ISSN 00405795, Theoretical Foundations of Chemical Engineering, 2012, Vol. 46, No. 1, pp. 1–7. © Pleiades Publishing, Ltd., 2012. Original Russian Text © V.V. Dil’man, V.V. Sokolov, N.N. Kulov, L.A. Yudina, 2012, published in Teoreticheskie Osnovy Khimicheskoi Tekhnologii, 2012, Vol. 46, No. 1, pp. 3–9.
Experience in Developing and Operating a HighIntensity Absorber for Process Gas Purification from Carbon Dioxide V. V. Dil’mana, V. V. Sokolovb, N. N. Kulova, and L. A. Yudinab a
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia b OOO Yang Inzhiniring, Moscow, Russia email:
[email protected] Received June 29, 2011
Abstract—Experience in developing and operating a highintensity largecapacity absorber for process gas purification from carbon dioxide in ammonia production is discussed. The use of scaleup and mathematical modeling techniques made it possible to avoid expensive and long commercial tests of the apparatus models in developing this absorber. DOI: 10.1134/S0040579512010034
The objective of this study is to discuss experience in developing the AM80 absorber and the results of its commercial operation.
INTRODUCTION Due to the complexity of chemical engineering pro cesses, similarity conditions for multiphase processes complicated by chemical conversions are generally incompatible with similarity conditions for transport processes. This makes it impossible to use physical modeling in developing new technological processes and apparatuses. At present, classical hydrodynamics methods are being replaced by approximate scaleup and mathematical modeling techniques for individual components of a technological process.
MAIN PROPOSITIONS OF THE APPROXIMATE MODELING OF GAS–LIQUID CHEMISORPTION PROCESSES Rigorous methods for the similarity theory of classi cal hydrodynamics make it possible to pass to any dimensions of a model [1]. However, these methods cannot be used to calculate equipment and design chemical productions due to the incompatibility of the time and space scales of the chemical and physical components of technological processes [2–4]. This required the development of approximate scaleup methods based on mathematical modeling. To imple ment approximate methods, it is necessary to study the kinetics of a process using small laboratory setups that operate under real conditions of the occurrence of tech nological processes and study the hydrodynamics of flows in fullscale benches that operate using an air– water system at atmospheric pressure and room temper ature. The approximate methods for modeling processes and apparatuses that were developed in [3–7] remain important at present. The rates of chemical conversions and phase transitions depend on the microscopic scales of the phenomena and are determined by the physico chemical properties of the medium. These rate can be found using small laboratory setups under real process conditions.
Methods of mathematical modeling of chemical reactors have made it possible to develop a transportable AM80 absorber for synthesis gas purification from car bon dioxide in ammonia production with a capacity of 1360 t/day without expensive and long commercial tests. An AM80 absorber for process gas purification from carbon dioxide that was designed (GIAP, Mos cow) and manufactured in Russia and the sizes of which allowed it to be transported by rail from a manufacturer to an installation point on a working floor is operating at an ACHEMA chemical plant (Jonava, Lithuania) in ammonia production. The AM80 absorber was developed using mathe matical modeling techniques that included only the laboratory studies of kinetics under real conditions of the occurrence of the process and bench tests of hydraulics using an air–water system at atmospheric pressure. 1
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The scaleup problem in chemical engineering is complicated by the multiphase nature of media, which can lead to the disturbance of the geometrical similarity of models with an increase in their size. For example, in tray columns with the crossflowing liquid and gas on trays, the velocity of the gas varies in inverse proportion to the square of the column diameter, and the liquid flow rate reduced to the overflow weir length varies in proportion to the column diameter to the first power. At a constant ratio of the flow rates L/G = const, an increase in the sizes of the column breaks the geometri cal similarity. An increase in the sizes of models gener ally leads to the deterioration of the conditions of flow supply to an apparatus, and the uniformity of flow dis tribution is disturbed, which reduces the separating capacity of the apparatus [8]. The problems of the applied hydrodynamics of gas– liquid chemisorption processes associated with the nonuniform turbulent flows of interacting media are very complex and cannot currently be solved without using empirical data. The hydrodynamics of twophase flows can be tested on fullscale models using the air– water system at atmospheric pressure and room temper ature, since it depends mainly on the inertial forces of the liquid phase. Of course, this does not exclude taking into account the effect of chemical conversions, inter phase transitions, and heat and mass transfer. Such scaleup methods are aimed at eliminating, rather than studying, all of the nonuniformities of the motion of gas–liquid media that are visually detected on hydrodynamic benches. The capacity of hydrodynamic benches should be sufficient to blow through a package of three plates to rectify the detected drawbacks of interplate interaction between flows. Tests are conducted over a wide range of air velocities, from minimum loadings which are of interest for startup regimes up to the flooding of trays, taking into account the value of the F factor, which makes it possible to judge the effect of dynamic loads from the gas phase. SPECIFIC FEATURES OF GAS–LIQUID CHEMISORPTION PROCESSES Gas cleaning by ethanolamine solutions is a typical chemisorption process, in which the molecules of the gas being absorbed react with the active components of the absorbent. Most reactions that occur in chemisorp tion are exothermic and reversible [8, 9]. The absorption of a gas depends on its physical solu bility, equilibrium constant, stoichiometric relation ships, and some other factors. The higher the absorp tion pressure, the slower the increase in solubility as the chemisorbent is consumed. The dependence of the sol ubility of the gas on its partial pressure in the apparatus
is substantially nonlinear. The efficiency of absorber operation depends on the dimensionless kinetic param eter R, which is a function of many variables of the chemisorption process [9]. The value of this parameter and its relationship with other criteria of chemisorption processes affect the rate of chemical conversions. This rate varies strongly along the height of the absorber, sharply decreasing from top to bottom, which is manifested in a slow increase in the concentration of carbonates in the falling liquid. An increase in the degree of carbonization of the liquid at the outlet of the absorber and its approach to the limit ing equilibrium value serves as an index of the cost effi ciency of the purification process, which increases as the equilibrium is approached. Another important characteristic by which the effi ciency of purification is judged is the concentration of carbon dioxide in a gas at the outlet of the absorber. Carbon dioxide poisons a catalyst for ammonia synthe sis. Its allowable concentration in the purified gas should not exceed 100 ppm. The purification of a process gas from carbon diox ide in the upper section of an absorber is limited by the rate of its physical absorption by the chemisorbent, and, in the lower section, it is limited by the occur rence of slow carbonization reactions. They are esti mated in terms of the reaction time τ r [9]. The modeling of the kinetics of purification was per formed using laboratory setups for gas cleaning under real process conditions. In experiments, the character istic regions of chemical conversions were identified, for each of which the reaction time τr was determined. In studying the kinetics of a gas–liquid chemisorp tion process, we used all of the known approaches to the development of its kinetic calculation, including those based on empirical mass transfer coefficients that gen eralize industrial experience in various nitrogen pro ductions [8, 9]. DESIGNING THE CONTACT DEVICE FOR AN AM80 ABSORBER In the first domestic largecapacity unit for ammo nia with a capacity of 1360 t/day, in an AM70 absorber for purification using methylethanolamine, highlayer sieve trays with the crosscurrent flow of a gas and liquid were used [8–10]. This apparatus was developed using empirical mass transfer coefficients for industrial appa ratuses with a lower production rate when scaleup methods had just begun to arise. It was assumed that large amounts of a liquid ensure the necessary degree of carbonization of the solution directed to the desorber [8, 9]. The height of the overflow weirs of the first three sieve trays in the lower section of the absorber was taken to be 1.6 m.
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Examination of the operation of the AM70 absorber has shown [11] that it ensures the necessary degree of process gas purification from carbon dioxide, but it has excessive dimensions and an excessive liquid holdup in the absorber, which unfavorably affects capi tal costs and the flow resistance of the absorber. The flow resistance of absorber trays is in good agreement with the observed additivity of the individual components of the total pressure drop when the gas passes through the bubbling layer on a sieve tray: (1) Δ P = Δ P1 + Δ P2 + Δ P3. These components are caused by the weight of the bubbling layer (ΔP3), dynamic resistance when a gas is blown through dry trays (ΔP1), and resistance associ ated with overcoming surface tension forces (ΔP2) [12]. The major part of the total resistance of an absorber with highlayer sieve trays is the resistance of the gas– liquid layer on a tray (ΔP3). This resistance consists of two parts: the resistance of the bubbling layer with a height equal to the height of an overflow weir and the resistance of the gas–liquid layer over the weir edge. The latter ensures the motion of a falling liquid from top to bottom along the absorber under the effect of gravity. The resistance of a gas–liquid layer on a tray is cal culated with the equation [13]
( )
2 ⎡ Q ⎤ ρ1 ΔP3 = ⎢Ψ Z 1 + 3 Ψ ⎥ . mb ⎦ ρ w ⎣
(2)
To determine the gas content ϕ of the bubbling layer (3) ϕ = 1 − ψ, we used the following equation: 0.2 ⎧ ⎫ U 2 ⎛ ρ1 ⎞ − 0.2 ⎜ ⎟ ⎪ ⎪ ⎪ ⎪ U ⎝ ρ2 ⎠ , (4) ϕ = 1 − K D K H exp ⎨ 0.75 ⎬ 0.95 ⎛ ρ1 ⎞ ⎪ ⎪1 + 0.00875 U 2 ⎜ ⎟ ⎪ ⎪⎩ U ⎝ ρ2 ⎠ ⎭
( )
where the quantities
( )
0.5 ⎧ U K D = 1 − exp ⎨−1.1 2 U ⎩ 0.7 ⎧⎪ ⎛ γ1 ⎞ ⎛ K H = 1 − exp ⎨0.405 ⎜ ⎟ ⎜ ⎝ γ2 ⎠ ⎝ ⎪⎩
gD⎫ ⎬, U2 ⎭ g Δ P3 ⎞⎫⎪ ⎬ U 2 ⎟⎠⎪⎭
(5) (6)
are the dimensionless coefficients that take into account the effect of the absorber diameter (KD) and the height of the bubbling layer (KH) on the gas con tent. The validity of this method for calculating the resis tance of highlayer sieve trays and the values of the gas
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content of bubbling layers using the above equations was experimentally verified on hydrodynamic ben ches [13, 14]. Visual observations of the structure of the twophase medium on AM70 absorber trays operating at high weir loading rates (170–300 m3/(m h) revealed hydro dynamic instability of the layer. This was manifested in the intensive swinging of the liquid, the ejection of the gas–liquid mixture, and a nonuniform distribution of bubbles and jets throughout the layer. Using a hydraulic bench, it was found that the maximum height of the gas–liquid layer on the individual sections of a tray exceeded the average value by a factor of 1.5. Highlayer trays with crossflowing gas and liquid ensured the necessary degree of purification from car bon dioxide. However, these trays were designed in such a way that they operated with excessively large quanti ties of a liquid on trays. This explains the high flow resis tance of the AM70 absorber. The design of the AM70 absorber cannot be con sidered successful. The major part of the cross section of perforated trays with the crosscurrent flow of a gas and liquid is occupied by a segmental downcomer. Passing to the countercurrent design of trays in the AM76 and AM80 absorbers made it possible to substantially improve the uniformity of flow distribution over the cross section of the column and to increase the fraction of the column area occupied by the bubbling layer [9– 11]. The potential of increasing the efficiency of process gas purification from carbon dioxide in comparison with the characteristics attained in the AM70 absorber was implemented to the greatest degree in the AM80 absorber. The countercurrent flow of a gas and liquid is widely used in packed columns and perforatedtray columns without downcomers [12]. However, the design of the countercurrent AM80 absorber has a number of spe cific features: it ensures a substantially higher throughput capacity than packed columns, since there is no friction of a gas and liquid against the packing; it facilitates the equalization of gas and liquid flows over the crosssectional area of the apparatus: gas flows cocurrently from bottom to top through perforation holes and then through a bubbling layer, and liquid flows cocurrently from top to bottom, while perforation holes and downcomers are uniformly distributed in the plane of trays; it makes it possible to implement interphase contact with a large liquid holdup for any given law of holdup distribution along the height of the absorber; it admits a wide range of apparatus loadings; it makes it possible to retain the geometrical similar ity of contact devices regardless of their sizes.
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Dimensions of largecapacity absorbers in ammonia production with a capacity of 1360 t/day D, m Absorber
H, m lower section
Notes
upper section
AM70
4.2
3.8
35.7
15 trays with the crosscurrent flow of a gas and liquid
AM76
4
3.8
51.8
15 trays with the countercurrent flow of a gas and liquid and 7 downcomers with a diameter of 0.6 m
AM80
3.2
3.2
46
15 trays with the countercurrent flow of a gas and liquid and 5 downcomers with a diameter of 0.6 m
One of the main requirements in developing the AM80 absorber was to ensure its transportation by rail from a manufacturer to the operating floor in a plant. This meant that the limiting diameter of the column was 3.2 m. It should be noted that none of the packed and plate absorbers that operate in the world for process gas purification from carbon dioxide in ammonia pro duction with a capacity of 1360 tons of ammonia per day has an absorber diameter smaller than 3.8 m, even at present. A TEC packed absorber has a diameter of 3.4 m, but there are two such apparatuses in one unit. A comparison of some characteristics of absorbers of var ious units is given in the table. The contact device of the AM80 absorber made it possible to replace two simultaneously operating packed absorbers with a single absorber equipped with countercurrent perforated trays (Fig. 1). The design and sizes of the AM80 absorber were found and checked by studies conducted on laboratory and hydraulic benches using mathematical modeling techniques, without intermediate commercial tests. The downflow of a liquid in the absorber occurs under the effect of gravity, and the countercurrent flow of a gas is ensured by centrifugal compressors. When the absorber operates, bubbling layers are kept over the tray perforation almost by the jets and gas bubbles that leave holes alone, since the surface tension forces of chemi sorbents can be nearly neglected at a perforation hole diameter of 14 mm. Gas and liquid flows are supplied to the working zone of the absorber through tubes with substantially smaller diameters than the column diameter. This cre ates a nonuniform initial distribution of flows over the cross section, which impairs the efficiency of apparatus operation [4–6, 15]. The countercurrent flow of a gas and liquid facili tates the equalization of gas and liquid flow rates over the cross section of the contact device. In this case, per forated trays act as equalizing grids, as in wind tunnels [15]. Bubbling layers on trays and downcomers that are
uniformly located in the cross section of trays also pro mote the equalization of the flow rate profile. The design dimensions of contact devices (the num ber of downcomers, plate spacing, plate perforation, etc.) were tested on hydrodynamic benches with full scale trays using the air–water system. The main objective of laboratory experiments was to estimate the reaction times τr that are necessary for the occurrence of reactions on different trays of the absorber. Using τr, we estimated the values of the liquid holdup and determined the heights of the overflow weirs of perforated trays. In designing a highintensity AM80 absorber, vari ous variants of its design were developed, including those containing trays with an additional zone of phase contact and two additional zones of phase contact [9, 16, 17]. EXAMINING THE OPERATING REGIME OF THE AM80 ABSORBER The examination of the apparatus was carried out jointly with the operating organization—Concern Achema Group UAB (Lithuania). The absorber is a plate apparatus with 15 special highintensity trays. The diameter of the apparatus is 3.2 m. At the moment of examination, the absorber operated at a nearly maximum capacity of 1580 t/day. An amine solution is supplied to the absorber (914 m3/h) at two points, specifically, onto the 15th tray from the bottom (610 m3/h) and onto the 8th tray from the bottom (304 m3/h). The composition and supply conditions of an amine solution that enters the absorber are the same for both of the streams. At the moment of examination, the temperature of the supplied amine was 47°С. The converted gas was fed to the absorber at a flow rate of 166.4 t/h at a temperature of 48°С and a pressure of 2.55 MPa. The concentration of СО2 in the supplied gas was 20.7 vol %. The concentration of
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D600
Tray no. 3
5
1000
D500
210
650 250
2100 D675
D2000
D3200
D600 1000
Tray no. 2 D675 2100
250
D675 210 D600 Tray no. 1
1000
Fig. 1. Perforated tray no. 2 in the AM80 absorber.
СО2 in the gas phase on the eighth tray was 0.8 vol %. The concentration of СО2 in the gas that was leaving the absorber was 80 ppm. The concentration of CO2 in the regenerated amine was 5 g/l. The concentration of CO2 in the saturated amine solution was 75 g/l.
A solution of 39% aqueous methyldiethanolamine solution with additives and activating agents was fed to the absorber. A general view of the absorber and desorber is pre sented in Fig. 2.
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Fig. 2. General view of the AM80 absorber (on the left) and desorber (on the right) on an operating floor (Jonava, Lithuania). JOURNAL OF CTHEORETICAL FOUNDATIONS OF CHEMICAL ENGINEERING Vol. 46
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EXPERIENCE IN DEVELOPING AND OPERATING A HIGHINTENSITY ABSORBER
CONCLUSIONS This study discusses experience in developing and operating a highintensity AM80 absorber for purify ing a process gas from carbon dioxide, which was designed at GIAP and manufactured at the USSR engi neering plants. The absorber operates at an ACHEMA chemical plant (Jonava, Lithuania) in ammonia pro duction. Its performance fully complies with design characteristics, and the apparatus has a stable operating mode, reliably ensuring a high production rate. The presented data are an example of the development and mastering of a complex apparatus that combines in a single housing the fine purification of a process gas and a high degree of carbonization of a chemical solvent. The used approaches and scaleup methods made it possible to develop a largecapacity absorber of a new design without intermediate expensive and long com mercial tests of apparatuses with a gradual increase in their capacity—from laboratory to industry. ACKNOWLEDGEMENTS Authors are grateful to Yu. Tunaitis, Technical Director of Concern Achema Group UAB, Lithua nia, for the materials concerning technological exa mination. NOTATION b—overflow weir perimeter, m; D—tray diameter, m; H—total height of the working section of an absorber, m; KD—dimensionless coefficient taking into account the effect of the absorber diameter on the gas content of a bubbling layer; KH—dimensionless coefficient taking into account the effect of the height of a liquid on a tray on the gas content of a bubbling layer; m—discharge resistance coefficient, m0.5/h; ΔP—flow resistance of a tray, mH2O; ΔP1—flow resistance of a dry tray, mH2O; ΔP2—resistance due to surface tension forces, mH2O; ΔP3—resistance of a bubbling layer, mH2O; Q—liquid flow rate, m3/h; R—dimensionless kinetic parameter; U—rise rate of a single bubble in a solution, m/s; U2—gas velocity related to the bubbling section of a tray, m/s; Z1—overflow weir height, m; ρ1—absorbent density, kg/m3; ρ2—gas density, kg/m3; ρв—water density, kg/m3;
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τD—characteristic time of diffusive transport of the components of a gas phase to the reaction site, s; τr—reaction time, s; ψ—dimensionless liquid content of the bubbling layer. REFERENCES 1. Loitsyanskii, L.G., Mekhanika zhidkosti i gaza (Fluid Mechanics), Moscow: Nauka, 1973. 2. Boreskov, G.K., Modeling of Chemical Processes, Vestn. Akad. Nauk SSSR, 1964, no. 5, p. 47. 3. Boreskov, G.K. and Slin’ko, M.G., Modeling of Chem ical Reactors, Teor. Osn. Khim. Tekhnol., 1967, vol. 1, no. 1, p. 5. 4. Masshtabnyi perekhod v khimicheskoi tekhnologii (Scale Up in Chemical Engineering), Rozen, A.M., Ed., Mos cow: Khimiya, 1980. 5. Kafarov, V.V., Osnovy massoperedachi (Fundamentals of Mass Transfer), Moscow: Vysshaya Shkola, 1979. 6. Rozen, A.M., Aksel’rod, L.S., and Dil’man, V.V., Some ScaleUp Problems in the Development of Mass Trans fer Apparatuses, Teor. Osn. Khim. Tekhnol., 1967, vol. 1, no. 4, p. 446. 7. Rozen, A.M. and Kostanyan, A.E., ScalingUp Effect in Chemical Engineering, Theor. Found. Chem. Eng., 2002, vol. 36, no. 4, p. 307. 8. Ochistka tekhnologicheskikh gazov (Purification of Pro cess Gases), Semenova, T.A. and Leites, I.L., Eds., Moscow: Khimiya, 1977. 9. Aksel’rod, Yu.V., Gazozhidkostnye khemosorbtsionnye protsessy. Kinetika i modelirovanie (Gas–Liquid Chemi sorption Processes: Kinetics and Modeling), Moscow: Khimiya, 1989. 10. Razrabotki GIAP. Katalog (Catalog of GIAP Develop ments), Cherkassy: Cherk. Filial GIAP, 1991. 11. Roshchin, B.E., Shenderov, L.Z., et al., Analyzing the Operation of an Industrial Absorber with HighLayer Sieve Trays, Khim. Promst., 1977, no. 6, p. 60. 12. Ramm, V.M., Absorbtsiya gazov (Absorption of Gases), Moscow: Khimiya, 1976. 13. Gazizulin, V.M. and Dil’man, V.V., Calculating the Static Pressure of a Gas–Liquid Layer on HighLayer Sieve Trays, Khim. Promst., 1981, no. 6, p. 49. 14. Dil’man, V.V. and Gazizulin, V.M., Gas Content of the Stabilized Section of a Bubbling Layer, in Trudy GIAP. Proizvodstvo organicheskikh produktov (GIAP Transac tions: Production of Organic Products), Moscow, 1981, p. 77. 15. Idel’chik, I.E., Aerogidrodinamika tekhnologicheskikh apparatov (podvod, otvod i raspredelenie potoka po sech eniyu apparatov) (Aerohydrodynamics of Technological Apparatuses: Flow Supply, Withdrawal, and Distribu tion over the Cross Section of Apparatuses), Moscow: Mashinostroenie, 1983. 16. Kochergin, N.A., Dil’man, V.V., Aksel’rod, Yu.V., et al., USSR Inventor’s Certificate no. 696646, Byull. Izobret., 1981, no. 22. 17. Aksel’rod, Yu.V., Shchedro, V.M., Gazizulin, V.M., et al., USSR Inventor’s Certificate no. 1012934, 1982.
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