CHEMICAL ENGINEERING ASSIGNMENT

Chemical Engineering Assignment



Chemical Engineering Assignment

Introduction

Styrene is known to be toxic to the central nervous system (CNS). This was the critical toxic effect upon which the American Conference of Governmental Industrial Hygienists (ACGIH)1 established the current threshold limit value. In 1995, the ACGIH evaluated data on styrene toxicity and recommended that the average occupational exposure for an 8-hour working day should not exceed 85 mg/m3 (20 ppm). This recommendation is based on human data indicating effects on the CNS at average concentrations exceeding 213 mg/m3 (50 ppm) styrene.

2,3 However, the conclusions suggesting that styrene causes functional CNS impairment at relatively low concentrations (B/500 mg/m3) are controversial.3 ] 10 Some authors4 ] 7 found impairment of colour vision, while Triebig et al.10 found no neurobehavioral effect in workers exposed to concentrations up to 1069 mg/m3. Moreover, it is not clear at present whether changes are more closely related to average styrene exposures or to high peak exposures.8,11 For example, Fallas et al.4 found impairment of colour vision among workers exposed to an average styrene concentration of 103.5 mg/m3 but some peaks mounted up to 1998 mg/m3.

From the literature data, it is difficult to draw up a coherent portrait of the acute neurotoxic effects of styrene, since many different tests and exposure procedures were used during acute inhalation studies in volunteers. It has been suggested that the number of self-reported symptoms would increase starting at acute exposure to 426 mg/m3 styrene, and possibly even below this threshold.12 There are no coherent results for the other measured variables, but it is likely that exposure to styrene must exceed 426 mg/m3 to observe some acute effects.

The exposures to styrene (99%, Aldrich chemicals, Milwaukee, WI) were carried out in a 18 m3 exposure chamber with a controlled dynamic environment.

Styrene concentrations were produced by dilution of styrene vapours in the pre-purified air entering the chamber at a flow rate of 4.5 m3/min.

Styrene concentrations were monitored continuously by Miran 1-A infrared analyzer and were confirmed every 2]/5 minutes using a gas chromatograph model CP- 3800 (Varian Canada, Montreal, Canada) equipped with a Megabore column (HP-1, 30 m3 /0.53 I.D., Hewlett-Packard, Montreal, Canada) and a flame ionization detector. The actual measured chamber concentrations did not differ substantially (9/3%) from the target concentrations.

The subjects were divided into nine groups of five persons or less. Exposure sessions (6 hours per day) took place once every two weeks for each group.

Each volunteer was successively exposed to five exposure scenarios during 6 hours:

Styrene is manufactured from ethylbenzene using one of two methods (ATSDR, 1992; Miller et al., 1994; EU, 2002). In the most common method, ethylbenzene is dehydrogenated using steam and an iron/zinc/magnesium oxide catalyst and the resulting styrene is purified under vacuum distillation. The second method involves oxidation of ethylbenzene to ethylbenzene hydroperoxide, which is reacted with propylene to yield propylene oxide and methyl phenyl carbinol. Using an acid catalyst, the carbinol is dehydrated to produce styrene.

Past or current manufacturers of styrene include BP Amoco ...