MACHEKE,TITSWALO
Q5074522
ICA 2016-17/HYSYS/CBE2019-N
LECTURER:DR SAMANTHA GOONERATNE
TEESIDE UNIVERSITY
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%, a very low amount of acetic anhydride was produced. The actual
conversion of ketene to acetic anhydride in the quench reactor was 21.9 %. This is because
the acetic acid is in excess and so it cannot completely react with ketene (limiting reagent) to
achieve a 100% conversion in the quench reactor. For that reason, as the mole ratio of acetic
acid to ketene in the reactor increases ,the actual percentage conversion of ketene to acetic
anhydride also increases as there is more ketene for the acetic acid to react with, as
demonstrated in figure 3.
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 4. Hence 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. The performance of the reactor can also can be improved
by increasing the length of the acetone cracker because 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 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 of 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 to the
system, there will be more methane in the fuel gas stream. As a result, the amount of
contaminants in the fuel is decreased.
AREA 4
Characteristics parameters of the short
cut distillation column.(Table 1.)
The Liquid streams from
the absorber and flush
drum are mixed and the
components
are
Light Key in the bottoms
separated in the short
(mole fraction of acetone ) 0.001
cut distillation column.
Heavy Key in distillate (mole 0.001
The
characteristic
fraction in acetic acid)
parameters of the short
Condenser pressure
101.3 kpa
cut distillation column
Reboiler pressure
101.3 kpa
are shown in the table
External reflux ratio
1.4098
above. They are the main set of elements that
Internal reflux ratio
1.007
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. Increasing 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 of ketene and acetic acid reacting in the liquid phase, as most of the acetic acid being
recycled is in the liquid phase. Therefore, increasing the percentage conversion of ketene to
acetic anhydride coupled with recycling of an additional amount of acetic anhydride in the
liquid phase back into the isothermal reactor increases 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 the actual conversion it only 21.9 % 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.
RESULTS
Figure 2 :Shows how the temprerature and %acetone conversion
varies with the tube legnth .
700
35
Temprerature ⁰C
680
30
660
620
20
600
15
580
%acetone
conversion
25
640
10
560
5
540
520
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
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 in the
quench reactor.
80
70
60
50
40
30
20
10
0
0
20
40
60
Temprerature °C
80
100
120
Figure 5: Shows how the Recycle ratios of the distillate overhead and bottoms affect
on % conversion of ketene actic anhydride in the isothermal reactor.
25
25
24,5
24
23,5
15
23
22,5
10
22
21,5
5
% conversion
% conversion
20
21
20,5
0
20
0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5 0,55 0,6 0,65 0,7 0,75 0,8 0,85 0,9 0,95 1
Recycle ratio
Recycle ratio of the bottoms
Recycle ratio of the distillate overhead