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Ind. Eng. Chem. Res. 2001, 40, 850-853
SEPARATIONS Failure Modes in Concentrated Absorbers during the Transition to Standby Operation Praveen Gunaseelan and Phillip C. Wankat* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907-1283
A dynamic sieve-tray model is used to investigate the effect of suddenly placing a concentrated absorber on standby by replacing the feed gas with inert gas. Model predictions show large desorption rates during the transient, resulting in a dramatic increase in gas flow rates, which leads to extremely high entrainment. The model also predicts a large pulse of solute in the vent gas and a large rise in the downcomer liquid level. The results indicate a strong possibility of design failure during the transient. We discuss methods to reduce the severity of the transient behavior. Introduction In a previous paper,1 we developed a dynamic sievetray model to study pressure effects in concentrated absorbers. The important assumptions in the model are equilibrium trays, completely mixed phases, and ideal gas behavior. Despite the limiting assumptions, the model was able to reliably predict the key features of concentrated absorber dynamics. The predictions of transient gas flow and pressure agreed qualitatively with theoretical predictions and experimental observations for analogous concentrated adsorption columns. The model was checked against limiting cases including dilute absorbers and isothermal absorbers. Our predictions for concentrated absorber start-up might also explain the failure of absorbers used for the emergency absorption of vent gas surges from reactors. In this paper we examine the process of placing an absorber on “standby” operation when the feed gas supply is interrupted. To prevent the liquid from draining out of the trays, the feed gas is replaced with an insoluble carrier gas. The resulting step decrease in solute concentration causes a liquid of high solute concentration to contact a gas of relatively low solute concentration. This will strip the solute from the liquid, causing a transient increase in the pressure and gas flow rates. Our objective is to delineate the mechanisms by which this can upset column operation.
Figure 1. Transient solute composition in gas for the test case. Feed gas switched from HCl to N2 at t ) 1700 s. Trays are numbered from top ) 1 to bottom ) 6 in all figures.
be based on the tray active area.3,4 This change does not affect the previous dynamic results much1 because entrainment is low during start-up. However, it does affect the steady-state solution and the dynamic results in this paper because entrainment is significant. Test Case
Model Equations The model has been described in a previous paper.1 We again study the aqueous absorption of HCl in a sieve-tray column with six trays. A correction was made to the entrainment correlation used in the model, which was taken from Ludwig.2
ew ) 3.08 × 105
( )( ) 73 vc σ S′
3.2
(1)
The earlier model calculated the superficial gas velocity vc in eq 1 based on the column area, whereas it should
For this study, we slightly modified the test case outlined in the previous paper.1 After the column is started and reaches a steady state, the feed gas solute concentration is drastically decreased (at t ) 1700 s) while the total feed gas flow rate is kept constant. All other process conditions (see Table 1) are kept constant during the step. The above process disturbance is of interest because there is a large change in the number of moles in the gas. Other disturbances (e.g., feed temperatures, feed liquid concentration, feed gas moisture content) were investigated and found to be uninteresting.
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Figure 3. Transient solute transfer rates for the test case. (Ntr is positive when solute is absorbed.)
Figure 2. Transient solute composition in liquid for the test case.
Discussion of Results
the large heat of absorption, there is significant evaporation of solvent (Figure 4). Interestingly, there is no condensation of solvent associated with solute desorption because the gas in the bottom trays contains very little water vapor. As a result, the heat of desorption is taken from the liquid, resulting in rapidly decreasing tray temperatures. (To confirm this, we ran a case in which half the feed gas after the step was replaced with steam. As expected, solute desorption resulted in condensation of steam, and the drops in the tray temperatures were smaller.) The large increase in the gas flow rates in the bottom trays (Figure 5) is primarily due to solute desorption. (It is expected that mass-transfer models would predict less severe transients.) In the upper trays, the decrease in gas flow due to the absorption of the solute stripped from the lower trays is overcompensated by the subsequent solvent evaporation, resulting in a net increase in the gas flow. The peak exit gas flow rate is 5 times
At t ) 1700 s, the feed gas (100% HCl) is replaced by an inert carrier gas (100% N2). Because there is no solute entering the column after the step, the solute concentrations in the column should eventually decrease to zero as the solute leaves the column with the exit streams. The model predicts this result (Figures 1 and 2). The transient solute concentrations for the upper trays (trays 1, 2, and 3) show a temporary increase before decreasing to zero. This occurs because the feed gas (100% N2) strips the solute from the bottom trays (Figure 3), and the desorbed solute is carried up the column and reabsorbed in the upper trays. Because the liquid concentration on tray 2 is lower than that on tray 3, tray 2 has a larger capacity for solute and absorbs solute at a higher rate. A part of the solute wave gets past tray 2 and is absorbed in tray 1. The solute transfer rates in the column eventually go to zero. Because of Table 1. Test Case Specifications
Feed Conditions gas feed stage pressure (bar) temperature (°C) component flows (kmol/s) before standby (t g 200 s) HCl water N2 during standby (t g 1700 s) HCl water N2 Initial Conditions in Column pressure drop per tray (Pa) tray temperatures (°C) HCl mole fraction in liquid transfer rates (kmol/sec) crest height (mm) downcomer liquid height (cm) number of trays tray diameter (m) tray spacing (m) hole diameter (mm) active area (m2)
800 55 0 0 20 35
liquid
6 ∼1.08 (variable) 0
1 1 100
0.05 0 0
0 0.3 0
0 0 0.05
0 0.3 0 Operating Conditions molar (L/G)feed feed temperature (°C) liquid gas feed liquid pressure (bar) heat removal (J/s)
Column and Tray Design Specifications 6 weir length (m) 1.25 weir height (m) 0.5 hole area (% active) 4.5 downcomer area (m2) 0.789 area under apron (m2)
6 100 0 1 0 0.875 0.05 0.1275 0.108 0.0219
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Figure 4. Transient solvent (water) transfer rates for the test case. (Ntrw is positive for solvent evaporation.) Figure 7. Transient entrainment rates for the test case.
Figure 5. Transient gas flow rates for the test case.
Figure 6. Transient liquid level in downcomer for the test case.
the inlet gas flow rate, and this pulse can mechanically damage the trays. Tray pressures and pressure drops both increase and cause a large rise in the downcomer liquid level (Figure 6), especially for the downcomer between trays 1 and 2, which almost overflows into tray 1. The high gas rates lead to severe entrainment (Figure 7). As expected, weeping is insignificant. The entrain-
Figure 8. Transient rates of solute loss in the vent for the test case.
ment rate is especially high for the top tray and is temporarily greater than the rate of feed liquid. The weight of liquid entrained from the top tray peaks at 1.4 kg/kg of gas, which is far beyond the level that is considered extreme2 (0.5 kg/kg of gas). The peak rate of solute loss in the entrained liquid (Figure 8) is more than 5 times the maximum recommended rate of solute loss. This “burp” of solute in the vent stream constitutes a design failure. (The curve for the maximum recommended rate of solute loss was obtained using the maximum recommended solute concentration in the vent gas, 0.03 wt %.5 The curve is not flat because of the variations in the transient gas flow rate.) Additional simulations showed that the test column is more vulnerable to failure when operated at higher capacities and when scaled down. For these simulations, downcomer overflow into the top tray occurred, which could cause downcomer flooding. It should be noted that the results discussed in this paper are in some ways specific to the HCl-water system, the magnitude of the concentration step, the operating L/G ratio, and the assumption of constant froth density. Columns operating at less than 60 mol % feed HCl do not respond dramatically to a step decrease. Lower values of L/G give a larger burp of solute, whereas at higher values, the burp is significantly reduced in magnitude. The value of S′ in eq 1 was calculated on the basis of Ludwig’s2 recommendation of a constant froth density
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of 0.4. Offline calculations of transient froth density, based on Kister’s6 correlation, predict an average froth density of 0.22 and predict large decreases in transient froth density at the instant of the step (t ) 1700 s). The lower froth densities would decrease S′ and hence increase entrainment. This indicates that we might be underpredicting the entrainment for the test case (Figure 7). Because the severity of the transient arises from the large step decrease, the large gas flow rates, and the resulting high entrainment rates, taking steps to prevent them will result in safer column operation. For existing columns for which design changes are inconvenient, transient behavior can be greatly improved by reducing the gas flow rates at the instant of the concentration decrease. After the solute is flushed out of the column, the gas flow rates can be restored so that the column is ready to be switched back to normal operation. For the test case, increasing the L/G ratio (by reducing gas flow from 0.05 to 0.02 kmol/s) at the instant of the step decreases the peak entrainment (0.03 kmol/s) and the peak solute loss in the vent (0.008 kg/ s) to acceptable levels and significantly reduces the peak downcomer liquid height (37 cm). There is a modest increase in the weep rates, but because the column is on standby, this is not a serious problem. The column response can be further improved by decreasing the solute concentration in two smaller steps instead of one large step, mimicking the procedure described for column start-up.1 At the price of a more complicated switching operation, the advantage of this method is that a less drastic reduction in gas flow rate (from 0.05 to 0.0375 kmol/s) is required to ensure safe column operation, and transient weeping is vastly reduced. More conservative design can solve the aforementioned problems in new columns or retrofits. Increasing the tray spacing will reduce entrainment rates without aggravating weep rates. Increasing the tray spacing from 50 to 70 cm brings the peak entrainment and solute losses in the vent to acceptable levels and solves the downcomer liquid level problem. The disadvantage is higher fixed costs. Installation of a mist eliminator at the vent will also help prevent the loss of solute with the entrained liquid. Used in conjunction with the previous recommendations, it will further reduce the risk of abnormal column operation during the transient. Summary and Conclusions Simulation results for aqueous absorption of HCl gas in a sieve-tray column reveal that a large step decrease in the feed gas concentration can result in phenomena that endanger safe column operation. These phenomena include large increases in gas flow rates, column pressure, entrainment rates, and downcomer liquid heights. Entrainment from the top tray can result in a spike of HCl in the vent stream.
In existing columns, these problems can be alleviated by decreasing the gas flow rate at the instant of the concentration step. The severity of the transient can be reduced further by using two small concentration steps instead of a single large step. These problems can also be addressed by providing additional tray spacing, if possible, and by decreasing gas velocities in the column. Installation of a mist eliminator at the vent will help prevent entrainment losses. In summary, concentrated absorbers with stable start-up and steady-state behavior can fail during the transition to standby operation. The possibility of column failure can be reduced by making changes at the design stage or by reducing the gas flow rates and using smaller concentration steps during the transition. Acknowledgment This research was partially supported by the Purdue Research Foundation. Nomenclature ew ) entrainment, kg of liquid entrained/kg of gas G ) molar gas flow rate, kmol/s L ) molar liquid flow rate, kmol/s Ntr ) transfer rate of solute across phases on equilibrium tray, kmol/s Ntrw ) transfer rate of solvent across phases on equilibrium tray, kmol/s S′ ) effective tray spacing, distance between top of foam and next tray above, m vc ) gas superficial velocity based on tray bubbling area, m/s σ ) surface tension in liquid, mN/m
Literature Cited (1) Gunaseelan, P.; Wankat, P. C. Dynamic Tray Model to Predict Start-Up Transients in Concentrated Absorbers. Ind. Eng. Chem. Res. 2000, 39, 2525-2533. (2) Ludwig, E. E. Mechanical Designs for Tray Performance. In Applied Process Design, 3rd ed.; Gulf Publishing Co.: Houston, TX, 1997; Vol. 2, pp 176-177. (3) Kister, H. Z.; Haas, J. R. Entrainment from Sieve Trays in the Froth Regime. Ind. Eng. Chem. Res. 1988, 27, 2331-2341. (4) Koziol, A.; Mackowiak, J. Liquid Entrainment in Tray Columns with Downcomers. Chem. Eng. Process. 1990, 27, 145153. (5) Gershon, S. Hydrochloric Acid. In Encyclopedia of Chemical Technology, 1st ed.; Kirk, R. E., Othmer, D. F., Eds.; Interscience: New York, 1952; p 666. (6) Kister, H. Z. Tray Design and Operation. Distillation Design; McGraw-Hill: New York, 1992; Chapter 6, p 319.
Received for review July 17, 2000 Accepted November 17, 2000 IE000669E
