RE: Cryogenic Grinding
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The term “Cryogenics” originates from Greek word which means creation or production by means of cold. As prices for energy and raw materials rise and concern for the environment makes safe waste disposal difficult and Costly, resource recovery becomes a vital matter for today’s business. Cryogenic grinding technology can efficiently grind most tough materials and can also facilitate Cryogenic recycling of tough composite materials and multi component scrap. The heart of this technology is the CRYO-GRIND SYSTEM. It employs a cryogenic process to embrittle and grind materials to achieve consistent particle size for a wide range of products. The cryogenic process also has a unique capability for recycling difficult to separate composite materials.
Cryogenic grinding is a method of powdering herbs at sub-zero temperatures ranging from 0 to minus 70°F. The herbs are frozen with liquid nitrogen as they are being ground. This process does not damage or alter the chemical composition of the plant in any way. Normal grinding processes which do not use a cooling system can reach up to 200°F. These high temperatures can reduce volatile components and heat-sensitive constituents in herbs. The cryogenic grinding process starts with air-dried herbs, rather than freeze-dried herbs.
CRYOGENIC GRINDING PROCESS
Since almost all materials embrittle when exposed to cold temperatures, cryogenic size reduction utilizes the cold energy available from liquid nitrogen to cool, embrittle and inert materials prior to and or during the grinding process. All materials which due to their specific properties at ambient temperatures are elastic, have low melting points, contain volatile or oily substances, have low combustion temperatures and are sensitive to oxygen, are ideal candidates for cryogenic size reduction.
Physical properties of liquid nitrogen is produced by the separation of air into its components in an air separation plant and is distributed in vacuum insulated transport vessels to the end user where it is stored in a vacuum insulated storage vessel till it is used. At atmospheric pressure liquid nitrogen is at a temperature of –320 deg F and possesses a latent energy content of 94 BTU/LB resulting in a total cooling energy content of 179.6 BTU/LB. Nitrogen is anon-flammable, non toxic and inert gas which makes up 78.09% of the air we breathe. It has the characteristics of an inert gas, except at highly elevated temperatures, and does not form any compound under normal temperatures and pressure. Drawn from the liquid phase, nitrogen generally has a purity of 99.998 % with a dew point less than – 100 deg F and is very dry.
Cryogenic Grinding System
When using the system, measurable and repeatable results are obtained for lab or productions calculations. Mills range in size from 7-1/2 HP to 200 HP. With our cryogenic grinding unit an understanding develops with interaction of equipment components and operating parameters. Factors such as consistent feed rate, precise temperature measurement, mill operating parameters and pressure control are critical to the evaluation of cryogenic grinding and cryogenic grinding systems.
CRYOGENIC GRINDING TECHNOLOGY
For pulverizing many materials, cryogenic grinding technology increases productivity and lowers power costs. Many elastic or "soft" materials are very difficult to pulverize, requiring long cycle times and high energy consumption. This combination decreased productivity and increased costs unnecessarily. Cryogenic grinding involves cooling a material below its embrittlement temperature with a cryogenic fluid, typically liquid nitrogen or, in certain applications, carbon dioxide. After cooling, the material is fed into an impact mill where it is reduced in size primarily by brittle fracture. This process has several benefits:
• Ability to process relatively "soft" or elastic materials that cannot otherwise be ground
• Increased throughput
• Reduced power consumption
• Smaller size particles
• Minimal loss of volatile components
• Lower capital investment
Probably the greatest benefit provided by cryogenic grinding is the ability to grind "soft" or elastic materials that otherwise could not be ground, or could be ground only with long cycle times and high energy use. By embrittling the material, fine powder or crumb can be obtained easily and with a minimum expenditure of energy. Because embrittled material grinds easily, the throughput for a given mill is substantially increased and less power is used per pound of material ground.
PROBLEMS WITH CONVENTION GRINDING
Loss of etheric oil
The applied energy gets dissipated in the form of heat (>99%) and hence the temperature in the grinding zone rises to more than 90oC resulting in loss of etheric oils whose boiling point vary from 50oC to 320oC. This results in the inferior quality of the ground product.
Clogging and gumming of the mill
Spices like nutmeg, clove, cinnamon, etc.., contain high level of fat while capsicum, chilli, etc, contain high moisture content. These cause clogging and gumming of mill thus affecting the throughput and quality of the ground product. High moisture content materials often stick to the parts of the mill.
Oxidation and related degradation:
Due to intimate cyclone effect of the air in the vicinity of grinding zone, aromatic substances in materials oxidize and become rancid. In addition the formation of fresh and exposed surfaces due to grinding, accelerates the process of oxidation.
Possibility of fine grinding of difficult spices
By use of low temperatures, the raw materials become brittle which causes fibers to shatter. Thus fibrous spices like ginger can be ground easily to finer particle size. High oil content spices like nutmeg can be ground easily. CryoGrinding can be employed for grinding green spices like chilies with no pre-drying and also with the retention of its original colour.
WORKING OF CRYOGRINDING PLANT
The spice to be ground is cleaned manually and fed in to the hopper. From the outlet of the hopper the spice enters in to the vibratory feeder, which is positioned with a small inclination towards the entry of the helical screw conveyor. The vibratory feeder has a provision to control the feed rate.
The helical screw conveyor has a total length of a metre where the fed material travels horizontally to the grinding mill. The screw conveyor is driven by a 0.75 KW drive with reduction gear and inverter control. Liquid nitrogen from a storage container is sprayed into the screw conveyor. The time of stay of spice in the conveyor can be adjusted by varying the speed of the drives. A censor monitors the temperature of the ground spice and the liquid nitrogen spray is optimized using automatic feed back control.
The grinding mill is driven by a 5.5 KW, 3 – phase,50 Hz motor. The mill is connected with the motor by a flat belt between the motor pulley and the mill pulley. Inside the mill, stud disc is mounted on the shaft. The circularly projecting studs fit between the similar projecting studs from the rear of the front door. The spice to be ground gets locked between the studs of the rotating disc and studs of the stationary disc. When the mill is running, the spice gets crushed between the studs and comes out through an optional sieve as a ground product.