Energy & Fuels 1999, 13, 459-464
459
Dewatering of Coal Tar by Heat Treatment Sei-Hun Yun and Chul Kim Department of Chemical Engineering, Ajou University, Suwon, Republic of Korea
Yeong-Cheol Kim Advanced Chemical Technology Division, Korea Research Institute of Chemical Technology, Taejon, Republic of Korea
Euy Soo Lee* Department of Chemical Engineering, Dongguk University, 3 Ga 26, Phil-Dong, Chung-Ku, Seoul, 100-715, Republic of Korea Received July 14, 1998. Revised Manuscript Received December 22, 1998
Coal tar was heat-treated at high pressures to separate water dispersed in it. Various samples of coal tar were bottle-tested at 383, 403, 423, and 443 K in dewatering closed systems. For fresh coal tar from coke ovens and for water-added coal tar, dewatering behaviors were studied under both saturated pressure and elevated pressure by adding noncondensable inert gas over saturated gas. The experimental data of the destruction and growth of aqueous dispersed phase were correlated with demulsification models by adjusting two model parameters. Coal tar tends to be easily dewatered when it is heat-treated under saturated pressure without preaddition of water. This work strongly suggests the existence of relationship between the phenomena of dewatering and boiling in coal tar.
Introduction Coal tar made from a coke oven is a very useful resource in the production of aromatic chemicals.1,2 Coal tar is obtained by quenching the coke oven gas by spraying cold water, while coke is made within the coke oven. Thus, the crude coal tar contains a lot of water, and its moisture content is varied according to the coke ovens themselves and their operating conditions.3 If the coke oven is old, heat energy is, in general, supplied in excess due to a low heating efficiency, which, in turn, leads to an increase of the amount of carry over. In this case, not only the solid content but also the water content of coal tar increases due to emulsification with scrubbing water.4-6 Various ionic compounds, inorganic salts, and hydrophilic organic compounds are dissolved in the aqueous phase dispersed in coal tar. Among these chemicals, in particular, compounds such as ammonia, chlorides, and cyanides raise severe corrosion problems in the downstream process of coal tar, especially in the * To whom all correspondence should be addressed. E-mail: eslee@ cakra.dongguk.ac.kr. (1) Austin, G. T. Shreve’s Chemical Process Industries, 5th ed.; McGraw-Hill: New York, 1984; Chapter 6. (2) McNeil, D. Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; John Wiley and Sons: New York; Vol. 22. (3) Aramaki, T.; Edakuni, T.; Mizuno, T.; Katahira H. Aromatics (Jpn.) 1989, 3/4, 10-17. (4) Kohata, Y.; Kitano, T.; Kurata, S. Chem. Econ. Eng. Rev. 1976, 8 (12), 144-146. (5) Kremer, H. A.; Cukier, S. Reduction of Ash Content of electrode binder Pitches by Dewatering. Presented in the International Coal Conference in Halifax, June 3, 1981, 1-8. (6) Knapton, J. K. Centrifugal Treatment of tar. TIS Report No. 0494, Category B; Tar Industries Services: Derbyshire, 1961; pp 1-12. (7) Muller, J. M. Method for Desalting and Dewatering Crude Coal Tar. U.S. Patent 3 151 055, 1964.
vacuum distillation unit.7-10 On the other hand, separation processes for the end products from coal tar become very complicated and energy consumption in excess is inevitable to stabilize the qualities of products and to meet customers’ strict specifications. Therefore, it is desirable to separate the aqueous phase in the upstream process as soon as possible from coal tar in which a large portion of corrosive and unnecessary chemicals are contained.11 Few methods have been studied on the dewatering from petroleum oil applicable in the stages of mining, transportation, storage, and pretreatment for petrochemical processing.12-16 Dispersed water can be removed by distillation and/or by a centrifugal field of high revolution. However, their operating costs are too high. The commercially adopted dewatering methods can be classified into two categories: simple heat treatment at high temperature and destabilizing the emulsion by adding demulsifying agents at around 360 K. The former method induces instability and the growth of droplets through the enhancement of droplet mobility (8) Coal Tar and Coal Tar Chemicals. SRI Report, 1976, Vol. 101. (9) Fear, R. C. CPC letter; Coal Processing Consultants Ltd., Jan. 1, 1983. (10) Lagassie, P. Light Metals 1992, 593. (11) Vshivtsev, V. G.; Smol’nikov, G. V.; Orlova, V. T. Coke Chem. 1991, 4, 36-40. (12) Dow, D. B. Oil Field Emulsions. U.S. Department of Commerce, Bulletin 250, 1926. (13) Shea, G. B. Practices and methods of Preventing and Treating Crude-Oil Emulsions. U.S. Department of Interior, Bureau of Mines, Bulletin 417, 1939. (14) Uran, Natl. Pet. News 1929, 51. (15) Steinhauff, F. Petroleum (London) 1962, 25, 294 and 335. (16) Bansbach, P. L. Oil Gas J. 1970, 52, 87.
10.1021/ef980155g CCC: $18.00 © 1999 American Chemical Society Published on Web 02/09/1999
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Figure 1. Schematic diagram of the pressure vessel.
and the change of physical properties by exposing the dispersed system to high temperature. However, this method is widely adopted because the process is very simple and the stable operating region is broad compared to that of the latter. As far as coal tar is concerned, three kinds of dewatering processes are commercialized: distillation,17 decanting,6 and centrifugation.6 Various demulsifying agents to dewater coal tar have been studied and commercialized.18-20 The dewatering behavior of coal tar was observed to vary sensitively with the origin and condition of coal tar production, in authors’ pretests for this study. Therefore, to obtain meaningful engineering data, it is necessary to understand the individual dewatering characteristics for many coal tar samples in addition to the general features. In this paper, for coal tar samples possessing typical properties found in those from wornout coke ovens, the experimental dewatering characteristics are summarized at various elevated temperatures under saturation pressure or pressurized conditions over saturated pressures, to obtain the general features in the dewatering of coal tars. The dewatering process of coal tar is divided into two steps: the growth to destruction of dispersed droplets by demulsification and macroscopic countercurrent transport of the aqueous phase and organic coal tar phase. From a rate-controlling viewpoint, the former demulsifying step is more significant.21 Demulsification occurs by successive and/or simultaneous steps of flocculation, film drainage, coalescence, and creaming.21,22 The physical chemistry of flocculation and coalescence can be understood based on collision models, and power (17) Frank, H. G.; Collin, C. Steinkohlentter: Chemie, Technologie, and Verwendung; Springer-Verlag: Berlin, 1968; pp 35-36. (18) Korobchanskii, V. I.; Akimova, L. N.; Pugch, V. G.; Prusova, D. A. Coke Chem. 1991, 5, 32-34. (19) Gnilokvas, O. P.; Chistova, G. Yu.; Butakova, A. V.; Perovskii, V. S.; Medvedev, N. G.; Anikin, G. Ya.; Andreev, L. M. Coke Chem. 1987, 5, 46-51. (20) Pavilcius, A. M.; Lindenberger, W. H. Method for Dewatering Coke Tar-Water Mixtures. U.S. Patent 4 139 451, 1979. (21) Lissant, K. J. Demulsification-Industrial Applications. In Surfactant Science Series; Marcel Dekker: New York, 1983; Chapter 1.
law types of correlations have been used to reproduce individual isothermal dewatering curves.23 In this paper, the applicability of self-consistent demulsification models to dewatering of coal tar was examined for the experimental data from bottle tests at relatively low temperatures and the model parameters were determined, which can be used in the design of dewatering processes and equipment. Experimental Section 1. Materials. Raw coal tar samples were taken downstream of commercial coke oven plants. Three different kinds of coal tar samples were used in this study from three different coke ovens: coal tar A, coal tar B, and coal tar C. Among these, coal tar C is a sample partly dewatered by the dehydrating unit of the decanter. The moisture content is known to vary, by weight, from 11.0% to 17.0% for coal tar A, from 16.0% to 19.0% for coal tar B, and 2.0% for coal tar C. The sulfur content also varies from 0.5 to 0.8 wt %, and the average sulfur content lies in the range of about 0.7%. The average viscosity of the coal tar is about 30 cp at 333 K. To quantify the moisture content in coal tar samples, an extra-pure grade of xylene (99% up) from Oriental Chemical Industries, Ltd. was used. The nitrogen used in the pressurization experiments was of carrier grade for gas chromatography. 2. Dehydrating Vessels. Capped tubes were used for dewatering experiments in a closed system at a relatively low pressure. The tubes are made of Pyrex glass, and the caps were made of polypropylene with a modified polystyrene seal. The internal volume of the tube was about 85 mL. The tubes inserted into the tube rack were immersed in an oil bath, where the temperature is precisely controlled within a deviation of 0.5 K around the set point. For dewatering experiments at high pressure over the limit of the capped tubes, another pressure vessel was used made of stainless steel 316, as shown in Figure 1. A J-type thermocouple was inserted into the vessel to measure the internal temperature, and a pressure gauge was attached for measurement of the system pressure. The temperature of the system (22) Krawczyk, M. A.; Wasan, D. T.; Shetty, C. S. Ind. Eng. Chem. Res. 1991, 30 (2), 367-375. (23) Spielman, L. A. Viscous Interactions in Brownian Coagulation. J. Colloid Interface Sci. 1970, 33, 4.
Dewatering of Coal by Heat Treatment
Figure 2. Dewatering of coal tar sample A-1 containing 16.7 wt % of initial water. was able to be controlled accurately with a maximum deviation of 1 K from the set points with the inserted thermocouple. The internal volume of the vessel was about 500 mL: 64 mm i.d. × 150 mm height. As shown in Figure 1, 13 mm of the middle part was made of Pyrex glass to monitor the dewatering process in the vicinity of macrointerface between the gas and condensed phases. Nitrogen gas was supplied from the bomb to pressurize in excess over the saturation pressure exerted at the experimental temperature through the pressure regulating valve and needle valve, and the system pressure was relieved by the valve attached in the vent line. The upper flanges were disassembled at every sampling time, and the samples were taken from the opened vessel. 3. Dehydration Test and Analysis of Moisture Content. Bottle tests in capped tubes were carried out at the saturation pressures corresponding to the experimental temperatures of 383, 403, and 423 K. Sixty milliliters of coal tar A was inserted in the capped tube and placed in an oil bath of constant temperature. The tubes were opened for sampling after heat treatment, and the changes of the moisture content in the coal tar samples were analyzed. The separated, upper aqueous layer was carefully removed by pipet, and the samples for analysis were taken. Special care was made to minimize the opening time of the caps, because the dewatering phenomenon may be affected by the volatilization of light components in coal tar. The upper water layer is preferentially removed, after cooling below 343 K. The degree of dewatering was calculated by analysis of the moisture content for coal tar taken at sampling time at each experimental temperature. The macroscopic phenomena of creaming and countercurrent transport of separated layers were not observable in the glass tubes due to the inherent poor transparency of the coal tar samples. As the maximum allowable temperature by the capped glass tube test was 433 K, the pressure vessel was used to perform the dewatering experiments at 443 K. About 400 mL of coal tar sample was placed inside the pressure vessel shown in Figure 1, and then internal air was removed by a vacuum pump. The experimental time was set to zero when the internal temperature reached the set point. At the predetermined sampling time, the vessel was opened and the sample withdrawn for analysis. As in the case of the tube tests, the aqueous upper layer was removed by pipet with care, then the moisture content was analyzed in the coal tar phase. The pressure vessel was evacuated, cleaned, and then refilled with the same coal tar to repeat the experiments at different dewatering conditions. This experimental procedure was repeated again for various coal tar samples with different histories. Thermal treatments above the saturation pressure were carried out within the pressure vessel. The vessel was filled
Energy & Fuels, Vol. 13, No. 2, 1999 461
Figure 3. Dewatering of coal tar sample A-2 containing 15.9 wt % of initial water.
Figure 4. Dewatering of coal tar sample A-3 containing 15.8 wt % of initial water. with coal tar, and the temperature of the system was raised to the set point. Then noncondensable nitrogen gas was finally added to pressurize over the saturation pressure at this temperature. To measure the moisture content of coal tar, azeotropic distillations were carried out with xylene added to the samples.24 Fractional dewatering χwater, defined below, was calculated by the change of the moisture content in coal tar sample before and after heat treatment
χwater ≡
i f Qwater - Qwater i Qwater
(1)
where Qiwater and Qfwater denote the quantities of water in coal tar samples before and after heat treatment, respectively.
Results 1. Effects of Temperature. The experimental results are shown in Figures 2-5 for the bottle tests of coal tar A. The points in the graphs are the experimental results at the specified conditions, and the lines are the correlations of the experimental observations with the demulsification models. As shown in Figures 2-5, (24) ASTM D95-74, Standard Test Method for Water in Petroleium Products and Bituminous Materials by Distillation, 1981.
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Figure 5. Dewatering of coal tar sample A-4 containing 13.6 wt % of initial water.
Figure 7. Dewatering of coal tar B at 423 K at and over the saturation pressure.
Table 1. Composition of Coal Tar A Samples coal tar samples (wt %) components
A-1
A-2
A-3
A-4
B
C
moisture quinoline insoluble ash
16.7 3.0 0.3
15.9 2.5 0.2
15.8 2.9 0.3
16.3 3.1 0.2
18.0 2.8 0.2
2.0 2.0 0.02
Figure 8. Dewatering of coal tar B at 443 K containing 18.0 wt % of initial water.
Figure 6. Dewatering of coal tar samples under the saturation pressure at 443 K.
four different samples of coal tar A, i.e., A-1, A-2, A-3, and A-4, were tested that were produced at the same coke oven but at different operating conditions. In Table 1, the typical moisture, quinoline insolubles, and ash contents for each sample are listed. Figure 6 shows the dewatering experiments for coal tar A, B, and C at 443 K and under saturation pressure, carried out in the pressure vessel illustrated in Figure 1. The moisture content of raw coal tar samples studied in this work was analyzed by the method described above, which gives 12.0 wt % for coal tar A, 18.0 wt % for coal tar B, and 2.0 wt % for coal tar C, respectively. In Figure 6, the solid lines are the simple connections of the successive experimental points and the broken lines are the average values of the experimental points belonging to the same experimental group. As shown in Figure 6, fractional dewatering of all of the coal tar samples was observed to reach its equilibrium value very quickly. In contrast to the experimental results from the bottle tests
in capped tubes, experimental errors were not negligible. This is inferred from the experimental conditions, such as high pressure, and the sampling step, which is tentatively disturbing the experimental conditions. In Figures 7-9, fractional dewatering was reached very close to the equilibrium, as in Figure 6. 2. Effects of Pressurization and Preaddition of Extra Water. The final values of dewatering are shown in Figure 7 for coal tar A with respect to the additional pressure above saturated. All the experimental points were obtained with the pressure vessel in the subcooled states at the elevated pressure by the addition of noncondensable nitrogen gas over saturation pressure at 423 K. Comparative dewatering behaviors of coal tar B at 443 K are shown in Figure 8 for the variations of system pressure and/or initial content of water before heat treatment. The difference in dewatering trends of coal tar C at 403 K is shown in Figure 9, with the changes of the system pressure and/or initial content of water. The solid lines in Figures 8 and 9 are the connections of successive experimental points belonging to the same experimental group, and the broken lines are their average values. Discussion The rate of dewatering on a macroscopic scale can be assumed to be determined by the rate of demulsification
Dewatering of Coal by Heat Treatment
Energy & Fuels, Vol. 13, No. 2, 1999 463 Table 2. Adjustable Parameters in Demulsification Models Used To Correlate the Dewatering Data of Coal Tar A model I samples
A1 (poise h K-1)
A-1 A-2 A-3 A-4
2.662 × 108 1.250 × 1010 1.438 × 1012 1.346 × 1016
model II A2 (K)
B1 (poise h K-1)
B2 (K)
9.074 × 104 1.032 × 105 1.181 × 105 1.456 × 105
1.126 × 107 2.075 × 108 1.015 × 1010 2.464 × 1013
8.344 × 104 9.288 × 104 1.051 × 105 1.311 × 105
to the Wegner-Muller model.25,26 The fractional dewatering by model II is given in eq 4
χwater ) 1 -
Figure 9. Dewatering of coal tar C at 403 K containing 2.0 wt % of initial water.
for the water-oil (w/o) emulsions of low viscosity and with an appreciable difference in densities between organic and aqueous phases.21 The lines in Figures 2-5 are the results of correlations with two kinds of demulsification models: model I and model II. In these correlations, the rate of demulsification was additionally assumed to be controlled by the initial rate of coalescence of the dispersed phases. Model I is based on the Smoluchowski-Stokes-Einstein collision model,25,26 and in addition to this, the energy barrier concept, E*, was adopted to coalescence. The resulting expression of the fractional dewatering, χwater, is given as follows
8kT E* n t exp kT 3η(T) 0 χwater ) 8kT E* 1+ n t exp kT 3η(T) 0
(
)
(
)
( ) ( )
(2)
(3a)
where
A1 ≡
8kn0 3
(3b)
E* k
(3c)
A2 ≡
(
)
kT t 1 + ξRan0 6πη(T)r
(4)
2
where ξ is the effectiveness in collision, Ra is the radius of action, and r is the radius of a droplet. As in model I, if the effectiveness in collision, ξ, is described in terms of the energy barrier in coalescence, then the resulting equation can be written as eq 5
(
ξ ) ξ0 exp -
E* kT
)
(5)
Therefore, model II expressed by eq 4 can be simplified to that possessing two adjustable parameters as
χwater ) 1 -
1
{
( )}
B2 T 1 + B1 t exp T η(T)
2
(6a)
where
where k is the Boltzmann constant, no is the number of dispersed emulsion droplets at the initial time, t is the time, and η(T) denotes the viscosity as a function of temperature. Equation 2 can be simplified with lumped adjustable parameters by combination of the multiplied or divided quantities within the same term to produce eq 3
A2 T A1 t exp T η(T) χwater ) A2 T 1 + A1 exp T η(T)
1
As shown in eq 3, model I includes two adjustable parameters. Model II is the combined Wegner-MullerStokes-Einstein model made by applying the StokesEinstein type of expression on the diffusion coefficient (25) Smoluchowski, M. V. Z. Phys. Chem. 1917, 92, 129. (26) Wegner, G.; Muller, K. W. Kolloid Chem. Beiheft 1928, 27, 223.
B1 ≡
kξ0Ran0 6πr
(6b)
E* k
(6c)
B2 ≡
To consider the temperature dependence of the viscosity, the same Vogel equation was used, which was empirically determined from a variety of viscosity measurements for many coal tar samples used in this study
(
η(T) ) C1 exp
)
C2 T + C3
(7)
where C1 ) 1.125 × 10-1 poise, C2 ) 1.672 × 102 K, and C3 ) - 2.671 × 102 K. As shown in Figures 2-5, the experimental trends of dewatering can be reproduced fairly well with both models. Model II seems to be more flexible than model I in describing the steep change of the moisture content at higher temperature. Table 2 lists the adjustable parameters of model I and model II which were used to correlate the dewatering data of coal tar samples. Table 3 lists the characteristic time values required to attain 50% water separation, τ1/2, estimated from the experimental data in Figures 2-5. Although a small difference is found according to the samples, the water content comes down to one-half of its initial value within ca. 10 h at 383 K, ca. 3 h at 403 K, and ca. 1 h at 423 K. It is obvious that an induction period exists in the initial
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Table 3. Separation Time (h) To Remove 50% of Water from Dispersed Phase by Heat Treatment under Saturated Pressure coal tar A temp (K)
A-1
A-2
A-3
A-4
383 403 423
8.8 3.5 0.5
9.3 3.5 1.0
8.0 3.0 0.5
13.5 2.0 0.2
stage of the dewatering curves in Figures 2-5. An induction period can be seen before the inflection point appearing in the beginning of the dewatering curve. During this induction period, it is suspected that the first steps for demulsification are propagated by thermal motion, such as collision, flocculation, film drainage, coalescence, and so forth. The existence of this induction period is more evident in the isothermal dewatering curves at 383 K, i.e., at the lowest temperature in this work. Although the induction period is considered as due to the thermal history of the sample before reaching its set point, it can also be considered to be due to other factors because its existence is more evident at low temperature. For example, it may result from the rheological properties of coal tar or from the influence of included colloidal particles. This complicated behavior is difficult to explain with the demulsification models. The rate of dewatering is very sensitive to the change of temperature, as shown in Figures 2-5. With a small increase of temperature, for example, about 10% in absolute scale, τ1/2 is observed to decrease abruptly about from 10% to 5%. As exhibited in Figure 6, the extent of water separation reaches to a final dewatering value within ca. 1 h when treated at 443 K. Figure 7 is the comparison of dewatering behaviors for coal tar B at 423 K and shows the separation of water near 100% within 10 min under saturated pressure. However, when additional pressure is applied over the saturated one, even the final value of fractional dewatering decreases to about 10%. Therefore, artificial external pressurization can be said to be unfavorable against dewatering operation. From this one can conjecture that the process of dewatering is closely related to the boiling phenomenon of volatile light components. In fact, when the dewatering experiment is carried out in an open system, a sharp decrease of the dewatering rate is frequently observed, due to the evaporation of light compounds such as naphthalene. In Figure 8, experimental results from three different sets of heat treatments are compared together with their experimental conditions at 443 K for coal tar B. With respect to the final value of fractional dewatering, heat treatment under saturated pressure with no preaddition of water can dewater coal tar more efficiently than the case under additional pressurization or premixing with additional water up to 50 wt %. If water is added before heat treatment, it is evident that the added amount of water cannot be removed with ease from coal tar. From the experimental data under artificial pressurization at 423 K shown in Figure 7 and at 443 K shown in Figure 8, the difference in the final values of fractional dewatering is observed to decrease with the increase of temperature. This characteristic trend again confirms the conjecture of the relationship mentioned above between the phenomena of dewatering and evaporation. For coal tar C, the dewatering behaviors are compared
by the system pressure and premixing with additional water in Figure 9. Heat treatment at high temperature under saturation pressure was also preferable in this case, rather than that with additional pressurization and that with additional pressurization and premixing with additional water, with respect to the final value of fractional dewatering. As shown in Figure 9, the final value of fractional dewatering at pressurized condition is much higher when coal tar is premixed with additional water in comparison with the case of no addition of water before heat treatments. It can be conjectured from this observation that water itself dispersed in coal tar can be considered as a light component working in the process of dewatering. When the moisture content of coal tar is 2.0 wt %, the amount of water as a light compound is insufficient to accelerate the rate of dewatering. This rate may be increased significantly by adjustment of the moisture content to 52 wt % with additional water. However, this addition of water is not desirable because the residual water in coal tar is still in a comparable amount even after dewatering by heat treatment. It is thought that the recycled stream of the low-boiling organic fraction downstream can be mixed with raw coal tar to make use of a boiling-induced dewatering process. Conclusion To understand the characteristics of the dewatering phenomenon in coal tar, bottle tests were made for several different coal tar samples. From this work, the following conclusions were derived: (1) With an increase of temperature, the rates of dewatering increase rapidly. The final values of fractional dewatering are attained within ca. 2 h at ca. 423 K. (2) The final fractional dewatering value depends mainly on the manufacturing condition of raw coal tar, the operating pressure in the heat treating system, and the initial moisture content in raw coal tar. (3) It is advantageous to separate water under saturation pressure rather than at a subcooled state by extra-pressurization or than with an increase of the moisture content by premixing with additional water, with respect to the rate of dewatering and the final value of fractional dewatering. Therefore, artificial pressurization and/or premixing with additional water should be avoided. (4) A relationship between the phenomena of dewatering and boiling seems to exist. Boiling of light components effects the viscosity of the oil phasesStokes’ Law. It is favorable to keep the content of low-volatile components as high as possible to accelerate the rate of dewatering and to minimize the final content of residual water in the heat-treated coal tar. (5) The dewatering behavior observed on a macroscopic scale can be analyzed by the demulsification possessing models. Some sets of model parameters were empirically evaluated, which can be referred to in the practical design of dewatering processes and equipment. Acknowledgment. The authors are indebted to Dr. K. E. Yoon of KOSCO Chemical Central Research Center for invaluable suggestions and encouragement during the experiments. This paper was partly supported by a research grant from Dongguk University. EF980155G
