Detailed process model description for electrolysis demo

Model specification


Intended audience

The present document is oriented to:


The scope of the present document is to describe the capabilities of the Electrolysis demo, for the process modeling of continuous alkaline electrolysis systems for hydrogen production.


Electrolysis kernel

In LIBPF® one kernel can support many process models, each as a different flowsheet type.

All the process models supported by a kernel share the same list of components and can use all LIBPF® embedded types plus the custom types registered by the kernel itself.

Type list

The Electrolysis kernel registers the following process models, based on the built-in LIBPF® FlowSheet type:

Type Name Description options Note
Elysim  Elysim  Alkaline electrolysis system for hydrogen production  STACK:multiReactions[0].H: Stack cell length  default model type
STACK:multiReactions[0].W: Stack cell width
STACK:multiReactions[0].nCells: Number of cells in the stack
S05:Tphase.mdot: Estimate for H2 side electrolyte recycle
S06:Tphase.mdot: Estimate for O2 side electrolyte recycle

Component list

The fluids to be processed are broken down in their constituents and represented as a mixture of basic components.

The components are defined using built-in LIBPF® basic types if possible, or with custom components.

More precisely the Electrolysis kernel defines the following component list (click on the component type to jump to the reference documentation for the component):

Type Name Description
purecomps::H2O water standard model for water
purecomps::H2 molecular hydrogen electrolysis product
purecomps::O2 molecular oxygen electrolysis product
KOH  potassium hydroxide  catalyst

Process description and scheme

The underlying process model is based on the publication: Mónica Sánchez, Ernesto Amores, David Abad, Lourdes Rodríguez, Carmen Clemente-Jul, “Aspen Plus model of an alkaline electrolysis system for hydrogen production”, International Journal of Hydrogen Energy, Volume 45, Issue 7, 2020, Pages 3916-3929 (doi: 10.1016/j.ijhydene.2019.12.027).


The following Process Flow Diagram should clarify the process.

Process Flow Diagram

The cell stack (STACK) is the heart of the system. Electricity is supplied to the cell stack to decompose water into hydrogen and oxygen: hydrogen is released at the cathode (S01) and oxygen is released at the anode (S02). Also certain side-reactions take place, the most important being the cross-flow of hydrogen to the anode.

Only about two thirds of the supplied electric power is electrochemically converted, the rest is released as heat in the cell stack, which has to be cooled to keep it at the correct operating temperature. The cooling of the cell stack is effected by circulating a large excess of electrolyte solution, separately at the two electrodes (S10 and S11), so that the water conversion per pass is effectively very low (in the range of 0.5%).

Due to the low water conversion per pass though the stack, the cathode and anode outlets (S01 and S02) do not contain only the produced gases, but also the excess circulating electrolyte. To recover the hydrogen (S03) and vent the oxygen (S04) so that the system pressure is kept low, the cathode and anode outlets are sent to two separate liquid-gas separation vessels (SEPH2 and SEPO2).

The degassed electrolyte is returned back to the stack by separate recirculation pumps (PUMPH2 and PUMPO2) and cooled in separate heat exchangers HXH2 and HXO2.

The water consumption due to the electrochemical reaction is compensated with a deionized water make-up (S12), so that the water hold-up in the circuit is constant.

Stream list

The streams are defined using built-in LIBPF® basic types.

Click on the type to jump to the reference documentation for the stream.

Type Name Description From To
StreamIdealLiquidVapor S01 H2 cathode outlet STACK catout SEPH2 in
StreamIdealLiquidVapor S02 O2 anode outlet STACK anout SEPO2 in
StreamVapor S03 H2 production SEPH2 vapor sink in
StreamVapor S04 O2 production SEPO2 vapor sink in
StreamLiquid S05 H2 side cathode electrolyte SEPH2 condensate PUMPH2 in
StreamLiquid S06 O2 side anode electrolyte SEPO2 condensate PUMPO2 in
StreamLiquid S07 Pumped H2 side cathode electrolyte PUMPH2 out HXH2 in
StreamLiquid S08 Pumped O2 side anode electrolyte PUMPO2 out HXO2 in
StreamLiquid S09 Cooled H2 side cathode electrolyte HXH2 out MIX in
StreamIdealLiquidVapor S10 Cooled O2 side anode electrolyte and anode inlet HXO2 out STACK anin
StreamLiquid S12 Water make-up source out MIX in
StreamIdealLiquidVapor S11 Cathode H2 inlet MIX out STACK catin

Unit list

The unit operations are defined using built-in LIBPF® basic types if possible, or with custom unit models.

Click on the type to jump to the reference documentation for the unit operation.

Type Name Description
Electrolyzer STACK Electrolysis stack
FlashDegasser<StreamIdealLiquidVapor> SEPH2 Hydrogen separator
FlashDegasser<StreamIdealLiquidVapor> SEPO2 Oxygen separator
Pump PUMPH2 Electrolyte circulation pump cathode side
Pump PUMPO2 Electrolyte circulation pump anode side
AirCooler HXH2 Electrolyte cooling cathode side
AirCooler HXO2 Electrolyte cooling anode side
Mixer MIX Electrolyte mixer

The Electrolyzer model is defined as a superclass of the built-in MultiExchanger model (a rating, optionally reactive multi-Stream heat exchanger), customized with three reactions:

The Aircooler model is defined as a superclass of the built-ins:

that allows to specify a COP (coefficient of performance) to have it compute the electrical consumption.

Model limitations

At this time the process model has a number of limitations:

In a subsequent version these limitations will be lifted.


Electrolysis demo

LIBPF® technology