the liquid–vapor contact time decreases, which diminishes the chance of vapor–liquid mass transfer. In steam rectification, the HETP values of both test systems increases with elevating vapor velocity (Fig. 7), but they change at different rate. The difference in mass transfer behavior of the two test systems may originate from their different chemical properties. The viscosity of 1,3,5-trimethylbenzene–1,2,4-trimethylbenzene system is much higher than its counterpart, therefore liquid water flow plays a predominant role in improving the mass transfer at low vapor velocity. However, the content of water vapor in steam rectification of 1,3,5-trimethylbenzene–1,2,4trimethylbenzene system is also much higher than that of its counterpart, thus at high vapor velocity the resistance of water vapor on mass transfer becomes more evident.
3.2.2.
The effect of liquid water on HETP
The experimental results of 1,3,5-trimethylbenzene–1,2,4trimethylbenzene system show that the separation efficiency of steam rectification with total water reflux is much higher than that without water reflux (Fig. 8). It clearly indicates that liquid water in the packed bed is markedly beneficial to mass transfer, especially at lower vapor velocity. The unique phenomenon can be attributed to the different nature of water and oil: water is insoluble in oil, and the surface tension of water is much higher than oil. Our results agree well with those of other researchers. Hoffmann et al. (2005, 2006) and Repke et al. (2007) have reported that water may form droplets on the oil film or rivulets below the closed oil film, and no evident overflow of the two liquid phases takes place. Ottenbacher and Hasse (2007) also found that the two phases did either flow more or less independently or that the aqueous phase was covered by the organic phase. The existence of a second liquid phase during distillation in packed column does not inevitably lead to a reduction of the separation efficiency (Repke and Wozny, 2002; Siegert et al., 2000). Siegert (1999) concluded from their experiments that the mass transfer area of the organic
Fig. 8 – The impact of liquid water on the HETP of steam rectification (1,3,5-trimethylbenzene–1,2,4-trimethylbenzene).
phase remains either unchanged or even increases because of the formation of ripples.
3.2.3.
The comparison with conventional rectification
Steam rectification can effectively reduce the distillation temperature. For two test systems, the bottom temperature can be lowered from 130.7 and 166.4 ◦ C of conventional rectification to 90.4 and 92.6 ◦ C of steam rectification. Water turbulence can promote the flowing of oil films, and increase effective mass transfer area in steam rectification. However, the lower temperature of steam rectification is also a disadvantage to mass transfer. The two factors mutually affect the mass transfer behavior of steam rectification. For noctane–p-xylene, the HETP of steam rectification is close to that of conventional rectification (Fig. 9). Whereas for 1,3,5trimethylbenzene–1,2,4-trimethylbenzene the mass transfer of steam rectification is much better than that of conventional rectification (Fig. 10). The viscosity of 1,3,5-trimethylbenzene and 1,2,4-trimethylbenzene is greatly higher than that of noctane and p-xylene (Table 1), thus water turbulence plays a more important role in enhancing effective mass transfer area of steam rectification.
Fig. 9 – The HETP of steam rectification and conventional rectification (n-octane–p-xylene).
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References
Fig. 10 – The HETP of steam rectification and conventional rectification (1,3,5-trimethylbenzene–1,2,4-trimethylbenzene).
4.
Conclusion
Steam rectification, multi-stage saturated steam distillation, can effectively reduce distillation temperature. It is a promising method for separating natural substances. For n-octane–pxylene and 1,3,5-trimethylbenzene–1,2,4-trimethylbenzene, the bottom temperature can be lowered 40 ◦ C and 75 ◦ C, respectively, by steam rectification. The flooding vapor velocity of steam rectification is about 2 times that of conventional rectification. However, at flooding point, the oil flux in steam rectification is lower than in conventional rectification. At lower vapor velocity water has little impact on pressure drop of the packed bed, however, near flooding point water plays a predominant role. The HETP of the packed bed in steam rectification increases with the elevation of vapor velocity. From the HETP data, we can contrast the mass transfer rate of the two systems in conventional and steam rectification. For noctane–p-xylene, the mass transfer rate in steam rectification is close to that in conventional rectification. Whereas for 1,3,5trimethylbenzene–1,2,4-trimethylbenzene, the mass transfer rate in steam rectification is much higher than in conventional rectification. Liquid water in packed bed may enhance mass transfer, especially for materials with higher viscosity. In a word, steam rectification can separate substances insoluble in water, at low temperature and with good mass transfer.
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