FIGURE 1. PROCESS FLOW DIAGRAM SHOWING A PRODUCTION OF ACETIC ANHYDRIDE FROM ACETONE
Vapour
outlet.2
(Fuel gas if
Mole fraction
of
acetone <1%)
Absorber
7 stages
Acetic acid
P-1 atm
T-30°C
Pure acetoneMass Flow(M.F)-8000 kg/h
Pressure (P)-1 atm
Temperature(T)- 30°C
P-1.6 atm
T-750°C
Mixer.1
Fresh acetic
acid
P-1 atm
T-30°C
Plug flow
reactor
Tube Diameter1.0m
P-1 atm
T-40°c
P-1 atm
T-60°C
Acetone conversion-20%
Compressor 1
Liquid stream 2.
P-1 atm
Isothermal
reactor
Mixer.2
P-1 atm
Heater.1
Vapour stream.2
P-1 atm
Vapour stream
.1
valve
Compressor. 2
Cooler.1
Compressor.3
Flash
Drum
Cooler 2
Liquid
stream.3
P-1 atm
Liquid
outlet.1
Mixer.3
Purge .1
Tee.1
99% overhead
recycled
Recycle.1
Overhead
0.1% mole fraction
acetic actid
Purge. 2
P-1 atm
Recycle.2
99% bottoms
recycled
Tee.2
Bottom
0.1% mole
fraction of
acetone
Short distillation column
INTRODUCTION
The purpose of the assignment was to simulate a simplified process for the production of
acetone to produce acetic anhydride via cracking of acetone to ketene where methane is
produced as a by-product followed by conversion reaction where ketene reacts with added
acetic acid to produce acetic anhydride. The conversion reaction occurs in the Isothermal
reactor using Hysys Aspen, following the specific design specification. This process is
demonstrated in the process flow diagram (figure.1).
ANALYSIS
AREA 1: ACETONE CRAC KER
Area one is where the process of cracking the acetone takes place in the plug flow reactor.
The tube length of the reactor has an effect on the temperature and the conversion of
acetone within the acetone cracker. As observed in figure 2, the conversion of acetone is
directly proportional to the tube length of the plug flow reactor and inversely proportional to
the temperature. Thus, as the length of the plug flow reactor increases, the conversion of
acetone increases and the temperature decreases. The reaction taking place in the reactor is
endothermic (because the acetone enters the plug flow reactor at 750 °C and leaves at a
temperature of 642°C). As a result, as more heat energy is taken in by the reaction, the
temperature in the plug flow reactor decreases because more acetone is being converted due
to the increase of the tube length as the volume of the reactor is increasing. Therefore, there
is more surface area for the reaction to take place.
Furthermore, the reactor length of 5.935 m and the temperature 645°C in the reactor is
needed to achieve a 20 % conversion acetone to ketene as shown in Figure 2.
AREA 2: QUENCH REACT OR
Acetic acid and ketene react in the isothermal reactor to form acetic anhydride. The ratio of
acetic acid to ketene entering the quench reactor is 7:1 therefore despite the expected
conversion being 100%, the actual conversion of ketene to acetic anhydride in the quench
reactor was 22 %, a very low amount of acetic anhydride is produced. This is because the
acetic acid is in excess meaning it cannot completely react with ketene (limiting reagent) to
achieve a 100% conversion in the quench reactor. For that reason, as the mole ratio acetic
acid to ketene increases in the reactor increases so does the actual percentage conversion of
ketene to acetic anhydride as there is more ketene for the acetic acid to react with, as
demonstrated in figure 4.
Moreover, the reaction takes place effectively in the liquid phase as the temperature
increases more reactants are in the gaseous phase so the amount of acetic anhydride formed
is limited therefore as the temperature of the quench reactor increases the % percentage
conversion of ketene to acetic anhydride decreases as demonstrated in figure 3. Therefore,
the performance of the quench reactor can be improved by decreasing the temperature of
the isothermal reactor as the percentage conversion of ketene to acetic anhydride increases
when the temperatures decrease. It can also can be improved by increasing the length of the
acetone cracker more acetone is converted to ketene as demonstrated in figure 2. As a result,
the ratio of acetic acid to ketene entering the quench reactor will be increased.
AREA 3: FUEL GAS RECOVERY
The vapour outlet of the flush drum is contacted with a fresh acetic acid stream in the
absorber, the vapour outlet of the absorber is used as a fuel gas. The fuel gas stream is made
out of 55.68% Methane and the rest of the stream is (0.81%) acetone, (6.60 %) acetic acid and
(38.65%) ketene. The primary component of the fuel gas is methane. There is a low amount
of acetic acid and acetone in the fuel so they may not hinder its use as fuel, however ketene
may hinder its use, as there is large amount of it within the fuel gas and will act as
contaminant.
Therefore, increasing the mole fraction of methane will decrease of the mole fraction of the
contaminants especially ketene making the fuel gas more sufficient to increase the amount
of methane in the system. The mole fraction methane can be increased by adding a fresh
stream of gaseous methane to the absorber and by increasing the recycle ratio of the
overheads of the shortcut distillation column as gaseous methane is recycled back system
there will be more methane in the fuel gas stream as a result, decreasing the amount
contaminants in the fuel.
AREA 4
Characteristics parameters of the short cut
distillation column
(Table 1.)
Light Key in the bottoms (mole
fraction of acetone )
0.001
Heavy Key in distillate (mole 0.001
fraction in acetic acid)
Condenser pressure
101.3 kpa
Reboiler pressure
101.3 kpa
External reflux ratio
1.4098
Internal reflux ratio
1.007
The Liquid streams from the absorber and flush drum are mixed and the components are
separated in the short cut distillation column. The characteristic parameters of the short cut
distillation column are shown in the table above. They are the main set of elements that
need to be defined to calculate the internal reflux ratio. While simulating the distillation
column in Hysys the external reflux ratio was set to 5 in order to calculate the minimum
reflux ratio. When the internal reflux ration is obtained it was multiplied by 1.4 to obtain the
new external ratio to meet the design specification.
The performance of the column is dependent on these parameters shown in table 1, it is
outlined by the optimum feed stage (9.951), the actual number of trays (14.938), minimum
number of trays (7.136), the temperature of the Reboiler (54.25⁰C) and condenser
(117.6⁰C), mass flows of the rectifying, stripping section, the mass flow of the condenser
duty and the mass flow of the Reboiler are calculated by Hysys.
The system performance is determined by its ability to convert acetone to acetic anhydride.
Increasing the recycle ratio of the column overhead has negative effect on the system
performance. Increases the recycle ratio in the overhead decreases the percentage
conversion of ketene to acetic anhydride (as demonstrated in figure 5 below) causing a poor
system performance as demonstrated.
In addition, Figure 5 also shows that increasing the recycle ratio of the bottoms increases
the amount ketene and acetic acid reacting in the liquid phase as most the acetic acid being
recycled is the liquid phase therefore increasing the percentage conversion of ketene to
acetic anhydride and an additional amount of acetic anhydride in the liquid phase is
recycled back into the isothermal reactor therefore increasing the amount of acetic
anhydride being produced.
CONCLUSION
In conclusion the simulation of the system was a success and met all the requirements of
the design specification despite the fact it only 21.74 % it did not meet the 100 % conversion
specification of ketene to acetic anhydride. Adjustments to the design specifications can be
made to improve the actual conversion which were found through analysing Hysys and are
mentioned in this report.
700
680
660
640
620
600
580
560
540
520
Effect of tube legnth on temprerature and %Acetone
conversion
35
30
25
20
15
10
5
0
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96
Tube Legnth (m)
%acetone conversion
Temprerature
%acetone
conversion
Temprerature ⁰C
RESULTS
Figure 2: shows the conversion and the temperature in the acetone cracker varies with tube
length
mole ratio of acetic acid to ketene and
% conversion
30
Figure 3: Shows how the % actual conversion
of ketene to acetic anhydride varys with the
mole ratio of acetic acid to ketene
% conversion
25
20
15
10
5
0
0
5
10
15
Mole ratio acetic acid to ketene
Temprerature of quench reactor and % converision
ketene to acetic anhydride
% Conversion
90
Figure 4: Shows how the % actual conversion of
ketene to acetic anhydride varys with the
temperature of the quench reactor.
80
70
60
50
40
30
20
10
0
0
20
40
60
Temprerature °C
80
100
120