Industrial Scale Alkaline Electrolysis
Plant Modeling
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Table of Contents
1
Introduction .......................................................................................... 1
2
Simulation Details .................................................................................. 1
3
2.1
Process Description ........................................................................... 1
2.2
Simulation Results ............................................................................ 3
Conclusion ............................................................................................ 7
ii
1 Introduction
Hydrogen is considered an energy carrier for a sustainable future when it is produced
from a renewable energy source. Today, the production of hydrogen through
electrolysis processes is quite low, since most of it is obtained from conventional
processes from fossil sources which are responsible for high greenhouse gas
emissions[1]. Then, to combat this problem, it is necessary to increase the hydrogen
production through use of clean technologies such as water electrolysis. Therefore,
water electrolysis is a key technology for splitting water into hydrogen and oxygen
by using renewable energy. For water electrolysis, there are three technologies
available: Alkaline water electrolysis (AEL), proton exchange membrane (or polymer
electrolyte membrane) electrolysis (PEMEL), and solid oxide electrolysis (SOEL) [2].
However, since the lifetime of alkaline water electrolyzers is higher and the annual
maintenance costs are lower compared to a PEMEL system, its use has been
preferred. In addition, the shortcomings have been gradually overcome by further
development[3].
In this sample case, the simulation of an industrial-scale alkaline electrolysis plant
under a steady-state regime is proposed. The results obtained are good and
representative, which allows use this work as a starting point for the realization of
more complex water electrolysis plant models using rigorous scenarios that imply the
use of severe operating conditions.
2 Simulation Details
2.1 Process Description
In this sample case, a model of an alkaline electrolysis plant operating in a steadystate regime is proposed in which both alkaline water electrolysis cell stack and
system are considered. The system indicated in the HYSYS model flowsheet below
(Figure 1) includes all components of the balance of the plant such as water supply,
gas-liquid separator vessels for oxygen and hydrogen streams, pumps and coolers.
The electrolyte used for the alkaline electrolysis process is potassium hydroxide
(KOH), whose amounts of deionized water and electrolyte are calculated by means
of the makeup unit operation. To maintain efficient operation, the system
temperature must be in the optimum range of 50 to 80°C for an electrolyte solution
with a concentration of 20 to 40 wt.%[4,5]. The KOH concentration specified in this
sample case is 25 wt.% at a calculated operating temperature of 72.82°C and 700
KPa pressure.
The alkaline electrolyzer is modeled using the rigorous dual feed stack with the
pressure drop calculated by the implemented correlation and the liquid head
management (see Figure 2), whose values of the main characteristics of the
electrolyzer operation and the geometry of the stack and membrane are given in
Table 1.
1
Figure 1: Industrial-scale alkaline electrolysis plant flowsheet.
Figure 2. Electrolyzer unit operation given in HYSYS.
2
For the correct mass balance of the aqueous KOH solution, the water equilibrium
reaction (2𝐻 𝑂 ⟺ 𝐻 𝑂 + 𝑂𝐻 ) and KOH dissociation reaction (𝐾𝑂𝐻 → 𝐾 + 𝑂𝐻 )
were considered.
Table 1. Parameters of the electrode and the membrane of the alkaline stack.
Alkaline stack
Parameter
Value
15000
411
36500
8
230
1.15 for anode
0.73 for cathode
Electric power
Voltage
Current
Number of stacks
Number of cells per stack
Charge transfer coefficient
unit
KW
V
A
-
Electrode geometry
Parameter
Active area (anode/cathode)
Width of channel (anode/cathode)
Length of channel (anode/cathode)
Thickness of channel (ta, tc) (anode/cathode)
Value
4.00
2.00
2.00
2.00
unit
m2
m
m
m
Membrane
Active area
Recombination reaction rate constant
4.00
2.778 × 10
m2
𝐾𝑔𝑚𝑜𝑙𝑒
𝑚 ∙𝑠
Hydrogen (Cat-out) and oxygen (An-out) produced in alkaline cell stack are led
with the electrolyte (KOH, 25%wt) to the liquid-gas separation vessels (Sep-H2
and Sep-O2, respectively), where the electrolyte is separated from the gas and
returned back to the stack by recirculation pumps (Pump-H2 for cathode circuit and
Pumo-O2 for anode circuit). Both KOH recycles (Recy-H2/KOH and Recy-O2/KOH)
pass through a heat exchanger (R-KOH-H2 Cooler and R-KOH-O2 Cooler,
respectively) to cool down the electrolyte before entering the mixer (Mix-Rcy). The
mixer outlet stream (To Makeup) enters the makeup unit described above, and the
makeup outlet stream (To Recycle) goes to a recycle unit whose outlet stream (Tto-S) enters a streams splitter to generate the alkaline cell stack feed streams (Anin and Cat-in). The hydrogen and oxygen are separated in the biphasic separation
vessels (Sep-H2 and Sep-O2 respectively) to eliminate the maximum amount of
condensate water.
The hydrogen purification stage consists of a splitter unit operation that ideally
allows 100% separation of hydrogen from water at 25°C and 1 atm pressure. This
additional purification stage is essential to have a hydrogen stream produced free
of water and other impurities it may contain.
2.2 Simulation Results
The temperature, pressure and mass flow of the alkaline aqueous solution entering
the alkaline electrolyzer were 72.82°C, 700 KPa and 1600000 Kg/h respectively.
3
The alkaline electrolyzer operates in adiabatic and non-isobaric conditions
considering an input power of 15,000 KW, an effective area of 4 m 2, 8 stacks and
a number of cells per stack of 230. Under such conditions, Table 2 shows the results
of pressure, temperature, mass flow rates and compositions of the inlet and outlet
streams of the alkaline electrolyzer, of the recirculation streams, and of the
hydrogen and oxygen streams of the separation section. The total amount of
hydrogen produced in the stack is 315.62 Kg/h, which goes to the hydrogen
separation section from the water, and to the purification stage. The amount of
hydrogen reached in the purification stage is 314,79 Kg/h, for a stack operating
input power of 15,000 KW, reaching pressure drops on the anode side and on the
cathode side of 23.76 and 23.95 KPa respectively, and a voltage efficiency of
82.44%. The operating temperature and pressure at the stack outlet are 74.5°C
and around 676.1 KPa. Such temperature and pressure results are similar to those
reported in the work of Sánchez et al[6].
Table 2. Composition of the matter flow at base-case operation conditions (72.82°C and 700 KPa)
Name
T
P
(°C)
(KPa)
Mass Flow
(
𝑲𝒈
𝑲𝒈
Composition (
𝒉)
𝒉)
H2 O
H2
O2
KOH
Water makeup
72.82
700
2986
2986
0
0
0
An-in
72.82
700
560000
420000
0
0
140000
Cat-in
72.82
700
1040000
780000
0
0
260000
An-out
74.50
676.2
582300
439750.37
0.0582
2505.44
139999.97
Cat-out
74.50
676.1
1018000
757428.52
315.62
0
260000.01
Sep-O2-R
74.50
676.2
579700
439689.65
0.0002
8.2143
139999.97
Sep-H2-R
74.50
676.1
1017000
757315.64
0.8344
0
260000.01
Recy-O2/KOH
74.52
736.2
579700
439689.65
0.0002
8.2143
139999.97
Recy-H2/KOH
74.52
730
1017000
757315.64
0.8344
0
260000.01
R-to-S
72.82
700
1600000
1200000
0
0
400000
O2-Prod
74.50
676.2
2558
60.72
0.058
2497.23
0
H2-Prod
74.50
676.1
427.7
112.88
314.79
0
0
H2-Pure
25
101.3
314.79
0
314.79
0
0
H2O-Pure
25
101.3
112.88
112.88
0
0
0
4
The following Table 3 shows the operating conditions of each of the equipment
considered in the flowsheet of the industrial-scale alkaline electrolysis plant.
Table 3. Operating conditions of pump, separator vessel and heat exchanger equipment.
Unit op.
Name
Parameter
Pressure drop
Duty (KW)
(KPa)
Pump-O2
60
10.55
Pump-H2
53.95
16.44
Separator-O2
0
0
Separator-H2
0
0
R-KOH-O2 Cooler
30.43
916.9
R-KOH-H2 Cooler
30
1579.44
Since the HYSYS electrolyzer model developed is applicable for a wide operating
range, a parametric study has been conducted, in order to investigate the influence
of temperature, pressure and total electric power on the global performance of the
alkaline electrolysis plant. The sensitivities used were 10%.
Figure 3. production of hydrogen as a function of the energy load.
5
The performance of the alkaline electrolyzer is shown in Figure 3, which is obtained
by varying the total electric power from 15,000 to 22,000 KW. The production rate
of hydrogen as a function of energy load (total electric power), provides an
operational perspective that sets the limits of production rate and load range [7].
The power consumed per stack was 1875 KW.
Another analysis of importance in hydrogen production is the behavior of the
voltage efficiency of the alkaline electrolyzer under the influence of the total
electrolyte resistance. The following Figure 4 shows this behavior when varying the
electric power of the electrolyzer from 15,000 to 22,000 KW. As we can see, the
efficiency of the stack is affected by the electrolyte resistance, decreasing from
90.7% to 78.8%.
Figure 4. Voltage efficiency of the alkaline stack as a function of total electrolyte resistance.
Also, in Figure 5 it can be seen that the temperature of the alkaline stack increases
as the input power increases. The highest temperature reached at the highest input
power in the stack used of 22000 KW is 75.83°C. It is important to mention that
temperatures above 80°C must be avoided by using an adequate cooling system
to prevent high degradation rates of electrode [4].
6
Figure 5. Profile of the system temperature as a function of the total electric power of the alkaline
electrolyzer.
3 Conclusion
The rigorous model of the industrial-scale alkaline electrolysis plant proposed in this
sample case shows that the electrochemical part of the model accurately predicts cell
voltage, hydrogen production, and efficiencies. This model can be used as a starting
point for the development of more sophisticated models. The HYSYS electrolyzer
model could be used as a useful tool to evaluate an alkaline electrolysis plant linked
to renewable energy sources, since the model is able to predict the performance of
the stack and auxiliary systems at different loads (energy input) under steady-state
operating conditions.
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References
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Alkaline Water Electrolysis. Frontiers in Energy Research, (5) 1, 1-12, (2017).
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Cost and Performance of Water Electrolysis: An Expert Elicitation Study. Int. J.
Hydrog. Energy. 42, 30470-30492, (2017).
[3] Seibel, C. and Kuhlmann, J.W. D.ynamic Water Electrolysis in Cross-Sectoral
Processes. Chem. Ing. Tech. 90, 1430-1436, (2019).
[4] Brauns, J. and Turek, T. Alkaline Water Electrolysis Powered by Renewable
Energy: A Review. Processes, 8 (2), 1-23, (2020).
[5] Tijani, A. S., Binti Yusup, N. A. and Abdol Rahim, A. H. Mathematical modelling
and simulation analysis of advanced alkaline electrolyzer system for hydrogen
production. Procedia Technology, 15, 799-807.
[6] Sánchez, M., Amores, E., Abad, D., Rodríguez, L. and Clemente-Jul, C. aspen Plus
modelo f an alkaline electrolysis system for hydrogen production. International
journal of hydrogen energy, 45 (7), 3916-3929, (2020).
[7] Varela, C., Mostafa, M. and Zondervan, E. Modeling alkaline water electrolysis for
power-t-x applications: A scheduling approach. International journal of hydrogen
energy, 46(14), 9303-9313, (2021).
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