Experimental Process of Selectively Separating a Mixed Gas of NH3

An experimental process of separating a mixed gas of NH3/CO2 ) 2 (mole ratio) contains a first section, which is a two-step series bubble absorber for...
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Ind. Eng. Chem. Res. 2003, 42, 2826-2831

SEPARATIONS Experimental Process of Selectively Separating a Mixed Gas of NH3/CO2 ) 2 in a High-Velocity Bubble Absorber Ping-Xiong Cai,* Cheng-Fang Zhang, Zhi-Sheng Zheng, and Shu-Jun Qin Research Institute of Chemical Technology, East China University of Science and Technology, P.O. Box 274, 130 Meilong Road, Shanghai 200237, People’s Republic of China

An experimental process of separating a mixed gas of NH3/CO2 ) 2 (mole ratio) contains a first section, which is a two-step series bubble absorber for selective absorption of NH3 under 80140 m/s inlet velocity, and a second section, which is a free NH3 recovery of the obtained aqueous solution followed by desorption of the solution. In the two-step series absorber, the total absorptivity of NH3 reaches 96.7% and the total absorptivity of CO2 is only 33.1%, so the exit content of CO2 for the second step absorber reaches 90.9% (by mole). Free NH3 of the solution is recovered to liquid NH3 with a purity of 99.9% under 1.7-1.9 MPa, and the remaining solution is then desorbed to NH3 and CO2, which is returned to the front first absorber for continuing separation. Then the discharged liquid contained 0.014% NH3 and 0.006% CO2; it can be returned to the absorption system to be used. From this experimental condition, the heat consumption for the separating process is estimated at 3.4 kg of steam of 1 MPa/kg of NH3. 1. Introduction During the process of the manufacture of melamine, a great deal of off-gas of NH3/CO2 ) 2 (mole ratio) is produced. In most cases the off-gas can only be used in the fertilizer plant as an aqueous carbamate solution. This is not possible when the melamine plant is far from a fertilizer plant. A good way that the off-gas can be separated into its components is as ammonia and carbon dioxide. However, the mixed gas of NH3/CO2 ) 2 (mole ratio) is in accordance with the constitution of ammonium carbamate; it cannot be separated by ordinary methods such as distillation, normal absorption, and adsorption. Therefore, the separation of the mixed gas of NH3/CO2 ) 2 has become a difficult problem of chemical technology. Though the mixed gas of NH3/CO2 ) 2 is so difficult to separate, we can use a specific absorption separation method that is carried out at high gas velocity; thus, dominant absorption of NH3 into the liquid phase and more CO2 remaining as the gas phase is realized. Therefore, BASF1 first proposed this novel separation process for its melamine off-gas, which uses different velocities of absorption between ammonia and carbon dioxide in water for separation of the components. Ripperger2 also involved this absorption into an aqueous solution that the ratio of NH3/CO2 can evidently increase in solution through a short contact time. Lai et al.3 also developed this method to separate NH3/CO2 mixed gas using nonequilibrium absorption. On the other hand, Wang et al.4 reviewed this special method * To whom correspondence should be addressed. Phone: 0086-21-64252386. Fax: 0086-21-64250884. E-mail: pxcai7080@ sina.com.cn.

for the separation of NH3/CO2 mixed gas through the venturi jet absorber. From this published literature, some work for separating the mixed NH3 and CO2 has been done; however, there is only a qualitative description of the process that cannot illustrate the influence factors, and no more effective experiment data are available. In the former paper of Cai et al.,5 a selective absorption of NH3 was studied for the separation of the mixed gas of NH3/CO2 ) 2 (mole ratio) by an intermittent bubble absorber at high inlet velocity. It has been demonstrated by the continuous bubble absorber6 that the selective absorption mechanism in which the absorption of ammonia is controlled by a gas film and the absorption of carbon dioxide is looked upon as a fast pseudo-first-order reaction; the best gas velocity of the nozzle is 120-140 m/s, and the insertion depth of the nozzle is 10 mm. In this paper, an experimental process containing a twostep series selective absorption and distillating separation of the solution is built, and then the mixed gas of NH3/CO2 ) 2 can be separated into liquid NH3 and gas CO2 through the experimental apparatus. 2. Experimental Apparatus and Procedure An experimental apparatus containing two-step series selective absorption and solution separation is shown in Figure 1. NH3 and CO2 were heated to 60 °C to avoid the crystal of ammonium carbamate and mixed sufficiently to the ratio of 2:1. Then the mixed gas entered the first-step absorber to be selectively absorbed by the solution coming from the second-step absorber. The exit gas is absorbed by the water coming from the NH2COONH4 desorption column in the second-step absorber. Through two-step series selective absorption, the residual gas, mainly CO2 washed by water and then

10.1021/ie0204282 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/07/2003

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Figure 1. Sketch of the experimental process for separation of a mixed gas of NH3/CO2 ) 2.

pure CO2, is gained. The absorption solution discharged through the liquid seal is pumped into the recovering column of free NH3, in which free NH3 is distilled out at 1.87 MPa and the liquid NH3 is gained from the top of the column after condensation. The 176 °C solution from the bottom of the free NH3 recovery column enters the desorption column of NH2COONH4. In the desorption column, the solution is desorbed to the mixed gas of NH3/CO2 ) 2 at 110 °C, which is cycled to the front first absorber for separation, and the desorbed liquid only containing a little NH3-CO2 is returned to the second absorber as water to use. In the experiment, the temperature of the absorbers and the column is controlled to (1 °C. After the system stabilized (at least 3 h), a sample is taken to analyze. The concentration of ammonia in the solution is determined by titration with a standard acid solution, and the concentration of CO2 is determined through an acid-decomposing method. The exit gas content is analyzed by absorption in a different standard solution. The absorber is the pivotal apparatus to selective absorption. Considering the requirement of high gas velocity and short contact, we choose the jet bubbling absorber. The absorber is made up of an absorption tube and a jet nozzle. The inner diameter of the absorption tube is 33 mm, and the total height is 200 mm, in which the height of the liquid is only 30 mm. In the center of the absorber tube, a jet nozzle, whose inner diameter is only 0.4-0.6 mm, is arranged at the tip of the inlet gas line. The nozzle size is small enough to gain a highvelocity gas stream while causing a pressure drop of 1020 kPa crossing one nozzle. When the mixed gas runs through the nozzle in which the insertion depth of the nozzle is 10 mm with a high gas velocity, the liquid is impacted by the bubble of high initial velocity; this will bring bubbling with high turbulence and little residence time and will result in the satisfactory effect of selective NH3 absorption.

The recovering column of free NH3 is made of stainless steel pipe, which has an inner diameter of 30 mm and a height of 2000 mm. It consists of a rectifying section and a stripping section of which 500 mm regular packing is respectively packed. The solution enters in the middle portion of the column and is uniformly spread by the liquid distributor. The heating coil provides heat at the bottom of the column, and the power is controlled by the voltage regulator. The temperature, pressure, and liquid height are also controlled. The desorption column of NH2COONH4 resembles the free NH3 recovery column, except the height is 1800 mm and it contains only the stripping section. 3. Experimental Results and Discussion 3.1. Selective Absorption by a Two-Step Series Absorber. The data of selective absorption for the mixed gas of NH3/CO2 ) 2 (mole ratio) by a two-step series absorber are showed in Table 1. Repeated tests are performed for each condition, and the results are identical. (1) Overall Performance. Generally, from the table, the effect of selective absorption was obtained as follows: the exit average content of CO2 for the secondstep absorber reaches 90.9% (mole content), and the remaining ammonia average content is only 9.1%; the total average absorptivity of NH3 reaches 96.7%; the total average absorptivity of CO2 is only 33.1%; the concentration of NH3 in the liquid phase is determined by the water flux, and the experimental concentration reaches 10.82 mol/L; the average value of NH3/CO2 in the exit liquid of the first-step absorber is 5.89, meaning that using the ordinary distillation methods can separate 66.9% of free NH3. Though the selective absorption of NH3 into an aqueous solution can be carried out at high gas velocity, CO2 is still absorbed by the NH3 solution in any

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 avg

no.

40.21 50.23 50.23 50.23 60.32 60.32 60.32 60.32 60.32 70.4 70.4 70.4 70.4 62.84 62.84 78.49 94.26 94.26 94.26 94.26 94.2 94.26 94.26 94.26 110 110 110 110 110

Q0 (mL/s)

0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

d1 ) d2 (mm)

1.21 1.2 1.2 1.18 1.18 1.18 1.18 1.0 0.813 1.2 1.2 1.0 0.82 1.18 1.19 1.12 0.82 1.0 1.1 1.1 1.2 1.18 1.18 1.45 0.83 0.92 1.12 1.2 1.35

QL (L/h)

80 100 100 100 120 120 120 120 120 140 140 140 140 80 80 100 120 120 120 120 120 120 120 120 140 140 140 140 140

u1 (m/s) 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 61 60 61 60 61 60 60 60 60 60 60 60

t1 (°C) 2.8113 3.490 3.513 3.619 4.321 4.281 4.256 4.987 6.078 4.967 4.909 5.836 7.102 4.495 4.422 5.899 9.504 7.865 7.201 7.143 6.561 6.702 6.748 5.501 10.827 9.887 8.213 7.720 6.879

NH3 0.525 0.618 0.620 0.642 0.707 0.701 0.690 0.837 1.041 0.807 0.802 0.970 1.240 0.787 0.772 0.987 1.770 1.401 1.218 1.198 1.090 1.113 1.123 0.881 2.104 1.789 1.458 1.303 1.108 5.85

5.36 5.65 5.67 5.64 6.11 6.11 6.17 5.96 5.84 6.15 6.12 6.02 5.73 5.71 5.73 5.98 5.37 5.61 5.91 5.96 6.02 6.02 6.01 6.24 5.15 5.53 5.63 5.93 6.21 51.1

50.5 50.2 50.8 50.6 50.4 50.4 50.3 50.6 51.0 51.7 51.0 51.4 51.8 50.8 51.0 51.1 52.1 51.9 51.6 50.7 51.1 51.0 51.0 50.4 53.0 52.0 51.6 51.2 50.3 48.9

49.5 49.8 49.2 49.4 49.6 49.6 49.7 49.4 49.0 48.3 49.0 48.6 48.2 49.2 49.0 48.9 47.9 48.1 48.4 49.3 48.9 49.0 49.0 49.6 47.0 48.0 48.4 48.8 49.7 58.1

first-step absorber liquid exit gas exit C (mol/L) (mol %) CO2 NH3/CO2 NH3 CO2 60.3 60.2 59.2 59.9 58.7 58.6 58.7 58.6 58.0 56.4 57.6 57.4 56.8 59.2 58.7 58.0 57.6 57.1 56.8 58.3 57.5 57.9 57.8 58.7 55.7 57.0 57.7 58.1 59.6 19.9

22.2 21.1 21.0 21.6 18.7 18.6 18.3 19.0 19.3 18.6 18.7 19.3 19.7 20.9 20.8 19.6 21.9 20.7 19.0 18.8 18.8 18.9 19.1 18.7 21.4 20.8 20.5 20.3 20.1

absorptivity (%) NH3 CO2

Table 1. Experimental Data of Selective Absorption in Two-Step Series Absorbers

44.46 60.51 60.83 60.47 70.71 70.82 71.01 72.96 71.87 94.0 93.1 91.5 86.70 44.58 44.06 54.15 62.20 66.01 66.96 65.84 66.4 66.14 66.13 65.61 77.76 77.08 76.6 76 74

u2 (m/s) 50 60 55 50 50 55 60 50 50 50 60 60 60 50 60 50 50 50 50 60 60 55 50 50 50 50 50 50 50

t2 (°C) NH3 1.080 1.310 1.370 1.412 1.724 1.687 1.659 1.929 2.357 2.105 1.984 2.340 2.880 1.769 1.739 2.380 3.777 3.203 2.987 2.818 2.650 2.702 2.751 2.202 4.436 3.987 3.310 3.109 2.679

0.206 0.235 0.239 0.244 0.293 0.289 0.285 0.340 0.421 0.334 0.328 0.381 0.508 0.306 0.297 0.392 0.683 0.557 0.514 0.502 0.452 0.458 0.464 0.356 0.879 0.715 0.586 0.499 0.401 5.82

5.25 5.58 5.73 5.80 5.88 5.83 5.83 5.67 5.60 6.30 6.05 6.14 5.67 5.78 5.85 6.07 5.53 5.75 5.81 5.61 5.86 5.90 5.93 6.18 5.05 5.58 5.65 6.24 6.68 9.1

6.5 10.0 8.4 5.1 6.4 8.7 10.0 11.6 13.7 6.0 8.8 11.0 11.7 7.0 8.9 7.5 12.2 9.9 8.0 9.8 9.3 8.3 6.6 6.1 15.0 11.9 9.3 7.3 6.6 90.98

93.5 90.0 91.6 94.9 93.6 91.3 90.0 88.4 86.3 94.0 91.2 89.0 88.3 93.0 91.1 92.5 87.8 90.1 92.0 90.2 90.7 91.7 93.4 93.9 85.0 88.1 90.7 92.7 93.4 38.5

second-step absorber liquid exit gas exit C (mol/L) (mol %) CO2 NH3/CO2 NH3 CO2

37.6 36.2 37.8 38.3 39.0 38.1 37.5 36.9 36.7 41.4 39.1 38.4 38.8 38.4 38.0 39.2 38.0 39.3 40.3 38.0 39.0 39.1 39.8 39.1 38.7 38.5 39.0 39.2 38.0 13.3

14.3 13.0 13.2 13.2 13.2 13.1 12.9 13.0 13.1 13.2 12.9 12.5 13.7 13.3 13.0 12.9 13.7 13.7 13.9 13.5 13.3 13.2 13.4 12.7 15.3 13.8 13.8 12.6 11.4 96.7

absorptivity (%) NH3 CO2

97.8 96.3 97.0 98.2 97.7 96.8 96.2 95.5 94.7 97.8 96.7 95.8 95.6 97.5 96.7 97.3 95.5 96.4 97.1 96.3 96.5 96.9 97.6 97.8 94.4 95.6 96.6 97.3 97.6 33.1

36.5 34.1 34.2 34.9 32.0 31.7 31.2 32.1 32.4 31.8 31.6 31.8 33.4 34.2 33.8 32.5 35.6 34.4 32.8 32.3 32.1 32.2 32.5 31.3 36.7 34.6 34.3 32.8 31.4

total absorptivity (%) NH3 CO2

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Figure 2. Effect of CNH3 of the first-step absorber on the remaining ammonia content: (b) u1 ) 120 m/s; ([) u2 ) 140 m/s.

case. It is found that average value of 33.1% CO2 is absorbed by the NH3 solution and only 66.9% of CO2 is separated. This means that there is 33.1% of NH3/ CO2 ) 2 (mole ratio) still in the solution and not separated. It is also found that the exit content of CO2 still contains 9.1% of NH3 by the two-step series absorber. On the basis of the analytical method of multistage complete mixing flow in reactors and the selective absorption mechanism,5 the total absorptivity of NH3 would reach an average of 99.6% and the total absorptivity of CO2 would be 34.12% if the three-step series absorber were used. The exit content of CO2 for the third-step absorber would reach 98.84%, and remaining ammonia content would be only 1.16%. (2) Influence of the Main Factors on the Performance. Upon careful examination of the table, the main influence factors on the remaining ammonia content and the total absorptivity of CO2 for the two-step series absorber are discussed as follows: (a) Influence Factors on the Remaining Ammonia Content of the Exit Gas of the Second-Step Absorber. Effect of the NH3 Concentration of the First-Step Absorber. An effect of the concentration of NH3 of the first-step absorber in the liquid phase on the remaining ammonia content is shown in Figure 2 when d1 ) d2 ) 0.5 mm. It can be seen that the remaining ammonia content increases with the concentration of NH3. This is because the equilibrium partial pressure of NH3 in solution increases with the concentration of NH3, thus inducing the remaining NH3 content to increase. Effect of the Temperature of the Second-Step Absorber. The effect of the temperature of the secondstep absorber on the remaining ammonia content is shown in Table 1. It can be seen that the remaining ammonia content increases quickly with an increase of the temperature of the second-step absorber. The exit content of NH3 for the second-step absorber will be enhanced by 2% when the temperature is enhanced by 10 °C. The temperature of the first-step absorber must be 60 °C to avoid ammonium carbamate crystallization; however, the temperature of the second-step absorber can be 50 °C because most of the ammonia is absorbed in the first-step absorber and cannot be solidified at 50 °C. Effect of the Gas Velocity of the Nozzle. As shown in Figure 2, the high gas velocity of the nozzle will help to decrease the remaining ammonia content. This is because the absorption of NH3 is controlled by a gas film and a high gas velocity favors selective absorption of NH3.

Figure 3. Effect of CNH3 of the first-step absorber on absorptivity of CO2: (9) u1 ) 80 m/s; (2) u1 ) 100 m/s; (b) u1 ) 120 m/s; ([) u1 ) 140 m/s.

(b) Influence Factors on the Total Absorptivity of CO2. Effect of CNH3 of the First-Step Absorber. The effect of CNH3 of the first-step absorber on the total absorptivity of CO2 is shown in Figure 3. Though the selectivity of NH3 absorption is high, the total absorptivity of CO2 increases quickly with an increase in the NH3 concentration obtained. This occurs because the CO2 reaction with NH3 increases with the concentration of NH3, so that absorptivity of CO2 is enhanced. Effect of the Gas Velocity of the Nozzle. As shown in Figure 3, the total absorptivity of CO2 decrease when the gas velocity of the nozzle is enhanced from 120 to 140 m/s. This occurs because the high gas velocity can increase the absorption of NH3 and accordingly decrease the absorption of CO2. 3.2. NH3 Recovery and Desorption of the Solution. The solution containing free NH3 and NH2COONH4 obtained by selective absorption is first distilled to the free NH3 recovery column, and the liquid NH3 is obtained by condensation at the top of the free NH3 recovery column; then the solution from bottom enters the NH2COONH4 desorption column, in which gas of NH3-CO2 is desorbed and recycled to the front of first absorber for continuing separation. The experimental conditions and obtained data for the NH3 recovery and desorption are listed in Table 2. At least 10 repeated samples are performed in every experiment, and the results listed are the average values. It is shown in Table 2 that most free NH3 is recovered and almost all of the NH3 and CO2 is desorbed through the NH2COONH4 desorption column; the purity of liquid NH3 is 99.9% at the top of the free NH3 recovery column under an absolute pressure of 18.7 MPa; the discharge liquid from the NH2COONH4 desorption column only contains 0.017% NH3 and 0.0067% CO2. 4. Evaluation of Heat Consumption in the Industrial Separation Process Figure 4 is a sketch of the recommended industrial separation process for separating mixed gas of NH3/CO2 ) 2 (mole ratio). The process consists of a three-step series selective absorber and a solution distillationdesorption system. There is a reboiler at the bottom of the free NH3 recovery column. The heat recovery of the discharge solution from the free NH3 recovery column can provide most of the heat needed. The whole separation process is divided into absorption, NH3 recovery, and desorption; the three parts conducted the heat consumption by energy balance according to the above experimental conditions.

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Figure 4. Sketch of the recommended industrial separation process for a mixed gas of NH3/CO2 ) 2 (mole ratio). Table 2. Experimental Conditions and Data of NH3 Recovery and NH2COONH4 Desorption free NH3 recovery column

NH2COONH4 desorption column liquid exit

column bottom no.

power (W)

t (°C)

1 2 3 4 5 6 7 8 9 10 avg

120 120 120 120 120 120 120 120 140 100 120

169 173 178 178 177.5 177 177 177 177 177.5 176.0

inlet solution

column top

p Q0 CCO2 NH3 content CNH3 (atm) (L/h) (mol/L) (mol/L) (mol/h) (%) 17 18 19 19 19 19 19 19 19 19 18.7

1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2

7.055 7.687 7.712 7.756 7.845 8.123 8.456 8.668 7.894 7.815 7.805

1.223 1.346 1.352 1.361 1.371 1.453 1.561 1.687 1.392 1.375 1.385

5.390 5.954 5.973 5.977 6.186 6.198 6.418 6.367 6.144 6.089 6.010

99.7 99.8 99.9 99.9 99.9 99.9 99.9 99.9 99.9 99.7 99.9

When 1000 mol of a mixed gas of NH3/CO2 ) 2 is separated, 666.7 mol of liquid NH3 and 333.3 mol of gas CO2 are recovered. The heat consumption of the free NH3 recovery column is 7027 kJ/kg of NH3, so the total heat consumption for the solution distillation system is 7027/kg of NH3. This corresponds to a consumption of 3.4 kg of steam of 1 MPa. The NH2COONH4 desorption column does not need any heat consumption because of the heat recovered from the discharge solution (176 °C). The heat cooled from the selective absorption system, condensers, and discharge liquid from the desorption column is 8178 kJ/kg of NH3. 5. Conclusion An experimental process of the selective separation mixed gas of NH3/CO2 ) 2 (mole ratio) has been studied

column bottom

content

power (W)

t (°C)

QL (L/h)

NH3 (%)

CO2 (%)

50 50 50 50 50 50 50 50 50 50 50

111 111 112 112 112 112 112 112 112 112 112

1.155 1.145 1.154 1.158 1.157 1.156 1.152 1.149 1.158 1.157 1.154

0.0102 0.0120 0.0135 0.0137 0.0142 0.0157 0.0161 0.0172 0.0148 0.0144 0.0147

0.0044 0.0053 0.0059 0.0061 0.0060 0.0072 0.0075 0.0082 0.0070 0.0064 0.0067

column top NH3 CO2 H2O (mol/h) (mol/h) (mol/h) 3.07 3.26 3.27 3.32 3.22 3.54 3.72 4.02 3.32 3.28 3.459

1.466 1.613 1.621 1.632 1.644 1.742 1.871 2.022 1.669 1.648 1.727

0.815 0.912 0.879 0.889 0.873 0.932 1.004 1.068 0.896 0.867 0.899

in high-velocity bubble jet absorbers. It can be concluded that the process is feasible, and the results are as follows: (1) It is enough to prove by two-step series bubble absorbers that the exit content of CO2 for the secondstep absorber reaches 90.9% and remaining ammonia content is only 9.1%. The total absorptivity of NH3 reaches 96.7%, and the total absorptivity of CO2 is 33.1%. (2) The remaining ammonia content of the exit gas in the second-step absorber increases with the NH3 concentration of the first-step absorber and the temperature of the second-step absorber but decreases with an increase of the gas velocity of the nozzle. Also, the total absorptivity of CO2 decreases with the obtained

Ind. Eng. Chem. Res., Vol. 42, No. 12, 2003 2831

concentration of NH3 and increases with the high gas velocity of the nozzle. (3) The most free NH3 can be recovered by distillation under 1.87 MPa, and the remaining NH2COONH4 would be desorbed through the desorption column. The purity of liquid NH3 obtained is 99.9%; the discharge liquid contains 0.014% NH3 and 0.0067% CO2. According to the above experimental results, the total heat evaluated is 7027 kJ/kg of NH3. This corresponds to consumption of 3.4 kg of steam of 1 MPa, and the total heat cooled needs to be 8178 kJ/kg of NH3. Nomenclature C ) concentration of i in liquid, mol/L d1 ) diameter of the nozzle of the first-step absorber, mm d2 ) diameter of the nozzle of the second-step absorber, mm h ) nozzle insertion depth, cm Q0 ) quantity of gas flow imported in the first-step absorber, NmL/s QL ) quantity of liquid flow, L/h p ) pressure of the free NH3 recovery column, atm t1 ) temperature of the first-step absorber, °C t2 ) temperature of the second-step absorber, °C u1 ) nozzle gas velocity of the first-step absorber, m/s u2 ) nozzle gas velocity of the second-step absorber, m/s VL ) volume of liquid in the absorption, m3 W ) heating power of the column, W

Literature Cited (1) Wilhelm, B.; Mutterstadt, H.; Gettert, M. Partial or complete separation of gas mixture containing ammonia and carbon dioxide. U.S. Patent 4,013,431, 1974. (2) Ripperger, W. The world melamine industry [J]. Nitrogen 1997, 228, 43. (3) Lai, Z. P.; Wang, J. F.; Deng, R. S.; Jin, Y. Study on the separation of NH3 and CO2 mixture using a nonequilibrium absorption process. Proceedings of the 9th National Conference on Chemical Engineering (NCCE’98), Qingdao, China, 1998; p 484. (4) Wang, M. K.; Cheng, F. H.; Meng, L. X. The new treating technology for melamine off gas. Chem. Ind. Eng. Prog. (China) 1999, 6, 59. (5) Cai, P. X.; Zhang, C. F.; Zheng, Z. S.; Qin, S. J. Selective Separation of NH3-CO2 (I) Mechanism and Influence Factor of Intermittent Absorption. J. East China Univ. Sci. Technol. (China) 2001, 2, 24. (6) Cai, P. X.; Zhang, C. F.; Zheng, Z. S.; Qin, S. J. Selective Separation of NH3-CO2 (II) Continuous Absorber Investigation and Mechanism Demonstration [J]. J. East China Univ. Sci. Technol. (China) 2002, 1, 38. (7) Ryo, K. Calculational method of effective ammonia concentration and partial pressure of carbon dioxide in NH3-CO2-H2O solution. Kagaku Kogaku 1964, 28, 625.

Received for review June 10, 2002 Revised manuscript received December 13, 2002 Accepted January 23, 2003 IE0204282