Benzene concentration

A European Directive, 85/210/EEC, limits benzene content to 5% by volume in all gasolines regular, premium, with or without lead. This level is easily achieved, since the average value in 1993 was less than 3%. in France, for example, average benzene concentrations of 1.7% and 2.6% were reported for leaded and unleaded premium fuels, respectively, in 1993. 

For the refiner, the reduction in benzene concentration to 3% is not a major problem it is achieved by adjusting the initial point of the feed to the catalytic reformers and thereby limiting the amount of benzene precursors such as cyclohexane and Cg paraffins. Further than 3% benzene, the constraints become very severe and can even imply using specific processes alkylation of benzene to substituted aromatics, separation, etc. 

Hydrocarbon Solvents. Most hydrocarbon solvents are mixtures. Few commercial hydrocarbon solvents are single compounds. Toluene is an exception. Hydrocarbon solvents are usually purchased and suppHed on specification. The most important specification properties are distillation range, solvency as expressed by aniline cloud poiat and Kauri-Butanol (KB) value, specific gravity, and dash poiat. Composition requirements such as aromatic content and benzene concentration are also important ia many appHcations. 

The identification of benzene is most easily carried out by gas chromatography (83). Gas chromatographic analysis of benzene is the method of choice for determining benzene concentrations in many diverse media such as petroleum products or reformate, water, sod, air, or blood. Benzene in air can be measured by injection of a sample obtained from a syringe directiy into a gas chromatograph (84). 

Colorimetric methods have led to the development of visual devices for measurement of benzene concentration. These visual detection tubes have been popular since the 1960s and have provided a simple and rehable method for evaluating ambient aromatic vapor contamination. These products are available from anumber of manufacturers such as Drager (Lubeck, Germany), Gastec (Tokyo, Japan), Kitagawa (Kawasaki, Japan), DuPont (Wilmington, Delaware, USA), and 3M (St. Paul, Minnesota, USA) (85). 

The odor perception threshold for benzene in water is 2 ing/L. The benzene drinking water unit risk is 8.3 x lO L/pg. Calculate the potential benzene intake rate (mg benzene/kg-d) and the cumulative cancer risk from drinking water with benzene concentrations at half of its odor threshold for a 30 year exposure duration. 

This residue is stirred at room temperature while 150 ml. of triethyl-amine [Ethanamine, A,A-diethyl] (Note 8) is added. Tricthylamine hydrobromide begins to precipitate soon after the addition is started, and the resulting slurry is refluxed gently for 15 minutes. It is then cooled to room temperature, and the insoluble material is removed by filtration and washed with benzene. Concentration of the combined filtrates under reduced pressure leaves an oily residue, which is dissolved in 600 ml. of ether. The ethereal solution is transferred to a 1-1. separatory funnel, washed with two 20-ml. portions of saturated aqueous sodium chloride, dried over anhydrous sodium sulfate, and filtered. Ether is removed with a rotary evaporator, and the resulting oil is distilled at reduced pressure. A forerun of 265 mg. is collected below 103° (22 mm,), and then 6.7 g. (70%) of 2A-pyran-2-one distils as a colorless oil, b.p. 103-111° (19-22 mm.) (Note 9). 

The benzenesulphonylation of benzene did not give a single kinetic form. Satisfactory second-order rate coefficients could be obtained provided that the initial benzene concentration was not small, when a small increase in the calculated rate coefficients as the reaction proceeds is observed. However, the values of the second-order rate coefficients are not independent of the initial concentration of reactants (Table 51). It was concluded that benzene reacts by both mechanisms ... 

It should be mentioned that the predicted curve at highest benzene level in Figure 13 agrees with classical kinetics (no diffusion-control). It is not clear therefore why measured data at even higher benzene concentrations do not agree with classical kinetics. There may be some subtle chemical interactions at these high solvent levels. Duerksen(lT) fomd similar effects with styrene polymerization in benzene and had to correct kp for solvent. 

Blood, tissues Homogenize, if tissue mix sample with acetone centrifuge concentrate saturate with sodium chloride evaporate organic layer cleanup on silica gel eluting with hexane-benzene concentrate GC/FPD <100 ppb No data EPA1980d... 

Example 10.4 A gas mixture with a flowrate of 0.1 m3 s-1 contains 0.203 kg m-3 of benzene. The temperature is 10°C and the pressure 1 atm (1.013 bar). Benzene needs to be separated to give a gas stream with a benzene concentration of less than 5 mg m-3. It is proposed to achieve this by adsorption using activated carbon in a fixed bed. The activated carbon is to be regenerated using superheated steam. The experimental adsorption isotherms cannot be adequately represented by Freundlich isotherms and, instead, can be correlated at 10°C by the empirical relationship ... 

Benzene chemisorption on platinum-alumina in the range 26°-470°C has been measured in a flow system by Pitkethly and Goble (7). A small dose of benzene was injected into a stream of inert carrier gas and transported to the reactor the effluent was then sampled repeatedly and analyzed by gas-liquid chromatography. Information concerning the adsorption and desorption of benzene was obtained from the shape of the subsequent benzene concentration versus time curves. Evidence was obtained for four types of adsorption of benzene ... 

Figure 21.2 shows how in the calculation results benzene is transported through the aquifer. The pulse of benzene migrates at the rate of groundwater flow, traversing the aquifer in ten years. As a result of biodegradation by the natural microbial consortium, however, the benzene concentration decreases markedly with time, compared to the non-reacting case. 

Often it is called, reasonably enough, benzene concentrate or aromatics concentrate. Benzene concentrate is about 50% benzene, plus some other C5 s, Ce s, and Cys. All of them boil at about 176°F, the boiling point of benzene. Since the boiling temperature of the benzene is so close to that of the other hydrocarbons in the concentrate stream, simple fractionation is not a very effective way of isolating the benzene from benzene concentrate. Instead, one of two processes is used to remove the benzene, solvent extraction process or extractive distillation. The two differ in the primary mechanism they use. One operates on a liquid-liquid basis, the other on a vapor-liquid basis. 

In the analogy for benzene, a batch of benzene concentrate is mixed with the solvent the benzene dissolves in the solvent the solvent separates naturally from the undissolved components the benzene-laden solvent is then drawn off and fractionated to separate the benzene. (This step is designed to be easy by selecting a solvent that has a boiling temperature much different from the benzene.) The fractionation products are solvent and benzene. 

In the incessant scramble to reduce capital and operating costs, chemical engineers adapted a related technique for removing benzene from benzene concentrate. For years, absorption, a gas/liquid extraction process, has been used for separations in refinery gas plants and natural gas plants. It only took a technique for using the special absorbents, the same ones used in solvent extraction, to reduce the complexity of the equipment and the processing costs. See Figure 2-5)... 

Separation of toluene from the other components can be by solvent extraction or extractive distillation, just as described in the benzene chapter. The boiling points of benzene and toluene are far enough apart that the feed to separation unit of choice can be split (fractionated) rather easily into benzene concentrate and a toluene concentrate. Alternatively, the separation unit can be thought of as aromatics recovery unit. Then an aromatics concentrate stream is fed to the solvent extraction unit, and, the aromatics outturn can be split into benzene and toluene streams by fractionation. Both schemes are popular. 

De Visscher et al. (1996) investigated the sonolysis of benzene and other monocyclic aromatic compounds in aqueous solution by 520 kHz ultrasonic waves. The experiments were performed in a 200-mL glass reactor equipped with a cooling Jacket maintained at 25 °C. At initial benzene concentrations of 3.38 and 0.45 mM, the first-order reaction rates were 0.00171 and 0.02308/min, respectively. 

Thomas and Delfino (1991) equilibrated contaminant-free groundwater collected from Gainesville, FL with individual fractions of three individual petroleum products at 24-25 °C for 24 h. The aqueous phase was analyzed for organic compounds via U.S. EPA approved test method 602. Average benzene concentrations reported in water-soluble fractions of unleaded gasoline, kerosene, and diesel fuel were 8.652, 0.349, and 0.200 mg/L, respectively. When the authors analyzed the aqueous-phase via U.S. EPA approved test method 610, average benzene concentrations in water-soluble fractions of unleaded gasoline, kerosene, and diesel fuel were lower, i.e., 1.107, 0.073, and 0.066 mg/L, respectively. 

Figure 19 Bay area refinery effluent treating block flow diagram. This refinery has a complicated wastewater treatment scheme because of the toxicity characteristics rule to separate streams with higher benzene concentrations for treatment in aboveground biotreater. (From Ref. 72.)...

Duffy and Nelson (1997) report during commutes in Sydney, Australia, that the benzene concentrations inside vehicles were 10-25 times those in ambient air and that the concentrations of 1,3-butadiene were more than 55-115 times greater. The source appeared to be primarily from the exhausts of surrounding vehicles. Similar enhancements of benzene and other VOCs such as toluene, ethylbenzene, and the xylenes in automobiles and buses have been reported in many countries, including Korea (Jo and Choi, 1996), Taiwan (Chan et al., 1993), and the United States (e.g., Chan et al., 1991a,b Lawryk and Weisel, 1996). 

---