Bronkhorst supplies a flow solution for a customer, adding odorant to natural gas in a continuous, controllable way. Commercial natural gas in the Netherlands must contain at least 18 mg of THT (tetrahydrothiophene) per cubic meter of gas. As natural gas is odorless by itself, the liquid THT acts as a “warning agent” in case of leakage. Accurate dosing of the odorant supply, THT, to the natural gas is critically important to ensure compliance and safety. By using a flow solution, consisting of Coriolis mass flow controllers, it’s possible to develop a continuous and accurate dosing system to supply the odorant to natural gas.
Traditionally, a pulse-wire technique adds THT to the gas on a regular basis, using a pump with a fixed stroke volume. Especially for small gas flows for biogas installations, in the range of 40-50 m3/hour, such a batch-wise injection may lead to liquid THT remaining in the gas lines. In this way, THT may not mix well with the gas, and it might have the wrong concentration since there is no control on the supply. By using the Coriolis flow solution, continuity and accuracy issues should not occur.
The key component of this new dosing solution for natural gas odorant is a Bronkhorst Coriolis mass flow controller (mini CORI-FLOW series). The mini CORI-FLOW injects a small but continuous liquid THT flow into the biogas stream. A master mass flow meter measures the biogas flow and sends a signal to the mass flow controller via a PLC. The flow controller sets a dosing value of 18 mg of THT per cubic meter gas and continuously injects the correct amount of THT.
In this configuration, nitrogen gas pressurizes a 40 liter THT storage tank. In addition, a pulsation damper removes pressure surges and guarantees stable control. This results in a precharge pressure upstream of the flow controller of 10 to 13 bars. At the same time, the pressure downstream of the device is typically 8 bars. As a result, the flow controller is set to a maximum dosing value of 40 mg/m3 .
Using this dosing solution, operators can dose THT very accurately, within a range of 1-2%. This is a major improvement in comparison with the traditional batch-wise method. Furthermore, continuous dosing results in a homogeneous THT/biogas mixture, which is a more efficient method of dealing with the amount of odorant as it avoids overdosing.
Another important aspect is the operational reliability of this solution. This refers to the ability to supply the natural gas odorant to the natural gas mains. Prior to entering the mains, a gas chromatograph analyzes the biogas composition. If the biogas doesn’t contains enough THT, the grid operator will shut off the supplier from the natural gas mains. By default, the monitoring of the dosing occurs by PLC, but it can also be done by means of the flow controller itself. In this solution, the dosage amount of THT is exactly known. Furthermore, an alarm will sound when the system does not detect any odorant. Moreover, it allows operators to predict when an empty THT storage tank needs replacement.
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