Ind. Eng. Chem. Res. 2008, 47, 4689–4696
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Equilibrium and Kinetic Studies on the Hydrolysis of Urea for Ammonia Generation in a Semibatch Reactor J. N. Sahu, K. Mahalik, A. V. Patwardhan, and B. C. Meikap* Department of Chemical Engineering, Indian Institute of Technology (IIT), Kharagpur, P.O. Kharagpur Technology, West Bengal, Pin - 721302, India
The present study is concerned with the methods and means to safely produce relatively small amounts (i.e., up to 50 kg/h) of ammonia. The equilibrium and kinetic study of urea hydrolysis was conducted in a semibatch reactor at atmospheric pressure to investigate the effects of reaction temperature, initial feed concentration, and time on ammonia production. This study reveals that conversion increases exponentially with increasing temperature but decreases slightly with increasing initial feed concentration of urea. Furthermore, the effect of time on conversion was also studied; it was found that conversion increases with increasing time. Using collision theory, the temperature dependency of the forward rate constant was determined, from which the activation energy of the reaction and the frequency factor were calculated. The activation energy and frequency factor of the urea hydrolysis reaction at atmospheric pressure were found to be 60.93 kJ/mol and 4.259 × 105 min-1, respectively. Introduction Ammonia is an extremely important chemical that has innumerable uses in a wide range of areas, including process, industry, and utility uses. The present study addresses the methods and means to safely produce relatively small amounts (i.e., up to 50 kg/h) of ammonia. Different application areas require the safe use of relatively small quantities of ammonia. For example, ammonia is used to increase of the efficiency of electrostatic precipitators for the removal of fly ash from the flue gas stream of boilers using fossil fuel.1–7 Other applications include (i) alleviation of the “blue plume” problems associated with the burning high-sulfur-content oil in boilers,8,9 (ii) flue gas conditioning applied to fabric filtration,10 and (iii) removal of the NOx contaminants contained in the flue gas stream from energy-producing boilers or combined-cycle systems in conjunction with selective catalytic reduction (SCR) and/or selective noncatalytic reduction (SNCR) systems.11–16 Ammonia for such uses is often delivered in the form of anhydrous ammonia or aqueous ammonia. Anhydrous ammonia is used in massive quantities worldwide for many industrial and agricultural purposes. Anhydrous ammonia is a gas at ambient temperatures and pressures, but it is normally shipped and stored as a liquid, either in pressure vessels at ambient temperature (e.g., at about 20 °C and a high pressure of about 16 × 105 Pa) or in refrigerated vessels at ambient or nearly ambient pressure (e.g., at about -33 °C).17 It is transported in bulk in ships, barges, and railroad tank cars and in tank trucks on public roads and highways. It is frequently stored in large quantities at industrial sites in populated areas and is frequently used as the working fluid in large refrigeration systems. It is now coming into wider use for the removal of NOx from flue gas at power generating stations, industrial heaters, or combined-cycle systems, often in urban areas. Anhydrous ammonia is an extremely hazardous, toxic, and volatile material. In the event of accidental discharge, it can cause immediate death to humans and animals and rapid death to trees and plants. Both anhydrous liquid ammonia and very * To whom correspondence should be addressed. Tel.: +91-3222283958. Fax: +91-3222-282250. E-mail:
[email protected] or
[email protected].
Figure 1. Schematic of the experimental setup for urea hydrolysis.
concentrated aqueous liquid ammonia exhibit deadly characteristics that substantially increase the risk of widespread injury and death in the case of a spill. The U.S. Occupational Safety and Health Administration (OSHA) has set a permissible exposure level of 50 ppm, as an 8-h time-weighted average. The effects of inhaling ammonia range from irritation to severe respiratory injuries, with possible fatality at higher concentrations. The U.S. National Institute of Occupational Safety and Health (NIOSH) has established an Immediately Dangerous to Life and Health (IDLH) level of 300 ppm for the purposes of respirator selection. Ammonia is corrosive, and exposure results in a chemical-type burn. Because ammonia is extremely hygroscopic, it readily migrates to moist areas of the body such as the eyes, nose, throat, and moist skin areas. Exposure to liquid ammonia also results in frostbite, as its temperature at atmospheric pressure is 2.2 °C. Specifically, upon sudden release to the atmosphere, as might occur in a train wreck or a traffic accident, ammonia forms a cloud made up of an aerosol fog of liquid ammonia droplets. Unlike gaseous ammonia, which, although toxic, is lighter than air and quickly dissipates to
10.1021/ie800481z CCC: $40.75 2008 American Chemical Society Published on Web 06/13/2008
4690 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008
Figure 2. HPLC analysis of the urea feed solution.
harmless concentrations, such a cloud of ammonia can persist for a long time. The cloud is typically heavier than air and tends to drift along the surface of the earth, i.e., the ground or the surface of a body of water. The cloud moves with the wind and can sweep over a total area (i.e., a “footprint”) that is much larger than the area covered by the cloud at any one moment. Contact with the cloud is instantly incapacitating, and a single breath can be fatal.18 In addition to the inherent danger of storing, transporting, and handling large quantities of ammonia, it is expensive in regard to safety aspects, insurance costs, specialized training, and the difficult of quantifying emotional costs of living and/ or working next to a such potential catastrophe. It is apparent that, if another, less hazardous commodity could transported or stored instead of ammonia and then be readily converted back to ammonia, the hazards associated with ammonia shipment would be considerably reduced. To some extent, attempts have been made to improve the supply of ammonia for NOx control in power-plant and industrial environments by substituting concentrated aqueous liquid ammonia for
anhydrous ammonia. Such a solution has achieved only limited success, for any number of reasons, including the high energy cost of transporting and vaporizing the water carrier; the relatively costly storage facilities; and the fact that, even though aqueous ammonia is safer to handle than anhydrous ammonia, it is still difficult and costly to handle in a safe manner. Urea is an ideal candidate to substitute for ammonia, as the method of urea-to-ammonia conversion is a hydrolysis process. Urea is a nontoxic chemical compound, and for the purposes of satisfying small requirements (i.e. up to 50 kg/ h), it presents essentially no danger to the environment, animals, plant life, and human beings. It is solid under ambient temperatures and pressures. Consequently, urea can be safely and inexpensively shipped in bulk and stored for long periods of time until it is converted into ammonia. It will not leak, explode, or be a source of toxic fumes; require pressurization; increase insurance premiums; require extensive safety programs; or be a concern to the plant, the
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Figure 3. Effect of temperature on conversion at different initial feed concentrations.
Figure 5. Effect of concentration on conversion at different temperatures.
which ammonia and carbon dioxide are produced, is strongly endothermic, with the result being that the reaction to release ammonia and carbon dioxide requires heat and quickly stops when the supply of heat is withdraw. Excess water promotes the hydrolysis reaction, the overall reaction for which is xH2O + NH2CONH2 f 2NH3 + CO2 + (x - 1)H2O
Figure 4. Temperature evolution of the reactor products for an initial feed concentration of 30 wt % urea to water.
community, or individuals who might be aware of the transportation and/or storage dangers of ammonia. The basic chemistry employed in the hydrolysis of urea, is a reverse of that employed in the industrial production of urea from ammonia and carbon dioxide and involves two reaction steps19,20 NH2CONH2 + H2O f NH2COONH4
(1)
NH2COONH4 f 2NH3 + CO2
(2)
The reaction in eq 1 in which urea is hydrolyzed to form ammonium carbamate, is mildly exothermic, whereas eq 2, in
(3)
The published information on the hydrolysis of urea for the production of ammonia is limited in detail and patented.21–29 However, no information is available in the literature regarding equilibrium and kinetic studies of urea hydrolysis for the production of ammonia. Therefore, we decided to study more thoroughly the phenomenon of urea hydrolysis for the production of ammonia in different application areas that require the safe use of relatively small quantities of ammonia. Experimental Techniques. Characterization of Urea. Urea and biuret were detected and quantified by HPLC (high-performance liquid chromatography).The HPLC equipment consisted of a Perkin-Elmer chromatograph series 200 UV/vis LC detector (Perkin-Elmer, Wellesley, MA) equipped with a series 200 isocratic pump and a rheodyne injector with a 20-µL loop; a variable-wavelength ultraviolet/visible detector was used. The equipment was controlled by Total Chrom software that controlled the solvent gradient, data acquisition, and data processing. The column was a reverse-phase Agilent column. Separation was performed in a C-18 column (250 mm × 4.6 mm diameter) with a 5-µm pore size and a UV detector set at a wavelength of 199 nm. The mobile phase used was 0.1 M phosphate buffer (pH 6.7) at a flow rate 1 mL/min, and the column temperature was 25 ( 1
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Figure 6. Effect of time on conversion at different temperatures for a constant 10 wt % solution.
Figure 8. Effect of concentration on reaction rate at different temperatures. Table 1. Kinetic Data for Urea Hydrolysis at Atmospheric Pressure
Figure 7. Effect of time on concentration at different temperatures.
temperature (°C)
k (min-1)
n (calculated)
n (average)
R2
140 150 160 170
0.008 0.014 0.019 0.027
0.94 0.89 0.99 0.99
1 1 1 1
0.99 0.982 0.958 0.964
°C. HPLC provides the following features: high resolving power, speedy separation, continuous monitoring of the column effluent, accurate quantitative measurements, repetitive and reproducible analysis using the same column, and automation of the analytical procedure and data handling. Reagents. All chemicals used in this study were from Merck (India) Ltd. and Qualigens Glaxo (India) Ltd. and were of analytical grade. Method of Experiment. A schematic of experimental setup is shown in Figure 1. The experimental setup mainly consisted of a reactor, heat exchanger, condenser, pump, feed tank, control panel, and product storage tank. The reactor had a capacity of 2000 cm3 and was made of stainless steel-316. A pump was used to feed the urea solution to the reactor at a controlled flow rate against a positive differential head between pump suction and discharge. A shell-and-tube-type condenser was used to exchange heat between the products (NH3, CO2, H2O) and cooling water. The system also had heat exchanger to cool the unreacted urea for recycling to the reactor, where tap water at room temperature without any pressure was used for cooling. In addition, the reactor also had two storage vessels: one for urea solution and another for product storage. The whole setup was built of stainless steel to prevent corrosion. The reactor contained two openings: one for feeding urea solution and the
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Then, the absorbing ammonia solution was removed, and its volume was measured. After the product had been absorbed by boric acid, three samples of 10 mL volume each were removed for titration. Three drops of methyl orange indicator were mixed in each sample, which was then titrated with hydrochloric acid. Boric acid is so weak that it does not interfere with the acidimetric titration. Then, by comparing the initial and final concentrations of ammonia, the equilibrium conversion was determines. Finally, the rate of the reaction was obtained from the slope of the time versus concentration data, plotted in terms of ln(CA) and ln(-rA). The slope and intercept give the order of the reaction (n) and the rate constant (k), respectively. The activation energy (E) and frequency factor (A) were calculated from an Arrhenius plot. Results and Discussions
Figure 9. Effect of temperature on the rate constant.
other for withdrawing the products to a condenser, where tap water was used to condense the gaseous products from the reactor. To measure the temperature and pressure of the reactor, a thermocouple and a pressure gauge were attached to the reactor through the control panel. A cooling coil was placed inside the reactor to cool the reactor when required; the same tap water was also used here as the coolant, and a control panel was used to control the required temperature. To supply the necessary heat, an electrical heating coil outside the reactor wall was applied at a controlled rate by means of the control panel. The control panel consisted of indicators connected by electrical wires, the main on/off switch, the electrical heater on/off switch, the alarm system, fuses, and a PID temperature controller. The whole setup was placed on a movable stand. The urea sample used to conduct the experiments was obtained from Nagarjuna Fertilizers and Chemicals Limited (Hyderabad, India). First, urea solutions of different concentrations (10, 20, and 30 wt % of urea to water) were prepared. In each case, the volume of the water was 1000 mL. Then, the solution of a particular concentration was fed into the feed tank through the feed hopper. A pump was used to feed the urea solution to the reactor at a controlled flow rate. Heat was supplied by heating the electrical coil outside the reactor wall at a controlled rate. The decomposition of urea takes place slowly starting at around 110 °C. As the reaction starts, the product, which is a gaseous mixture of ammonia, carbon dioxide, and water vapor, goes through the condenser. In the condenser, the water vapors are condensed, and the gaseous products are cooled, by circulating tap water at room temperature without any pressure through the condenser. Then, the product was stored in the product storage tank and collected in a beaker with boric acid solution, which is an absorbing material for ammonia solution. Boric acid solution was prepared by dissolving 4 g of boric acid in 100 mL of warm distilled water.
Physical and Chemical Characterization of the Urea Samples. The Nagarjuna-brand urea sample was collected from near a local market at Kharagpur, India. It is a common nitrogen fertilizer used by Indian farmers. The physical appearance is white and granular, and it is highly soluble in water. The pH of the 10 wt % urea solution in water was 7.82. The urea sample was characterized to determine the purity before conducting the experiments. Specifically, it was characterized by HPLC to determine the impurity level. The HPLC analysis of the feed urea sample is shown in Figure 2. The chromatograph indicates the presence of urea and biuret in the inlet feed solution of urea and shows that the Nagarjuna urea contains 2.89% biuret and has a purity of 94.32%. Study of Equilibrium Conversion. Equilibrium experiments were conducted on the hydrolysis of urea at atmospheric pressure in a semibatch reactor at different temperatures and initial feed concentrations. From the initial and final concentrations, the conversion was calculated in order to determine the effects of temperature and initial feed concentration on the equilibrium conversion were studied. Effect of Temperature on Conversion. It can be seen from Figure 3 that the conversion is a function of temperature. Specifically, conversion increases exponentially with increasing temperatures. For 10% urea solution, the conversion increased from 2.12% to 17.07% when the temperature was increased from 110 to 180 °C in 10 °C intervals. Similar trends were observed for the 20 and 30 wt % urea initial feed solutions, and the conversions were 10.81% and 9.58% respectively at 180 °C. Also, it can be concluded that the initial reaction rate is low and that the reaction becomes rapid at around 130 °C. At this temperature, the production of ammonia is higher than that at lower temperature. The product composition (number of moles of a single product/total number of moles of all products) obtained from the reactor with 30 wt % urea feed inlet solution is shown in Figure 4 for different temperatures. It can be seen from the figure that the production of ammonia and carbon dioxide increased markedly and the production of excess water decreased as the temperature was increased. Effect of Initial Concentration on Conversion. The conversion was slightly affected by the initial concentration of urea fed to the reactor. It can be seen from Figure 5 that, with increasing initial concentration of urea, the conversion decreases slightly. For each initial concentration, the temperature was varied from 110 to 180 °C. For a constant temperature of 180 °C, the conversion decreased from 17.07% to 9.58% when the initial feed concentration was increased from 10 to 30 wt % urea. Similarly, for the other temperatures, the decreases in
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Figure 10. HPLC analysis of the solids precipitated from the reactor solution.
equilibrium conversion when the initial feed concentration of urea was increased from 10 to 30 wt % were as follows: 110 °C, from 2.12% to 1.76%; 120 °C, from 2.5% to 2.32%; 130 °C, from 3.25% to 3.03%; 140 °C, from 5.34% to 3.87%; 150 °C, from 6.97% to 4.96%; 160, from 8.52% to 6.72%; and 170 °C, from 12.24% to 7.87%. This is due to the fact that solutions with higher concentrations of urea have less excess water and, therefore, a lower likelihood of achieving the hydrolysis reaction. Because the reaction is favored by higher concentrations of water, solutions with lower weight percentages of urea have higher conversions. Study of Reaction Kinetics. Experiments were also conducted on the hydrolysis of urea at atmospheric pressure in a semibatch reactor at different temperatures and concentrations in order to investigate the kinetics. Again in this case, the conversion was determined from the initial feed concentration and the final concentration of urea. Assuming no backward reaction, the rate constant for the forward reaction was calculated. Because excess water was used in the reaction, the concentration of water was neglected in the kinetic analysis. Let -rA be the rate of reaction, CA0 be the
initial concentration, and CA be the concentration at any particular time. The rate of reaction for the forward reaction can be written as dCA ) k(CA)n dt Taking the natural logarithm of both sides of eq 4 gives -rA ) -
ln(-rA) ) ln(k) + n ln(CA)
(4)
(5)
Hence, plotting ln(CA) on the abscissa and ln(-rA) on the ordinate should give a straight line whose y intercept is represented is the rate constant and whose slope is the order of the reaction. Effect of Time on Concentration. From the concentrationtime data, a kinetic study has performed. It was observed that the conversion increased with time. For the 10 wt % urea initial feed concentration, the conversion increased from 12.24% to 78.96% at a constant temperature 170 °C for a reaction time of only 50 min. Similarly, for the temperatures of 140, 150, and 160 °C, the conversions at 50-min reaction time were 36.32%,
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51.52%, and 63.85%, respectively. This trend is shown by Figure 6, where it can also be seen that the time required to achieve higher conversions is longer at 140 °C and shorter at 170 °C. The effect of rate on concentration at a fixed temperature is shown in Figure 7. As the time increased, the concentration decreased, and it was highest in the case of higher temperature. From Figure 8, it is observed that the slope of the plot is approximately equal to 1. This indicates that the forward reaction is first-order. Moreover, the intercepts of Figure 8 give the values of forward rate constant, which is a function of temperature. It can be seen from the figure that, as the temperature was increased from 140 to 160 °C, the forward rate constant increased from 0.0080571 to 0.0274359 min-1. The variation with temperature is very small because of the fact that there was no stirring and no catalyst. Table 1 lists the values of the rate constant with temperature. From the above results, it was concluded that the order of the forward reaction is close to 1 and that the rate constant increases with increasing temperature. Therefore, the temperature dependency of the forward rate constant was explained by Arrhenius theory. The rate equation can be written as -rA ) kCA
(6)
where rA is the rate of the reaction, k is the forward rate constant, and CA is the concentration of reactant. According to Arrhenius theory, the temperature dependency of the forward rate constant can be written as k ) Ae-E/RT
(7)
where A is the frequency factor and E is the activation energy. From Figure 9, the activation energy and frequency factor were determined to be 60.93 kJ/mol and 4.259 × 105 min-1, respectively. Characterization of the Reactor Liquor Samples. Detailed analysis of reactor liquid samples was performed for the system at 170 °C with an inlet feed concentration of 10 wt % urea. The pH of the reactor liquid sample was 8.02. The reactor liquid typically contained 3-4% urea, 0-5% higher urea derivatives, and 1-2% ammonia. At temperatures above 120 °C, any ammonium carbamate in the liquid immediately decomposed to ammonia and carbon dioxide, and hence, very small concentrations (1-2%) of ammonium carbamate were present in the reactor liquor. The remainder was water. The HPLC analysis of the reactor liquor sample is shown in Figure 10. This analysis indicates the presence of urea, biuret, and other new products. The amount of urea in the reactor liquor is lower than that in the inlet feed urea solution, but the biuret percentage is higher, which might be due to the formation of higher urea derivatives, as new peaks were also observed. Conclusion The objective of this work was to study the hydrolysis of urea for the generation of ammonia in a semibatch reactor. Conclusions from the present study are as follows: Urea samples (Nagarjuna brand) were collected from the market, and characterization was performed by HPLC. It was found that the urea contained 2.89% biuret and had a purity of 94.32%. From an equilibrium study at atmospheric pressure, it can be concluded that the equilibrium conversion increases with increasing temperature and reaches a maximum value of 17.07% for an initial urea concentration of 10 wt % and a temperature
of 180 °C. The equilibrium conversion decreases with increasing initial feed concentration. From kinetic study at atmospheric pressure, it was also found that the forward reaction is a pseudo-first-order reaction with a rate constant that varies from 0.0080571 to 0.0274359 min-1 within the temperature range of 140-170 °C. The activation energy and frequency factor of the urea hydrolysis reaction at atmospheric pressure were determined to be 60.93 kJ/mol and 4.259 × 105 min-1, respectively. Finally, it can be concluded that the hydrolysis of urea to form ammonia and carbon dioxide behaves as a first-order reaction with respect to urea. Also, it can be concluded that the initial reaction rate is low and that the reaction becomes rapid at around 130 °C. At this temperature, the production of ammonia is higher than that at lower temperature. Further, the reaction rate constant is a function of temperature, and similar work is in progress to investigate this dependence in an effort to increase the conversion in the hydrolysis of urea. The initial results imply that the hydrolysis of urea for the manufacturing of ammonia is a suitable technique for safe utilization in coalfired thermal power plants. Acknowledgment The authors gratefully acknowledge the National Thermal Power Corporation (NTPC), New Delhi, India, for the financial support of this research. Nomenclature A ) frequency factor (min-1) CA ) final concentration of urea solution (mol/L) CA0 ) initial concentration of urea solution (mol/L) E ) activation energy (kJ/mol) k ) forward rate constant (min-1) n ) order of the forward reaction R ) ideal gas law constant (kJ/kg · mol · K) rA ) rate of reaction T ) reaction temperature (K) t ) reaction time (min)
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ReceiVed for reView March 26, 2008 ReVised manuscript receiVed April 16, 2008 Accepted May 2, 2008 IE800481Z
