Q: My aromatic plant consumes hydrogen during startup. A dedicated line supplies hydrogen to the plant. I would like to install an instrument to measure the mass flow of the gas for billing purposes. What kind of flow measurement technology should be used?
Since the plant is in operation, the hydrogen line should not be cut. So I'm looking for a mass flowmeter that does not require cutting the line. On the other hand, clamp-on devices I’ve seen don't have enough accuracy for billing purposes.
G.F. Hekmat / firstname.lastname@example.org
A: The short answer is that clamp-on sensors are not suited for accounting applications. For that, you need an in-line instrument, such as a Coriolis flowmeter, with accuracy better than ±0.5% of actual reading (AR) over a 100:1 range, or a thermal mass flowmeter with accuracy of ±1% AR to ±2% full scale (FS) over a 5:1 range. The accuracy of clamp-on, ultrasonic meters is 3-5% FS, which is no good. If your flow rate is fairly constant (within, say, 2:1) and the measurement variation is mostly due to density changes, you can add a nuclear densitometer to the clamp-on unit, and expect to get a ±5% AR accuracy over a 2:1 range.
Keep in mind that hydrogen gas usually contains some air, which is 14 times heavier than hydrogen. So, if it's not measured and corrected for, the mass flow measurement it will read high.
The importance of hydrogen flow measurement is increasing. Some expect that, in a couple of generations, our present fossil-nuclear energy economy will be replaced by a solar-hydrogen one. They believe our energy source will become solar energy, and it will be stored and transported in hydrogen. Whether this will happen or not, we don’t know, but we do know the market for hydrogen flowmeters is already rising. One of the newer applications is in the fuel cell industry, which serves both electric cars and solar homes, where it provides electricity during the night. Fuel cells operate by generating electricity, while oxidizing hydrogen into water (Figure 1).
The specific energy of a kilogram of hydrogen (143 MJ/kg) is three times higher than that of gasoline (46.9 MJ/kg). The cost of a kilogram of hydrogen is around $5, and it is expected to level off at $2-4. In electric cars, hydrogen is stored in fuel tanks at around 10,000 psig (700 barg). One kilogram of hydrogen can provide the same driving range as one gallon (3.79 liters) of gasoline. In fuel cell-operated electric cars, the fuel tanks can contain 5 kilograms of hydrogen, which can give a driving range of up to 600 kilometers (375 miles). Refueling at hydrogen fuel stations takes about 5 minutes.
In the U.S., the National Institute of Standards and Technology (NIST) specifies the accuracy requirements for the measurement of gasoline at fuel pumps. They usually use piston-type positive-displacement pumps having ±0.3% AR accuracy, meaning that the actal volume of a 10-gallon purchase could vary between 9.97 gallons and 10.03 gallons at the delivery temperature of the gasoline.
For hydrogen, NIST mandates that filling stations (Figure 2) selling the fuel be able to dole out the gas at least at ±2% AR accuracy (20 grams per kilogram). In addition, NIST Handbook 44, which has been adopted by all states, stipulates that hydrogen will be sold by the kilogram. A modified version of this requirement (S&T Item 339-2) has widened the current accuracy tolerances, and it's not yet entirely clear what the final numbers will be. In short, this area of standardization is still in the development stage and our process control profession can contribute to helping to get it right.
As of today, the number of hydrogen fueling stations in the world is reported to be 760, including 35 in the U.S. (32 in Catifornia), but these numbers are increasing very fast. Building these stations is expensive (about $1 million each) and their refueling precision is debatable. While the pumps usually display the sales as accurate to within ±0.01 kg, I'm not sure if they really are. When refueling, the hydrogen is charged into the car's storage tanks, which are designed to hold the gas at 10,000 psig (700 barg). The availability of such high-pressure mass flowmeters is limited, but they do exist and their accuracy ranges from better than 0.5% AR to about 2% FS (Figure 3).
Naturally, one could also use the ideal gas law (PV = mRT) to determine the mass of hydrogen in the fuel tank at any one time (m = PV/RT), where m = mass, P = absolute pressure, V = volume of the fuel tank, R = the gas constant and T = absolute temperature. Using this method would also be feasible because, if the tank is being refilled with pure hydrogen, the tank volume and the gas constant are known (V and R) and the hydrogen pressure and temperature (P and T) can be accurately measured. To my knowledge, this method of mass flow measurement has not yet been used, but it could.
Béla Lipták / email@example.com
A: While there are many flowmeters that are capable of measuring hydrogen gas, only the Coriolis flowmeter corrects for density variations resulting from both temperature and pressure changes. All other flow measurements are volumetric, and must be corrected for variations in pressure and temperature. Coriolis is the only technology sufficiently accurate to be used in custody transfer operations, such as your application.
Dick Caro / RCaro@CMC.us
A: I recommend that you consider a Coriolis mass flow meter for this application. For additional details and technical guidelines, please see www2.emersonprocess.com/siteadmincenter/PM%20Micro%20Motion%20Documents/Hydrogen-Gas-Benefits-WP-00539.pdf.
Raj Binney / firstname.lastname@example.org
A: I concur with Bela – use Coriolis. Note, however, the extreme sensitivity of hydrogen measurements to the presence of any contaminants. One percent of, for instance, oxygen is equal to 8% hydrogen. You can't rely on the Coriolis meter to determine the mixture density/composition as you could in liquid phase. So if you're purchasing 95% hydrogen on the basis of its hydrogen content, most of your measurement is the contaminants. This is a matter that hydrogen producers have traditionally been careful to conceal in their contracts.
Ian H. Gibson / email@example.com
A: You can definitely use a Coriolis mass flowmeter. Check with Micro Motion. I've used it for measuring flow as low as 8 gm/ hr hydrogen for gas-phase polymerisation reactors.
Debasis Guha / Debasis_guha71@yahoo.com