Among the petroleum installations at Abqaiq, Saudi Arabia, is a big processing plant known as a crude-oil stabilizer, and, figuratively speaking, you might say that it's main job is to get rid of the "rotten eggs."
"Rotten eggs" means hydrogen sulfide gas (H2S)—they share a common odor. If you've ever cracked a bad egg into an omelet, you'll get the idea.
This gas is contained in the crude oil as it comes from the wells. It not only has a vile odor, it's also poisonous—it can kill you if you inhale it. And it's corrosive. If the crude is "wet"—that is, if it contains water—the H2S and the H2O will react to form H2SO4—sulfuric acid, a heavy oily liquid that can eat its way through a steel pipeline or storage tank.
Hydrogen sulfide has to go. And one of the Aramco plants that does this job at Abqaiq is the world's largest crude-oil stabilizer: it can process up to 950,000 barrels of crude daily. That's equal to 39,000,000 gallons—a pretty fair batch in anybody's kitchen.
The stabilization process—basically a form of partial distillation—does two jobs at the same time: it sweetens "sour" crude oil (removes the hydrogen sulfide) and reduces vapor pressure, thereby making the crude safe for shipment in tankers. Vapor pressure is exerted by light hydrocarbons, such as methane, ethane, propane, and butane, changing from liquid to gas as the pressure on the crude is lowered. If a sufficient amount of these light hydrocarbons is removed, the vapor pressure becomes satisfactory for shipment at approximately atmospheric pressure.
All the crude produced in Saudi Arabia—except for that of the offshore Safaniya Field in the Persian Gulf—is "sour." At ground level the pressure may be as high as 1,000 pounds per square inch (or "1,000 psi," as the engineers say). It must be reduced considerably before it reaches the stabilizer, so it's sent first to a gas-oil separator plant, or "GOSP." There are eleven of these in the Abqaiq area.
Now, the gas can't be allowed to "blow off" all at once. If it did, a considerable amount of liquid would also be lost—something like shaking a bottle of soda pop before uncapping it.
The gas is released in stages in a series of drums or columns, known as separators, before it reaches a spheroid where the pressure is cut down to about 2 psi. By now, most of the light hydrocarbon gases have been removed, but the gas is still "sour." The next step is to pump it to the sour-crude storage tanks at the stabilizer to await processing.
Booster pumps push the oil from the storage tanks to the top of one of the stabilizer columns. It enters the column and starts to flow down through a series of "bubble trays." Near the bottom, the down-flowing oil is channeled to a reboiler that heats it. By the time it reaches the bottom section of the stabilizing column, it's hot. Gases contained in the crude—both hydrogen sulfide and light hydrocarbons—begin to boil off at this temperature. They rise to the top, while the heavy crude remains at the bottom.
The rising gases, incidentally, perform a useful function on their way up the column. Being hot, they supply preliminary heating to the incoming sour crude, and also "strip out" some of the gases contained in it.
When the hot gases leave the column, they're piped to what's known as a "scrubber." Its purpose is to recover most of the liquid hydrocarbons contained in the gas mixture. The leftover gases—mostly hydrogen sulfide—are ignited and flared off.
Plans are underway, however, for installation of new facilities which will permit utilization of these gases in Aramco's oil conservation program. The gases that come off the top of the stabilizer columns will be combined with spheroid gases, compressed and liquified, and piped to the 'Ain Dar area. There, they will be combined with high-pressure gases from the 'Ain Dar gas-injection plant and injected into the underground reservoir.
This injected combination serves as a solvent that "flushes out" crude oil and also helps to maintain reservoir pressure, thus permitting a higher percentage of the crude oil to be recovered.
The stabilized crude oil goes on to the cooling unit, where enormous fans reduce the temperature. Why? Because if the oil remained warm, all the lighter fractions of crude oil would evaporate. Hie cooler the oil, the more stable it is.
The Abqaiq stabilizer can process 950,000 barrels of crude oil in summer, but only about 860,000 in winter. Most crude oil has some water in it when it comes from the reservoir. In warm weather the water vaporizes easily and is taken off at the gas-oil separator before stabilization. But in cold weather it remains in the crude that is pumped to the stabilizer and causes "foaming," which reduces the efficiency of the process. "Foaming" is similar to the action that occurs when you shake water and vinegar to make a salad dressing. Ordinarily the oil floats free above the vinegar, but shaking them together produces a bubbly emulsion, a frothy mixture that will not separate for quite a while.
As in other modern oil-processing installations, practically everything is operated automatically from the control room. There are only five men on each shift: a head operator and four assistants, all Saudi Arabs.
By the time the stabilizer has finished its job, the hydrogen sulfide that was in the oil has been almost entirely removed—reduced to an infinitesimal ten parts per million.