Hydrocarbon Compression

Archive for the ‘Design Considerations’ Category

Stripping Gas Still Temperature

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A higher temperature in the top of the still column can increase glycol losses due to excessive vaporization. The boiling point of water is 212°F and the boiling point of TEG is 546°R The recommended temperature in the top of the still column is approximately 225°F. When the temperature exceeds 250°F the glycol vaporization losses may become substantial. The still top temperature can be lowered by increasing the amount of glycol flowing through the reflux coil.

If the temperature in the top of the still column gets too low, too much water can be condensed and increase the reboiler heat load. Too much cool glycoi circulation in the reflux coil can sometimes lower the still top temperature below 220°F. Thus, most reflux coils have a bypass to allow manual or automatic control of the stripping still temperature.

Stripping gas will have the effect of requiring reduced top still temperature to produce the same reflux rate.

Written by Jack

September 21st, 2009 at 12:25 am

Glycol Circulation Rate

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When the number of absorber trays and lean glycol concentration are fixed, the dew-point depression of a saturated gas is a function of the glycol circulation rate. The more glycol that comes in contact with the gas, the more water vapor is stripped out of the gas. Whereas the glycol concentration mainly affects the dew point of the dry gas, the glycol rate controls the total amount of water that can be removed. The minimum circulation rate to assure good glycol-gas contact is about two gallons of glycol for each pound of water to be removed. Seven gallons of glycol per pound of water removed is about the maximum rate. Most standard dehydrators are designed for approximately three gallons of glycol per pound of water removed.

An excessive circulation rate may overload the reboiler and prevent good glycol regeneration. The heat required by the reboiler is directly proportional to the circulation rate. Thus, an increase in circulation rate may decrease reboiler temperature, decreasing lean glycol concentration, and actually decrease the amount of water that is removed by the glycol from the gas. Only if the reboiler temperature remains constant will an increase in circulation rate lower the dew point of the gas.

Written by Jack

September 21st, 2009 at 12:24 am

Glycol Dehydration Stripping Gas

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The lean glycol concentration leaving the reboiler can be lowered by contacting the glycol with stripping gas. Often, wet gas that is saturated with water vapor at ambient temperature and 25 to 100 psig is used. At 25 psig and 100°F this gas is saturated with 1,500 Ib/MMscf of water vapor. At atmospheric pressure and the temperatures in the reboiler the gas can absorb over 100,000 Ib/MMscf.

In most situations the additional fuel gas required to heat the reboiler to increase lean glycol concentration is less than the stripping gas required for the same effect. Thus, it is normally desirable to use stripping gas only to increase lean glycol concentration above 98.5 to 98.9%, which can be reached with normal reboiler temperatures and normal back pressure on the still column. If the glycol circulation rate must be increased above design on an existing unit and the reboiler cannot reach desired temperature, it is often possible to use stripping gas to achieve the desired lean glycol concentration.

Figure 8-12 shows the effects on the glycol purity of stripping gas flow rate for various reboiler temperatures, assuming the gas is injected directly into the reboiler. Greater purities are possible if stripping gas contacts the lean glycol in a column containing one or more stages of packing before entering the reboiler.

 Glycol Dehydration Stripping Gas

Written by Jack

September 21st, 2009 at 12:22 am

Glycol Reboiler Pressure

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Pressures above atmospheric in the reboiler can significantly reduce lean glycol concentration and dehydration efficiency. The still column should be adequately vented and the packing replaced periodically to prevent excess back pressure on the reboiler.

At pressures below atmospheric the boiling temperature of the rich glycol/water mixture decreases, and a greater lean glycol concentration is possible at the same reboiler temperature. Reboilers are rarely operated at a vacuum in field gas installations, because of the added complexity and the fact that any air leaks will result in glycol degradation. In addition, it is normally less expensive to use stripping gas. However, if lean glycol concentrations in the range of 99.5% are required, consider using a reboiler pressure of 500 mm Hg absolute (approximately 10 psia) as well as using stripping gas. Sometimes the addition of a vacuum will help extend the range of an existing glycol system

Written by Jack

September 21st, 2009 at 12:20 am

Glycol Reboiler Temperature

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The reboiler temperature controls the concentration of the water in the lean glycol. The higher the temperature the higher the concentration, as shown in Figure 8-11. Reboiler temperatures for triethylene glycol are limited to 400°F, which limits the maximum lean glycol concentration without stripping gas. It is good practice to limit reboiler temperatures to between 370°F and 390°F to minimize degradation of the glycol. This effectively limits the lean glycol concentration to between 98.5% and 98.9%.

When higher lean glycol concentrations are required, stripping gas can be added to the reboiler, or the reboiler and still column can be operated at a vacuum.

 Glycol Reboiler Temperature

Written by Jack

September 21st, 2009 at 12:17 am

Glycol Concentration

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The higher the concentration of the lean glycol the greater the dewpoint depression for a given glycol circulation rate and number of trays.

Figure 8-9 shows the equilibrium water dew point at different temperatures for gases in contact with various concentrations of glycol. At 100°F contact temperature there is an equilibrium water dew point of 25°F for 98% glycol and 10°F for 99% glycol. Actual dew points of gas leaving the contactor will be 10°F to 20°F higher than equilibrium.

Figure 8-10 shows that increasing the lean glycol concentration can have a much greater effect on dew-point depression than increasing the circulation rate. To obtain a 70°F dew-point depression a circulation rate of 6.2 gaVlb at 99.95%, 8.2 gal/lb at 99.5% or in excess of 12 gal/lb at 99% is required.

The lean glycol concentration is determined by the temperature of the reboiler. the gas stripping rate, and the pressure of the reboiler. Glycol concentrations between 98 and 99% are common for most field gas units.

 Glycol Concentration

 Glycol Concentration

Written by Jack

September 20th, 2009 at 7:03 pm

Lean Glycol Temperatures

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The temperature of the lean glycol entering the contactor has an effect on the gas dew-point depression and should be held low to minimize required circulation rate. High glycol losses to the gas exiting the contactor may occur when the lean glycol temperature gets too hot. On the other hand, the lean glycol temperature should be kept slightly above the contactor gas temperature to prevent hydrocarbon condensation in the contactor and subsequent foaming of the glycol. Most designs call for a lean glycol temperature 10°F hotter than the gas exiting the contactor.

Written by Jack

September 20th, 2009 at 6:09 pm

Glycol Contactor Trays

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The glycol and the gas do not reach equilibrium on each tray. A tray efficiency of 25% is commonly used for design. That is, if one theoretical equilibrium tray is needed, four actual trays are specified. In bubble cap towers, tray spacing is normally 24 in.

The more trays the greater the dew-point depression for a constant glycol circulation rate and lean glycol concentration. By specifying more trays, fuel savings can be realized because the heat duty of the reboiler is directly related to the glycol circulation rate. Figure 8-8 shows how the number of trays can have a much greater effect on dew-point depression than the circulation rate.

The additional investment for a taller contactor is often easily justified by the resultant fuel savings. Most contactors designed for 1 Ib/MMscf gas are sized for 6 to 8 trays.

 Glycol Contactor Trays

Written by Jack

September 20th, 2009 at 6:06 pm

Glycol Contactor Pressure

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Contactor pressures have little effect on the glycol absorption process as long as the pressures remain below 3,000 psig. At a constant temperature the water content of the inlet gas decreases with increasing pressure, thus less water must be removed if the gas is dehydrated at a higher pressure. In addition, a smaller contactor can be used at high pressure as the actual velocity of the gas is lower, which decreases the required diameter of the contactor.

At lower pressure less wall thickness is required to contain the pressure in a given diameter contactor, therefore, an economic trade-off exists between operating presssure and contactor cost. Typically, dehydration pressures of 500 to 1,200 psi are most economical.

Written by Jack

September 20th, 2009 at 6:01 pm

Glycol Dehydration Inlet Gas Temperature

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At constant pressure, the water content of the inlet gas increases as the inlet gas temperature increases. For example, at 1,000 psia and 80°F gas holds about 34 Ib/MMscf, while at 1,000 psia and 120°F it will hold about 104 Ib/MMscf. At the higher temperature, the glycol will have to remove over three times as much water to meet a pipeline specification of 7 lb/MMscf.

An increase in gas temperature may result in an increase in the required diameter of the contact tower. As was shown in separator sizing , an increase in temperature increases the actual gas velocity, which in turn increases the diameter of the vessel.

Inlet gas temperatures above 120°F result in high triethylene glycol losses. At higher gas temperatures tetraethylene glycol can be used, but it is more common to cool the gas below 120°F before entering the contactor. The more the gas is cooled, while staying above the hydrate formation temperature, the smaller the glycol unit required.

The minimum inlet gas temperature is normally above the hydrate formation temperature and should always be above 50°F. Below 50°F glycol becomes too viscous. Below 60°F to 70°F glycol can form a stable emulsion with liquid hydrocarbons in the gas and cause foaming in the contactor.

There is an economic trade-off between the heat exchanger system used to cool the gas and the size of the glycol unit. A larger cooler provides for a smaller glycol unit, and vice versa. Typically, triethylene glycol. units are designed to operate with inlet gas temperatures between 80°F and 110°F.

Written by Jack

September 20th, 2009 at 5:59 pm