2.7 Material and Energy Balance
Mass and energy balances are carried out for each of the units shown in Figure 2.1.
Synthesis gas
High Temperatur e Shift reactor
Low Temperatur e Shift reactor
Absorber
PSA system
Figure 2.1 Schematic Process Flow Diagram
In order to complete the material and energy balance effectively and clearly, the simulation of the process is applied by using computer software package named HYSYS. Each of the unit will be explained with the reference of the HYSYS simulation.
2.7.1 High Temperature Shift Reactor (HTS Reactor) 2.7.1.1 Mass balance
The Figure 2.2 shows that the inlet and outlet stream properties of high temperature shift reactor.
Figure 2.2: The mass balance in HTS reactor
4
The Figure 2.2 shows that the mass flow rate of the inlet stream, cooler 1, is 1.18 x 10 kg/hr in the pressure of 1000kpa and temperature of 400 The outlet vapour stream, High Temp Vap, 4
mass flow rate is 1.18x10 kg/hr whereas mass flow rate of liquid stream is 0 kg/h. It indicates that there is not any liquid come out from the reactor. Due to the high operation temperature, 400, all the water component of the inlet stream is evaporated to the gas form and thus, there is no liquid outlet flowing out from the reactor.
4
Mass in = Mass flow rate of Cooler 1 = 1.18 x 10 kg/hr 4
Mass out = Mass flow rate of High Temp Vap = 1.18 x 10 kg/hr
2.7.1.2 Energy Balance
The reaction is operated adiabatically with no heat loss in the industrial scale where the temperature increases along the length of the reactor. The Figure 2.3 shows the heat flow of the inlet and outlet stream in high temperature shift reactor.
Figure 2.3: The Energy balance in HTS reactor
The WGS reaction is slightly exothermic and the heat of reaction at 25°C is – 41 41 kJ/mol based on water in a vapor state which the CO is react with the steam to produce CO2 and H2. Moreover, the pretreatment of HT shift catalyst is carried out by partially reducing the Hematite (Fe 2O3) to
Magnetite (Fe3O4) using the addition of the process gas mixtures to activate the catalyst [Rhodes et al., 1995].
This also converts any CrO3 present in the catalyst to Cr 2O3. The reactions are
3Fe2O3 + H2 → 2Fe3O4 + H2O
∆H
= -16.3 kJ/mol
3Fe2O3 + CO → 2Fe3O4 + CO2
∆H = +24.8 kJ/mol
The total heat of the reaction reac tion in the reactor = - 41 kJ/mol-16.3 kJ/mol +24.8 kJ/mol = -0.1 kJ/mol 7
Energy in = 9.646 x 10 kJ/hr Energy supplied in the reactor = -0.1 kJ/mol x Molar flow of High Temp Vap V ap = -0.1 kJ/mol x 569.9 mol/hr = -56.99 kJ/hr. 7
Energy out = 9.646 x 10 kJ/hr
The exothermic reaction of 56.99 KJ/hr is comparative much lesser than the inlet stream energy and it does not show in HYSYS precisely. The heat flow of the outlet stream after the reactor is not increasing obviously because the input feed syngas contain a large amount of energy which 4
is -6.403 x 10 KJ/mol..
2.7.2 Low Temperature Shift Reactor (LTS Reactor) 2.7.2.1 Mass balance
Figure 2.4 show that the inlet and outlet stream properties of low temperature shift reactor.
Figure 2.4: The mass balance in LTS reactor
4
The Figure 2.4 shows that the mass flow rate of the inlet stream, cooler 2, is 1.18 x 10 kg/hr, however, the temperature of the inlet gas is cooled down to 100. Therefore, some of the water components are being liquefy in 98.12 after the reactor.
4
Mass in = Mass flow rate of Cooler 1 = 1.18 x 10 kg/hr Mass out = Mass flow of Low Temp Vap + Mass flow of Low Temp Liq 4
4
= 1.141 x 10 kg/hr + 396.1 kg/hr = 1.18 x 10 kg/hr
2.7.2.2 Energy Balance
As mention in section 2.7.1.2, the WGS reaction is exothermic with the heat value of 41 kJ/mol based on water in a vapor state. The reactor is assumed to be adiabatic operation with no heat loss. The Figure 2.5 shows the heat flow of the inlet and outlet stream in low temperature shift reactor.
Figure 2.5: The Energy Balance in LTS reactor
Similar to the HT catalyst, the LT catalyst needs to be activated and hence the catalyst is exposed to the process stream with dilute H 2 [Rhodes et al., 1995]. The CuO is reduced to copper by the following reaction and the catalyst gets activated:
CuO + H2→Cu + H2O ∆H = - 80.8 kJ/mol
The total heat of the reaction in the reactor = -41 kJ/mol- 80.8 kJ/mol = -121.8 kJ/mol
7
Energy in = -9.646 x 10 kJ/hr Energy supplied to the Low Temp Vap = -121.8 kJ/mol x Molar flow of Low Temp Vap = -121.8 kJ/mol x 548.0 mol/hr = -66,746.4 kJ/hr
Energy supplied to the Low Temp Liq Liq = -121.8 kJ/mol x Molar flow of Low Temp Liq = -121.8 kJ/mol x 21.96 mol/hr = -2674.728 kJ/hr 7
Energy out = -66,746.4 kJ/hr -2674.728 kJ/hr - 9.646 x 10 kJ/hr 7
= - 9.646 x 10 kJ/hr
2.7.3 Absorber 2.7.3.1 Mass balance
The Figure 2.6 shows the mass balance in the absorber column.
Figure 2.6: The Mass Balance in Absorber
The absorber is used to remove the carbon monoxide and carbon dioxide from the AbsorberFeed stream. Fresh water is fed into the system in the 20 and 600kPa. The Absorbergas stream contains trace amount of carbon monoxide and carbon dioxide after the absorber column. Mass in = Mass flow of Fresh water + Mass flow of AbsorberFeed 4
4
= 1.2 x 10 kg/hr +1.141 x 10 kg/hr 4
= 2.61 x 10 kg/hr
Mass out = Mass flow of Absorbergas + Mass flow of Waste Water 4
= 551.2 kg/hr + 2.285 x 10 kg/h 4
= 2.61 x 10 kg/hr
2.7.3.2 Energy balance
The Figure 2.7 shows the energy balance in the absorber column.
Figure 2.7: The Energy Balance in Absorber
A high heat flow of the input stream is fed into the absorber, thus, the waste water contains high heat flow after the absorber. The waste water stream could be recycling to other equipment as an energy stream, which is in 25 and 200kPa.
Energy in = Heat flow of Fresh water + Heat flow of AbsorberFeed 8
8
= -1.909 x 10 kJ/hr -1.041 x 10 kJ/hr 8
= -2.95 x 10 kJ/hr
Energy out = Heat flow of Absorbergas + Heat flow of Waste Water 4
8
= -8.837 x 10 kJ/hr kJ/hr - 2.945 x 10 kJ/hr 8
= -2.95 x 10 kJ/hr
2.7.4 Pressure Swing Adsorption (PSA) 2.7.4.1 Mass balance
The Figure 2.8 shows the mass balance in the Pressure Swing Adsorption column. A mathematical model including sets of mass and energy balance was shown for developing a dynamic model for PSA process with the following assumptions: (1) The flow pattern is described by the axially dispersed plug flow model (2) Thermal equilibrium is assumed between fluid and particles (3) The diffusivities are assumed to be constant; (4) The gas phase behave as ideal gas mixture.
Figure 2.8: The Mass Balance in PSA
PSA processes are generally carried out with packed adsorption columns. The dynamic behavior of an adsorption column is governed governed by the coaction of adsorption kinetics, adsorption equilibrium, and fluid dynamics, and its understanding is vital for process modeling and analysis.
Mass in = Mass flow of Absorbergas = 551.2 kg/hr
Mass out = Mass flow of Purified Hydrogen + Mass flow of Wasted Gas = 541.3 kg/hr + 9.874 kg/h = 551.2 kg/hr
2.7.4.2 Energy balance
In general, temperature difference is neglected and thermal equilibrium is assumed between the bulk gas phase and adsorbent particle. Moreover, heat transfer in the axial direction by thermal conduction is often negligible unless the operation is adiabatic at a very high flow rate. Based on these assumptions, Energy balance is calculated. The Figure 2.9 shows the energy balance in the Pressure Swing Adsorption column.
Figure 2.9: The Energy Balance in PSA
Energy in = Heat flow of Absorbergas 4
= -8.837x 10 kJ/hr
Energy out = Heat flow of Purified P urified Hydrogen + Heat flow of Wasted Gas 4
4
= -28.89 kJ/hr -8.832 x 10 kJ/hr = -8.837x 10 kJ/hr
Overall Process Mass Balance
Stream no.
S-2
Flow Rate ( kg/hr)
1.18x 10
Temperature (°C)
400
Component (%)
Pressure (kPa)
4
Stream no.
S-4
Flow Rate ( kg/hr)
1.18x 104
Temperature (°C)
100
13.21
CO2
26.17
CO
7.73
H2O
26.66
CO2
31.65
H2
33.96
H2O
21.18
H2
39.44
200
S-6
Flow Rate ( kg/hr)
1.18x 10
Temperature (°C)
CO
Component (%)
Stream no.
Component (%)
Pressure (kPa) Pressure (kPa)
4
20
Stream no.
S-11
38.99
Flow Rate ( kg/hr)
541.3
H2O
13.84
Temperature (°C)
H2
46.78
CO
0.39
CO2
200
Component (%)
200
Pressure (kPa)
Syngas
Low Temperature Shift Reactor
High Temperature Shift Reactor
Absorber
S-8
Stream no.
S-9
Flow Rate ( kg/hr)
1.141
Flow Rate ( kg/hr)
396.1
Temperature (°C)
25
Component (%)
Pressure (kPa)
CO
0.00
CO2
0.08
H2O
0.00
H2
99.92
200
Component (%)
Pressure (kPa)
0.01
CO2
0.71
H2O
99.28
H2
0
101.3
Pressure Swing Adsorption
Stream no.
Temperature (°C)
25 CO
Stream no.
S-10
Flow Rate ( kg/hr)
9.874
Temperature (°C)
25
25
CO
0
CO
0.24
CO2
0
CO2
22.91
H2O
0
H2O
76.86
H2
100
H2
0
200
Component (%)
Pressure (kPa)
101.3
Unit Mass and Energy balance High Temperature shift reactor
Details
From No
Stream Weight ( kg/hr)
Mass inlet Mass outlet Mass
Accumulation
Balance
Balance
0
Details
Heat
From No
Stream Heat energy (kJ/hr)
Generation
(Supply)
Energy Balance
Heat Consumption
Balance
0 Details
From No
Stream Weight ( kg/day)
Mass inlet Mass Balance
Mass outlet
Accumulation Balance
0