Chemical Technology July 2015

CONTROL & INSTRUMENTATION

Calculation of the methane content at the wastewater treatment plant in the Ara Region Bern, Switzerland

The molar mass (M) of a gas can be determined according to the fol- lowing formula which is implemented in the OPTISONIC 7300.

γ = Adiabatic index, pre-set value into the flowme- ter electronics; an average value of 1,31 is entered for the CO 2 /CH4 mixture. R = Molar gas constant, approx 8,3145 J mol–1 K–1. T actual = Gas temperature in Kelvin, can be measured via a 4–20 mA input on the converter VoS = Sound velocity of the gas

M= y RT actual VoS 2

Figure 2: Composition of biogas depends on the molar weight

The methane content can be calculated from the molar mass, considering the following: The molar weight of CO 2 (44) Themolarweight ofmethane (16) The molar weight of water (18) The amount of water in the gas depends on the temperature. For a water saturated gas the amount of water is known.

The methane content is measured with an accuracy of 2 % (including uncertainty of the temperature measurement of 1 %).

Figure 1: Velocity of sound/methane content at 30°C

Figure 3: Amount of water in a gas depending on the temperature

7300. The device offers a series of new functions for biogas measurement while remaining completely competitive as regards procurement cost. The measuring instrument, which functions according to the time transit differential method, guarantees flow measurement with a high degree of long term stability regardless of the gas composition. The solution is mainte- nance-free and immune to deposits. Thanks to its special signal converter design, the OPTISONIC 7300 also features considerably higher signal strength. The NACE approved Grade 29 Titanium transducers provide excellent corro- sion resistance against H2S. An enhanced digital signal processor (DSP) also allows for better detection of small acoustic signals strongly dampened by high CO 2 content. In addition, its transducer and transducer pocket design make it insensitive to liquid water. In this way old sources of error are eliminated from the start. The OPTISONIC 7300 does not have limitations such as regular recalibrations, pressure loss or limited flow range. In addition, to make various measurements comparable, the standard volume can be measured following the input of temperature and pressure using the OPTISONIC converter. The measuring device also features an integrated calcula- tion of the methane content (see box above). Practical experience with the OPTISONIC has shown after a short period of time how stable and accurately biogas measurements can now be performed. Exactly how operators of biogas plants can use this to their advantage is clearly illustrated by the application example of the Ara- bern wastewater treatment plant shown in the box above. z

with high water content in the past. In the case of these measuring devices, the high CO 2 content typically present in biogas flows had a negative effect on the measur- ing stability since the molecular structure of the carbon dioxide can significantly decrease the dispersion of the ultrasonic signals. In addition, the cost to purchase these devices is still comparatively high, which often made the use of ultrasonic flowmeters unattractive in the past.

Nevertheless, based on their design, ultrasonic flowmeters have always been particularly well suited to biogas appli- cations as they guarantee a full transit without loss of pressure or any negative effect on the flow and they can cover a wide measuring range. Innovative ultrasonic flow measuring technology for biogas Much has been learned in measuring technology from the experience with biogas applications. This is especially true in the area of ultrasonic flowmeasuring technology. Through investment, research and development, leading measuring technology manufacturers have managed to produce a new generation of ultrasonic flowmeters that specifically targets the high requirements of biogas applications. The fact that ultrasonic technology has helped make measuring the flow of biogas manageable while remaining affordable is illustrated, for example, by Krohne’s new OPTISONIC

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Chemical Technology • July 2015