GAS PROCESSING ENGINEERING

2- Hydrate

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It is a result of the hydrogen bond that water can form hydrates. The hydrogen bond causes the water molecules to align in regular orientations. The presence of certain compounds causes the aligned molecules to stabilize and a solid mixture precipitates.

The water molecules are referred to as the “host” molecules, and the other compounds, which stabilize the crystal, are called the “guest” molecules, the guest molecules are more often referred to as “formers.” The hydrate crystals have complex, three-dimensional structures where the water
molecules form a cage and the guest molecules are entrapped in the cages.

The stabilization resulting from the guest molecule is postulated to be due to van der Waals forces, which is the attraction between molecules that is not because of electrostatic attraction. As described earlier, the hydrogen bond is different from the van der Waals force because it is due to strong electrostatic attraction, although some classify the hydrogen bond as a van der Waals force.

Another interesting thing about gas hydrates is that there is no bonding between the guest and host molecules. The guest molecules are free to rotate inside the cages built up from the host molecules. This rotation has been measured by spectroscopic means. Therefore, these compounds are best described as a solid solution.

The formation of a hydrate requires three conditions:

The right combination of temperature and pressure. Hydrate formation is favored by low temperature and high pressure.
A hydrate former must be present. Hydrate formers include methane, ethane, and carbon dioxide.
A sufficient amount of water – not too much, not too little.

The exact temperature and pressure depends on the composition of the gas. However, hydrates form at temperatures greater than 0°C (32°F), the freezing point of water.

To prevent hydrate formation, one merely has to eliminate one of the three conditions stated previously. Typically, we cannot remove the hydrate formers from the mixture. In the case of natural gas, the hydrate formers are the desired product. Therefore, we attack hydrates by addressing the other two considerations.

Other phenomena that enhance hydrate formation include:

Turbulence

High Velocity

Hydrate formation is favored in regions where the fluid velocity is high. This makes choke valves particularly susceptible to hydrate formation. First, there is usually a significant temperature drop when natural gas is choked through a valve due to the Joule-Thomson effect. Second, the velocity is high through the narrowing in the valve.

Agitation

Mixing in a pipeline, process vessel, heat exchanger, etc. enhances hydrate formation.

2. Nucleation Sites

In general terms, a nucleation site is a point where a phase transition is favored, and in this case the formation of a solid from a fluid phase. An example of nucleation is the deep fryer used to make French fries in fast-food restaurants throughout the world. In the fryer, the oil is very hot but does not undergo the full rolling boil because there are no suitable nucleation sites. However, when the potatoes are placed in the oil, it vigorously boils. The French fries provide an excellent nucleation site.

3.Free-Water

No, this is not a contradiction to earlier statements. Free-water is not necessary for hydrate formation, but the presence of free-water certainly enhances hydrate formation.
In addition, the water-gas interface is a good nucleation site for hydrate formation. The items in the previous list enhance the formation of a hydrate, but are not necessary. Only the three conditions given earlier are necessary for hydrate formation.

Another important aspect of hydrate formation is the accumulation of the solid. The hydrate does not necessarily agglomerate in the same location as it is formed. In a pipeline, the hydrate can flow with the fluid phase, especially the liquid. It would tend to accumulate in the same location as the liquid does.
Usually, the accumulations of the hydrates cause the problems. In a multiphase pipeline, the accumulations block line and plug and damage equipment.

Often, pigging is sufficient to remove the hydrate from the pipeline. Pigging is the process of inserting a tool (called a “pig”) into the line. Modern pigs have many functions, but the main one remains pipeline cleaning. The pig fits tightly into the line and scrapes the inside of the pipe. It is transported along the line with the flow of the fluid, and by doing so removes any solids (hydrate, wax, dirt, etc.) from inside the line. The pigging can also be used to remove accumulations of liquids.

However, the pigging must be scheduled such that the accumulations of hydrates do not become problematic. Usually, pigging is not used to clean hydrates from a line. Other measures are more commonly used to deal with hydrates. Another benefit of pigging is the removal of salt, scale, etc., which is important for the proper operation of a pipeline. It also means that potential nucleation sites for hydrate formation are removed.

SEPARATION

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TYPES OF SEPARATORS

Separators are usually characterized by orientation as vertical or horizontal. They may be further classified as two-phase (gas-liquid) or three-phase (gas-liquid-liquid). Horizontal separators can be single- or double-barrel and can be equipped with sumps or boots.

Vertical Separators

Vertical separators are usually selected when the gas-liquid ratio is high or total gas volumes are low.
In a vertical separator, the fluids enter the vessel through an inlet device whose primary objectives are to achieve efficient bulk separation of liquid from the gas and to improve flow distribution of both phases through the separator. Liquid removed by the inlet device is directed to the bottom of the vessel.

separator

The gas moves upward, usually passing through a mist extractor to remove any small entrained liquid droplets, and then the vapor phase flows out of the vessel. Liquid removed by the mist extractor is coalesced into larger droplets that then fall through the gas to the liquid reservoir in the bottom. The ability to handle liquid slugs is typically obtained by increasing vessel height to accommodate additional surge volume. Level control is normally not highly critical and liquid level can fluctuate
several inches without affecting the separation performance or capacity of the vessel. Except for knockout drum applications, mist extractors are normally used to achieve a low liquid content in the separated gas in vessels of reasonable diameter.

Typical vertical separator L/D ratios are normally in the 2–4 range.
As an example of a vertical separator, consider a compressor suction scrubber. In this service the vertical separator:

Does not need significant liquid retention volume
A properly designed liquid level control loop responds quickly to any liquid that enters, thus avoiding tripping an alarm or shutdown
The separator occupies a small amount of plot space

Horizontal Separators

Horizontal separators are most efficient when large volumes of liquid are involved. They are also generally preferred for three-phase separation applications. In a horizontal separator, the liquid that has been separated from the gas moves along the bottom of the vessel to the liquid outlet.

The gas and liquid occupy their proportionate shares of the shell cross-section. Increased slug capacity is obtained through shortened retention time and increased liquid level.

Horizontal separators have certain advantages with respect to gravity separation performance in that the liquid droplets or gas bubbles are moving perpendicular to the bulk phase velocity, rather than directly against it as in vertical flow, which makes separation easier.

In a double-barrel separator, the liquids fall through connecting flow pipes into the external liquid reservoir below. Slightly smaller vessels may be possible with the double-barrel horizontal separator, where surge capacity establishes the size of the lower liquid collection chamber.

Typical L/D ratios for horizontal separators normally fall in the range of 2.5–5.

As an example of a horizontal separator, consider a rich amine flash tank. In this service:

There is relatively large liquid surge volume required for longer retention time. This allows more complete release of the dissolved gas and, if necessary, surge volume for the circulating system.
There is more surface area per liquid volume to aid in more complete degassing.
The horizontal configuration handles a foaming liquid better than vertical.
The liquid level responds slowly to changes in liquid inventory, providing steady flow to downstream equipment.

Hydrate

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1- Introduction

In its most general sense, a hydrate is a compound containing water. For example, there is a class of inorganic compounds called “solid hydrates,” ionic solids where the ions are surrounded by water molecules and form crystalline solids. hydrates are composed of a small molecule and water.

Hydrates are crystalline solid compounds formed from water and smaller molecules. They are a subset of compounds known as clathrates or inclusion compounds. A clathrate compound is one in which a molecule of one substance is enclosed in a structure built from molecules of another substance.

Even though the clathrates of water, the so-called hydrates, are the focus of this work, they are not the only clathrate compounds. For example, urea forms interesting inclusion compounds as well.

Although hydrates were probably encountered by others earlier, credit for their discovery is usually given to the famous English chemist, Sir Humphrey Davy. He reported of the hydrate of chlorine in the early 19th century. In particular, he noted that the ice-like solid formed at temperatures greater than the freezing point of water, and that the solid was composed of more than just water.

Davy’s equally famous assistant, Michael Faraday, also studied the hydrate of chlorine. In 1823, Faraday reported the composition of the chlorine hydrate.

Although his result was inaccurate, it was the first time the composition of a hydrate was measured.

Throughout the 19th century, hydrates remained an intellectual curiosity. Early efforts focused on finding which compounds formed hydrates and under what temperatures and pressures they would form. Many of the important hydrate former were discovered during this era.

Among the 19th-century hydrate researchers who deserve mention are the French chemists Villard and de Forcrand, who measured the hydrate conditions for a wide range of substances.

However, it would not be until the 20th century that the industrial importance of gas hydrates would be established.

Hydrates

In combination with water, many of the components commonly found in natural gas form hydrates. One of the problems in the production, processing, and transportation of natural gas and liquids derived from natural gas is the formation of hydrates. However, the importance of natural gas hydrates was not apparent in the early era of the gas business.

During that time in the natural gas business, gas was produced and delivered at relatively low pressure. Thus, hydrates were never encountered. In the twentieth century, with the expansion of the natural gas industry, the production, processing, and distribution of gas became high-pressure operations.

Under pressure, it was discovered that pipelines and processing equipment were becoming plugged with what appeared to be ice, except the conditions were too warm for ice to form. It was not until the 1930s that Hammerschmidt clearly demonstrated that the “ice” was actually gas hydrates. Moreover,

the hydrates were a mixture of water and the components of natural gas.

In the petroleum industry, the term hydrate is reserved for substances that are usually gaseous at room temperature. These include methane, ethane, carbon dioxide, and hydrogen sulfide. This leads to the term gas hydrates and to one of the popular misconceptions regarding these compounds. It is commonly believed that non-aqueous liquids do not form hydrates. However, liquids may

also form hydrates. An example of a compound that is liquid at room conditions, yet forms a hydrate, is dichlorodifluoromethane (Freon 12).

The water molecule

Many of the usual properties of water can be explained by the structure of the water molecule and the

consequences of this structure.

Of particular interest to us is that the structure of the water molecule leads to the possibility of hydrate formation. we will demonstrate that water does indeed have some unusual properties.

The Shape of the Water Molecule and the Hydrogen Bond

Virtually all of the unusual properties of water noted earlier can be explained by the shape of the water molecule and the interactions that result from its shape.
The water molecule consists of a single atom of oxygen bonded to two hydrogen atoms, as depicted in the above Figure. In the water molecule, the bond between the oxygen and hydrogen atoms is a covalent bond, which is essentially a shared pair of electrons. The angle between the two hydrogen atoms in the water molecule is about 105°.
These electrons induce negative charges on the oxygen molecule and a small positive charge on the hydrogen atoms. The induced electrostatic charges on the molecule. Thus, the water molecules will tend to align with a hydrogen molecule lining up with an oxygen.

This aligning of the hydrogen and oxygen atoms is called a “hydrogen bond.” The hydrogen bond is essentially an electrostatic attraction between the molecules.
It should be noted that each water molecule has two pair of unbond electrons and thus has two hydrogen bonds—two water molecules “stick” to each water molecule. The hydrogen bond is only 1/10 or 1/20 as strong as a covalent bond, which is what holds the oxygen and hydrogen atoms together in the water molecule, but this is still strong enough to explain the properties discussed earlier.

The hydrogen bonds are particularly strong in water although they are present in other substances, such as the alcohols. It is for this reason that the normal boiling points of the alcohols are significantly larger than their paraffin analogues.

When the water molecules line up, they form a hexagonal pattern. This is the hexagonal crystal structure discussed earlier. From elementary geometry, it is well known that the angle between the sides of a regular hexagon is 120°, which is greater than the 105° angle in the water molecule. This seeming paradox is overcome because the hexagonal pattern of the water molecules is not planar. The hexagonal pattern of the water molecules in the ice crystal is shown in the next Figure. In this figure, the circles represent the water molecules and the lines the hydrogen bonds.

Definitions

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Definitions of Words and Terms Used in the Gas Processing Industry

absorber
A tower or column that provides contact between natural gas being processed and a liquid solvent.
absorption
The operation in which one or more components in the gas phase are transferred to (absorbed into) a liquid solvent.
acid gas

The hydrogen sulfide and/or carbon dioxide contained in, or extracted from, gas or other streams.
adsorbent
A solid substance used to remove components from natural gas being processed.
adsorption
The process by which gaseous components are adsorbed on solids because of their molecular attraction to the solid surface.

API Gravity
An arbitrary scale expressing the relative density of liquid petroleum products. The scale is calibrated in degrees API, calculated by the following formula:

associated gas
Gaseous hydrocarbons occurring as a free-gas phase under original oil-reservoir conditions of temperature and pressure. atmospheric pressure The pressure exerted on the earth by the earth’s atmosphere. A pressure of 760 mm of mercury, 29.92 inches of mercury, or 14.696 psia is used as a standard for some measurements. State regulatory bodies have set other standards for use in measuring the legal volume of gas. Atmospheric pressure may also refer to the absolute ambient pressure at any given location.
blowdown
The act of emptying or depressuring a vessel. This may also refer to discarded material, such as blowdown water from a boiler or cooling tower.
bubble point
The temperature at a specified pressure at which the first stable vapor forms above a liquid.
compressibility factor
A factor, usually expressed as "Z," which gives the ratio of the actual volume of gas at a given temperature and pressure to the volume of gas when calculated by the ideal gas law.
compression ratio
The ratio of the absolute discharge pressure from a compressor to the absolute intake pressure. Also applies to one cylinder of a reciprocating compressor and one or more stages of a rotating compressor.
condensate
The liquid formed by the condensation of a vapor or gas; specifically, the hydrocarbon liquid separated from natural gas because of changes in temperature and pressure when the gas from the reservoir was delivered to the surface separators. In a steam system it may be water that is condensed and returned to the boilers.
critical density
The density of a substance at its critical temperature and critical pressure.
critical pressure
The vapor pressure of a substance at its critical temperature.
critical temperature
For a pure component, the maximum temperature at which the component can exist as a liquid.
dew point
The temperature at any given pressure, or the pressure at any given temperature, at which liquid initially condenses from a gas or vapor. It is specifically applied to the temperature at which water vapor starts to condense from a gas mixture (water dew point), or at which hydrocarbons start to condense (hydrocarbon dew point).
distillation
The process of separating materials by successively heating to vaporize a portion and then cooling to liquefy a part of the vapor. Materials to be separated must differ in boiling point and/or relative volatility.
dry gas
(1) Gas whose water content has been reduced by a dehydration process. (2) Gas containing little or no hydrocarbons commercially recoverable as liquid product. Gas in this second definition preferably should be called lean gas.
flash point
The lowest temperature at which vapors from a hydrocarbon liquid will ignite. See ASTM D-56.
fractionation
Generally used to describe separation of a mixture of hydrocarbons into individual products based on difference in boiling point and/or relative volatility.
gas constant (R)
The constant multiplier in the Ideal Gas Law. Numerically, R=PV/T, if V is the volume of one mole of an ideal gas at temperature T and pressure P.
gas injection
The injection of natural gas into a reservoir to maintain or increase the reservoir pressure or reduce the rate of decline of the reservoir pressure.
gas lift
A method for bringing crude oil or water to the surface by injecting gas into the producing well bore.
gas-oil ratio (GOR)
The ratio of gas to liquid hydrocarbon produced from a well. This may be expressed as standard cubic feet of gas per barrel of stock tank liquid.
gas processing
The separation of constituents from natural gas for the purpose of making salable products and also for treating the residue gas to meet required specifications.
gas processing plant
A plant which processes natural gas for recovery of natural gas liquids and sometimes other substances such as sulfur.
hydrate
A solid material resulting from the combination of a hydrocarbon with water under pressure.
Joule-Thomson effect
The change in gas temperature which occurs when the gas is expanded at constant enthalpy from a higher pressure to a lower pressure. The effect for most gases at normal pressure, except hydrogen and helium, is a cooling of the gas.
lean gas
(1) The residue gas remaining after recovery of natural gas liquids in a gas processing plant. (2) Unprocessed gas containing little or no recoverable natural gas liquids.
LNG (liquefied natural gas)
The light hydrocarbon portion of natural gas, predominately methane, which has been liquefied.
LP-gas (liquefied petroleum gas)
Predominately propane or butane, either separately or in mixtures, which is maintained in a liquid state under pressure within the confining vessel.
LRG (liquefied refinery gas)
Liquid propane or butane produced by a crude oil refinery. It may differ from LP-gas in that propylene and butylene may be present.
LTX (low temperature extraction unit)
A unit which uses the cooling of a constant enthalpy expansion to increase liquid recovery from streams produced from high pressure gas condensate reservoirs. Also called LTS (low temperature
separation) unit.
Mcf
An abbreviation for one thousand cubic feet of gas.
MMcf
An abbreviation for one million cubic feet of gas.
mercaptan
Any of a homologous series of compounds of the general formula RSH. All mercaptans possess a foul odor.
natural gas
Gaseous form of petroleum. Consisting predominately of mixtures of hydrocarbon gases. The most common component is methane.
NGL (natural gas liquids)
Natural gas liquids are those hydrocarbons liquefied at the surface in field facilities or in gas processing plants. Natural gas liquids include ethane, propane, butanes, and natural gasoline.
recovery
That percent or fraction of a given component in the plant feed which is recovered as plant product.
recycle
Return of part of a process stream to a point upstream from where it was removed to enhance recovery or control.
reflux
In fractionation, the portion of condensed overhead returned to the column to enhance achievable purity of the overhead product.
reflux ratio
A way of giving a relative measurement to the volume of reflux. Usually referred either to the feed or overhead product.
relief system
The system for safely relieving excess pressure to avoid exceeding equipment design pressure.
sour gas
Gas containing undesirable quantities of hydrogen sulfide, mercaptans, and/or carbon dioxide. It also is used to refer to the feed stream to a sweetening unit.
specific gravity
The ratio of the mass of a given volume of a substance to that of another equal volume of another substance used as standard. Unless otherwise stated, air is used as the standard for gases and water for liquids, with the volumes measured at 60°F and standard atmospheric pressure.
sweet gas
Gas which has no more than the maximum sulfur and/or CO2 content defined by (1) the specifications for the sales gas from a plant; (2) the definition by a legal body. Also, the treated gas
leaving a sweetening unit.

GAS INTRO

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History of Natural Gas

Although naturally occurring gas has been known since ancient times, its commercial use is relatively recent. In about 1000, B.C., the famous Oracle at Delphi, on Mount Parnassus in ancient Greece, was built where natural gas seeped from the ground in a flame. Around 500 B.C., the Chinese started using crude bamboo “pipelines” to transport gas that seeped to the surface and to use it to boil sea water to get drinkable water.

The first commercialized natural gas occurred in Britain. Around 1785, the British used natural gas produced from coal to light houses and streets. In 1816, Baltimore, Maryland used this type of manufactured natural gas to become the first city in the United States to light its streets with gas.

Naturally occurring natural gas was discovered and identified in America as early as 1626, when French explorers discovered natives igniting gases that were seeping into and around Lake Erie. In 1821, William Hart dug the first successful natural gas well in the U.S. in Fredonia, New York. Eventually, the Fredonia Gas Light Company was formed, becoming the first American natural gas distribution company.

In 1836, the City of Philadelphia created the first municipally owned natural gas distribution company. Today, U.S. public gas systems number more than 900, and the Philadelphia Gas Works is the largest and longest operating public gas system in the U.S.

During most of the 19th century, natural gas was used almost exclusively as a source of light, but in 1885, Robert Bunsen's invention of what is now known as the Bunsen burner opened vast new opportunities to use natural gas. Once effective pipelines began to be built in the 20th century, the use of natural gas expanded to home heating and cooking, appliances such as water heaters and oven ranges, manufacturing and processing plants, and boilers to generate electricity.

Natural Gas Today

Today, natural gas is a vital component of the world's supply of energy. Natural gas currently supplies more than one-half of the energy consumed by residential and commercial customers, and about 41 percent of the energy used by U.S. industry. It is one of the cleanest, safest, and most useful of all energy sources.

Ninety-nine percent of the natural gas used in the United States comes from North America. Because natural gas is the cleanest burning fossil fuel, it is playing an increasing role in helping to attain national goals of a cleaner environment, energy security and a more competitive economy. The two million-mile underground natural gas delivery system has an outstanding safety record.

Liquefied natural gas (LNG) is beginning to play a more prominent role in the overall gas supply picture. Although about one percent of the natural gas consumed in this country is currently imported as LNG, it is estimated that our nation's imports of LNG will grow to approximately 7 or 8% by the end of this decade. This will require more than the four LNG facilities that currently exist.

OIL INTRO

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The History of Petroleum

OIL PRODUCTION

Various theories have been advanced over the years as to the origin of petroleum proposing an animal, vegetable, mineral and even meteoric origin. Today, however, most scientists believe that oil and gas originated in plant and animal matter that accumulated in fine grained sediments at the bottom of ancient seas many millions of years ago. This theory suggests that oil originated as the remains of countless organisms that either lived in the sea or were deposited there with mud and silt from prehistoric rivers and streams.

The remains of these ancient plants and animals were transformed into oil and gas by bacterial action with heat and pressure resulting from deep burial beneath other sediments.

It seems, then, that the story of petroleum began hundreds of millions of years ago. But when did humans first learn to use petroleum?

Petroleum Used by Ancient Civilizations

No one knows exactly when humans first used petroleum. It is known, however, that ancient peoples worshiped sacred fires fueled by natural gas seeping to the surface through pores and cracks.

The use of thick gummy asphalt to waterproof boats and heat homes was recorded as long ago as 6000 BC.

About 3000 BC the Egyptians used asphalt in the construction of the pyramids, to grease the axles of the Pharaoh's chariots, as an embalming agent for mummies and in medicinal preparations.

In early North America, Indians skimmed oil from the surface of springs using it to waterproof canoes and in the making of war paints. They taught the colonists to use oil for healing and self-styled "doctors" bottled "Seneca oil" or "rock oil" and sold it as a cure-all.

When whale oil, the main source of lamp fuel in the early 1800’s, became scarce, a new source was needed.

Inventors began to distill illuminating and lubricating oils from natural petroleum seeps and from coal.

Despite the varied and inventive uses being recognized, petroleum was still thought to be a substance of only minor importance. Some thought it was suitable only for use as a lubricant, patent medicine, asphalt and kerosene (for lamps) and nothing else. Others even argued that removal of petroleum from beneath the earth's surface would put out the fires of Hades. But changes were occurring in the world. Changes that would make the continued development and growth of the petroleum industry inevitable.

The First Oil Wells

For thousands of years, the only sources of petroleum had been surface seeps or tar pits. These sources were not very productive, so certain individuals decided to look for oil underground, by drilling.

In 1858, one such individual, a 39 year old carriage maker from Hamilton, Ontario, named James Miller Williams made the first major commercial oil discovery in North America at Oil Springs, Ontario.

Drilling in "gum beds" in Lambton County, 25 km southeast of Sarnia, he struck oil at a depth of only 18 metres. Williams refined the oil he produced and sold the product as lamp oil. In the following

year, Colonel Edwin L. Drake discovered oil in Titusville, Pennsylvania by drilling to 21 meters. This discovery signaled the birth of the modern petroleum industry in the United States.

The Oil Boom Begins

As a result of the oil discoveries of the 1850's, numerous refineries were built to turn crude oil into kerosene for lamps and into lubricating oils for the machines of the industrial revolution. Oil began to replace coal as the fuel for steam engines.

The invention of the gasoline engine (1885) and diesel engine (1892) allowed inventors of horseless carriages to adopt a new power source to replace steam and electricity. Gasoline which had previously been considered a useless by-product of the distillation of crude oil would now take on a new importance as advancements in technology made possible the mass production of the automobile in early years of the twentieth century.

In 1903, at Kitty Hawk, North Carolina, the Wright brothers utilized petroleum to realize what could only be imagined by previous generations - flight. Since that first flight at Kitty Hawk, North Carolina, petroleum has fueled every type of aircraft from Lindberg's "Spirit of St. Louis" to today's jumbo jets.

The Present

Today, we are well into the space age, but petroleum is still the dominant source of energy used by humankind. This is especially true in the transportation industry.

This versatile fuel that is so easily transported and poured into the fuel tanks of our cars and planes has also been largely responsible for the rapid technological advances we have experienced in the past 100 years.

It was less than 500 years ago, in 1519, that Ferdinand Magellan set out with five ships on one of the greatest expeditions in human history; to circumnavigate the world. This voyage cost the lives of most of Magellans' men, including his own, and took three years to complete. Today, it would not be considered a great accomplishment to go around the world, using modern air travel, on a three week vacation. Indeed the world has become a much smaller place, largely due to petroleum - that smelly, oily substance that had its beginnings in ancient seas millions of years before men and women walked the earth.