T. BALL, Emerson, Minneapolis, Minnesota
The electrolysis process is used in many different industries, including metal extraction, zinc and copper refining, electroplating, semiconductors and green H2 production. This process uses liquid in which electrodes (positively and negatively charged materials that react when a current is sent through them) are suspended. At the end of the process, a different material or set of materials is produced. Both the electrodes and the liquid are vital to electrolysis: the role of measurement in this context is to ensure that the correct amount and type of liquid is used. The efficacy of various types of measurement devices in achieving this depends on both the intended end product and the composition of the initial liquid. The process for making chlorine gas (Cl2), for example, uses a water and salt (NaCl) solution to produce Cl2, H2 gas, and sodium hydroxide (NaOH). It is one path to H2, albeit not a green H2 option. This article will focus solely on the production of green H2.
Green H2 production, the process of producing H2 without involving the use of CO2-emitting fossil fuels, is achieved by using electrolysis to split ultra-pure water (H2O) into H2 and oxygen (O2). The energy needed to provide the requisite electrical current differs based on what natural energy sources (solar and wind being two examples) are available.
Accurate accounting of a material used—in the case of green H2 production, water—requires reliably accurate measurements. Suppose that a device capable of delivering such could also wirelessly transmit data on flow, mass balance, and material coming in and going out: this device would make it easier for a facility to report on water usage, costs, environmental impact and any other metric the facility is required to report. The same holds true for the salable products at the process’s end: they need to be measured accurately to ensure the correct amounts are flowing from the process and are stored, sold and transported.
The following quote ought to give an idea of how much water is used in the creation of H2.
“…For every kg of hydrogen produced, 9 kg of water must be consumed. Therefore, 2.3 Gt of hydrogen requires 20.5 Gt, or 20.5 billion m3, per year of freshwater, which accounts for only 1.5 ppm of Earth’s available freshwater, an amount smaller than what is currently consumed by fossil fuel-based energy production and power generation1.”
Water used, however, does not necessarily equate to water permanently consumed. The paper quoted above also explains that H2 returns H2O as its waste product when consumed as a fuel. The water lost in the production process is therefore returned to general circulation, if not to the H2 production facility.
Four types of flowmeters can be used at various points of the electrolysis process: the best choice for installation will depend on the facility’s budget and where in the process measurements are desired. The flowmeter options are as follows: vortex flowmeters, differential pressure (dP) flowmeters, Coriolis flowmeters and magnetic flowmeters. The following are the relevant details and best practices regarding each.
A proven, widely used technology, the vortex flowmeter (FIG. 1) can be used for all measurement points. It is a relatively affordable option and comes in dual or quad configurations to optimize safety. The vortex flowmeter must be of a smaller size than the pipe to optimize the flow range, and a minimum flow rate must be maintained for the meter to work correctly. It is safety integrity level (SIL) rated for safety loops and systems, which can significantly benefit H2 production. In green H2 electrolyzers, vortex flowmeters are often used for the H2O feed line, as the H2O is typically deionized or ultra-pure. They can also be used to measure the H2 gas and O2 gas products.
FIG. 1. Vortex flowmeter with transmitter.
These flowmeters (FIG. 2) have a broad measuring range, can operate wirelessly without external power and come in a wide range of designs to suit the application. Selecting the proper dP range and the correct size of dP flowmeter primary element are both key to achieving an accurate flow measurement with suitable flow range coverage. This type of meter is approved for some custody transfer measurements by third parties. It can, however, create permanent pressure loss depending on the primary element selected for it. Some of the elements are prone to plugging or clogging, but this drawback is mitigated by the inclusion of onboard diagnostics, including plugged impulse line detection, that help reduce unplanned process upsets. DP flowmeters are also available with SIL ratings and are often utilized in green H2 electrolyzers. DP flowmeters can be a good choice for all of the flows on a green H2 electrolyzer: the SIL certification can have value on H2O feed lines, and this type of meter is a good option for the H2 and O2 product flows as well. The most important action is selecting the right primary element and transmitter pressure range to ensure accurate and reliable measurement over the entire flow range.
FIG. 2. A dP flowmeter with transmitter.
Considered the most accurate flow measurement devices on the market, Coriolis flowmeters (FIG. 3) provide direct mass flow for simplified mass balance and density measurement. Multiple advanced diagnostics come with the meter to verify meter performance and health. It can be used on all flow measurements and has a broad rangeability with highly accurate measurement across the entire flow range. Coriolis flowmeters do, however, generate a permanent pressure loss. Coriolis flowmeters can handle most applications that use a H2 feed–particularly deionized or ultra-pure H2O streams–brine solutions, NaOH product, Cl2 product and H2 product.
FIG. 3. Coriolis flowmeter with transmitter.
These flowmeters are highly accurate and support a wide range of materials for process compatibility, limiting the need for exotic metallurgy. The magnetic flowmeter (FIG. 4) is also cost effective and has a broad rangeability with minimal impact on accuracy until velocities drop below 1 ft/sec. However, the fluid flowing through a magnetic flowmeter must be conductive, which rules out the ultra-pure or deionized H2O commonly used in green H2 production. These meters also cannot be used to measure gases such as H2 or Cl2: they are therefore poorly suited to the endpoint of the electrolysis process.
FIG. 4. Magnetic flowmeter with transmitter.
There are three key measurement points during the green H2 generation process. The first is the feed H2O line into the electrolyzer. Since this H2O is deionized or ultra-pure and is often used to cool the electrolyzer as well, an SIL rated flowmeter device is typically required. This makes a vortex or dP flowmeter the best choice under most circumstances, but a Coriolis flowmeter can be used instead if direct mass and density measurements are desired.
When it comes to measuring the H2 produced, the best measurement device is going to be a Coriolis flowmeter. In addition to its overall accuracy and reliability, it is the best choice to deal with the idiosyncrasies of H2 custody transfer. H2 is typically stored at high pressure to ensure it remains in a gaseous state, and this high pressure adds a layer of complexity and safety concerns to the preparation of H2 for transport and endpoint consumer dispensing alike. Vortex and dP flowmeters are less expensive than their Coriolis counterparts and can be used in this capacity, but often do not have the accuracy or approvals desired for H2 custody transfer.
The final measurement point concerns the produced O2. Vortex and dP flowmeters are both excellent solutions here. They provide good flow measurement performance while minimizing leak points and, potentially, pressure loss (depending on the primary element selected for the dP flowmeter, if such is used). Coriolis flowmeters can also be used in this application and have value if a direct mass measurement is desired or if the flow measurement is a custody transfer point.H2T
a Emerson Rosemount’s vortex flowmeter with transmitter
b Emerson Rosemount’s differential pressure flowmeter with transmitter
c Micro Motion Elite Coriolis flowmeter with transmitter
d Emerson Rosemount’s magnetic flowmeter with transmitter
TREVER BALL is Director of Product Marketing for Rosemount magnetic flowmeter products at Emerson. He has more than 20 yr of experience with process instrumentation, specializing in magnetic flowmeters.