Oil refining: removing water and salts

After that long introduction, it's time to look at how an oil refinery works. The refinery oust separate the various components of crude oil into specific petroleum prod­ucts such as gasoline or lubricating oil. Unfortunately, the crude oil that arrives at the refiner) rarely contains the right assortment of molecules for the products the refinery wanes to produce. Thus the refinery must usually modify the molecules it receives so char they fit its products. This purification and modification is an enormous task and requires a large facility.

The refinery's first job is to remove water and salt from the crude oil. These con­taminants are of no use to the refinery. Fortunately, water and hydrocarbons don't mix well because their molecules don't bind to one another strongly. The molecules in water cling to one another with hydrogen bonds, while the molecules in oil hold onto one another only with weaker van der Waals forces. When you put the two liquids together, the water molecules stay bound to water molecules and the oil molecules stay bound to oil molecules. They don't mix.

What ultimately makes oil and water so immiscible is the strength of the hydrogen bonds between water molecules. It takes far too much energy to separate water mol­ecules for them to mix with the oil molecules. If you pour water and oil into a glass, the less dense oil floats on top of the water and a sharply defined interface forms between the oil and the water.

The water molecules at this interface are special. While the water molecules below them can form hydrogen bonds with neighbors in all directions, the water molecules at the interface have only oil molecules above them. These surface water molecules cling par­ticularly tightly 10 one another and they create an inward tension along the water's surface. A surface tension of this type appears whenever one material ends and another begins.

Surface tension is particularly strong in water because water molecules attract one another so strongly. Surface tension always acts to minimize a liquid's surface area. The surface of the liquid behaves like an elastic membrane, stretching when you exert forces mil but always snapping back to a taut, smooth shape.

Surface tension squeezes raindrops into tiny spheres and turns the surface of a calm lake into a trampoline for water bugs. Surface tension minimizes the surface area be­tween the water and the oil by making the interface flat and level. But if you cover the glass and shake it hard, the interface will stop being flat. Instead, the glass will become filled with droplets of oil in water and water in oil. You will have formed an emulsion, a situation in which droplets of one liquid are suspended in another.

Surface tension will quickly minimize the surface area of each droplet by making it spherical. But the emulsion will contain more surface area overall than it did before you shook the glass. Because it can further reduce the surface area by reducing the number of droplets, you will see the droplets touch and coalesce. Each time two droplets merge, their combined surface area goes down. Eventually all of the droplets will have joined together and the oil and water in the glass will have separated completely.

But the smallest droplets don't merge together easily. They experience large drag forces as they try to move through the surrounding liquid and travel extremely slowly. It takes a long time for them to find other droplets with which to coalesce. In thick, gooey crude oil, tiny water droplets form an emulsion that takes almost forever to settle. In fact, various chemical impurities in the petroleum actually surround the water droplets, so that they can't touch and coalesce. As a result, getting the water out of crude oil is quite difficult.

Refineries usually break the emulsions by heating the oil and passing it through settling tanks or filter columns. At elevated temperatures (90 to 150 єC), water's surface tension decreases and the water droplets are able to merge together more easily. In fact, the energetic water molecules bounce around so vigorously that they have trouble stay­ing together at all. To keep molecules of water or oil from becoming gaseous, the hot crude oil must be kept under pressure. Heat also reduces the crude oil's viscosity and the water molecules are able to settle more easily.

As it settles, the water collects the salt molecules in the crude oil Since salts are composed of electrically charged ions, they only dissolve in liquids that bind well with charged particles. Water molecules are polar and do a good job of dissolving most salts. Because hydrocarbon molecules are nonpolar-they have no electrically charged ends-they rarely dissolve salts. So the salts accumulate in the water as it settles to the bottom of a tank or filter column.

The smallest water droplets still have trouble settling out of the crude oil. Gravity and buoyant forces are sometimes just too weak to overcome drag forces. Many refin­eries use electrostatic precipitators to pull the water droplets through the oil. Since oil doesn't conduct electricity, it behaves like very thick air. Charged particles injected into the crude oil quickly attach themselves to water droplets and these electrically charged water droplets are pulled through the oil by electric fields.