by Wesley Sund, Brooks Instrument, LLC
December 17, 2011

The details of a mass-based chemical injection system for an application that requires critical dosing of a trace chemical into a continuous flowing fluid.

In 1937, a natural gas leak at a school in Texas killed 237 students. The gas leak in the school was not detected even though it had been leaking for a long time. To solve the problem of detecting odorless hydrocarbon gaseous fuels, laws were passed that required natural gas and propane to be odorized, so they can be detected by the average human nose at 1/5 the lower explosive limit in air.

Ethyl mercaptan is required to be added to propane to meet DOT requirements for

Ethyl mercaptan is required to be added to propane to meet DOT requirements for safe transportation and usage.

Propane is primarily sourced as a byproduct of the production of natural gas. To maintain the heating value of natural gas within a narrow range, heavier components (natural gas liquids) are removed from the natural gas near the production well. The liquids are sent to an additional plant where these liquids are separated for higher value uses. Propane is sold as a fuel and is distributed throughout the country via pipeline or rail car.

Mercaptans are sulfur-containing analogs of alcohols (oxygen replaced by sulfur), and the human nose is sensitive to these chemicals. A skunk's smell, for instance, is a mixture of methyl and butyl mercaptans. For propane, the most common mercaptan used for odorization is ethyl mercaptan, which matches the vapor pressure of propane but is also resistant to decomposition. Ethyl mercaptan must be added to propane to meet Department of Transportation standards for over-the-road shipment at a minimum of 1 pound per 10,000 gallons of propane.

Propane Odorization

To efficiently handle the distribution of propane from a single pipeline across a wide geographical area, pipeline storage terminals are built along the pipeline. At periodic intervals, propane is withdrawn from the pipeline and stored in large horizontal cylinders. The delivery window to withdraw from the pipeline is fixed, and it is critical that all delivery equipment be in good operating condition to take the delivery. Any equipment failure can interrupt or stop delivery. Lost propane deliveries equals lost sales . . . resulting in lost profits for the terminal operator.

Propane is odorized as the propane is withdrawn from the pipeline. Odorant injection equipment operating perfectly is critical to the terminal operation's financial health. This is the primary reason why a propane terminal operator in the upper Midwest invested in a more accurate, reliable and consistent propane odorization system.

Positive displacement dosing pumps have been the traditional method for metering odorant into propane. A flowmeter measures the propane delivered to the terminal, and a control system commands the metering pump to periodically inject a known quantity of mercaptan in the propane at a frequency to meet the desired dosing rate. The stroke detector on the pump sends a signal back to the control system as confirmation that odorant is being injected.

If after a number of commands to stroke the pump there is no signal indicating stroking, the terminal control system will stop delivery of propane. Problems with the injection pump might put the delivery of propane into jeopardy.
Metering pump system seals must be replaced on a periodic basis. Pump-based systems are typically set to overdose to compensate for any inaccuracies, with the result being higher operating costs to purchase odorant.

  • A better solution was sought that would provide the following benefits:
  • Higher accuracy to minimize odorant consumption
  • No moving parts for reliable operation, minimal maintenance
  • Documentation of actual odorant dosing rates

Mass-Based Odorant Injection

A liquid mass flow controller based on Coriolis technology was selected as a technology that would provide these desired benefits. The operating concept of a mass flow controller-based odorization system is simple. The odorant injection control system takes a reading from the propane delivery meter and calculates the flow set point required to meet the odorant mass/propane volume ratio specification. The flow controller is sized to meet a wide variation in propane delivery rates experienced from summer to winter.


 Mass flow injector designed to be mounted in hazardous environment associated wi

 

Mass flow injector designed to be mounted in hazardous environment associated with propane terminal operations.

The key technology in the liquid mass flow controller is the Coriolis sensor tube. Coriolis sensor technology has been used in process flow metering since the 1970s, but only recently has the technology been reduced in size to meet the lower flow requirements for injection of chemicals like odorant.

Coriolis mass flow measurement technology is simply a momentum metering device to determine fluid mass flow rate. A vibrating tube acts as the sensor. The momentum of the fluid flowing through the tube will change the shape of the tube as it vibrates. Detectors are used to measure the change in shape and, when calibrated, a linear relationship exists between the degree of tube twist and the fluid's mass flow rate.

Pages