Ind. Eng. Chem. Res. 1999, 38, 3105-3114
3105
A Novel Process for Diethanolamine Recovery from Partially Degraded Solutions. 2. Process Analysis Majid Abedinzadegan Abdi and Axel Meisen* Department of Chemical and Bio-Resource Engineering, The University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
The performance of a separation process for the purification of contaminated amine solutions is described. The process uses multistage distillation and an inert carrier liquid (hexadecane). A distillation column (50-mm i.d., filled to a height of 250 mm with stainless steel DE-Pak 1/4-in. packing) was employed to confirm the predictions made with the ASPEN process simulator and using the physical property data presented in part 1. Very good separation efficiencies were obtained under vacuum conditions for impurities typically found in contaminated diethanolamine solutions. The results are compared with conventional single-stage flash distillation. Introduction In part 1,1 we outlined a process based on multistage distillation and an inert liquid to recover alkanolamines from contaminated solutions. The vapor-liquid equilibrium studies on mixtures of diethanolamine (DEA) and bis(hydroxyethyl)piperazine (BHEP) indicated that although the volatilities of the components are close, the differences are likely sufficient for efficient separation in a multistage process. The need for an inert carrier liquid (e.g., hexadecane) was also described in part 1, and the present paper therefore examines the performance of multistage DEA reclamation processes under different operating conditions. The ASPEN process simulator was used. Distillation under Total Reflux Conditions Distillation experiments were performed using a packed column equipped with a total reflux condenser to evaluate the DEA purity achievable in the top product of a distillation column with a given number of theoretical stages. These results also provide a means of verifying the accuracy of the nonrandom two liquid (NRTL) model predictions presented in part 1.1 Experimental Apparatus and Procedures. The distillation equipment consisted of a 300-mL flask, a glass column (50-mm i.d., 200-mm high) which could be filled to various heights with stainless steel DE-Pak 1/ -in. packing (Distillation Engineering, Livingston, NJ) 4 and a reflux condenser. An electric heating mantle controlled with a Variac supplied the heat required for distillation. The vacuum was drawn using a two-stage rotary vacuum pump (model E2M5, Edwards High Vacuum International, U.K.), and the pressures were measured at the top and bottom of the column using a diaphragm vacuum gage (model l-68801-53, Cole Parmer Instruments Co., Chicago, IL). Type-K thermocouples were placed in the liquid of the reboiler and just below the entrance to the reflux condenser. The column, reboiler, and condenser were insulated using glass wool and insulating tape. The reflux condenser was designed in such a manner that samples could be withdrawn from * Corresponding author. Tel.: 604-822-6708. Fax: 604-8226003. E-mail:
[email protected].
the top of the column. The reboiler flask had a discharge joint and a stopcock at the bottom for liquid sampling. To withdraw samples, a vacuum was applied to the sampling points. To moderate boiling, relatively large quantities of glass beads (almost 1/3 of the flask volume) were added to the flask. A liquid distributor was placed on top of the packing to ensure good distribution of the totally refluxed top product. Samples from the top and bottom of the distillation column were analyzed using gas chromatography. For samples containing two liquid phases, a combination of gravimetry and gas chromatography was used to determine the inert liquid content of the samples. The DEA-BHEP and DEA-hexadecane systems were used to determine the number of theoretical stages of the distillation column. The column was packed to a height of 120 mm. Then, 250 mL of a test sample consisting of DEA saturated with BHEP at 80 °C was charged into the boiling flask. The samples from the top and bottom section of the column were withdrawn, and the temperatures were recorded when the column reached steady-state conditions. Results and Discussion. The number of theoretical stages for a packing height of 120 mm is given in Table 1; Figure 1 shows the equilibrium diagram for the BHEP-DEA system at 1.1 kPa. Experiments with the DEA-hexadecane system showed that a single stage could change the DEA concentrations from the azeotropic values at the top to very small values at the bottom. The analysis of the samples with very low DEA concentrations at the bottom proved difficult, and temperatures were therefore used to infer the bottom concentrations. As seen from Table 2, total reflux experiments conducted under vacuum and with solutions containing DEA, BHEP, and THEED [tris(hydroxyethyl)ethylenediamine] showed that the top product contained no detectable amounts of THEED. However, the BHEP/ DEA ratios varied with the composition of the initial charge and the packing height. As expected, better DEA purities in the top product are achieved with taller columns. For the pressure range examined (1.3-13.3 kPa), no significant variations in BHEP separation were observed. Therefore, the results are reported for selected pressure values. Table 3 provides a comparison between the experimental results and the predictions obtained
10.1021/ie9804531 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/13/1999
3106 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 Table 1. Results of DEA-BHEP and DEA-Hexadecane Separation Experiments for the Approximate Number of Theoretical Stages (Column, 50-mm i.d. × 120-mm High; Packing, DE-Pak, 1/4-in. Stainless Steel) DEA mole fraction
temp., °C
binary system
top
bottom
top
bottom
pressure,a kPa
stages
DEA-BHEP DEA-Hexadecane DEA-Hexadecane
0.9941 0.4935 0.4825
0.9581 0.0007b 0.0027b
146.6 171.5 138.9
153.6 185.4 151.2
1.1 6.7 1.3
3-4 2 2
a Pressures measured at the column top; less than 0.3 kPa pressure drop along the column. b Calculated from temperature readings with the ASPEN simulator.
Table 2. Experimental DEA, BHEP, and THEED Separations Achieved with a Distillation Column Operating under Total Reflux Conditions (Column, 50-mm i.d.; packing, PE-Pak, 1/4-in. Stainless Steel; Pressure, 1.3-13.3 kPa (Absolute Pressure); Column Pressure Drop, Max. 0.1-0.3 kPa; Initial Charge, ∼250 mL) column height: 120 mm stream feed
xDEA xBHEP xTHEED XBHEP/DEA XTHEED/DEA
top product
bottom product
BHEP separation, %a a
b
parameter
Undetectable.
run 1
run 2
column height: 250 mm
run 3
run 4
run 5
run 6
0.855 0.078 0.067 0.091 0.078
0.612 0.243 0.145 0.397 0.237
0.878 0.065 0.057 0.074 0.065
0.823 0.071 0.106 0.086 0.129
0.660 0.207 0.133 0.313 0.202
0.911 0.045 0.044 0.049 0.048
xDEA xBHEP xTHEED XBHEP/DEA XTHEED/DEA temp., °C pressure, kPa (abs)
0.988 0.012 N.D.b 0.012 0.000 187.0 6.7
0.975 0.025 N.D. 0.026 0.000 172.5 4.0
0.991 0.009 N.D. 0.009 0.000 203.4 12.7
0.998 0.002 N.D. 0.003 0.000 184.5 6.4
0.997 0.003 N.D. 0.003 0.000 169.7 3.2
0.999 0.001 N.D. 0.001 0.000 204.0 12.9
xDEA xBHEP xTHEED XBHEP/DEA XTHEED/DEA temp., °C
0.849 0.081 0.070 0.095 0.082 191.5
0.597 0.252 0.151 0.422 0.253 183.4
0.873 0.068 0.059 0.078 0.068 208.1
0.798 0.081 0.121 0.102 0.152 189.6
0.611 0.236 0.153 0.373 0.250 179.6
0.899 0.051 0.050 0.057 0.056 209.1
87.5
93.9
88.4
99.0
99.3
98.2
(BHEP/DEA)bottom - (BHEP/DEA)top 100 (BHEP/DEA)bottom
and the experimental results. The slight discrepancies between the measured and calculated temperatures at the bottom are likely due to the overheating of the thermocouple in the flask due to conduction of heat from the heating mantle to the thermowell where the thermocouple is placed. As seen from Table 4, somewhat lower separation efficiencies for BHEP and DEA were obtained when hexadecane was introduced. Because of the low mutual solubility characteristics, the presence of the inert carrier (hexadecane) provided an inert dilution which reduced the absorptive capacity for higher boiling components (e.g., BHEP) in the vapor phase. For the examined pressure range of 1.3-13.3 kPa, no significant variations in BHEP separation were observed. As shown by Table 4, a high degree of BHEP removal was achieved. Table 5 shows the results of the ASPEN simulation for the distillation column. Fewer than three theoretical stages are present when the column is filled to a height of 120 mm.
Figure 1. Equilibrium diagram for the DEA-BHEP system at 1.1 kPa (broken lines show bottom (x) and top (y) compositions).
from the ASPEN simulator. According to the simulation results, between three and four theoretical stages are present when the column is filled to a height of 120 mm. Very good agreement was obtained between the ASPEN predictions of the temperatures at the top of the column
Process Analysis with the ASPEN Process Simulator The ASPEN process simulator was used to study the performance of the proposed distillation process in recovering DEA from partially degraded solutions. Detailed process design calculations and energy balances can also be performed using the simulator, but they fell outside the scope of the present work. To model
Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 3107 Table 3. Comparison of Experimental and Theoretical (ASPEN Simulation) Results for DEA, BHEP, and THEED Separation Experiments Conducted under Total Reflux (Run 1: Pressure ) 66.7 kPa (Absolute Pressure); Height ) 120 mm. Run 2: Pressure ) 4.0 kPa (Absolute Pressure); Height ) 120 mm. Run 3: Pressure ) 12.9 kPa (Absolute Pressure); Height ) 250 mm) ASPEN top product
experimental
run
parameter
bottom product
N)3
N)4
N)5
top
bottom
1
xDEA xBHEP xTHEED XBHEP/DEA temp., °C
0.849 0.081 0.070 0.095 189.53
0.9808 0.0192 3.9 × 10-7 0.0196 187.00
0.9897 0.0103 9.1 × 10-10 0.0104 186.91
0.9943 0.0057 3.9 × 10-7 0.0057 186.87
0.988 0.012 N.D.a 0.012 187.0
191.5
2
xDEA xBHEP xTHEED XBHEP/DEA temp., °C
0.597 0.252 0.151 0.422 180.70
0.9628 0.0371 9.4 × 10-7 0.0385 173.47
0.9802 0.0192 2.1 × 10-9 0.0196 172.87
0.9896 0.0105 4.3 × 10-12 0.0106 172.41
0.975 0.025 N.D. 0.026 172.5
183.4
6
xDEA xBHEP xTHEED XBHEP/DEA temp., °C
0.651 0.175 0.174 0.269 205.87
0.9978 0.0022 5.6 × 10-15 0.0022 203.85
0.9987 0.0013 1.4 × 10-17 0.0013 203.85
0.9992 0.0008 8.9 × 10-21 0.0008 203.85
0.999 0.001 N.D. 0.001 204.0
209.1
a
Undetectable by GC.
Table 4. Experimental DEA, BHEP, and THEED Separations Achieved in the Presence of Hexadecane under Total Reflux Conditions (Column i.d., 50 mm; packing type and size, DE-Pak 1/4-in. Stainless Steel; Pressure Range, 1.3-13.3 kPa (Absolute Pressure); Column Pressure drop, Max. 0.1-0.3 kPa; Initial Charge Volume, ∼250 ML) height: 120 mm stream feed
top product
bottom product
BHEP separation, % a
height: 250 mm
height: 450 mm
parameter
run 1
run 2
run 3
run 4
run 5
run 6
xDEA xBHEP xTHEED xHexadecane XBHEP/DEA xDEA xBHEP xTHEED xHexadecane XBHEP/DEA temp., °C pressure, kPa (abs) xDEA xBHEP xTHEED xHexadecane XBHEP/DEA temp., °C
0.126 0.003 0.011 0.860 0.024 0.485 0.006 N.D.a 0.509 0.012 163.5 5.6 0.116 0.004 0.011 0.869 0.034 173.9
0.067 0.007 0.006 0.920 0.104 0.609 0.006 N.D. 0.385 0.010 187.8 13.1 0.057 0.008 0.007 0.928 0.140 195.4
0.141 0.007 0.016 0.836 0.050 0.518 0.009 N.D. 0.473 0.017 158.0 4.3 0.104 0.008 0.019 0.869 0.077 174.3
0.066 0.014 0.010 0.910 0.212 0.571 0.016 N.D. 0.413 0.028 173.7 7.3 0.046 0.016 0.012 0.923 0.348 180.2
0.063 0.010 0.011 0.916 0.159 0.549 0.003 N.D. 0.448 0.005 172.9 7.2 0.049 0.012 0.012 0.927 0.245 179.2
0.101 0.009 0.014 0.876 0.089 0.545 0.001 N.D. 0.454 0.002 166.5 5.5 0.090 0.011 0.016 0.883 0.122 174.1
66.1
75.1
77.8
91.9
98.9
98.4
Undetectable.
a process with the ASPEN process simulator, the appropriate thermodynamic model must be identified and its parameters specified. The NRTL adjustable parameters based on the work reported in part 1 are presented in Table 6. The required physical property data for process simulation are available in the ASPEN data banks or were calculated by functional group estimation methods. The performance of a single-flash distillation unit as well as the proposed process will be discussed in the subsequent sections. Flash Distillation. Precise operating conditions, feed characteristics, and product compositions have not been published for the Canadian Chemical Reclaiming Ltd. (CCRL) process. It is therefore difficult to compare the results of computer simulations with plant data. However, it is clear that a single-stage flash process with a high recovery of DEA cannot give good separation of close boiling impurities. Table 7 contains the results of
single-stage flash calculations for a typical feed and a molar vapor-to-feed ratio of 0.99-1 (see Table 8 for typical input information for a flash separator). Figure 2 shows the DEA recovery and the BHEP/DEA mole ratio in the vapor phase as a function of the molar vapor-to-feed ratio. As the ratio of the vapor product to the feed increases, the DEA recovery increases, but the BHEP/DEA mole ratio approaches that of the feed and consequently less separation occurs. Although the BHEP/ DEA ratio in the vapor phase can be reduced to a minimum value of approximately 0.035, more than 90% of DEA would be lost (see Figure 2). It should be noted that contaminated DEA solutions typically contain various amounts of solid material (dissolved or suspended). Full recovery of DEA by a single-stage flash separation is almost impossible since some liquid must be left in the bottom product to carry the solids out of the process. In addition to poor removal
3108 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 Table 5. Comparison of Experimental and Theoretical (ASPEN Simulation) Results for DEA, BHEP, and THEED Separation Experiments Conducted in the Presence of Hexadecane and under Total Reflux (Run 1: Pressure ) 5.6 kPa (Absolute Pressure); Height ) 120 mm. Run 4: Pressure ) 7.3 kPa (Absolute Pressure); Height ) 250 mm. Run 5: Pressure ) 7.2 kPa (Absolute Pressure), Height ) 450 Mm) ASPEN run
parameter
1
xDEA xBHEP xTHEED xHEXADECANE XBHEP/DEA temp., °C xDEA xBHEP xTHEED xHEXADECANE XBHEP/DEA temp., °C xDEA xBHEP xTHEED xHEXADECANE XBHEP/DEA temp., °C
4
5
a
top product
bottom product 0.116 0.004 0.011 0.869 168.31 0.046 0.016 0.012 0.926 177.74 0.049 0.012 0.012 0.927 176.80
N)3
N)4
0.5021 0.0038 2.7 × 10-7 0.4941 0.0076 167.05 0.4913 0.0176 6.5 × 10-7 0.4911 0.0358 173.65 0.5062 0.0066 1.3 × 10-9 0.4872 0.0130 173.08
0.5041 0.0016 4.2 × 10-10 0.4943 0.0032 167.01 0.5043 0.0086 1.4 × 10-9 0.4871 0.0171 173.54 0.5105 0.0034 2.9 × 10-12 0.4861 0.0067 173.04
N)5
top
0.5099 0.0043 2.9 × 10-12 0.4857 0.0084 173.51 0.5126 0.0018 6.2 × 10-15 0.4856 0.0035 173.03
0.485 0.006 N.D.a 0.509 0.012 163.5 0.571 0.016 N.D. 0.413 0.028 173.7 0.549 0.003 N.D. 0.448 0.005 172.9
bottom
173.9
180.2
179.2
Undetectable by GC.
Table 6. NRTL Adjustable Parameters for Binary Mixtures Containing DEA, Degradation Products, Water, and Hexadecane
Table 8. Typical Input Information for the Flash Separator and Distillation Column of the Proposed Process
component
unit
1
2
a12
DEA BHEP BHEP DEA BHEP THEED THEED THEED THEED water
water DEA water hexadecane hexadecane water DEA BHEP hexadecane hexadecane
12.4298 -1.2522 5.5313 -5.4080 -3.8173 -0.1549 -2.5678 13.7973 3.4546 -3.1771
a21
b12
-4.6697 -3685.53 1.9612 33.255 -2.3338 -1400.14 -4.3107 4583.84 -5.5447 2922.46 -1.7745 -854.58 1.9890 1213.54 -8.3323 -6080.17 -3.2864 146.78 -0.7838 6252.20
b21
R
1177.12 136.48 480.85 2871.64 3277.39 1812.43 -957.10 3585.20 2166.59 2748.65
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2
feed
vapor product
liquid product
temperature, °C pressure, kPa
20.0 101.3
162.0 13.3
162.0 13.3
flow rate, conc.,a flow rate, conc.,a flow rate, conc.,a kg/h wt % kg/h wt % kg/h wt % 177.25 20.49 675.60 36.87 60.00 910.21a
19.47 2.25 74.22 4.05
174.87 19.87 675.12 1.15
20.08 2.28 77.51 0.13
100.00
871.01a
100.00
2.14 0.62 0.39 35.74 60.00 38.89a
characteristic
input
pressure, 13.3 kPa (absolute) 13.3 vapor product/feed mole ratio 0.990 algorithm option three phase
distillation column pressure, kPa (absolute) (block COLUMN) number of theoretical stages feed tray locationa reflux ratio distillate/feed mole ratio algorithm pressure drop, kPa/stage distillate liquid fraction
13.3 8 5 2.2 0.745 three phase 0.14 1.0
Stages are numbered from top to bottom (condenser ) stage
1).
parameter
DEA BHEP water THEED solids total
flash separator (block FLSH)
a
Table 7. Stream Summary for a Single-Stage Flash Process Separating DEA, BHEP, water, THEED, and Solids as Predicted by the ASPEN Simulator; Molar Vapor Product/Feed Ratio ) 0.99
a
experimental
5.50 1.59 1.01 91.90 100.00
Excluding solids.
of BHEP, some DEA would therefore also be lost with the impurities. Flash and Fractional Distillation. The proposed process described in part 1 is represented by the flow sheet generated by the ASPEN simulator and is shown in Figure 3. The process mainly consists of a flash separator and a distillation column. The feed is mixed with a stream of inert liquid which had previously been warmed in a heater. The resulting mixture (designated MIX-FEED) has a composition which may be calculated from the information given in Table 9. For the case
shown, the feed flow rate and composition are selected to be similar to those of the CCRL process. The mixture is then introduced into the flash separator where solids and part of the degradation products are separated as the underflow. The vapor product from the flash separator is cooled and passed to the distillation column. Table 9 provides information on the important process streams for a typical simulation. For the stated feed composition and inert liquid flow rate, almost 98% of DEA is recovered. Similarly, almost 91% of BHEP and 100% of THEED are removed from the DEA. Because the existence of an azeotrope in the DEA-hexadecane system, some hexadecane is present in the top product of the distillation column. However, it is readily separated in the separator (SEP1) because of its very small solubility in aqueous DEA at ambient temperatures. The primary input data for the flash separator are the operating pressure and the vapor product/feed ratio (V/F). A three-phase algorithm is selected for the flash separator. The V/F ratio, at a pressure which provides a safe temperature for DEA (99%) recovery for DEA is obtained. The input data for the aforementioned simulation run are summarized in Table 8. Depending on how the process is simulated, the input data for the distillation column
Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 3109
properties of the mixtures in the flash section of the process is neglected; therefore, their mass flows are not included in the stream tables. Between 3 and 6 wt % of solid materials (dissolved or suspended) typically exists in contaminated amine solutions. The hexadecane flow rate can be adjusted so that the liquid product leaving the bottom of the flash separator contains sufficient amounts of hexadecane to remove the solid contained in the feed. Effect of Operating Conditions on Process Performance
Figure 2. BHEP/DEA ratio and DEA recovery in the vapor phase for a single-stage flash separator as a function of molar vapor product-to-feed ratio (absolute pressure: 13.3 kPa; feed mole fractions: DEA ) 0.0427, water ) 0.9495, BHEP ) 0.0030, and THEED ) 0.0048).
may vary. Desirable product purities and DEA recoveries can be reached by varying the number of theoretical stages, distillate-to-feed mole ratio, feed tray location, and the reflux ratio. Dissolved and suspended solids were regarded as inert materials, and their effect on the equilibrium
To investigate the importance of various parameters on the performance of the process and to determine the appropriate operating conditions, a sensitivity analysis was performed. The inert liquid flow rate was varied to examine the DEA recovery and BHEP separation for the process. Figures 4 and 5 show the simulation results for the flash separator (block FLSH) as a function of hexadecane in the feed. As is evident from Figure 4, the BHEP/DEA ratio in the vapor product of the flash separator is almost identical to that of the process when no inert liquid is present. As shown by Figure 4, increasing the hexadecane content of the feed stream to the flash separator does not significantly reduce the BHEP/DEA mole ratio in the vapor product of the flash separator. The hexadecane flow rate can therefore be kept low. However, the flow should be sufficient to provide a continuous stream to remove nonvolatile impurities and
Figure 3. Flow diagram of the proposed process generated with ASPEN.
3110 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 Table 9. Stream Summary for the Proposed Process as Predicted with the ASPEN Simulator with Conditions Selected to Correspond to Those Typical for the CCRL Process (for Stream IDs, see Figure 3) FEED
INERT
PRODUCT
WASTE
temperature, °C pressure, kPa vapor fraction
stream ID
20.0 101.3 0.0
200.0 13.3 0.0
20.0 13.3 0.0
60.0 13.3 0.0
flow rate, kg/h DEA BHEP water THEED hexadecane solids
195.537 30.818 729.012 44.633 0.0 60.000
0.474 0.480 0.006 8.480 4000.0 0.000
191.423 2.877 728.815 0.000
