Issue link: https://geospatial.trimble.com/en/resources/i/1415432
60 Spring 2019 more than 2 billion m 3 of vapours discharged in 2015 worldwide contained over 2 million t of VOCs. The chemistry behind vapour control Liquid crude oil has more or less the same vapour pressure of gasoline, i.e. the same tendency to evaporate. Since crude oil is not a standardised product (hundreds of qualities exist), it has a very wide distillation curve, from low initial boiling point (IBP) to very high final boiling point (FBP). In practice, this translates into the presence of light components not found in gasoline vapours, in particular methane, ethane and hydrogen sulfide (H 2 S). Table 1 illustrates the difference between gasoline and crude oil vapours, as well as between different qualities of crude. H 2 S is a colourless, toxic gas. It can be detected by its odour at concentrations as low as 0.005 ppm. Concentrations above 50 ppm are harmful to human health; those above 800 ppm are lethal. H 2 S presence in vapours during loading of crude oil may vary from 10 ppm (sweet crudes) to 1000 ppm (sour crudes), depending on the quality of the crude. Therefore, it is one of the most important emission components to control and reduce. Vapour control technologies can be grouped under two basic categories: vapour recovery and vapour destruction. Carbon adsorption, a vapour recovery process, involves VOC adsorption onto activated carbon followed by vacuum regeneration and ultimately VOC absorption back to the liquid phase (Figure 1). It has been recognised by several environmental agencies worldwide as a best demonstrated technology (BDT) for recovering VOC from gasoline vapours. This is mainly due to low CAPEX, OPEX and its simplicity of operation. But the presence of light components and H 2 S in crude oil vapours makes recovery by carbon adsorption ineffective at controlling VOC emissions. This is because these components have a low boiling point (i.e. they are light) and are not effectively captured by the activated carbon, resulting in unacceptable VOC emissions. These issues set new challenges in vapour control for the storage and distribution sector and establish the basis for further technological developments. Figure 1. Process diagram of a standard carbon adsorption VRU. Figure 2. Process diagram of a VCU. Table 1. Composition of the vapours of gasoline and two qualities of crude Composition Gasoline Stablised crude from Middle East Crude from Southern Italy Inert gases (% vol.) 58.1 68.0 58.3 Methane (% vol.) 0.0 0.3 2.4 Ethane (% vol.) 0.0 0.3 3.6 Propane (% vol.) 0.6 7.9 4.8 N-butane (% vol.) 17.4 2.2 6.2 I-butane (% vol.) 6.1 10.2 1.3 N-pentane (% vol.) 7.1 2.9 8.3 I-pentane (% vol.) 7.7 4.5 6.9 C6+ (% vol.) 3.0 3.7 8.2 H 2 S (ppm) 0 <100 350