Electrolysis and Electrolyzer Pressure Control - Equilibar

Electrolysis and Electrolyzer Pressure Control

Back pressure regulators for electrolysis

  • schematic of Equilibar BPR with single dome pilot pressure for PEM stack control

What is electrolysis? Electrolysis uses an electric current to energize a chemical reaction that will not occur naturally. Water electrolysis is an electrochemical process using electricity to split water into hydrogen and oxygen. Carbon dioxide electrolysis is the reduction of CO2 into CO. Chlor-Alkali electrolysis combines salt and water to generate chlorine, caustic soda and hydrogen (2 NaCl + 2 H2O = Cl2 + 2 NaOH + H2). These reactions can occur in an electrolysis cell or an electrolyzer unit containing an electrolytic medium between an anode and cathode. Pressure control during electrolysis is an important consideration and depends on the electrolyzer technology. Equilibar products are widely used in electrolyzer and fuel cell applications. Contact us for more information.

Electrolyzer Technology

Alkaline water electrolysis is the oldest and most advanced methodology for water separation but newer systems are rising in popularity. Technologies using Polymer Electrolyte Membrane stacks, also called Proton Exchange Membranes or PEM, were designed for higher efficiency water electrolysis than the earlier alkaline solution electrolyzer. PEM electrolysis systems may be designed as a balanced pressure system, where O2 and H2 operate at the same pressure, or as a differential pressure system where O2 and H2 operate at different pressures. They typically operate at a higher pressure than alkaline systems, giving them the advantage of minimizing the need for a second stage mechanical compression to pressurize for storage. In either the differential or balanced pressure designs, pressure regulation of the O2 and H2 gas streams is extremely important to the efficiency and life of the PEM electrolyzer.

In CO2 electrolysis, research is ongoing to find efficient electrolyzer technology to use at large scale. Solid oxide electrolysis cells (SOECs), molten carbonate electrolysis cells (MCECs), and gas diffusion electrodes (GDE) are all being studied. Electrolyzers assisted by catalysts, creating a multilayer electrolyzer stack is also under investigation. CO2 electrolyzer technology is emerging as an important contributor to carbon reduction by using CO2 and H2O to create synthetic fuels.

In Chlor-Alkali electrolysis, the anode and the cathode are separated by an ion-exchange membrane. Hydrogen gas forms at the cathode and chlorine gas forms at the anode. The NaOH caustic solution remains and is collected and often concentrated. Differential pressure across the membrane is an important design consideration.

What are the benefits of the Equilibar electrolyzer pressure control solutions?

Equilibar Wide Cv turndown (100:1 or greater) exceeds the range of traditional globe or ball valves

Equilibar BPRs are often used to control electrolysis processes at a range of flows and pressures, high or low, with extreme precision. The unique multi-orifice design of Equilibar BPR allows for stable pressure control across a wide Cv range and can easily handle mixed phase (liquid-gas) flow or varying pressures on the outlet of the BPR.

Equilibar BPRs can handle high dP across the valve without issue

Oftentimes in production electrolyzer systems, it is advantageous to have minimal pressure drop across the BPR, with the pressure downstream being as high as possible to reduce the need for pumping up the gas to a higher pressure for storage or usage. Equilibar BPRs, when sized properly for the required Cv, can maintain stable pressure control with either minimal pressure drop across the BPR, or with a large pressure drop, depending on the case. In contrast, traditional control valves and spring-loaded BPRs often have difficulty operating with a very small dP across the valve.

If instead, the system requires a large pressure drop across the valve, which would normally cause cavitation or otherwise damage traditional single-seat valves, Equilibar BPRs can also handle that scenario. The unique throat geometry of the Equilibar valves with multiple parallel flow paths has proven to avoid cavitation issues (both pitting and noise).

Equilibar valves can be made from materials to handle aggressive conditions

The simplicity of the Equilibar design also allows us to easily tailor the materials of construction for the challenges of high pressure electrolysis. The regulator body can be machined from high nickel containing alloys like brass, bronze, or Monel to be used with high pressure pure oxygen. For alkaline electrolysis, the diaphragm and O-ring materials can be chosen to be compatible with the caustic electrolyte solution. Equilibar valves can be cleaned for oxygen service upon request as well.

The Equilibar back pressure regulator adjusts instantaneously to maintain pressure

The pressure control of the Equilibar regulator is not reliant on an external transducer and PID loop, so the BPR reacts nearly instantaneously to changes in flow and adjusts its Cv accordingly to maintain the desired pressure. This precise pressure control is particularly important when dealing with sensitive proton exchange membranes which will rupture if the dP across them is too high.

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Ways to use Equilibar back pressure regulators in electrolysis systems

PEM Electrolyzer differential pressure control

Equilibar BPRs can be used to control the outlet gas pressure on oxygen and hydrogen lines coming from a PEM stack. By putting an Equilibar BPR on the gas outlet of the anode and cathode side of PEM stack, precise dp (differential pressure) can be maintained across the stack at all times. Why use an Equilibar BPR for PEM stack pressure control? Because an Equilibar BPR can control high pressures, can process high temperature and mixed-phase fluids, and is easily integrated into electronic control system with an E/P or I/P pilot regulator.

schematic of PEM Electrolyzer differential pressure control using Equilibar valves

Equilibar back pressure regulators control PEM stack dP

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PEM Electrolyzer control downstream of separator

Equilibar BPRs can also be used downstream of a separator or liquid dropout. In some systems it is advantageous to put the BPR downstream of a separator, as opposed to directly downstream of the PEM stack, so that pressure in the DI water loop is maintained. Advantages of using an Equilibar for this setup are:

  • Equilibar BPRs can handle high dP across the valve without causing damage or poor performance issues.
  • The Equilibar BPR will isolate the system from downstream pressure fluctuations (e.g. potentially varying gas pipeline pressure in production facility) and maintain stable control even as this outlet pressure varies.
schematic of PEM stack control using Equilibar valves after the separator or liquid dropout

Place an BPR downstream of a PEM separator so that pressure in the water loop is maintained

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Use a single setpoint on an Equilibar BPR to maintain PEM stack pressure differential

The dome-loaded control of our BPRs also means that that it is possible to control the pressure on both the cathode and anode side of the electrolyzer with one pilot pressure regulator connected to the dome of two BPRs, ensuring that the pressure on both sides of the PEM closely match each other. The schematic below is an example of such a system.

schematic of Equilibar BPS with one electronic pilot regulator for matching pressure control on water electrolyzer

One electronic pilot pressure regulator controls both the anode and cathode sides of the water electrolyzer

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schematic of alkaline water electrolysis

Alkaline electrolysis system using Equilibar BPR for H2 and O2 pressure control

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GSDM8 high pressure 1 inch back pressure regulator

Equilibar GSDM8 back pressure regulator for higher flow electrolyzer pressure control

image of EPR 3000 controlling pilot of Equilibar H3P high pressure back pressure regulator

Equilibar LF back pressure regulator for low flow electrolysis pressure control

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Equilibar technology used for electrolysis to generate oxygen in space

Flow cell electrolysis, which is used for spaceflight applications, uses a flowing water electrolyzer to generate enough oxygen from water to keep up with the crew’s metabolic requirements. Many of the current electrolyzer designs require a downstream gas-liquid separation process, presenting challenges in a microgravity environment. One complication is two-phase flow caused by bubble generation in the electrolyzer. Researchers at the University of Colorado at Boulder proposed an alternative electrolyzer configuration and built a prototype using an Equilibar® vacuum regulator.

Read the case study in the link to the right.

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Equilibar back pressure regulators used in “artificial photosynthesis” to make power from renewable sources

Our partners at Pressure Control Solutions (PCS) in the Netherlands worked with Evonik and Siemens on a project where the two companies combined CO2 electrolysis and gas fermentation in an “artificial photosynthesis” process they call The Rheticus Project. The project links the companies’ innovations in fermentation and electrolysis to make power from renewable energies.

PCS was called upon to find a solution for this project where a range of precision pressure setpoints was required. The customer found the Equilibar BPR technology to be “superior to conventional valve technologies”. Additionally, with the solution PCS designed, the customer was able to avoid crystallization issues related to salt in the electrolysis process. Read more in the case study on PCS website.

What are the products from water and CO2 electrolysis used for?

Energy researchers are optimizing water electrolysis for producing hydrogen as an energy source from renewable resources. Wind turbines and PV cells can be used as electricity sources for the electrolyzer to produce hydrogen for hydrogen fuel cells. The goal is to increase the use of renewable energy and decrease carbon emissions.

Oxygen gas produced in the reaction may be used for a variety of applications, including breathable O2 for space travel and submarines.

Carbon Monoxide is used in a pure form as a source for many industrial and fine chemicals or together with H2 in synthetic gas. When CO2 and H2O are reduced in a co-electrolysis process, syngas is produced and used to make fuels. It’s a win-win process taking CO2 from the atmosphere and using it to make renewable fuel.

About Equilibar back pressure regulators

The Equilibar® back pressure regulator is dome-loaded and, with its novel multiple orifice design, delivers instantaneous reliable control in the most challenging applications. The dome-loaded design means it requires a fluid (typically air or nitrogen) pressure applied to the dome of the BPR matching the desired inlet pressure. This setpoint pressure can be manually or electronically controlled. Learn how Equilibar back pressure valves work.

Our team of experienced engineers offer detailed guidance in choosing the pressure control products for a specific condition. Special considerations are taken into account for oxygen rich processes seen in electrolysis.

Contact an Equilibar engineer to discuss your application in more detail.

Equilibar EVR used for electrolysis in space for oxygen generation

Case Study: Prototype electrolyzer for generating oxygen in space

 

Artificial photosynthesis using Equilibar BPR

Case Study: Artificial Photosynthesis using electrolysis and fermentation

 

 

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