25
Table 4.4: Separation Process Stream Summary. This table provides temperature, pressure, vapor fraction, and component flow rates for all streams in the separation section of the process flowsheet.
Stream S
‐116 S
‐117 S
‐118 S
‐119 S
‐120 S
‐121 S
‐122 S
‐123 S
‐124 Temperature °F
85 86
86 400
418 318
317 295
307
Pressure psia 25
150 130
125 98
95 90
70 73
Vapor Fraction 1
1 1
1
Total Flow lbhr 48596
48596 48596
48596 23846
24750 24750
19472 5278
Component Flows lbhr
Propane 3651
3651 3651
3651 3651
3651 3651
Oxygen 3
3 3
3 3
3 3
Propylene 380
380 380
380 380
380 380
Acrylic Acid 26702
26702 26702
26702 23844
2858 2858
1024 1834
Water 16419
16419 16419
16419 16419
16419 13036
3383
Nitrogen 130
130 130
130 130
130 130
Acetic Acid 1003
1003 1003
1003 1
1001 1001
941 60
Carbon Dioxide 308
308 308
308 308
308 308
Dowtherm Stream
S ‐125
S ‐126
S ‐127
S ‐128
S ‐129
S ‐130
S ‐131
S ‐132
WASTE Temperature °F
374 371
415 100
298 287
265 264.
236
Pressure psia 68
68 68
63 65
67 65
60 25
Vapor Fraction
1 1
1 1
1 0.3
Total Flow lbhr 5278.
1469 25314
25314 3809
13819. 5653
5653. 17628
Component Flows lbhr
Propane 834.
2817. 2817.
834
Oxygen 3
3
Propylene 75
305 305.
75
Acrylic Acid
1834 1443
25286 25286
392 913.
110. 110.
1305
Water 3383.
25 25
25 3358
11202. 1834.
1834. 14560
Nitrogen 130.
130.
Acetic Acid 60
1 3
3 59
783 158.
158. 842
Carbon Dioxide 12
296. 296.
12
Dowtherm Stream
DOWTHERM ‐3 DOWTHERM‐4 DOWTHERM‐5 DOWTHERM‐6
Temperature °F 458
299 203
198
Pressure psia 30
25 20
15
Vapor Fraction Total Flow lbhr
385000 385000
385000 385000
Component Flows lbhr Dowtherm
385000 385000
385000 385000
26
27
Section 5
Process Description
28
5.1 Reactor Section
The conversion of propane to acrylic acid through propylene requires contacting the
paraffin with a mixed metal oxide catalyst at approximately 750°F. Since the reaction is highly
exothermic, a fixed bed reactor configured as a shell and tube heat exchanger was used, with Dowtherm A as the coolant. The flowsheet for the reactor section can be seen in Figure 4.3, page
22.
Reactor The reactor dimensions were chosen to ensure both adequate volume for chemical
conversion and adequate surface area for sufficient cooling. Based on patent data, the fixed bed reactor, R-101, needs 160 lb catalyst for a space velocity of 1662 hr
-1
. Given the required feed flow rate, the minimum reactor volume to ensure this space velocity is 670 ft
3
. The actual reactor volume is 700 ft
3
, allowing 30 ft
3
to be used to adjust the reaction extent with inert ceramic beads or additional catalyst. These beads will also help ensure operating temperature conditions are
more consistent with researched conditions. The minimum required surface area for cooling is 11,800 ft
2
, which is safely under designed reactor surface area of 16,755 ft
2
. The required cooling surface area is based on an estimated overall heat transfer coefficient of 100 Btuft
2
-hr
and a log-mean temperature difference of 100°F.
Table 5.1: Reactor Specifications
Reactor Specifications
Number of Tubes 4000
Tube Length ft 8
Inner - Tube Diameter in 2
Pressure Drop psi 6
Based on the number of tubes, tube length, and tube diameter, the resulting pressure drop across the reactor is calculated to be approximately 6 psi using the Ergun equation. The feed to
29
the fixed bed reactor is pre-heated to 740°F using the reactor effluent, at 780°F, as a heat source.
In order to control the reaction, feed conditions are set such that oxygen is the limiting reactant.
Dowtherm Cooling Due to the high operating temperature of the reactor and the highly exothermic nature of
the reaction, it is necessary to cool the reactor with a Dowtherm stream. The Dowtherm enters
the reactor at approximately 198°F and exits at about 738°F. After exiting the reactor, the
Dowtherm is used as a heat source for the reboiler of tower D-101, heat exchanger HX-105, and the reboiler of tower D-104, where it provides 54,100,000 Btuhr, 28,000,000 Btuhr, and
13,800,000 Btuhr of heat, respectively. This brings the Dowtherm temperature back down to
203°F. Additional heat losses through the piping system brings the temperature of the Dowtherm back down to the 198°F. In order for the Dowtherm to flow through the piping and three heat
exchangers, it is pressurized to 45 psia using a set of 20 pumps setup in parallel.
30
5.2 Separation
The separation scheme for this process involves one flash drum and four distillation towers. The flowsheet for the separation section of the process can be seen in Figure 4.4, page
24. The main challenge was to isolate acrylic acid from water and acetic acid since these three chemical species form three separate azeotropes: wateracrylic acid, wateracetic acid, and
acrylic acidacetic acid form an azeotrope. Rather than trying to break the azeotropes, this separation scheme avoided them by performing the separation in a specific order. The first tower
D-101 recovers a pure stream of acrylic acid. The second tower D-102 removes most of the water and acetic acid from the remaining acrylic acid, which is recovered in the third tower D-
103. The fourth tower removes the remaining propane, which is recycled, from the water and acetic acid which exits as wastewater in addition to the distillate from the third tower.
Flash Drum The reactor effluent S-111 was first sent to a flash drum F-101, where virtually all the
propane, oxygen, nitrogen, carbon dioxide, and propylene exits as vapor and the acrylic acid exits as liquid. About 70 of the water and acetic acid exits in the liquid phase and the balance
exits in the vapor phase. The vapor from the flash drum is recycled back as reactor feed, while the liquid is fed to the first distillation tower D-101.
Tower 1 D-101 separates acrylic acid from the reaction byproducts and leftover reactants. About
75 of the acrylic acid produced is recovered in the bottoms S-120 of this tower. The propane, propylene, carbon dioxide, nitrogen, acetic acid, and water leaves in the distillate S-121, along
with the remaining 25 of the acrylic acid.
Tower 2 D-102 removes propane, propylene, carbon dioxide, and nitrogen from the acrylic acid. It
also removes some of the water and acetic acid from the acrylic acid. The distillate from D-101 is the feed to this tower. About 82 of the unrecovered acrylic acid leaves in the bottoms S-
124 and the rest leaves in the distillate S-123. All the propane, propylene, carbon dioxide, and
31 nitrogen leave this tower in the distillate. About 78 of the water leaves in the distillate along
with 87 of the acetic acid, while the balance leaves in the bottoms.
Tower 3 D-103 separates acrylic acid from the remaining water and acetic acid. All the water and
acetic acid entering this tower leaves in the distillate S-129. About 85 of the acrylic acid entering this tower is recovered in the bottoms S-126 and the balance leaves in the distillate.
Tower 4 D-104 separates the water and acetic acid, which leave as bottoms S-130, from the
propane, propylene, carbon dioxide, and nitrogen, which are recycled S-131. About 90 of the acrylic acid that enters this tower leaves as bottoms. All the carbon dioxide and nitrogen leave
this tower in the distillate S-131. About 80 of the propane and propylene leave in the distillate, while the remaining 20 leave in the bottoms as wastewater. About 85 of the water
and acetic acid that entered this tower leave in the bottoms and the balance leaves in the distillate.
Recovery of Acrylic Acid About 28,550 lb acrylic acid exit the reactor R-101 each hour and enter the flash drum
F-101, which is the first separation unit. The first product stream S-120 has about 23,800 lb of acrylic acid and the second product stream S-126 has about 1,450 lb acrylic acid. This gives
an overall acrylic acid recovery of about 89. About 5 exits the process in wastewater WASTE.
