PC program developed for estimating pipeline drying time

Abdulghani A. Al-Farayedhi
King Fahd University of Petroleum & Minerals
Dhahran, Saudi Arabia

• Equations box [273,114 bytes]
• Example problem data [74,304 bytes]

A computer program has been developed that enables process engineers and designers to estimate the total drying time required for removing water from pipelines.

The program (PIPE.DRY) determines the total amount of water to be removed from a pipeline and the total time required to dry the pipeline. It also estimates the volume of the desiccant and the size of the bed required for the operation.

Design procedures involve calculations for volume and mass of water in the pipeline, volumetric as well as mass flow rate of dry air, moisture content of saturated air and dry air, and total drying time in hours and days. These procedures also involve calculations for total water adsorbed, volume of desiccant, and size of bed.

The thickness of the water film, the volumetric flow rate of dry air, the dew point temperature of the dry air, and the length of the pipeline, all influence the time required for drying the pipeline. The useful design capacity of the desiccant and its density influence the size of the bed.¹

The equations box contains the required equations and terms; the nomenclature box provides definitions.

The analysis is based upon the following assumptions:

• The dry air and the moisture-containing air obey all the laws of ideal gases.
• The pipe-wall temperature is assumed to be the same as the ambient temperature.
• Water-film thickness on the pipeline wall is uniform.
• The dew point temperature inside the pipe is the same as the pipe-wall temperature.
• The influence of the temperature drop of the air as a result of moisture evaporation from the pipe wall on the drying time is neglected.
• The ratio of the length of the bed to the diameter of the bed is assumed as 4.

Pipeline drying

Saudi Arabia has several ports and oil-export terminals connected by pipelines to oil fields and gas-processing plants.

For example, crude oil is piped to Yanbu from the Abqaiq oil fields in the east through a 1,200-km pipeline.² A 20-in. (50-cm) OD, 380-km pipeline has been laid from Dhahran to Riyadh to improve refined-product distribution.

Pipeline drying is a growing problem for the operation of pipelines transporting petroleum products and natural gas.

Hydrostatic testing is usually conducted in the oil and gas industry to check the pipe strength and leak tightness.³ The water that remains in a pipeline after the test is normally removed by a pig, a foam plastic cylinder or sphere a little wider than the pipeline diameter.

After pigging operation, a very thin layer of water, usually 50-100 ?m thick, remains on the inner surface of the pipeline. Drying the inside surface of the pipeline is necessary to prevent the formation of hydrates with hydrocarbons.

The white granular scales caused by this mechanism block equipment and connecting lines. Moreover, carbon dioxide and hydrogen sulfide in conjunction with water vapor enhance corrosion in pipelines and valves.⁴

Low dew point air

The water content in air is commonly expressed in terms of a humidity ratio (gram of water/kilogram of dry air).

Another useful method of indicating the water content in air is in terms of the dew point: the temperature to which the air must be cooled before it becomes saturated with water vapor. Dew point is a more direct indication of dehydration effectiveness than absolute water content.

Water that remains in the pipeline can be removed by any of the following techniques: ventilative drying, vacuum drying, absorptive drying, or use of nitrogen, methanol, and natural gas.⁵

For economic, technical, and safety reasons, the last three methods have generally proven unsatisfactory. The ventilative drying process (OGJ, June 11, 1984, p. 114) offers the following advantages:

• The equipment cost is relatively low.
• It does not require skilled personnel.
• It enables attaining very high drying levels.
• It does not require the availability of natural gas and inert gas.

In this discussion, the process of ventilative drying is investigated for its simplicity and practicality. The process is based on passing dry air through the pipeline for a period of time.

The principle of drying is simple. As air with a low dew point blows into the pipeline, moisture is absorbed in the dry air stream because it has a low partial pressure of water vapor.

The performance of ventilative drying depends mainly on the potential for mass transfer: the difference in water-vapor pressure between the water film on the inner side of the pipeline and the dry-air stream. A pipeline's drying time decreases as the potential of mass transfer increases.

Several techniques can produce low dew point air:

• Direct cooling
• Compression followed by cooling
• Absorption
• Adsorption.

To obtain very low dew point air, additional dehydration by absorption or adsorption is required in direct cooling and compression followed by cooling. ⁶

Fig. 1 [65,072 bytes] presents a psychrometric of the dehumidification process for direct cooling. In this method, air passes through the cooling coil where the dry-bulb temperature of the moist air decreases, while the moisture content remains constant.

As moisture begins to condense out of the air onto the cooling coil, the dry-bulb temperature continues to decrease, resulting in a simultaneous decrease in the moisture content or dew point temperature (Line 1-2 in Fig. 1). Although dehumidification by refrigeration is quite effective at high temperatures, it becomes less effective at lower temperatures (0-5° C.) because the condensed moisture will freeze on the cooling coil.

For example, a drop of 1° C. in the temperature of saturated air at 7° C. removes less than one-third of the amount of water vapor that a corresponding drop removes from 29° C. saturated air.

Absorption, adsorption

Some granular solids have an affinity for water. When air flows through a bed of such solids, the water is retained on their surfaces in a process called adsorption.

Water vapor may also be removed from air when it is exposed to certain liquids that have an affinity for water. When water vapor is removed by this process, the operation is called absorption.

The liquid or solid with an affinity for water and used in either process is the desiccant.

Each process has particular advantages, disadvantages, and degree of usefulness; all produce low dew point air.

Fig. 1 also presents the desiccant dehumidification process. This system, with no heat added or removed, will then be adiabatic (Line 1-3 of Fig. 1).

The process increases the dry-bulb temperature of the moist air. For solid-desiccant materials, this increase is a result of the heat of adsorption that consists of the latent heat of vaporization of the adsorbed liquid plus an additional heat of wetting.

In liquid-desiccant systems, air passes through a cooling coil or through a contact surface such as cooling-tower packing that has been wetted with liquid desiccant such as lithium chloride, lithium bromide, calcium chloride, or triethylene glycol.

The desiccant absorbs moisture from the air, making the liquid solution more dilute. The weak desiccant is sent through a heater and sprayed into a reactivation air stream.

The reactivation air carries away water vapor given off by the warm desiccant so that the reconcentrated desiccant can be used for ventilative drying again. It is possible to achieve an equilibrium water dew point of -60° C. with triethylene glycol.

The disadvantages of liquid-desiccant systems are corrosion, weight, and the need to control liquid sloshing and entrainment.

Solid systems

In solid-desiccant systems, air is circulated through a bed of absorptive material such as silica gel, alumina, molecular sieves, lithium chloride, or zeolite. The actual outlet dew point depends on the desiccant chosen and design of the unit. The dew points are achievable with desiccants ( Table 1 [15,799 bytes]).

Silica gel desiccant is widely used in industrial drying processes and hence the same is considered for this study.

Silica gel is a hard, granular, porous product made from the gel precipitated by acid treatment of sodium silicate solution. Its moisture content before use varies 4-7%; it is used principally for dehydration of air and other gases.

Solid-desiccant materials are arranged in a variety of ways in desiccant-dehydration systems. A large desiccant surface area in contact with the air stream is desirable, and a way to bring regeneration air to the desiccant material is necessary.

Three types of solid-desiccant dehumidifiers are available commercially:

1. Rotary-bed dehumidifier
2. Desiccant wheel dehumidifier
3. Dual-bed packed tower dehumidifier.

In a horizontal rotating-bed dehumidifier, granules of desiccant are held in perforated rotating trays which rotate between the process and regeneration air streams. Like the horizontal rotating bed, the desiccant wheel rotates continuously between the process and regeneration air streams.

The wheel is constructed by placement of a thin layer of desiccant material on a plastic or metal support structure. The support structure, or core, is formed so that the wheel consists of many small parallel channels coated with desiccant.

The channels are small enough to ensure laminar flow through the wheel. Some kind of sliding seal must be used on the face of the wheel to separate the two streams.

Typical rotation speeds are between 6 and 20 rotations/hr. These two types of dehumidifiers are commonly used for commercial space-conditioning applications.

For the present study, a packed-bed configuration is chosen. Fig. 2 [72,828 bytes] shows the schematic of a dual-bed packed tower dehumidification system suitable for pipeline-drying applications. As the ambient (process) air passes through one tower, the reactivation air is routed through the other tower.

When the desiccant in the first tower becomes saturated, the air streams are switched. The drying system must be designed with a blower system to handle the friction losses of the pipeline as shown in the figure.

Air-to-air heat exchangers may be added to cool the warm air leaving the process section of the unit and preheat the air entering reactivation. Either heat pipes or plate-type heat exchangers may be used for this purpose.

Air filters are an important component of solid-desiccant systems. Dust or other contaminants can interfere with the adsorption of water vapor and quickly degrade the system performance.

The capacity of a desiccant for water is expressed normally in mass of water adsorbed per mass of desiccant. The useful capacity of a desiccant is defined as the design capacity that recognizes loss of desiccant capacity with time as determined by experience and economic considerations and the fact that all of the desiccant bed can never be fully used.⁷

Example

In this example, a ventilative drying system that uses a solid desiccant (silica gel) as the adsorbent to produce the dry air is used to estimate the drying time required for a pipeline operating under the input data shown in the accompanying example box.

PIPE.DRY estimates the total time required to dry the pipeline for the given operating conditions, length of the pipeline, velocity of dry air, and physical properties. The program also estimates the volume of the desiccant required.

The diameter of the bed is calculated based on the assumption that the ratio of the length of the bed to the diameter of the bed is 4.

The example box shows the input data and computer output with the dry air flow rate of 4,210.75 kg/hr and the moisture-removal capacity of the dry air of 32.53 kg/hr. The total drying time required for drying the pipeline is 204.47 hr or 8.52 days.

Acknowledgments

The author is grateful for the financial support and facilities provided by the King Fahd University of Petroleum and Minerals.

References

1. Syed, Younus Ahmed, Gandhidasan, P., and Al-Farayedhi, A.A., "Pipeline drying using dehumidified air with low dew point temperature," Applied Thermal Engineering, Vol. 18, No. 5, 1998, pp. 231-44.
2. Arab Oil & Gas Directory, published by Arab Petroleum Research Center, Paris, 1997.
3. LaCasse, G.A., and Ingvordsen, T., "Desiccant drying of gas pipelines," Material Performance, Vol. 27, September 1988, pp. 48-51.
4. Aitani, A.M., "Sour natural gas drying," Hydrocarbon Processing, April 1993, pp. 67-73.
5. Gorislavets, V.M., and Sverdlov, A.A., "Numerical investigation of the process of ventilative drying of a pipeline," Journal of Engineering Physics, Vol. 60, 1991, pp. 615-23.
6. Ikoku, Chi U., Natural Gas Engineering, Tulsa, PennWell Publishing Co., 1980, p. 147.
7. Campbell, John M., Gas Conditioning and Processing, 6th Edition, Vol. 2, Campbell Petroleum Series, 1984, p. 341.

Editor's note: OGJ subscribers in the U.S. may obtain a copy of the PIPE.DRY program by sending a blank 3.5 diskette formatted to MS DOS and a self-addressed, postage-paid or stamped return diskette mailer to:

Pipeline/Gas Processing Editor
Oil & Gas Journal
1700 West Loop South, Suite 1000
Houston, TX 77027-3006

OGJ subscribers outside the U.S. may send the diskette and return mailer without return postage to the same address. This offer expires on June 30, 1999.

The Author