Mixing in the oil and gas industry

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The oil and gas industry includes the processes of exploration, extraction, upgrading, refining, transporting and marketing oil products. The largest volume products of the industry are fuel oil, <gasoline> and feedstock for bulk and fine chemicals [1]. There are two types of crude oil: one is <conventional> and the other is <unconventional> to which oil sand resource belongs. There are an estimated 1.5 and 170.4 billion barrels of established conventional and bitumen crude oil remaining as of 2009 in Alberta [2]. A typical production flow of conventional oil is shown in Figure 1. The extraction of heavy oil (bitumen) from oil sands conserves is shown in Figure 2; the rest of the flow is the same as the conventional crude oil.



Figure 1           Product Flow of Crude Oil [4]



Figure 2           Mining and Extraction of Bitumen from Oil Sands [9]
Mixing is involved in every step of the oil industry from exploration to marketing products. While drilling oil and gas wells, drilling fluid is applied. The fluid consists of a mixture of clay and a stabilized water-in-oil emulsion. The emulsion is prepared batchwise by dispersing water in oil in agitated tanks. The main functions of drilling mud include providing hydrostatic pressure to prevent formation fluids from entering into the well bore, keeping the drill bit cool and clean during drilling, carrying drill cuttings out of the well and suspending the drill cuttings while drilling is paused and the drilling assembly is brought in and out of the hole. Thus, mixing technology plays an important role in the exploration stage [3].
Meanwhile, mixing is important for product sampling in the pipeline transport. When crude oil is sampled to determine its water content before its custody is transferred to refineries, the water has to be uniformly dispersed across the cross-section of pipes. Thus, a mixer system has to be installed upstream of the sampler. Adequate mixing should create a good dispersion but still allow water to easily settle in downstream storage tanks. Optimum mixing can add a high value for refineries as even a sampling error of 0.1% can cost refineries about $250, 000 per medium-sized tanker [4].
Viscous crude oil has become an increasingly important source of hydrocarbons around the world. Transportation of these viscous crudes from the source to the refinery is a challenge because existing pipelines were designed for less viscous crudes. One available technology is to mix crude oil into water with an emulsifier to form high oil content oil-in-water emulsions with a dramatic decrease in viscosity and pressure drop. The pumping cost for these emulsions is similar to the lower viscosity conventional crude oils [5, 6].
Mixing is used control sludge accumulation in crude oil storage tanks. Crude oil usually carries a certain amount of bottom sludge and water. As this sludge is heavier than crude oil, it settles in storage vessels at terminals and refineries. Excessive sludge accumulation can occur in tanks with poor mixers and at low ambient temperatures. Once the sludge is settled on the tank floor, it hardens and cannot be removed by normal pumping. The tank has to be taken off-line and cleaned, which is hazardous, expensive, time consuming and requires sludge disposal [4].
Furthermore, mixing is extensively used in unit processes of gas treating, extracting, upgrading and, refining. <Natural gas> often contains a high concentration of <CO2> and <SO2> which make it unsuitable for direct use as fuel gas. Conventional processes reduce these concentrations by absorbing CO2 and SO2 in an amine solution in a packed bed contactor. Although these towers provide the maximum driving force for <mass transfer>, they are very large. Therefore, on <offshore platforms> <static mixers> can be the favored choice because they provide perfect <plug flow>, a large number of stages, good radial mixing, high mass transfer coefficient, and are smaller in size and weight [4].
<Propane> deasphaleting is another example of solvent extraction. In this case a static mixer is employed to mix <asphaltene> and part of propane before feeding into the extractor; it significantly reduces the size of the extractor.     
Desalting, the first step in refineries, removes salt from crude oil before it is sent to the downstream units. This is carried out by first mixing a demulsifier in the pipe followed by emulsifying fresh water in an in-line mixer. The water-in-oil emulsion is broken in an electrostatic separator to produce oil with negligible salt and water. Optimum mixing of fresh water is desired for satisfactory desalting. Poor mixing can lead to carry over of salt in the crude; over mixing can result in the formation of a stable emulsion and poor separation [4].
In many process units with a <continuous stirred-tank reactor (CSTR)>, such as fluid coking (Figure 3), visbreaking, <fluidized catalytic cracking> and ebullated bed hydroconversion, mixing plays a vital role. For example, the yield of products is determined by the feed properties, the temperature of the fluid bed, and the residence time in the bed. The fluid bed reduces the residence time of the vapor-phase products and provides excellent heat transfer that allows the reactor to operate at higher temperature. These factors generally give a lower yield of coke and high yield of gas oil and olefins [8].

Figure 3           Process Schematic of Fluid Coking [8]
Most products of refineries need to be blended with additives prior to marketing. Base lube oils are fed to a tank with agitators or blenders with additive concentrations prescribed by the formula for each product and  are mixed according to the precise weight prescription in order to achieve the required properties (Figure 4) [10, 11].
Mixing systems used in the oil and gas industry play very important roles. However, they are often designed based on limited data and experience and may be inadequate. Design guidelines are needed to achieve good process performance and reliability [4].



Figure 4           Lube Oil Bleeding [10]