Don't Let Condensate Return Chemistry Bite YouBrad Buecker, process specialist, Kiewit Engineering and Design | September 10, 2015
At many industrial facilities, steam generated for use in process heat exchangers is recovered as condensate and returned to the boilers. Water recovery can provide significant savings with regard to makeup treatment, equipment size and operating costs. However, plant operators and technical personnel may not pay enough attention to condensate return quality, which can lead to severe problems in the steam generators. This article discusses some of the primary contaminants that may return with condensate, highlights the difficulties these contaminants cause and outlines control treatment methods.
Let’s start with a case history. A number of years ago, a colleague and I were asked to visit an industrial chemicals plant that produced several phenol-derivative compounds. Phenols are important organic chemicals. Steam production for the plant primarily came from four, 550 psi superheat units. Plant personnel informed us that they had to replace each of the boiler superheaters every 1.5 to 2 years due to excessive scale formation.
Upon inspecting a failed superheater in a laydown area, we observed deposits ⅛” to ¼” thick. Additional inspection showed foam issuing from the saturated steam sample lines. Subsequent examination of chemistry data supplied by the plant’s water treatment chemical vendor indicated that at times, total organic carbon (TOC) in the condensate return reached 200 parts per million (ppm). The limit for boilers of this pressure is 0.5 ppm.  The excessive organic contamination of the boiler water caused foaming in the drums, which induced solids carryover to the superheaters. Compounding the problem was that lab facilities at the plant were rudimentary, with a staff not trained in steam generation chemistry.
A variety of contaminants may enter condensate return depending upon the chemical processes at the plant. Lubricating oils are one such possibility. These compounds can coat system internals, form char on boiler waterwall tubes and cause problems similar to those just outlined. If the steam is cooled in heat exchangers with raw water on the other side, any number of contaminants can enter via tube leaks. These may include: the hardness elements, calcium and magnesium; potentially corrosive anions such as chloride and to a lesser extent sulfate; silica; large organic molecules; suspended solids; and others.
Virtually all of these impurities become more problematic at the temperatures encountered in the steam generator. Calcium will react with alkalinity to form calcium carbonate scale. Chloride in particular can concentrate underneath iron oxide deposits on boiler waterwall tubes to form acids and hydrogen gas that weaken and destroy the tube metal. At elevated pH, silica will combine with magnesium to form a silicate scale that is difficult to remove. Generally increased dissolved solids allow solutions to become more corrosive due to their ability to support the electron and ion transfer common to all corrosion mechanisms.
Moreover, water that is returned to even moderate-pressure steam generators should have impurity concentrations in the low ppm range if not the parts-per-billion range.
So what are the common techniques to accomplish condensate return cleanup? For suspended solids removal, filtration is an obvious choice and this technology has improved in recent years. The membrane techniques of micro- and ultra-filtration (MF and UF, respectively) have become popular for many applications. A common design is based on hollow-fiber membranes encased within pressure vessels.
The membranes are capable of removing even sub-micron solids. UF has become popular in the potable water industry as it will filter viruses, which are not always completely killed by chlorine treatment.
For dissolved solids removal, a potential option is ion exchange (IX). In this process, water flows through vessels containing millions of small plastic beads that contain exchange sites. If the system is set up in true demineralizer fashion, the cation resin will exchange hydrogen ions (H+) for cations such as sodium, calcium and magnesium in the water, while anion resin exchanges hydroxyl ions (OH-) for alkalinity, chloride, silica and sulfate. H+ and OH- react to form water.
An aspect that may limit or require modification to membrane and IX treatment of condensate return is temperature. Some anion resins may degrade at temperatures above 100oF (38o C). High temperatures can also damage common MF/UF membrane materials. So, heat exchange may be necessary to lower the condensate return temperature, or more exotic materials (such as ceramic membranes) may be needed.
One focus for us here is to consider removal of organic compounds, as many industrial processes utilize, modify, or produce organic chemicals, such as petroleum refining, petrochemical production, pharmaceuticals synthesis, and so forth. For removal of large organic compounds or organic molecules that have an electrical charge, activated carbon may be a suitable choice.
Granular activated carbon is prepared by heating an organic material (such as coal or even coconut shells) in the absence of air. The particles that eventually emerge from the process contain many pores, and thus the material when installed as a bed in a treatment vessel has enormous surface area. The carbon captures large organics by adsorption, and, depending upon the amount of impurities in the stream, may last for a year or longer before exhaustion. Once exhausted, the most common step is to remove the material and replace it with fresh media. Activated carbon has been a popular choice for makeup treatment over the years.
However, what about processes that may introduce small organics to condensate return such as what was outlined in the initial case history? A modified version of IX is possible in these cases.
While the resin beads resemble those utilized for IX, they contain no exchange sites. Rather, the organic compounds adsorb onto the resin. This is a similar process to activated carbon organic removal, but the resins can be formulated to adsorb much smaller organic compounds.
Another potentially attractive aspect of this technology is that the resin, upon exhaustion, can be regenerated. Hot air or even steam can be blown through the bed to dislodge the adsorbed materials. This stream then can be processed to recover the organic compounds. So, unlike activated carbon that has to be replace regularly, the resin can be used repeatedly.
Additional aspects of condensate return will be the subject of an upcoming series of articles on liquefied natural gas production. As will be discussed, in some cases, for very small and non-polar organics such as the alkanes, it may be possible to extract the compounds via mechanical deaeration.
- Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers, The American Society of Mechanical Engineers, New York, NY, 1994.
Brad Buecker is a process specialist with Kiewit Power Engineers, in Lenexa, Kan. He has 35 years of experience in, or affiliated with, the power industry, much of it in chemistry, water treatment, air quality control and results engineering positions with City Water, Light & Power in Springfield, Ill., and Kansas City Power & Light Company’s La Cygne, Kan., station. He has a B.S. in Chemistry from Iowa State University with additional course work in fluid mechanics, energy and mass transfer balances and advanced inorganic chemistry. He is a member of the ACS, AIChE, ASME, CTI, and NACE. Buecker is also a member of the ASME Research Committee on Power Plant & Environmental Chemistry and the program planning committee for the Electric Utility Chemistry Workshop.