James Curley CHEG 4142 10/24/16
CHALLENGE #2 PROBLEM 1 A tower packed with 1-in. (25.4 mm) ceramic Intalox saddles is to be built to treat 25,000 ft3 of entering gas per hour. The ammonia content of the entering gas is 2 percent by volume and contains 98 vol.% air (79 vol. % nitrogen and 21 vol.% oxygen). Ammonia free water is used as absorbent. The temperature of the entering gas and the water is 68°F; the pressure is 1 atm. (a) If the design pressure drop is 0.5 in. H20 per foot of packing, and we want 0.5 vol. % ammonia in the liquid stream leaving the absorber, what does the inlet liquid flow rate need to be? Try starting with the number of stages set to 7 then vary to 10 and 12 stages. Guess a liquid inlet flowrate and see how the volume fraction of ammonia leaving the absorber in the liquid changes. c hanges.
THEORY The numbers below were for calculating the flow rates of the Incoming gas and liquid streams based on the compositions specified. Gas stream: 25000 ft3/hr
Std. volume fraction ammonia: 0.02 Std. volume fraction air: 0.98 Std. volume fraction N2: 0.98*0.79 = 0.7742 Std. volume fraction O2: 0.98*0.21 = 0.2058
Figure 1. Absorber unit in Aspen Plus.
APPROACH So, my approach to this problem was to setup the absorber with the specifications, and then use multiple liquid flow rates and observe how the volume fraction of ammonia changes in the liquid out stream. I used NRTL as the thermodynamics package. I used 1-IN generic ceramic INTX packing. I assumed the 1
packing height was 100 ft (it was found that packing height was not sensitive on ammonia absorption, so this assumption is reliable). In order to specify the pressure drop as the problem statement says, I inputted the pressure profile such that the top stage was at 1 atm, and at each foot of packing, 0.5 in-water was dropped, resulting in a bottom pressure of 0.8772 atm. 50 in-water would therefore be the t otal pressure drop using the assumption of 100 ft of packing (figure 3).
0.5 ℎ = 0.00122793 100 ∗ 0.5 ℎ = 0.122793 = 1 = 1 − 0.122793 = 0.8772
RESULTS Table 1. Various inlet liquid flow rates and subsequent volume fraction of ammonia in liquid out stream. No. Stages 7
10
12
Flow Rate (ft 3/hr) 150 300 220 150 300 220 150 300 220
XNH3,out 0.00736 0.00369 0.00503 0.00736 0.00369 0.00503 0.00736 0.00369 0.00503
Table 1 shows a summary of the results in the absorption column at various stages and flow rates. It was found that 220 ft3/hr as the liquid inlet flow rate r esulted in an absorption of 0.5% ammonia. Several flow rates were iterated, and as you can see a slower flow rate has more ammonia in the outlet, and a faster flow rate has less ammonia in the outlet. Furthermore, it was found that the number of stages did not change the absorption of ammonia. Packing height was changed to many values from 10 ft to 1000 ft in order to observe a change in XNH3,out at different number of total stages. No change was observed. Theoretically, however, more stages would result in more absorption of ammonia provided it does not reach equilibrium. For instance, if we ran a simulation with 7 stages at a flow rate which yielded a certain amount of absorption, increasing the number of stages would allow for it to come closer to equilibrium. Figures 2 through 5 show the input specifications used to simulate this column using 12 stages. Everything was the same in the 7 and 10 stage inputs except for specifying the number of stages to the appropriate number.
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Figure 2. Absorber unit specifications.
Figure 3. Pressure configuration.
Figure 4. Volume fractions of components in the column with 12 stages.
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Figure 5. Results summary for 12 stages and 220 ft3/hr liquid inlet flow rate.
PROBLEM 2 If an absorption column packed with 1-in. Berl saddles is repacked with 1-in. metal Pall rings, what will be the major changes in operating characteristics?
Figure 5. Berl saddle vs. Pall ring (http://www.separationprocesses.com/Operations/Fig035a.htm).
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Figure 5.1. Table of packing characteristics from Unit Operations of Chemical Engineering; McCabe, Smith and Harriott.
As you can see from the chart, there are different characteristics between the two packing designs. For instance, the bulk density of the Pall rings is much lower (30 lb/ft3) than that of the berl saddle (45 lb/ft3). This is mostly due to the shape of each, such that the berl saddle allows for more to be packed in per unit volume of the column. Next, the total area for the pall rings is also smaller at 63 ft 2/ft3 compared to the berl saddle’s 76 ft 2/ft3. The porosity for pall rings is much higher at 0.94, compared to the berl saddle 0.68. Furthermore, the pressure drop packing factor for the berl saddle is nearly twice that of the pall ring, and the mass transfer coefficient of t he pall ring is slightly higher than the ber l saddle.
WHAT THIS MEANS Since the pressure drop factor for pall rings is much higher, it will result in a larger pressure drop at flooding. The berl saddle does have a higher mass transfer coefficient which would increase t he tendency for the sour gas to transfer into the liquid phase. Higher porosity or voidage will result in a higher efficiency of removing the sour gas, such as CO2. With these characteristics in mind, it is probable that replacing the berl saddles with metal pall rings would result in a more efficient operation and more absorption of sour gas into the liquid phase.
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PROBLEM 3 The flowsheet for the pilot plant for CO2 capture by MEOH includes an absorber, a flash tank, a stripper, and so on. However, only the absorber data are reported. The sour gas enters the bottom of the absorber, contacts with lean MEOH solvent from the top counter-currently and leaves at the top as sweet gas, while the solvent flows out of the absorber at the bottom as the rich solvent with absorbed CO2 and some other gas components. (a) Plot the liquid temperature profile (b) Plot the compositions of the components at each stage; Analyze the composition profiles.
THEORY
Figure 6. Absorber unit in Aspen Plus. Figure 6 shows the aspen setup for the absorption component described in the system.
APPROACH My approach to this problem is to generate mole fraction vs. stage graphs for the absorber and analyze its validity and efficacy, using the given specifications and operating conditions. The objective is to remove the sour gas from the inlet gas stream using the liquid absorbent, methanol. I expect the liquid mole fractions of the sour gas to increase as the stage increases, thereby signifying the mass transfer from the gas to liquid phase. I used Peng-Robinson equation of state.
Figure 6.1. Rating specifications for the absorber. 6
RESULTS
Figure 7. Liquid mole fraction vs. stage for all components in t he absorber. Figure 7 shows all of the components in the absorption process. This graph is a good visualization for what is going on. I set up my simulation so that the liquid inlet, methanol, enters at the above stage, stage 1, and the inlet gas at the on-stage at stage 10. The methanol starts at a liquid mole fraction of nearly 1, and as the gas gets absorbed it goes down to about 0.937 because some of the gas is absorbing into the liquid. You can see that by the end that the components that make up the gas inlet make up about 6.3% of the liquid phase on a mole basis.
Figure 8. Liquid mole fraction of carbon dioxide and hydrogen sulfide vs. stage in the absorber. Figure 8 is examining the carbon dioxide and hydrogen sulfide which are the primary concern for absorption. This absorber is somewhat effective at removing CO 2 and H2S from the gas phase, where CO2 makes up nearly 6% of liquid and H2S makes up 0.25% (which is a lot considering the inlet gas flow rate of H2S was only 0.001 lbmol/hr, with an inlet liquid flow of 8.29 lb mol/hr).
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Figure 9. Temperature profile vs. stage in the absorber. Figure 9 shows the temperature profile throughout the absorber. The first stage has a starting temperature of around the inlet temperature of liquid at about -34°F. This temperature comes into contact with the vapor and as the co unter-current streams meet the temperature incre ases exponentially to about -16°F in the liquid phase.
Figure 10. Stream results for the absorber unit in Aspen Plus. Figure 10 shows the overall stream results for the absorber in Aspen Plus. Note that there is trace amount of H2S flow in the outlet sweet gas stream, and the CO2 in the sweet gas stream also decreases slightly.
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For each problem: • Theory – list relevant equations along with any relevant assumptions (20 points) • Solution Approach – clearly outline a process for solving the problems (30 points) • Discussion and Results – Do your results make sense? If so, why? It not, why not? If you don’t have results, what could have gone wrong? What could you change to potentially obtain results? (50 points)
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