Butadiene Selective Measurements Using NEO BENZ

Introduction

1,3-Butadiene is a toxic gas used in the manufacture of polymers like rubber and acrylonitrile-butadiene-styrene (ABS) plastics. It is also a component in many fuels and oil refinery processes. Classified by IARC as a known human carcinogen, it has a low OSHA PEL of 1 ppm and an ACGIH TWA of 2 ppm. 1,3-Butadiene is recognized as a Highly Reactive Volatile Organic Compound (HRVOC) due to its potential to readily form ozone, leading to its regulation by TCEQ in parts of the Houston-Brazoria-Galveston Ozone Non-Attainment Area. Therefore, measuring butadiene selectively is important for various reasons, especially when it is present with less toxic compounds.

Achieving Butadiene-Specificity

The NEO BENZ selectively measures 1,3-butadiene (C4H6) in chemical mixtures through two processes:

1. The 9.8 eV lamp removes response to aliphatic compounds of 5 carbons or less.
2. The Butadiene Filter Tube adsorbs nearly all compounds of 6 carbons or more.

Low-molecular weight olefins such as ethylene, propylene, butenes, vinyl chloride, and butadiene pass through the filtering tube and are measured. Table 1 indicates that most tested compounds do not interfere with butadiene measurements. Acrylonitrile and styrene do not respond, nor do hydrocarbons like hexane, propane, or methane. Among the BTEX components of gasoline, only benzene shows a slight response. Vinyl chloride passes through the tube, while trichloroethylene does not significantly interfere.

Table 1. Response of the NEO Butadiene system to various possible interferences

* Methane or propane concentrations over 1% quench the response of butadiene and other VOCs.

Butadiene Linearity

Figure 1 illustrates that butadiene response as a pure vapor is linear up to at least 5 ppm. The detection limit is approximately 0.05 ppm.

[Graph showing Butadiene Linearity: Reading (ppm) on the y-axis from 0 to 6, and Butadiene Concentration (ppm) on the x-axis from 0 to 6. A line with a positive slope passes through these points, indicating linearity.]

Fig. 1. Butadiene response as a pure vapor is linear to 5 ppm.

Butadiene Screening and Measurement

The NEO Butadiene is initially used for screening by making continuous VOC measurements without a filtering tube. If a reading of concern is detected, a tube is broken open and inserted into the tube holder. The temperature is selected (see temperature effects below), and a reading is initiated, taking 3 minutes at room temperature. The single reading is displayed, and the user can then choose to continue with a STEL measurement or remove the tube and return to continuous butadiene screening.

[Image of the NEO BENZ instrument with a filter tube inserted.]

STEL: Short-Term Exposure Limit

STEL is the average concentration over any 15-minute interval. After the initial 3-minute reading, the user can continue sampling for STEL by measuring for an additional 12 minutes. To extend the working life of the tubes, the NEO operates at maximum flowrate during initial sampling and then switches to the lowest pump speed for the STEL reading.

Tube Discoloration, Humidity Effects and Re-Use

Butadiene filtering tubes are designed for a single measurement. They do not change color when adsorbing VOCs, so there is no visual indication of tube capacity usage. A tube can be re-used for a second, and possibly third, measurement if previous readings were close to zero. However, if a re-used tube shows a significant response, the results are suspect, and a new tube should be used. The same tube can be used for initial zero and span calibration, as the zero gas should contain no VOCs. Tubes do not significantly absorb water vapor but may exhibit moderate capacity losses in high-humidity environments. Tubes can be broken open up to a few hours before use, but it is recommended to discard them if left open for more than one day.

Temperature Effects

Readings are not instantaneous, requiring a few minutes for butadiene to saturate the tube and stabilize. The required sampling time depends on the ambient air temperature and the tubes, as shown in the table below. Most measurements are performed in the 20-30°C range, requiring a 3-minute sampling time. Lower temperatures yield a higher final response, while higher temperatures yield a lower final response.

Butadiene Tube Measurement Time vs Temperature

Calibration

Zero and span calibration should be performed using a butadiene tube at a temperature close to the expected measurement temperature to compensate for temperature effects on timing and response magnitude. Performing calibration at room temperature may result in a 20% error for measurements at temperature extremes. For typical conditions, 5 ppm butadiene gas is recommended for calibration. For highly accurate work at concentrations below 2 ppm, using a 2 ppm butadiene standard gas, available from various calibration gas suppliers, is recommended.

Isobutylene Calibration

If butadiene is unavailable, isobutylene can be used. For 10 ppm isobutylene, set the span value to 7.4 ppm, or for 5 ppm isobutylene, set the span value to 3.7 ppm. This adjustment compensates for the response difference between isobutylene and 1,3-butadiene. Calibration should be performed using a butadiene tube for zero and span, and at a temperature as close as practical to the expected measurement temperature.

Tips on Opening Tubes

Step 1: Insert tube fully into ceramic tip breaker and rotate to etch glass.

Step 2: Pull breaker away slightly from etch mark.

Step 3: Grip tube close to end and snap off tip.

[Image sequence showing how to open a gas detection tube.]

Tube Hazards and Disposal

Butadiene filtering tubes contain no toxic or corrosive chemicals and can be disposed of in regular trash after taking precautions for sharp glass edges. Unlike benzene tubes, Butadiene tubes may remain in the tube holder for extended periods with the pump on without damaging the instrument. They only require replacement when their capacity for VOCs is exhausted.