Naphtha benzene content

A tank containing 1500 m3 of naphtha is to be blended with two other hydrocarbon streams to meet the specifications for gasoline. The final product must have a minimum research octane number (RON) of 95, a maximum Reid Vapor Pressure (RVP) of 0.6 bar, a maximum benzene content of 2% vol and maximum total aromatics of 25% vol. The properties and costs of the three streams are given in the Table 3.5. 

However, the most economical and octane effective route to reduce benzene is to eliminate benzene and benzene precursors from the naphtha reformer feed by pre-fractionation in an up-stream splitter and then send these precursors to an isomerization unit. With this approach, benzene will be saturated and isomerised to a mixture of higher octane saturated cyclics. Depending on the feed benzene content, various solutions can be applied to handle the heat load from the saturation of benzene. These heat management solutions entail saturation of the benzene in the iso-... 

Gasoline is usually produced as a blend of several petroleum streams that boil in the range of naphtha. A typical gasoline might contain 50% by volume of cracked naphtha with benzene content between 0.5 wt% and 2.0 wt% and 25% by volume of catalytically reformed naphtha with benzene content between 1 wt% and 3 wt%. Estimate the cost per gallon of gasoline of reducing the final benzene content to 0.62% by volume. Compositions of other components in the naphtha streams can be found in the patent literature. 

Solvents commonly used in the industry have been shown to contain benzene. Elkins, et al. found that from time to time analyzed solvents for benzene content showed anywhere from 1% to 2% benzene. In that paper, which was published in 1956, the authors state that the TLV value from a regulatory point of view, at that time, was 35 ppm compared with 100 ppm previously. Aeeording to their calculations, they found that a benzene content below 3.5% will be neeessary, for instance in solvents containing naphtha, hexane, and toluene, otherwise the permissible level for benzene vapor will be exceeded over 3 5 ppm, which... 

The amount of benzene produced in a reformer will depend on the composition of the feed. Every crude oil has naphtha with different PNA (paraffin, naphthene, aromatics) content. In commercial naphtha trading, the PNA content is often an important specification. High naphthene and aromatic content would indicate a good reformer feed. High paraffin content would indicate a good olefin plant feed. 

The development of nylon by DuPont in 1938 generated the initial big commercial interest in cyclohexane as they settled on its use as their preferred raw material. In the period right after World War II, the manufacture of nylon grew for a while at 100% annually, quickly overwhelming the availability of cyclohexane naturally available in crude oil. The typical crude oil processed in U.S. refineries at the time had less than 1% content of cyclohexane. Ironically, since cyclohexane leaves the crude oil distillation operation in the naphtha, it was usually fed to a cat reformer, where it was converted to henzene. As it turned out, with so many other precursors also being converted to benzene in the cat reformer, benzene became a good source for cyclohexane manufacture. 

On the w hole. these analyses show that gas oil steam cracking produces fewer light produas than the treatment of naphtha, and more heavy products which display a higher aromatics content. Hence the Cj- 200 C cut boosts the BTX (Benzene, Toluene, Xylenes) (majority benzene concentration. Similarly, fuel oil (fraction above 200 C) displays a more pronounced aromatic character. This feature makes it incompatible with straight-run distillation fuel oils. The mixture causes the deposition of asphaltenes and other... 

Figure 1. The content of a) C5 b) and c) C7 components in the naphthas as a function of the initial boiling point (IBP). CP=cyclopentane MCP=methylcyclopentane C7CP=dimethyl- and ethyl -cyclopentane CH=cyclohexane MCH=methylcyclohexane BEN=benzene TOL=toluene.

Other test methods are available. Content of benzene and other aromatics may be estimated by spectrophotometric analysis (ASTM D-1017) and also by gas-liquid chromatography (ASTM D-2267, ASTM D-2600, IP 262). However, two test methods based on the adsorption concept (ASTM D-2007, ASTM D-2549) are used for classifying oil samples of initial boiling point of at least 200°C (392°F) into the hydrocarbon types of polar compounds, aromatics, and saturates and recovery of representative fractions of these types. Such methods are unsuitable for the majority of naphtha samples because of volatility constraints. 

Catalytic reforming converts low-octane heavy naphtha into a high-octane product (reformate) that is an excellent gasoline blend stock. Reformate has a high content of BTX (benzene-toluene-xylene), so it also serves as a great source of aromatics for petrochemical plants. 

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