Above is an 850,000-Btu/h flameless catalytic system.

Since the mid-20th century, water bath heaters have been the most common way to heat process fluids – particularly liquid hydrocarbons – and to prevent freeze damage to critical metering and regulation equipment. Now, a perfect storm of waste stream prevention, the environmental risks that accompany hazardous chemicals, and, increasingly, corporate environmental initiatives, is causing change.

Other factors in the mix include pressures from homeowners associations regarding the odor and noise inherent in water baths, security concerns regarding open flames, and the use of acknowledged carcinogens in populated areas.

One possible alternative to water baths, known as glycol heaters, or WEG (water/ethylene glycol) systems, is based on a technology that has been around since the 1950s. But only since 2005, however, has it been widely applied to heating critical industrial pipeline products, from water, to natural gas/LNG, to the propane that is injected into diesel to make super-diesel fuels.

The flameless catalytic process was patented and commercialized by Bruest Catalytic Heaters, Independence, Kan. The key was an oxidation-reduction reaction that converts natural gas into three components: infrared energy, CO₂ and water. This produced heat with no open flame, and no ethylene glycol or other chemical charge.

Once the first few systems were proven, their popularity spread. There are, as of February, 2010, more than 120 large-scale systems in use, protecting industrial and utility pipelines, compressor stations and gas storage fields.

Perhaps most notably, unlike water baths, catalytic infrared is a direct heating method, which can translate into lower operating costs. A catalytic pipeline heater generating infrared energy has an average heat transfer efficiency of 70 percent, compared to published water bath transfer efficiencies averaging 40 to 50 percent. In an application using 1.4 million Btu/h, the fuel savings with a catalytic infrared system, compared to a glycol water bath, is approximately $63,000 annually, based on the February 2010 value of natural gas.

The key to this efficiency is in surrounding the heat exchanger with catalytic infrared energy, which is directly absorbed. The system requires just two heat transfers: infrared to heat exchanger, and heat exchanger to the fluid or gas. By contrast, water bath devices can 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 fluid or gas.

Other factors typically used to compare the two systems include installation-related items, ongoing operational expense, maintenance and safety. Table 1 shows data based on a 700,000-Btu/h system, and compares the cost to own and operate the catalytic heater versus a water bath glycol heater sized to meet that duty.

Installation

Enlarge this picture Table 1: The above data is a cost comparison of a 700,000-Btu/h system operating with a catalytic or water bath glycol heater.

A facility, by law, cannot have an open flame near an area where material or substances that come under Group D, Division 1 or 2 classifications are present. This is a huge classification group that includes hundreds of common industrial substances. Because there is no open flame in a catalytic heating system, it can be installed in Division I and II areas, rather than outside the halo. This can further cut operational costs, particularly in areas that experience cold weather, for applications where heated gas must otherwise move 15 feet or more, continuously losing heat energy in the process.

The footprint of a catalytic system can be comparatively small, though the foundations required for installation vary. 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 feet tall, are required for the water bath; there are no stacks for a catalytic system, and the system itself is under 11 feet in height. No containment ring is needed.

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. Also, its greater viscosity 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 1,200 gallons of fluid at threshold temperature, which is between 100ºF and 120ºF at idle times, and 190ºF during system operation.

The systems' respective turndown ratios are also in stark contrast, at 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 only activated as needed, to maintain the natural gas at a set temperature. Once the desired temperature is established, the programmable logic controller adds heat when required, and turns it off otherwise.

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 shutdowns 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 designated a hazardous substance under Section 3(b) of the Federal Hazardous Substances Act (its MSDS is located online at www.jtbaker.com/msds/englishhtml/E5125.htm). Exposure 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 EPA's Toxic Substances and Disease Registry, Division of Toxicology. With proper management, it is a safe substance, but its toxic properties make it a liability for any facility using it.

Environmental Issues: effluent, chemical release, air, noise

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 NO_X. Peripheral to these hard issues are those that can potentially generate headlines and headaches for business, e.g. 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 releases are potential threats. Ethylene glycol use requires permitting, and water baths (including low-NO_X units) release significant quantities of VOCs and NO_X. As for noise, there are now noise ordinances covering thousands of incorporated and unincorporated areas.

This last consideration, though not as strictly regulated, is a primary factor in recent market trends. Most Americans believe they have a right to be free of excessive or obnoxious noise pollution, and is a leading complaint in neighborhoods, according to the Census Bureau. As of February 2010, municipalities in 31 states have placed limits on noise-emitting equipment and activity. The distinctive fire tube roar of water bath heaters has often been the target of homeowner groups, activists and municipalities.

In contrast, silent catalytic heating does not require the same level of permitting, and generates virtually no VOCs or NO_X. The byproducts of the catalytic heating process are water and CO₂, which could be a potential liability in the future.

It is rare when an old technology can be replaced with one that is nearly as well established. That is happening now as many water baths have reached the point that replacement, or major overhaul is required. Notably, stimulus dollars are funding some of this replacement activity: the environmental and energy use advantages of catalytic pipeline heating has made them eligible for subsidies in some instances. PE