2. Product Information and Characteristics

2.4.3. Standard Inhibitor Levels

The recommended practice is to maintain the inhibitor (TBC) levels between 10-50 ppm, but higher inhibitor levels may be required depending on the storage conditions, process or specific user requirements. Table 4 below presents typical protection (in days) for styrene when both the inhibitor and oxygen concentrations are above the recommended minimum levels. Styrene users are advised to have a TBC concentrate/solution on-site to restore the TBC level once it falls below 10 ppm. It is stressed, in particular, if styrene is stored under a nitrogen blanket, that users should inject air or oxygen, mix thoroughly, and measure at proper intervals the inhibitor and oxygen levels to ensure homogenous distribution and assurance of the recommended level.

Table 4: TBC Depletion in Styrene (if stored under air)

Temperature	Depletion Days/1 ppm	TBC Content		Shelf Life
°C t		[initial] ppm	[end] ppm	Assured days
25	11	15	10	55
30	7	15	10	35
40	1.5	15	10	7
Note: Oxygen inhibits polymer formation, TBC controls the oxygen depletion rate.

The depletion of oxygen in styrene is not only dependent on the temperature but also on the TBC level added (see Figure 6). At low TBC levels oxygen depletion is rapid and the inhibition system will quickly become ineffective. These tests verified that polymer formation begins once the oxygen content falls below the desired minimum of 10-15 ppm.

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