PC program estimates TEG circulation rate, number of trays

P. Gandhidasan, A. A. Al-Farayedhi, Ali A. Al-Mubarak
King Fahd University of Petroleum & Minerals
Dhahran

A computer program has been developed that enables process engineers and designers to estimate the circulation rate of lean triethylene glycol (TEG) needed to dry natural gas in glycol dehydrators that use a tray column.

The program (TEG.TRAY) determines the total amount of water vapor to be removed from the natural gas to achieve the required exit-gas water content and the circulation rate of lean TEG required for this purpose.

It also estimates the number of trays required for the glycol dehydrator along with the column diameter.

Presented here are the calculations behind the program along with an example.

Replaces hand calculations

Design procedures involve calculations for the mole fraction of water in both streams, the activity coefficient for water in TEG-water solutions, the equilibrium constant for water in TEG-water system, the maximum possible amount of water absorbed, the absorption efficiency, the absorption factor, and the circulation rate of lean TEG required in liters/kilogram of water absorbed.

These procedures also involve calculations for number of trays required and the dehydrator or the absorber diameter.

In order to carry out these calculations, several charts and tables must be referred to, and a tedious hand calculation performed.

The advantage of TRAY.TEG is that it will replace the use of tables, charts, and hand calculations, and it is faster. Here, the results of the computer program are compared with data available in the literature to show that the program accurately predicts the circulation rate of lean TEG and the diameter of the tray column.

The concentration of lean TEG, the volumetric flow rate of natural gas, the pressure and temperature of the natural gas, the tray efficiency, and the required exit-gas water content, all influence the number of trays required and the circulation rate of TEG.

The equation box contains the required equations and terms; the nomenclature box provides definitions. The conservative approach is used for calculating the circulation rate of lean TEG with the following assumptions:

The gas volume is constant at each point in the absorption tower.
The gas entering the bottom tray is 1 mole/unit time. This permits calculation from the absorption factor of the number of moles of lean TEG entering the top tray per unit time.
The density of TEG and the gas are 1.12 and 0.0416 kg/l., respectively.
The wet gas entering the bottom of the tower is saturated.
The ranges of operating pressure and temperature of the natural gas are 2-10 MPa and 20-40° C., respectively.
The standard pressure (Ps) and temperature (Ts) are 100 kPa (absolute) and 15° C., respectively. The corresponding number of moles in a million standard cubic meters/day (MMscmd) of the gas is 1,739 kmol/hr.
The range of lean TEG varies from 97 to 99.85%.
The molecular mass of water and TEG is 18 and 150, respectively.
Flooding velocity is 80%, for calculating the gas superficial velocity.

Natural-gas drying

Natural gas contains many impurities, the most undesirable being water vapor. ¹ All natural gases contain water vapor to some degree. The natural gas must be dried before combustion or transmission for long distance through the pipelines for the following reasons:

Gas hydrates plug equipment and pipelines.
Water vapor decreases the combustion temperature and hence lowers the combustion efficiency.
Natural gas containing water is corrosive particularly when CO₂ and H₂S are present.
Slug-flow conditions caused by condensation of water vapor in natural gas.
Water vapor increases the volume and decreases the heating value of natural gas; this reduces line capacity.
A water dew point requirement of a sales-gas-contract specification ranges 32.8 to 117 kg/million standard cu m (MMscm).

Dehydration systems

Water content of natural gas is indirectly indicated by the dew point: the temperature at which the natural gas is saturated with water vapor at a given pressure.

There are different methods for dehydrating gases, but only two major types of dehydration equipment are in current use for natural gas:²

Solid-desiccant dehydrator
Liquid-desiccant dehydrator.

Each method has its own advantages and disadvantages for its usefulness in the field of applications.

Solid-desiccant dehydrator

Such solids as silica gel, molecular sieves, zeolite, and others that have an affinity for water vapor are called the solid desiccant.

When natural gas flows through a bed of such granular solids, the water is retained on the surface of the particles of the solid material. This process is called adsorption.

While one adsorber is dehydrating, the other adsorber is being regenerated by a hot slipstream of inlet gas from the regeneration gas heater. There are some operating problems with solid-desiccant dehydration systems that include the following:

Space adsorbents degenerate with use and eventually require replacement. The amount of water vapor adsorbed per regeneration decreases with continued use.
Loss in capacity may be accelerated by contaminants (like compressor cylinder oil) collecting and being deposited in desiccant beds.
A tower must be regenerated, cooled, and readied for operation as another tower approaches exhaustion. With more than two towers involved, this operation is relatively complicated.
Unloading towers and recharging them with new desiccant should be completed well ahead of the operating season. In the interest of maintaining continuous operation when most needed, this may require discarding desiccant before the end of its normal operating life.
Sudden pressure surges may upset the desiccant bed and channel the gas stream with poor dehydration.
If a plant is operated at greater than its rated capacity, pressure loss will increase and some attrition may occur.

Liquid-desiccant dehydrator

Water vapor may be removed from natural gas by bubbling the gas counter-currently through certain liquids that have an affinity for water; the operation is called "absorption."

A liquid-desiccant dehydration system has the following advantages over a solid-desiccant dehydration system:

The dehydration process is continuous rather than batch or intermittent. Desiccant make-up process is easier than emptying and refilling solid-desiccant towers.
Installation cost is about half that of solid-desiccant systems.
Pressure drop in the absorber is lower than that in the adsorber.
Less heat is needed for regeneration for each kilogram of water removed.
Liquid desiccants are more resistant to contaminants.

A number of liquids, such as calcium chloride, lithium chloride, lithium bromide, and glycols can absorb water from natural gas. The following are the general requirements of liquid desiccants with particular reference to dehydration of natural gas: ³

It must be highly hygroscopic and noncorrosive.
Viscosity must be low and solubility sufficiently high over a considerable temperature range to ensure no solidification.
It should not form precipitates with gas constituents.
It should be easily regenerated to a high concentration.
It should be nonsoluble in liquid hydrocarbons.
It should be relatively stable in the presence of sulfur compounds and carbon dioxide under normal operating conditions.

Among these desiccants, glycol is generally preferred, and several of them come close to meeting all of the previously stated criteria. Diethylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (TREG) possess suitable traits.

DEG is somewhat cheaper to buy but has a larger carry-over loss, offers less dew point depression, and regeneration to high concentration is more difficult.

TREG is both more viscous and more expensive than the other glycols. The only advantage is that its lower vapor pressure reduces absorber carry-over loss. It may be used in those relatively rare cases in which glycol dehydration will be employed on a natural gas whose temperature exceeds about 50° C.

Bubble-cap trays

Almost 100% of the glycol dehydrators use TEG4 because of low equipment and operating costs, high thermal stability, low vaporization losses, and efficient regeneration at high reboiler temperature.

The total quantity of water transferred from the natural gas to TEG depends on the quality of the contact between the two fluids (interfacial area, mass transfer coefficient) and the driving force which moves the system more towards a state of thermodynamic equilibrium (OGJ, Nov. 26, 1984, p. 100).

In the TEG dehydrator, the water vapor is removed from the natural gas by intimate contact with TEG. The gas flows counter-currently to the glycol. The contacting is usually performed in packed or tray towers.

Although the random packing towers have been used for gas-liquid contact in a glycol absorber, it is not generally recommended3 because one can encounter liquid distribution problems. If undue foaming occurs, the tower can flood at lower-than-normal gas rates and cause excess glycol losses.

With tray towers, the choice is between valves and bubble-caps. The former is more efficient at design capacity, but at lower flow rates glycol "weeping" may produce unsatisfactory water dew points.

Bubble-caps certainly are a safe choice in service where widely fluctuating gas flow rates are anticipated. Although bubble-cap trays are less efficient than valve trays (25% vs. 33%), they are preferred because they are suitable for viscous liquids and low liquid/gas flow ratios.

Fig. 1 [69,555 bytes] presents a flow diagram for a typical TEG dehydrator with bubble-cap trays.5 Pressure and temperature of the inlet gas control the water content and the amount of water to be removed.

High pressures and low temperatures are the preferred operating conditions because they reduce water content, equipment size, and fabrication costs.6 Note that cooling TEG increases foaming tendency; an inlet-gas temperature of less than 15° C. is not recommended.

In order properly to design a unit, one must calculate the lean TEG circulation rate required to pick up from the gas the needed amount of water necessary to meet the outlet-gas water content specification.

The glycol circulation rate must be adequate for good distribution of the glycol in the inlet gas and to maintain the rich glycol concentration.

Although a number of circulation rates are possible, the minimum feasible one should be used because as the circulation rate increases, so does operating cost.

The required size of a glycol circulating pump can be readily determined from the glycol circulation rate and the maximum operating pressure of the absorber.

Example

In this example, a bubble-cap tray column that uses TEG as the liquid desiccant to produce dry gas is used to estimate the lean TEG circulation rate needed to dry 1 MMscmd of gas.

It also estimates the number of trays and the column diameter required for a dehydrator operating under the input data shown in the accompanying example box.

TEG.TRAY estimates the circulation rate required to dry 1 MMscmd of gas for the given operating conditions to achieve the required exit-gas water content.

For the present study, the pressure and temperature of the inlet gas and the lean TEG concentration are fixed. The program also estimates the column diameter and the number of trays required.

The example shows the input data and computer output with the gas volumetric flow rate of 0.168 cu m/sec (605 cu m/hr) and the TEG flow rate of 2,443.87 kg/hr. The number of trays required is four; the circulation rate of TEG is 2,182 l./hr. The circulation rate of TEG for each kilogram of water absorbed is 53.76 l., and the tray column diameter is 97.76 cm, or approximately 1 m.

Hand calculation is available in the literature³ to calculate the circulation rate of lean TEG needed to dry 1 MMscmd of gas for the same input data. The hand calculation estimates the circulation rate of TEG for each kilogram of water absorbed as 55.5 l., whereas the program predicts 53.76 l.

Further, it is reported³ that the hand calculation yields a circulation rate slightly higher than the more rigorous calculation. This comparison makes clear that the program predicts the circulation rate of lean TEG accurately. It is indeed reliable and faster.

The tray column diameter is also estimated for comparison with the program prediction using the chart available in the literature⁴ for the same gas flow rate and its pressure, and it yields 40 in. (1 m) as predicted by the program.

Acknowledgments

The authors are grateful for the financial support and facilities provided by the King Fahd University of Petroleum & Minerals.

References

Grosso, S., Fowler, A.E., and Pearce, R.L., "Dehydration of natural and industrial gas streams with liquid desiccants," in Drying '80 (1980), Vol. 1, A.S. Mujumdar, ed., Washington, pp. 468-74.
Ikoku, Chi U., Natural Gas Engineering, Tulsa, PennWell Publishing Co., 1980, p. 147.
Campbell, John M., Gas Conditioning and Processing, 6th Edition, Vol. 2, Campbell Petroleum Series, 1984, p. 295 and p. 309.
Manning, W.P., and Wood, H.S., "Guidelines for glycol dehydration design-Part 1," Hydrocarbon Processing, January 1993, pp. 106-14.
Petroleum Extension Service, Plant Processing of Natural Gas. Austin, The University of Texas Press, 1974.
Manning, W.P., and Wood, H S., "Guidelines for glycol dehydration design-Part 2," Hydrocarbon Processing, February 1993, pp. 87-92.

Editor's note: Any OGJ subscriber worldwide may obtain a copy of the TEG.TRAY program by sending an e-mail request to TEG.TRAY Program ( [email protected]). Please include your name, company name with physical mailing address, and contact telephone and telefax numbers. Alternatively, OGJ subscribers may send a blank 3.5-in. diskette formatted to MS DOS and (for U.S.-based subscribers) a self-addressed, postage-paid or stamped return diskette mailer to: Pipeline/Gas Processing Editor Attn: TEG.TRAY Oil & Gas Journal 1700 West Loop South, Suite 1000 Houston, TX 77027-3005 USA OGJ subscribers outside the U.S. may send the diskette and return mailer-without return postage-to the same address. This offer expires after Sept. 31, 1999.

The Authors

P. Gandhidasan is associate professor of mechanical engineering at King Fahd University of Petroleum & Minerals, Dhahran. He joined the department in February 1992. He holds a PhD (1979) in mechanical engineering from the Indian Institute of Technology, Madras. Gandhidasan served as a lecturer in mechanical engineering at the University of the West Indies, St. Augustine, Trinidad, from 1978 to 1984 and then as a senior lecturer from 1984 to 1992.
Gandhidasan received Fulbright/Laspau award for 1984-85 which he spent at Texas A&M University, College Station, as a post-doctoral fellow in mechanical engineering. And he served as visiting associate professor in mechanical engineering at Texas Tech University, Lubbock, 1990-91.

Abdulghani A. Al-Farayedhi is assistant professor and chairman of the mechanical engineering department at King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia. He joined the department as a graduate assistant in 1976 after receiving a BS in mechanical engineering, obtained an MS in 1979, and served as a lecturer in the department. In 1987, he received a PhD in thermal sciences from the University of Colorado, Boulder.