The Need for Gas Feed
Chlorine disinfection aids flour and other milling applications
Although any grain—including rice, oats, barley, corn, millet, sorghum and wheat—can be ground into flour, most of the world’s flour is produced from wheat. During the flour milling process, water is used to temper the wheat grain and an oxidant is used to bleach the flour. One of the most prevalent oxidants used, both in a wet and dry process, is chlorine gas.
Using a wet process, the wheat is tempered once it is ready to be conditioned for milling. The tempering process consists of storing the wheat in tempering bins of chlorinated water for up to 24 hours. The chlorinated water serves to prevent mold formation in the bins and on handling equipment.
After the wheat has been cleaned using reciprocating screens, separators and countercurrent aspirators to remove impurities, the chlorinated water is added to toughen the bran and mellow the inner endosperm. This starts the wheat’s conditioning process, during which the initial breakdown of enzymes within the wheat takes place in preparation for milling. The addition of this chlorinated water makes parts of the kernels separate more cleanly and easily, limiting their shattering during the grinding process.
With a dry process, the dry flour product often is bleached to achieve refined white flour after the milling of the wheat, which consists of breaking, sifting, purifying and reduction grinding. The finished flour flows through a device that releases the chlorine bleaching/maturing agent in measured amounts. This process rapidly dupli- cates the oxidation that occurs when flour is allowed to naturally age, resulting in whitened flour with improved baking characteristics.
Gas Feed Solutions
One of the safest and most cost-effective ways to disinfect the wet tempering process water and oxidize the dry bleaching process with gas chlorine is through the use of an all-vacuum chlorine gas feed system, such as the Capital Controls Series 200. The chlorine gas feed system can be supplied in a direct cylinder-mounted, ton container or manifold-mounted vacuum regulator design, depending on the disinfection requirements of the mill.
Use of a vacuum-operated manual or built-in switchover vacuum regulator ensures a constant supply of chlorine during unattended operation. The ejector produces a vacuum through the use of a booster pump and injects the chlorine into the pressurized water main, which, in turn, provides the necessary disinfectant or oxidizing agent to the milling plant operation. A PVC plastic “tee” or multiple “tees,” with appropriate vacuum fittings, may be added to the vacuum line to allow for additional feed points—for instance, the denaturing flume(s), tempering tank(s) and bleaching operation(s). The vacuum regulators are made available with optional loss-of-chlorine alarm contacts designed to alert operational personnel to a high vacuum condition, indicating a loss of gas.
Figure 1 depicts the typical installation of direct cylinder-mounted chlorine vacuum regulators with automatic switchover capability.
Operation in the Tempering Process
To meet the disinfection requirements of the tempering process, the all-vacuum chlorine gas feed system is mounted directly to the water main via a corporation stop assembly.
Piping is connected from the water main to the booster pump suction, then from the discharge of the booster pump to the ejector. A piece of rubber hose normally is used at this point to facilitate service on the inlet side of the ejector, and the corporation stop allows for removal of the ejector for complete servicing. Polyethylene vacuum tubing connects the vacuum regulators, switchover module and remote meter panel(s) with the ejector.
Regardless of the vacuum regulator size, location or mounting, it will be equipped with its own vent connection. Vent lines should never be manifolded, yet each vacuum regulator should have its own separate independent vent to a safe area where a discharge of the chlorine gas being dispensed can be tolerated. Ideally, the vent line should be made of polyethylene tubing or PVC plastic pipe and slope downward from the vacuum regulators to provide a positive drain, preventing moisture buildup in the vent line. Each vent line should be terminated with the open end pointing downward, and an insect screen should be placed over the end of
the tubing to prevent insect intrusion.
Determining the actual chlorine feed rate in the tempering process can be calculated using the following formula: ppd = gpm x 0.012 x dosage (mg/L or ppm).
- - ppd = pounds per day chlorine feed rate
- - gpm = gallons per minute (U.S.)
- - 0.012 = constant
- - dosage mg/L = milligrams per liter
- - dosage ppm = parts per million
During the bleaching process, the system’s chlorine flowmeter is sized to allow for a range of adjustment to the chlorine feed rate. Figure 1 depicts a system schematic for a flour bleaching system. This shows that another chlorine feed point may be included to the all-vacuum system by adding a remote meter panel and ejector. The process also depicts applications in which chlorine gas is injected into the inlet of an agitator using a vacuum-producing, air-operated ejector to accomplish the bleaching process. A compressor or blower is used to produce the volume and air pressure required to produce a suitable vacuum. In this type of system, the proper amount of pressure and air flow is required to produce a stable vacuum for proper system operation.
An air filter protects the compressor or blower and keeps debris from entering the ejector’s venturi nozzle. Because air temperatures entering the ejector can measure more than 130°F, an air cooler and chlorinated polyvinyl chloride construction ejector are utilized for long service life.
In the milling process, proper disinfection of the incoming plant well water and wheat/grain in the denaturing flume, along with the tempering process and bleaching of flour to a high-grade white all are important tasks. All-vacuum chlorine gas feed systems have proven to be effective tools for the disinfection needs of the flour and other milling industries.
http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/ST-010-4300-4.jpgFigure 1. Flour Bleaching System Schematic