Closed Water Circuit

Biocides are added to the wet end process to prevent slime formation. Introduction of neutral or alkaline sizing instead of acidic papermaking, the closed water circuits, and the increasing proportions of recycled paper have required changes in biocide types in order to control different microbial populations. 

Recent environmental regulations which essentially require closed water circuits make the problems of mineral fines buildup especially severe. Similar difficulties are associated with the disposal of refuse fines. 

It is easy to find everyday objects that show signs of corrosion. A used automobile consists essentially of different metals, plastics, paints, metallic coatings, and so forth, which have been exposed to a variety of aggressive conditions. These materials are exposed to the action of atmospheric agents under conditions of high temperature, are incorporated in closed water circuits, or are subject to mechanical wear. As a result, we see many signs of corrosion in cars, chiefly on the bodywork and in the exhaust system, and especially after years of use. 

For a closed water circuit, the condensation of water at 313 K from both fuel-ceU off-gases is necessary. The catalytic burner is therefore fed with a fuel-air mixture at 313 K. This temperature is fairly low for ignition of the combustible species in the fuel-gas mixture. 

Dazomet s spectrum of activity is attractive for application of the compound in a number of industrial systems, e.g. as a slimicide in closed water circuits, paper machine systems, as a broad spectrum microbicide which prevents fungal blooms in metalworking fluid systems, or is suitable for the wet state preservation of a wide range of formulations, products for which the incorporation of a powder microbicide is necessary included. Compatibility with anionic, cationic and non-ionic surfactants increases formulation opportunities and improves effectiveness. However, there are limitations poor water solubility, instability, release of H2S and accordingly coloration by reaction with heavy metal salts. 

Most of the wastewater from German pulp and paper mills is treated biologically, either in municipal treatment plants (18% of production volume) or in in-mill plants (74% of production volume). 4% of the annual paper volume is produced in mills with a totally closed water circuit which means that these mills are absolutely effluent free [2]. 

Fig. 10.4 Scheme of a closed water circuit [6]. (a) Paper machine, (b) stock preparation plant, (c) storage, (d) save-all, (e) fresh water,... 

No open or closed water circuit is homogeneous in terms of materials, i.e. contains only components in aluminium or in stainless steel or cuprous alloys. Materials for the components of a circuit are selected according to feasibility, cost and availability criteria. For radiators of a central heating installation in aluminium alloy, the boiler may be in stainless steel, the pumps in stainless steel or cuprous alloys, and the piping in ordinary steel or copper. The same applies to water distribution circuits. 

Closed-Mill Concept. The closed-mill concept, or water circuit closure, has been studied by the pulp and paper industry for many years. In some parts of the paper manufacturing process, up to 98% of the water is recycled within the process, eg, the wet end of the paper machine. However, in the pulp mill, especiaUy kraft mills, effluents are produced owing to the need to purge from the system various metals that come in with the wood, as weU as organic by-products from the pulping process, additives, and especially chloride ions that originate in the bleach plant. 

Closed Air Circuit Water Cooled (CACW) The motor s interior hot air forms one part of the closed air circuit that is circulated by the motor s internal fans. A separate heat exchanger is mounted on top of the motor as the cooling water circuit. This forms the second cooling circuit. 

Closed Air Circuit Air Cooled fCACA) Thiscooling system is the same as for CACW except that, instead of water, air flows through the top-mounted heat exchangers. See Figures 1.21 and 1.22. 

A pressurizer operating within the range of approximately 2,450 to 2,750 psig maintains pressure within the closed-loop circuit. The pressurizer contains the cold spray water system, which is typically supplied at around 550 °F (288 °C), and the electric heater-bank system. These systems maintain and control primary-coolant water pressure and temperature. The high-temperature water produced in the primary... 

Although close attention to all aspects of the chemistry of the various water and steam-water circuits and the impurities they contain is clearly vital, control is more than simple analysis. And apart from the smallest HW boilers (and LP steam boilers with low MU requirements), control of water chemistry typically requires that some form of external conditioning is required, not merely internal treatment. 

Polysilicates, incorporated in various formulations that may include P-P04 or molybdate and phosphonates/TTA/polymer, have found application for smaller, light duty, open cooling systems and closed-loop circuits, especially where naturally soft or ion-exchange-softened makeup waters have been used. The reported effectiveness of such programs has been mixed. 

There is a wide range of metals used in the construction of closed-loop circuits and the often considerable differences in the nature of the circulating water (ranging from deionized water to glycol mixtures to brine water). As a consequence, it is not possible to devise a single water treatment formulation that satisfies the requirements of all closed-loop circuits and their operators. 

Rich solvent from the bottom of the EDC is routed to the solvent-recovery column (SRC), where the aromatics are stripped overhead. Stripping steam from a closed-loop water circuit facilitates hydrocarbon stripping. The SRC operates under vacuum to reduce the boiling point at the column base. 

Because ozone is only slightly soluble in water and is unstable in nature, readily decomposing into ordinary oxygen, only a portion of that applied is actually utilized in the sterilization process. Part of the ozone is not absorbed by the water and passes off the top of the contact columns. For instance, in routine treatment at Saint-Maur 0.5 gram of ozone per kilogram of air is present at the column outlet in winter and 1 gram per kg. in summer. This is 20 to 40% of the amount applied thus, in 1 day up to 160 kg. of ozone might be dispersed into the atmosphere around the plant if a closed air circuit were not used. 

The SMBR plant is equipped with several sensors the temperature of the columns and the incoming streams are kept at around 60 °C by a closed heating water circuit that is controlled by a thermostat. Concentrations in the product line and in the recycling loop are measured online using a combination of a density measurement unit and a polarimeter (Jupke et ah, 2002). The pressure in the recycling loop (i.e. at the inlet of the recycling pump) is maintained constant using a P-controller that varies the extract flow rate. 

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