Solar-heating system studied for heavy-oil pipelines

Here is how the Helitherm system would look on a pipeline. A photograph of a 7-ft segment of 8-in. line equipped with the system (left foreground) has been electronically enhanced to simulate how a longer segment would appear (Fig. 1).

Modeling of a solar-powered, heat-tracing system for pipelines suggests it can be successful as a nonintrusive pour point depressant or drag-reducer, according to Solar Systems Pty. Ltd., Hawthorn, Vict., Australia.

The company developed the Helitherm system, which is patented in the U.S., Europe, and Australia.

In simulation models, the company used data provided by two operators, Petroleos de Venezuela (Pdvsa) and the Fauji oil terminal, Karachi. For Pdvsa, Solar Systems tested the Helitherm system for throughput enhancement of heavy crude oil; for the Fauji terminal, heat tracing of a heavy-fuel-oil pipeline.

Thermal diode; prototypes

Depending on process requirements and economics, the following have been techniques used to enhance throughput and to heat trace pipelines specifically when transporting heavy, high pour point crude oil or product: ¹

Heating the crude to a high temperature at the inlet to the pipeline and allowing it to reach its destination before cooling below the pour point. The pipeline may or may not be insulated.
Pumping the crude at a temperature below the pour point.
Adding a hydrocarbon diluent such as a less waxy crude or a light distillate.
Injecting water to form a layer between the pipe wall and the crude.
Mixing water with the crude to form an emulsion.
Processing the crude before pipelining to change the wax crystal structure and to reduce pour point and viscosity.
Heating both crude and pipeline by steam tracing or electrical heating.
Injecting paraffin inhibitors.
Injecting drag-reducing agents (DRAs).

An alternative to these is the Helitherm technique installed on pipelines to increase daytime and night time throughput, maintain product temperature at 50° C. greater than ambient, reduce pumping system operating pressure, and reduce or eliminate steam injection and electrical heat tracing.

Fig. 1 shows a simulated Helitherm pipeline. Fig. 2 [72,500 bytes] shows the principle of operation of Helitherm. The system can also be used on tanks.

Solar-powered heat tracing for new or existing pipelines relies on an integrated thermal diode that traps sunlight and keeps in heat. Because the pipe is fully insulated, heat loss is minimized and temperatures rise, says the company. The passive system requires no power.

Helitherm is based on three fundamentals of heat transfer-a thermal diode, convection suppression, and radiation suppression-that operate through transparent/opaque insulation and spectrally selective coatings.

The thermal diode is the heart of the system, says Solar Systems, and functions through three subcomponents, the combination of which produces the thermal diode effect:

Transparent insulation material (TIM): Polymer-based insulation transparent to the solar spectrum and opaque to infrared radiation. TIM forms a part of pipe annulus and lets in solar energy but prevents heat loss.
Opaque insulation (poly urethane, Rockwool, or others) reduces heat loss from the system and forms the remainder of the pipe annulus.
Spectrally selective coating: The coating, according to Solar Systems, is a "proprietary radiant energy trap [with] high absorptivity to solar radiation and low emissivity to infrared radiation that suppresses radiation heat loss."

Solar radiation is absorbed on the pipeline surface with the coating and transferred as heat to the fluid.

The daytime increase in the temperature of what is in the pipeline depends upon the thermophysical properties of the fluid, optical/thermal characteristics of the insulation, the ambient temperature, and solar radiation.

Helitherm senses when useful energy is available at the receiver and transfers this heat to the pipeline.

Solar Systems developed a computer program to predict the performance of a Helitherm pipeline for any geographical location in the world. Once the meteorological parameters-solar-radiation data, ambient temperatures, wind speed, pipe orientation-and the thermophysical properties of the fluid are fed to the computer, it can predict the minimum, average, and maximum temperatures attained by the pipeline for the entire year.

This enables the sizing of the thickness and aperture angle of TIM and opaque insulation in order to meet the fluid temperature range specified by the pipeline operator.

Field tests of various products and pipe sizes conducted since 1983 have laid the foundation for a rigorous mathematical simulation model.

Between 1994 and 1998, monitoring and evaluation in the field of 12 prototypes at mid and tropical latitudes on pipelines that transported Bach Ho crude oil (pour point 36° C.) provided data for accurate correlation of the model.

The design model was used in two feasibility studies based on product data supplied by the oil companies.

Heavy-oil pipeline

The first study was conducted at the request of Pdvsa for throughput enhancement of a heavy crude oil pipeline. Pdvsa superintendent of technology for petroleum and natural gas Rafael J. Paz provided the technical data.

The bare pipeline flow rate was 269 b/d. With a Helitherm retrofit, total enhancement in throughput was estimated to be 46 b/d, a 17% increase.

Summary of results is shown in Table 1 [73,008 bytes]. Following are the conditions:

Daytime

- Average radiation for the Lagunillas site in Maracaibo: 375 w/sq m
- Average daytime ambient temperature: 28° C. (82.4° F.). (Average daily solar radiation on horizontal plane for Maracaibo = 16.2 mega Joules/sq m-day. For approximately 12 sunshine hr, the average radiation = 375 w/sq m.)
- Average wind velocity: 0 m/sec. (Solar Systems says that because of the absence of published information, wind velocity was assumed to be zero. Actual wind velocity, which may vary between 0 and 5 m/sec, has a significant effect in cooling bare pipe. A Heiitherm-coated pipeline, however, is unaffected.)
- Throughput (24-hr basis): 269 b/d
- Daytime duration: 12 hr

Night time

- Average temperature: 26.6° C. (80° F.)
- Average wind velocity: 0 m/sec
- Average night-time duration: 12 hr

Conclusions (Fig. 3a [157,410 bytes])

- Daytime enhancement in throughput: (22 bbl/hr - 19 bbl/hr) x 12 = 36 bbl
- Night-time enhancement in throughput: (4.2 bbl/hr - 3.4 bbl/hr) x 12 = 10 bbl
- Total enhancement in throughput: 36 bbl + 10 bbl = 46 b/d. (This estimate may vary between 42-88 b/d, says Solar Systems.)
- Increase in Pdvsa's turnover: 46 b/d x 365 days x $13/bbl = $218,270.

Table 2 [89,798 bytes] presents the pipeline parameters from the simulation.

Heat tracing

Based on data provided in May 1993 by Promet Private Ltd., Singapore, Helitherm was one solution proposed for heat tracing a 36-in. heavy-fuel-oil pipeline at the Fauji oil terminal, Karachi.

The goal was at all times to maintain the oil temperature at greater than the pour point of 35° C. The average ambient temperature for Karachi is 26.2° C., varying between 45° C. in summer and 3° C. in winter.

The Helitherm system proposed to heat trace this pipeline, provided a 120° TIM pipe section facing the sun and 240° of opaque insulation to reduce the heat loss. During a no-flow condition (stagnation) with Helitherm system installed, the oil's temperature would be greater than 60° C.

Temperature drop in an insulated pipeline is negligible at high flow rates.

The average yearly fuel oil temperature, simulated with the Helitherm system installed, experienced by the pipeline is 78.9° C., 52.7° C. greater than the yearly average ambient temperature in Karachi (Fig. 3b).

Fig. 3b shows the variation in pipeline stagnation temperatures throughout the year with the Helitherm simulator. Table 3 [90,522 bytes] shows the product and pipe parameters and summary of simulation results.