Patent Application
20070081573
This invention relates to a process for inhibiting the formation of biofilm and/or removing biofilm deposited on an enhanced tube of a heat exchanger in contact with an aqueous system. The process comprises feeding an oxidant selected from the group consisting of chlorine dioxide and stabilized bromine into an aqueous system such that the oxidant comes into contact with the enhanced tube.
In water treatment systems that utilize a heat exchanger, it is desirable to reduce the capital costs of the heat exchanger by reducing the physical size of the heat exchanger, while maintaining the same heat transfer area. In order to accomplish this objective, some or all of the “smooth tubes”, traditionally used in heat exchangers, are replaced by “enhanced tubes”.
When water, or some other liquid, is used as the coolant, it circulates inside the enhanced tube. In these systems, the enhancements are found inside the tube. The internal enhancements are typically helical flutes or fins, which increase the surface area of the heat exchanger, and thus promote more efficient heat exchange. By using the enhanced tubes to increase the surface area of the heat exchanger, it is possible to decrease the size of the heat exchanger.
The problem with using heat exchangers that have enhanced tubes is that the enhancements promote the precipitation of solids from the aqueous stream in contact with the enhanced tubes, which provides an ideal environment for the growth of biofilm. Thus, the enhancements may become fouled with a biofilm rather quickly and to such an extent that most or all of the benefits of associated with using the enhanced tubes are neutralized.
Biofouling is an extremely complex phenomenon in open recycling cooling systems. Biofouling of heat transfer equipment occurs when microorganisms attach to the surface of heat transfer equipment. Biofouling results in the deposit of a “biofilm” which is comprised of any or all of the following classes of materials: biomass, scale, corrosion products, insoluble materials (silt, etc.), and/or water formed sludge. Factors that influence the biofouling of enhanced tubes are well-known from the literature related to heat exchangers containing enhanced tubes. Numerous attempts have been made to adapt standard treatment chemistries for use with heat exchangers containing enhanced tubes to eliminate or reduce biofouling, but unfortunately results have been inconsistent. Therefore, there is a need for a process, which inhibits the formation of biofilm on an enhanced tube and/or removes biofilm deposited on an enhanced tube.
All citations referred to in this application are expressly incorporated by reference.
This invention relates to a process for inhibiting the formation of biofilm and/or removing biofilm deposited on an enhanced tube of a heat exchanger in contact with an aqueous system. The process comprises:
feeding an effective amount of an oxidant selected from the group consisting of chlorine dioxide and stabilized bromine into said aqueous system in contact with a heat exchanger containing one or more enhanced tubes, such that the oxidant inhibits the formation of biofilm on the enhanced tube and/or removes biofilm deposited on the enhanced tube.
It was surprising that the use of chlorine dioxide or stabilized bromine as the oxidant inhibits the formation of biofilm on the enhanced tube and/or removes biofilm deposited on the enhanced tube. Experimental tests indicate that the use of other oxidants was not as effective as chlorine dioxide or stabilized bromine. It was also surprising that chlorine dioxide or stabilized bromine could be used effectively without using a biodispersant. And it was surprising that these effects were more pronounced on enhanced tubes than on smooth tubes.
Based on the data generated from experiments, it is evident that the described process is a non-corrosive process, which effectively removes biological and biologically entrapped foulants from enhanced tubes without jeopardizing the integrity of the tubes. Chlorine dioxide or stabilized bromine effectively attacks the root cause of deposit formation on the surface of heat transfer equipment, which is biofouling. When deposits caused by biofouling are released, other foulants are also released from the surface of the heat transfer equipment.
Experimental data showed that chlorine dioxide was the most chemically effective and cost effective oxidant, because bifouling could be inhibited and/or reduced more effectively when chlorine dioxide was used as the oxidant without a dispersant. Concentration rates and treatment times could be reduced when chlorine dioxide was used as the oxidant.
An enhanced tube is a tube used in a heat exchanger that increases the surface area of the heat exchanger when compared to the use of a similar tube, which is smooth, in the heat exchanger. Typically, the internal enhancements to the enhanced tube are grooves, ribs, fins, and the like, which typically form helical flutes and the like.
Although the heat exchanger containing one or more enhanced tubes may be found in a variety of places through which water flows and is treated, e.g. open recirculating cooling water systems with cooling towers, once-through cooling water systems and closed cooling water systems. Heat exchangers that may employ enhanced tubes include, but are not limited to chillers and condensers on refrigeration machines, compressor intercoolers and aftercoolers, and various chemical, petrochemical and hydrocarbon process streams. Typically, the tubes in any individual heat exchanger will be of the same type, either smooth or enhanced, but there is no design limitation to prevent the mixture of tube types in the same heat exchanger. However, in any given cooling water circuit, there may be multiple heat exchangers some of which have enhanced tubes, while others have smooth tubes.
Chlorine dioxide or stabilized bromine is used as the oxidant. Stabilized bromine is hypobromite stabilized by the addition of a halogen stabilizer such as a sulfamate.
Although the chlorine dioxide can be generated by any of the methods known in the art, it is typically generated on site by reacting sodium chlorite with gaseous chlorine or acidified sodium hypochlorite as follows:
(1) Gaseous chlorine Generation
2NaClO2+Cl2→2Cl O2+2NaCl
(2) Acidified sodium hypochlorite Generation
2NaClO2+NaOCl+2HCl→2ClO2+3NaCl+H2O
Specific practices for generating chlorine dioxide according to these methods are well known in the art, including the electrolytic/membrane process. Examples of generation equipment suitable for use in the process are described in U.S. Pat. Nos. 4,013,761 and 4,147,115. Various means may be used to control the delivery of sodium chlorite and other chemicals to the generator. Such means include, but are not limited to, variable rate pumps, valves, eductors and metering devices. The precursor chemicals are typically pumped or educted into motive or dilution water. Then they are mixed in a mixing chamber or sent directly to the reaction chamber of the chlorine dioxide generator. Mixing the precursor chemicals first provides better yields of chlorine dioxide.
The chlorine dioxide or stabilized bromine may be added to the aqueous system either continuously or in batch feed over one or more hours. Typically batch feed is used. The residual amount of chlorine dioxide or stabilized bromine present in the aqueous system containing the heat exchanger with enhanced tubes is typically from 0.05 ppm to 1.0 ppm, preferably from 0.10 ppm to 0.25 ppm depending on the level of microbial contamination in the aqueous stream.
Biodispersants may also be fed to the aqueous system in order to eliminate or reduce the deposition of biofilm on the enhanced tubes of the heat exchanger. Examples of biodispersants include terpenes, nonionic esters, nonionic ethoxylates, amide-based dispersants, as well as any nonionic, cationic or anionic surfactant. They are all efficacious, however the active concentration and contact time varies between these chemistries.
Although the pH of the aqueous system treated may vary over wide ranges, the pH of the aqueous system typically ranges from 6.0 to 9.5, more typically from 6.6 to 9.2.
Although the oxidation-reduction potential (ORP) of the aqueous system treated may vary over wide ranges, the ORP of the aqueous system typically ranges from −100 to +250 without the addition of any oxidant, during the feed of chlorine dioxide, the ORP of the aqueous system will be raised to a range of from +350 to +650, typically to a range of +400 to +500. The ORP is typically measured with an ORP monitor such as those supplied by Rosemont, Honeywell, Foxboro, etc.
The pH, ORP, and other parameters of the system can be monitored, preferably continuously and automatically, by methods well-known in the water treatment industry. It is particularly useful to control the feed and feedrate of chlorine dioxide by systems comprising ORP technology analyzers, sensors, and a Master Control Unit. See, for example, U.S. Pat. No. 5,227,306, which describes how to proportionally feed ClO2 to an aqueous system. The total control system consists of a Master Control Unit, which is a PLC (programmable logic controller)-based system comprising control logic and an operator interface. Any PLC and operator interface-based system can be used.
Abbreviations and Definitions:
Dispersant A ENV IROPLUS® 8100, a biodegradable terpene dispersant, sold by Drew Industrial of Ashland Inc.
Dispersant B DREWSPERSE® 739, a nonionic ester dispersant, sold by Drew Industrial of Ashland Inc.
Dispersant C DREWSPERSE® 1930, an amide-based dispersant, sold by Drew Industrial of Ashland Inc.
Dispersant D DREWSPE RSE® 738, a nonionic ethoxylated dispersant, sold by Drew Industrial of Ashland Inc.
Stabilized bromine STABROM 909, a hypobromite stabilized by the addition of a halogen stabilizer such as a sulfamate, sold by Albemarle.
The Examples will illustrate specific embodiments of the invention. These Examples are not intended to cover all possible embodiments of the invention, and those skilled in the art will understand that many variations are possible without departing from the scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. In this application all units are in the metric system and all amounts in milligrams per liter, unless otherwise expressly indicated.
In order to test the effect of various biodispersants and/or oxidants on the formation of biofilm on an enhanced tube, an automated, laboratory scale cooling tower system comprising the following components was constructed: (a) a circulating pump, (b) a small water-to-water heat exchanger, which is used to supply a heat load to allow the tower water to concentrate, and (d) a modified, laboratory version of Ashland's online P.U.L.S.E. analyzer cooling water monitor1, containing one enhanced tube2 and one smooth tube.
The operating parameters of the cooling water system and the make-up water tested are set forth in Table I. During these tests, a portion of the circulating water was bypassed to the tower basin to maintain the cold water temperature (temperature drop across the cooling tower with the limited heat input). The concentration of circulating water was maintained by conductivity control. In addition, fouling factor, pH, and ORP were constantly monitored.
1 The modified version of the P-U-L-S-E® fouling monitor (see U.S. Pat. Nos. 4,339,945, Re. 33,346, 4,346,587, and Re. 33,468) was designed to more effectively measure the biofouling of enhanced tubes found in a heat exchanger in contact with an aqueous system. The fouling tendency of the fluid on the enhanced tube is evaluated by the passage of water from the aqueous system tested through the heat transfer test assembly under controlled rates of flow and heat output from the heating element. A side stream from the aqueous stream tested, which is in contact with the enhanced tube, is diverted to the modified heat transfer test assembly in order to measure the biofilm on the enhanced tube in the contact with the aqueous system tested. The significant modification of the P-U-L-S-E® fouling monitor involved replacing the smooth tube the P-U-L-S-E® fouling monitor with an enhanced tube. Measurement of temperatur drops between the tube in the test assembly and the aqueous system tested are correlated with the formation and reduction of fouling. The modified P-U-L-S-E® fouling in monitor was designed to emulate the same tube-side flow conditions.
2 The enhanced tube with externally enhanced with helical flutes.
Control
(Preparing a Contaminated Enhanced Tube)
In order to determine the effect of various biodispersants and/or oxidizing agents on the formation of biofouling deposits on the smooth tubes and enhanced tubes having flutes (grooves between adjacent ribs). The enhanced tube was the controlling test. That is, since the two tubes were running in parallel in the same test rig, the runs started and stopped at the same time and were exposed to the same cooling water. The test heat exchanger was exposed to cooling water, which was inoculated with pseudomonas aerigenosa (ATCC 27853), a known slime forming aerobic bacterium, in order to contaminate the enhanced tube with biofilm.
Nutrient broth was added to the tower daily at a dose of 50 mg/L. Chemistry and data associated with this test run are provided in Table II. Although biofouling was observable by the naked eye by the seventh day into the run, it was actually detected as an increasing trend on the third and fifth day by the enhanced tube. The enhanced rod fouled at a linear rate of 0.439 m2-°K/Watt-sec. The tests were run until the enhanced rod flutes (grooves between adjacent ribs) were filled with foulants and there was a significant trend differentiating the enhanced tube from the smooth tube. All of the runs lasted 20 to 36 days, which is within the typical range (14 to 45 days) for biofilm grow-up on surfaces and P-U-L-S-E fouling runs.
Aerobic Bacteria added - Pseudomonas aerigenosa - 4.55 × 106 CFU/mL
Difco Nutrient Broth (37.5% Beef Extract + 62.5% Peptone) - 50 mg/L/day
Tube velocity, m/sec - 1.6-1.68
Skin Temperature ° C. - 37.8
(Use of Dispersant A to Treat the Aqueous System)
In this example, Dispersant A was used to treat the aqueous system to determine its effect on reducing the deposit of biofilm on a fouled enhanced tube and smooth tube. The water chemistry used for the experimental runs is listed in Table III.
Note:
Aerobic Bacteria, nutrient broth, tube velocity and skin temperature data remain the same as shown in Table I
After the contaminating the water with aerobic bacteria, Dispersant A was slug fed at 150 ppm for 12 days.
(Use of bromine as Oxidant)
In this example, bromine was used as the oxidant, and its effect on reducing the deposit of biofilm on a fouled enhanced tube and smooth tube was investigated. The water chemistry used for the experimental runs is listed in Table III.
Note:
Aerobic Bacteria, nutrient broth, tube velocity and skin temperature data remain The same as show in Table I
After the contaminating the water with aerobic bacteria, bromine alone was slug fed for 4 hours per day to +550 mV ORP for nine days.
(Effect of using bromine with different biodispersants on reducing the deposit of biofilm on a fouled enhanced tube and smooth tube)
In this example, the use bromine as the oxidant with different biodispersants and their effects on reducing the deposit of biofilm on a fouled enhanced tube and smooth tube were investigated. The water chemistry used for these experimental runs is also listed in Table III.
After the contaminating the water with aerobic bacteria, bromine oxidant was slug fed 4 hours per day to maintain +475 to +550 mV ORP. Then Dispersants A, B, C and D were separately slug fed into the aqueous system in the amounts set forth in Table IV.
(Use of chlorine as Oxidant)
In this example, chlorine was used as the oxidant, and its effect on reducing the deposit of biofilm on a fouled enhanced tube and smooth tube was investigated. The water chemistry used for the experimental runs is listed in Table III.
After the contaminating the water with aerobic bacteria, biodispersant A was slug fed into the cooling tower at 150 ppm daily. After 12 days, both the enhanced and smooth rods continued to foul, though at a slower rate.
Beginning on the 13th day, chorine was slugged over a 4 hour period/day to maintain an ORP of +450 mV. The fouling factors on both tubes dropped as illustrated in
At this point, (
For the final 25 days, Dispersant A was again slug fed at 150 mg/l/day along with the daily slug of chlorine. Clean-up rates of the remaining biofilm were 82% for the enhanced rod.
(Use of Stabilized bromine as Oxidant)
In this example, stabilized bromine was used as the oxidant, and its effect on reducing the deposit of biofilm on a fouled enhanced tube and smooth tube was investigated. The water chemistry used for the experimental runs is listed in Table III.
After the contaminating the water with aerobic bacteria, stabilized bromine was slug fed for 4 hours per day without a dispersant. Initially, the oxidant slug feed was performed to maintain +325 mV ORP, which did not halt biofilm growth on the heat transfer rod (
The data again confirm that the enhanced tube fouls more rapidly than the smooth tube. The data also indicate that use of stabilized bromine alone is effective at inhibiting and/or removing biofilm, particularly if the ORP of the oxidizing aqueous system is at least about 425 mV. Thus, it is possible to effectively inhibit and/or remove biofilm from the enhanced tube of the test apparatus using stabilized bromine alone without a biodispersant.
(Use of chlorine dioxide as Oxidant)
In this example, chlorine oxide was used as the oxidant, and its effect on reducing the deposit of biofilm on a fouled enhanced tube and smooth tube was investigated. The water chemistry used for the experimental runs is listed in Table III.
After the contaminating the water with aerobic bacteria, chlorine dioxide was slug fed for 4 hours/day to maintain an ORP of 400 mV.
The results of the test are shown in
Summary of Test Results
Simulation of cooling waters under fouling conditions was achieved with the dynamic pilot system in the laboratory. Experimental investigations provided information regarding the fouling behavior of enhanced tubes compared to smooth bore tubes, the effect of using different oxidants, dispersants, and combinations thereof. The following observations were noted:
This application claims the benefit of U.S. Provisional Application Ser. No. 60/666,750 filed on Mar. 31, 2005, entitled Heat Transfer Test Assembly for an Apparatus for Monitoring Fouling of Aqueous Systems, the contents of which are hereby incorporated into this application.
| Number | Date | Country | |
|---|---|---|---|
| 60666750 | Mar 2005 | US |
