Neutralizing Aminesin Boiler Steam and Humidified Indoor Air

1.1. Humidification Methods

Air humidity has an essential impact on health and productivity. The correct humidity level in the respiratory air prevents the mucus membrane from drying out and reduces the risk posed by microorganisms and the occurrence of specific symptoms of illness. The right amount of humidity in indoor air provides comfort to the body, improves health, and increases the durability of materials [1].

There are mainly two ways of humidifying indoor air: adiabatic and isothermal. Adiabatic humidification injects fine water droplets into the air, and the heat of the surrounding air causes the water to evaporate. There is no contribution of external thermal energy. Isothermal humidification, however, requires an external energy source to change the state of water from liquid to vapor [2]. Direct injection of steam from a central boiler plant is commonly used in many commercial and industrial facilities as a means of isothermal humidification [3], which is also adopted by the Central Utility Plant (CUP) at the National Institutes of Health (NIH).

1.2. Clean Steam and Utility Steam

Steam is produced in various ways depending on the application. ‘Clean’ steam is used in thepharmaceutical and health care industries where sterilization iscritical. It is also needed inclean rooms in pharmaceuticalmanufacturing plants forthe manufacture of sterile compounds or ingestible medications. A secondary generator produces ‘clean steam’ using the heat from the central plant steam line.By using ‘clean’steam, manufacturers know there willbe no boiler additives, volatiles, andparticulates that could taint, blight,or contaminate the production process. In addition, ‘clean’ steam is often used notonly to remove contaminants, but alsoto ensure the quality control of criticalattributes such as dryness, superheat,and production of non-condensablegases, all of which could adverselyaffect the process and equipment. However, it is costly to use ‘clean’ steam due to prior purification of water and high quality non-corrosive conduit components that are very expensive [3].

Direct steam injection is a type of humidification method which applies the utility steam produced by the central plant steam systems. A steam/condensate system issubject to corrosion due to the carbondioxide (CO₂) presence.‘Utility’ steam from a conventionalboiler contains anti-corrosion chemicals (amines) that help prevent equipment failure, increase reliability, and distribution system life [3]. The amines injected in the steam usedfor humidification are carried viahumidifiers into room air, where theyare inhaled andingested via hand-to-mouth contactof surface deposits. These amines havebeen implicated, mostly based onanecdotal evidence, as the causative agents for adverse health effects such as eye, upper respiratory, and skin irritations on humans and animals. The NIH vivarium applies direct injection of steam produced by the Central Utility Plant. Therefore, it is necessary to monitor the amine concentration in the humidified air for the safety of research staff and animals.

1.3. Source of CO₂ in Steam

When the generated steam leaves the boiler through piping to transfer heat to a heat exchanger or steam jacket, it condenses in the pipes, which tends to run to the bottom of the pipe and is carried along with the steam flow. In many industrial plants, this condensate may gather numerous impurities in the process, such as suspended solids, oil and grease, and dissolved mineral salts. A very common impurity is carbon dioxide that may produce from the breaking down of bicarbonates/carbonates in the boiler, and it can convert to the carbonic acid in the condensate and lower the condensate pH. Typically, condensate pH is monitored by measuring the pH of the condensed steam at different locations of the steam distribution system, such as immediately after steam generation before amine injection, after amine injection, and at different other locations of the steam distribution system. The amount of CO₂ that dissolves depends upon the percent of the steam that condenses and the degree of subcooling. The addition of 1 ppm CO₂ to a system of pH 7 will change the pH to 5.49. If left untreated, acidic condensate corrodes condensate return piping, pipe threads, nipples, and elbows, which will eventually lead to leaks, equipment deterioration and failure [4].

1.4. CO₂ Corrosion on Steel Piping

Carbon dioxide released into the steam due to thermal decomposition of boiler water alkalinity will eventually dissolve in the steam condensate. The dissolved CO₂ in the condensate is highly corrosive and can cause significant damage to steel piping. The following reaction would occur between CO₂ and condensate.

(1)

Carbonic acid would form as a product. As a result, the pH of condensate would be depressed. Carbonic acid would react with iron and releases hydrogen.

(2)

(3)

Consequently, metal loss would occur. The corrosion process usually happens at the bottom of the pipe, and steam traps. The corrosion is influenced by the temperature and pH. Studies showed that the thickness of the iron corrosion product layer decreases with the increasing temperature. The corrosion product also varies in several forms, with iron carbonate as the main product followed by limited amounts of Fe₃C or Fe₃O₄ at higher or lower temperature [5]. Further study showed that the corrosion rate of carbon steel is the highest at pH of 4.0 [6].

1.5. Properties of Neutralizing Amines

Condensate systems are chemically treated to reduce metal corrosion. Generally, a combination of neutralizing amines, and oxygen scavenger-metal passivators are used for this purpose.It takes one mole of amine to neutralize one mole of carbonic acid. When feeding the amine to the feed water system, it should be fed downstream of the deaerating equipment. The neutralizing amine is volatile and carried out with the steam in the condensate to react with carbon dioxide. Three factors are considered for amine blending, namely basicity (signifies by base hydrolysis constant, K_b), neutralizing capacity, and distribution ratio (DR, i.e., [concentration in vapor phase]/[concentration in liquid phase]). The greater the value of K_b, the more basic is the amine. The neutralizing capacity of amine is inversely proportional to the molecular weight of amine, and positively correlated to the number of amine groups in the molecule. DR indicates if an amine has a short run, medium run, or long run distribution.Low DR amines will concentrate in the condensate providing good tank protection,but its concentration will diminish as it travels down the steam line. A high DR amine will remain in the steam and travel further down the system and provide better protection at distant locations [7].

The four most common neutralizing amines are ammonia, cyclohexylamine (CHA), diethylaminoethanol (DEAE), and morpholine (MORPH). Fig. (1a-b) shows graphic comparisons of the above properties for some common amines. Fig. (1b) also includes the distribution ratio of CO₂ at 10 psig and 150 psig within the pH range of 7.5-9.5. It provides a relative vapor/liquid (V/L) distribution ratio comparison among different amines and CO₂. It also indicates that CO₂ gas spreads easily throughout the steam system, forming carbonic acid almost everywhere. As the steam condenses back to its liquid phase, the high DR amines traveling further down the system also return to their aqueous phase, neutralize the CO₂ in condensate and prevent corrosion [8-11].

1.6. Typical Amine Control Limits

The Code of Federal Regulations is shown in Table 1 [12-15]. The Federal Agencies regulating the amine concentration includes Food and Drug Administration (FDA), Occupational Safety and Health Administration (OSHA), American Conference of Governmental Industrial Hygienists (ACGIH), and National Institutes for Occupational Safety and Health (NIOSH). The FDA has approved MORPH,CHA, and DEAE for use in steam generation systems. The CUPfollows FDA control limits for DEAE(15 ppm) and CHA (10 ppm)under FDA 21 CFR 173.310, Title 21 CFR 184.1139 [3].

1.7. National Institutes of Health Central Utilities PlantCyclohexylamine Inline Measurement

The CUP of NIH currently injects a 50/50 neutralizing amine blend of CHA and DEAE. The CUP adds amines at twoseparate locations in-house at the steam distribution line after generation. The CUP has installed a neutralizing amine sensor in building 10CRC (Fig. 2) that reads specifically CHA concentration in parts per million (ppm). The Pyxis CHA analyzer is an online device that uses a reagent-based fluorescence technique to measure the CHA concentration in a steam condensate sample. The reagent reacts with CHA to form a fluorescent substance. The fluorescent substance formed is measured by an online fluorometer.The panel requires access to electrical service (110 VAC), cooling water, and a steam condensate sample line [16]. The CUP monitors the level of CHA after condensing the steam at building 10CRC of the steam distribution system. Fig. (2a) shows the image of the analyzer installed at one of the facility building. The analyzer takes a sample of the condensed steam and have the sample react with reagent to generate fluorescence readings. The readings are recorded by the analyzer and monitored by the engineers at the central utility plant. The analyzer dimensions and layout are shown in Fig. (2b). Fig. (2c) shows how the sample flow was controlled in the analyzer. The analyzer also has a sample port for taking steam condensate samples manually.

1.8. Background and Objectives of the Study

The NIH has 11 buildings encompassing 300,000 square feet of animal housing and laboratories space at the Bethesda Campus. Approximately 100,000 animals, including rodents, rabbits, primates, carnivores and ungulates are managed by the animal facility personnel on campus. Humidity control is required in all animal facilities for the well-being of laboratory animals. The direct injection of steam from a central boiler plant is the most economical humidification method. However, the health of facility personnel is a primary concern for using utility steam for humidification purposes. The focus of this study is to address the concern by measuring the concentration of CHA and DEAE and show that the concentrations of the aforementioned corrosion inhibitor chemicals are well below the FDA limit to cause any human health effects. To the best of the authors’ knowledge, the study performed in this paperhas not been done elsewhere in the recent decades and would be of general interest to the scientific community.