Flameless catalytic process improves pipeline heating

For many years, water bath indirect heaters have been the most common way of heating pipelines and preventing freeze damage to critical metering and regulation equipment. Now, a “perfect storm” that includes maintenance requirements, the risks inherent in using hazardous chemicals, and, increasingly, corporate environmental policies, is changing this.  

Other factors in the mix include growing concerns about odor and noise from homeowner’s associations and individuals; security concerns regarding open flames; and the presence of chemicals that are known carcinogens.

The alternative to water baths, (or “glycol heaters,” as they’re often called) is based on a technology that’s been around nearly as long. Only since 2005, however, has it been applied to large scale pipeline heating projects. Once the first few systems were proven, however, its popularity spread. There are, as of this writing, more than 120 large-scale systems used for the protection of distribution and transmission lines, compressor stations, and gas storage fields.

This “new” alternative is a flameless catalytic process patented and commercialized by Bruest Catalytic Heaters, Independence, Kansas. The catalytic process is an oxidation reduction reaction which converts natural gas into three components:  infrared energy, CO2 and water. There is no open flame, and no ethylene glycol or other chemical charge. Perhaps most notably, catalytic infrared is a direct, rather than indirect, heating method, which translates into substantially lower operating costs.

Specifically, in field operation, a catalytic pipeline heater generating infrared energy has an average heat transfer efficiency of 70%, compared to the widely-published water bath transfer efficiency of 40-50%. This 30% advantage can save thousands of dollars each year in operating costs.  

Taking the example of an application where the duty is 1,400,000 BTUH, the fuel savings with a catalytic heater, compared to a glycol water bath, is approximately $63,000 annually, based on the value of the natural gas. The device easily pays for itself during the first few years, and the fuel savings will continue to boost the owner’s profit line for many years to come.    

The key to the catalytic heater’s high heat transfer efficiency is the way in which it uses infrared energy. By surrounding the heat exchanger with catalytic infrared energy that is absorbed directly, system operation requires just two heat transfers: infrared to heat exchanger, and heat exchanger to gas. By contrast, water bath devices involve four separate heat transfers: from flame to the fire tube inside the solution; from the fire tube to the ethylene glycol; from the ethylene glycol to the tube bundle; and from the tubes to the gas.

Other factors typically used to compare the two systems include several installation-related items, ongoing operational expense, maintenance, safety, and environmental issues. For purposes of discussion, here are data based on a 700,000 BTUH duty, which compares the cost to own and operate the catalytic heater, and a water bath glycol heater sized to meet that duty (Table 1.)

 

Installation

The installation checklist begins with siting. The new catalytic heater has a small footprint, and uses less space than a water bath of equal capacity. This is a difference reflected in both real estate cost and recurring taxation expense. There is also permitting to consider. The catalytic heating system can be installed in Division I and II areas; water baths must, for safety reasons, be installed outside the classified area “halo.” The reason is simple – you can’t have an open flame near high pressure gas piping. This fact also drives up operational costs, particularly in cold weather, when heated gas must move 15 ft. or more, continuously losing heat energy in the process.    

The foundations required for installation vary as well. A 1,200-gallon water bath, filled with fluid, weighs approximately 30,000 lbs. The total weight for a comparably sized catalytic heater will be under 10,000 lbs.  

Stacks, each generally 15 to 20-ft tall, are required for the water bath. In contrast, there are no stacks for a catalytic system, and the system itself is below 11 ft in height. This is particularly important when installation is in a residential area. A containment ring is required for a water bath in virtually all instances; none is needed for a catalytic system. Finally, the water bath will require an initial 1,200-gallon chemical charge of ethylene glycol mix which costs (at late-2009 prices) just over $12,000.  

Concerns over the toxicity of ethylene glycol have persuaded some users to shift to propylene glycol. Toxicity is lower, and heat transfer is more efficient. However, propylene glycol is less effective at freeze point depression. Its greater viscosity also increases head loss in the system and accelerates pump wear. In addition, the cost of propylene glycol is even higher than ethylene glycol. And if a changeover is made, the ethylene glycol must be dumped, because the two cannot be combined. There is no chemical charge to contend with in a catalytic heating system.  

Operation

Differences in operating cost primarily stem from the minimum fuel consumption rate for each system, expressed in cu ft/hr (the maximum fuel use, in cu ft/hr, is similar.) Water baths require the constant burning of fuel to maintain those 1,200 gallons of fluid at threshold temperature, which is between 100º and 120ºF at all times, and 190ºF during system operation. The systems’ respective turn-down ratios are also in stark contrast. It is 2:1 or 4:1 for the water bath, and 8:1 or 16:1 for the catalytic system
The high turndown ratio for the catalytic technology is a function of its zoned design. Catalytic system automation allows these zones to be activated only as needed to maintain the natural gas at a set temperature. Once the desired temperature is established, the PLC adds heat when required, and turns it off otherwise. This is done automatically, without operator intervention.  

Maintenance

The maintenance involved with water bath operation primarily involves four considerations: corrosion management, burner maintenance, tube maintenance, and chemical replenishment.

Corrosion is a continuous challenge for all components in systems that use a water-based ethylene glycol or propylene glycol mix. Oxygen, heat, metallic impurities, sulfates and chlorides all promote corrosion, causing shut-downs and shortening the life of the system. Also, as glycols degrade from their exposure to heat, they produce organic acids that lower the pH of the fluid. The result is corrosion that’s more aggressive than water. In addition, high pressure gas weakens the tubes, and tube bundles that are immersed in ethylene glycol are highly vulnerable to corrosion, which worsens with the application of heat.    

Catalytic pipeline heaters do not have corrosion issues. There are no flame burners or tube bundles to maintain. Catalytic systems also have a design advantage in that, unlike water baths, they do not have corrosion-prone components, or problems with related leakage. And there are no chemicals to replenish to compensate for boil-off.  

Safety

Worker safety is an issue where water bath devices and catalytic heaters contrast sharply.  As discussed, water bath heaters use ethylene glycol, a poisonous alcohol which is designated a hazardous substance under Section 3(b) of the Federal Hazardous Substances Act. Exposure to ethylene glycol from glycol heaters (or any source) can damage the central nervous system, heart and kidneys. It can also damage red blood cells and bone marrow. Exposure to vapors can cause nausea and vomiting, pulmonary edema and central nervous system depression. Exposure to ethylene glycol in heated or mist form has produced coma. Ethylene glycol has been found in at least 34 of the 1,416 National Priorities List sites identified by the Environmental Protection Agency, according to the U.S. Agency for Toxic Substances and Disease Registry, Division of Toxicology.

Environmental

The environmental issues of most concern when companies are acquiring new process equipment generally include whether or not hazardous chemicals are used, and the potential consequences of accidental spills (particularly if there is a chance of incidental contact with drinking water). Other items of concern include whether permitting is required, and whether the process generates VOCs and/or NOx. Peripheral to these “hard issues” are others that are important, because they make headlines – and headaches – for business. An example of these is noise pollution.

To address these individually, there is always environmental risk where large quantities of ethylene glycol are used, perhaps more so in unmanned facilities, where both inadvertent and malicious chemical release is an ongoing threat. Ethylene glycol use requires permitting, and water baths release both VOCs and NOx. As for noise, there are now noise ordinances covering thousands of incorporated and unincorporated areas.