cryogenic grinding

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CRYOGENIC GRINDING

Presented by:NINGTHOUJAM MANDA DEVI

M.Tech ( 2nd Year )Reg. no: J4-01245-2013

Processing and Food Engineering Department

CAET, Junagadh


Extension

What is Grinding

Grinding is a process of reducing the size of solid materials by mechanical action, dividing them into smaller particles.

Perhaps the most extensive application of grinding in the food industry is in the milling of grains to make flour, but it is used in many other processes.

The grinding of corn for manufacture of corn starch, the grinding of sugar and the milling of dried foods, such as vegetables


Grinding process

Grinding size reduction is achieved by fracturing the materials

The material is stressed by the action of mechanical moving parts

Time also plays a part in the fracturing process and it appears that material will fracture at lower stress concentrations if these can be maintained for longer periods of time.

The energy required depends upon the hardness of the material and also upon the tendency of the material to crack - its friability


Size reduction equipment can be divided into two classes –• Crushers : The major action is compressive, breaking large pieces of

solid material into small lumps. • Grinders: Grinders combine shear and impact with compressive forces,

reduce crushed feed to powder form.

Force for reduce the size of fooda)Compression forcesb)Impact forcesc)Shearing(or attrition) forces

-Both the magnitude of the force and the time of application affect the extent of grinding achieved.- For efficient grinding, the energy applied to the material should exceed, by as small a margin as possible, the minimum energy needed to rupture the material.

GRINDING EQUIPMENT


Fig. 1: Jaw crusher Fig. 2: Gyratory crusher

Crushing head

Funnel shaped casing

Stationary jaw

reciprocating jaw


Fig.3 : Hammer mill Fig. 4 : Plate mill

Circular plates

Axis of rotation


Fig. 5: Roller mill

Both Rollers rotates at same speed- compression is the primary force

If at different speeds- shearing and compression are the primary forces

If the rolls are grooved, a tearing or grinding component is introduced

Coarse grooves provide less size reduction than fine grooves do


Fig.6: Ball mill


PROBLEMS FACED IN CONVENTIONAL GRINDING PROCESS

High heat generation.

Introduction of tensile residual stress.

Less tool life.

Oxidation

Clogging and gumming of the mill

Loss of enteric oil in spice grinding.


CRYOGENIC GRINDING


INTRODUCTIONThe word “ CRYOGENICS’ originates from the Greek word

“cryo”, which means creation (or) production by means of cold.

It deals with low temperatures as low as below −150 °C or 123 K to absolute zero.

Cryogenics is the study of low temperatures and behavior of materials under these low temperatures.

A person who studies elements that have been subjected to extremely cold temperatures is called a CRYOGENICIST.


Cryogens

The extremely low temperature are produced by using substances called “ cryogens” such as liquid nitrogen and liquid helium.

All cryogenic liquids are gases at normal temperatures and pressures. So, these gases must be cooled below room temperature to liquefy them.

They have boiling points below -150°C. (Carbon dioxide and nitrous oxide, which have slightly higher boiling points are sometimes included in this category).


There are several cryogenic liquids such as Nitrogen, helium, neon, argon, krypton, hydrogen, methane and liquefied natural gas etc..

Liquid Nitrogen are the most commonly used.

Cryogens are stored in vessels called as Dewar flask which provides good insulation

Different cryogens become liquids under different conditions of temperature and pressure, but all have two properties in common: they are extremely cold, and small amounts of liquid can expand into very large volumes of gas

Cryogens


Liquid NitrogenNitrogen gas makes up the major portion of the atmosphere

(78.03% by volume). Inert, colourless, odourless, non-corrosive, non-flammable liquid Extremely cold (boiling point -195.8°C, 77°K). Similar appearance to water. Volume of expansion liquid to gas (at 15°C, 1 atm.) is 682 , i.e. 1

litre of liquid nitrogen evaporated to form 682 litres of nitrogen gas.


The Dewar flask

Fig. 7: Dewar flask


Sir James Dewar (20 September 1842 – 27 March 1923) was a Scottish chemist and physicist.

Invention of the Dewar flask

The man who first liquefied hydrogen.

Sir James Dewar


CRYOGENIC GRINDING

Also known as freezer milling/ freezer grinding / cryomilling.

is the act of cooling/chilling a material and then reducing it to smaller particle size

Also a method of powdering herbs at sub-zero temperatures ranging from 0 to -70°F

All materials embrittle when exposed to low temperature

Utilizes the cooling effect of liquid nitrogen to embrittle materials prior to and or during the grinding process


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.

But cryogenic grinding process does not damage or alter the chemical composition of the plant in any way.

Materials which are elastic in nature, having low melting points, low combustion temperatures , sensitive to oxygen can be ideally machined by cryogenic grinding process

CRYOGENIC GRINDING


GENERAL APPLICATIONS OF CRYOGENIC GRINDING

Cryogrinding of steel:

The large amount of heat is generated during machining/grinding at high speed and feed rate raises the temperature at the cutting zones excessively.to overcome this problem liquid nitrogen is fed to the grinding spot.

Thermoplastics and thermo sets:

To which nylon, PVC, polyethylene, synthetic rubber are commonly used in powder form, but not limited to, a variety of applications such as adhesives, powdered coatings, fillers and plastic sintering and moulding.


Adhesives and waxes;

To avoid the pliable and sticky of certain materials which is unable in conventional grinding

Explosives;

To grind the explosives materials below their ignition temperature

Spices;

To overcome the volatility of etheric oils ( gives the taste and smell of spices)


Spices are the most important constituents of Indian food, not only in household purposes but also in hotels, restaurants, food processing industries etc.

Spices like Pepper, cinnamon, chilly, Ginger, Cumin seed, Nutmeg, clove etc., have a characteristic taste and aroma and medicinal values.

These characteristic qualities are essential in them to have their value as ‘spice’.

These qualities exist in them due to the presence of etheric oils within it. (boiling points ranging down to 50oC)

SPICES


PROBLEMS WITH CONVENTIONAL GRINDING OF SPICES

1. Loss of etheric oilThe 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 ranges down to 50oC. This results in the inferior quality of the ground product.

2. 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.


3. 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.

PROBLEMS WITH CONVENTIONAL GRINDING OF SPICES


CRYOGENIC GRINDING PROCESS

The cryogenic grinding system consists of two main units, namely…

1)Precooling unit and 2)Grinding unit.

1). Precooling unit

The cryogenic precooler is a cooling device made up of a screw conveyor enclosed in a properly insulated barrel and a system to introduce liquid nitrogen into the barrel, thereby providing refrigeration (liquid and cold gas) within the system.

The function of the cryogenic precooler is to remove the heat from the material before it enters the grinder.


1). Precooling unit

It consists of a screw conveyor assembly, an air compressor, a liquid nitrogen (LN2) dewar , a power transmission arrangement and control panels.

reduce the temperature of the seed below its brittle point as well as the freezing point of its oil, before it enters the grinder.

The temperature of the precooler and the feed rate to the grinder are control to minimise the loss of quality of the final powdered material.

Consumption of liquid nitrogen and the operating cost are important considerations and matters of concern for a cryogenic precooling system.

The liquid nitrogen losses can be minimized to a great extent by proper consideration of the design and insulation of the precooler.


The main engineering considerations which can be adopted in the design and development of a cryogenic precooler are:

1. Retention time of the seed in the liquid nitrogen and gaseous zone should be accurately proportioned so that the available refrigeration could be utilised at its optimum level.

2. Appropriate insulation should be used such that losses to the ambient could be minimised.

3. Various components of the precooler should be arranged in such a manner that dismantling and cleaning could be easier.

4. Cooldown losses should be reduced by keeping the machine size and structural components to a minimum

1). Precooling unit


2). Grinding unit

The operation of grinding was performed by impact and attrition.

The grinder was operated by an electric motor.

The ground powder was collected in the collector pan from an outlet and the nitrogen vapour let out.


PROCESS FLOW CHART OF CRYOGENIC GRINDING OF SPICES

Cleaning of spices

Cleaning of spicesLoading in Hopper through Vacuum ConveyorFeeding in cryogenic screwChilling of spices with liquid nitrogenFeeding of spices in Pin MillGrinding of Spices in Mill at low temperature andinert atmoshereExtracting low temperature from grounded spicesPacking of Spices in Liquid Nitrogen (inert)atmosherePDF created with pdfFactory trial versionwww.pdffactory.com

Loading in Hopper through Vacuum Conveyor

Feeding in cryogenic screw conveyor

Chilling of spices with liquid nitrogen

Feeding of spices in Grinders

Grinding of Spices at low temperature and inert atmosphere

Extracting low temperature from grounded spices

Packing of Spices


Fig. 8: Schematic diagram of cryogenic grinding system

Feeding of Manually cleaned materials to be ground

LN2 from the storage tank is spray into the cooling conveyor

Ground powdered product are collected in collecting bin

Vaporized LN2 is sucked by a centrifugal blower

and fed back to the mill, and cyclic process is

continued


ADVANTAGES OF CRYOGRINDING WITH LIQUID

NITROGEN IN SPICES

1. Higher retention of etheric oils- Due to lower operating temperatures, the etheric oils will be retained in the product almost to the original level.

2. Prevention of oxidation and rancidity - The heat developed during grinding will be absorbed by liquid nitrogen and converted to vapour and eliminates the possibility of oxidation.

3. Increased throughput and power saving- Due to usage of liquid nitrogen, the raw material becomes brittle. This in turn keeps the oil and moisture content in the crystallized condition during grinding and avoids clogging. Required less power to crush.


4. Finer particle size

5. Reduction in microbial load- By cooling with liquid nitrogen, some bacteria’s may become dormant.

6. Possibility of fine grinding of difficult spices – - By use of low temperatures, the raw materials become brittle

which causes fibers to shatter - fibrous spices like ginger can be ground easily to finer particle

size. - High oil content spices like nutmeg can be ground easily - can grind green spices like chilies with no pre-drying and also

with the retention of its original colour.

ADVANTAGES OF CRYOGRINDING WITH LIQUID

NITROGEN IN SPICES


Traditional Grinding System Cryogenic Grinding System

The heat is developed inside the grinding mill

Temperature below 0oC inside the grinding mill

The heat, which is developed during grinding, leads on one hand to evaporation of the essential oil and on the other hand, heat sensitive fats are melted.

Negligible loss of volatile component

This in turn can lead to the grinding elements become greasy (oily) and even harms the machine by blocking it.

Not in Cryogenic process

High energy consumption Low energy consumption

High capacity motors are required to grind the material

Low capacity motors are required to grind the material

No control on particle size Particle size are under controlled

TABLE 1: DIFFERENCE BETWEEN TRADITIONAL AND CRYOGENIC GRINDING SYSTEM


Table 2: Superiority of the whole cryogenically process over standard grinding process spices w.r. t

Essential Oil Content

Spices

Ungroundedessential oilcontent(ml. Per gm.)

%

Cryogenic Grindedessential oilcontent(ml. Per am.)

%

Standard Groundedessential oilcontent(ml. Per qm.)

%

Turmeric 5.5 100 5.5 100 3.5 64

Coriander 0.6 100 0.6 100 0.4 67

Black Pepper 2.9 100 2.9 100 1.5 52

Cumin Garam 3.5 100 3.5 100 1.6 46

Garam Masala 4.6 100 4.6 100 2.0 43

http://www.cryofoods.com/what-is-cryogenic.asp, accessed on 28-11-14


Table 3: Superiority of cryogenically processed wheat flour over wheat Grain

Comparative Chart of Nutritional Values, App. Per 100 gm of Whole Wheat and Wheat Flour

Parameters Whole WheatCryogenically Ground

Flour Report Result % Report Result %

Energy 364 K cal. 100% 364 K cal. 100%

Protein 11.77 Gm 100% 11.77Gm 100%

Carbohydrates 75.17 Gm 100% 75.17 Gm 100%

Fat 1.83 Gm 100% 1.83 Gm 100%

Saturated fat 0.36 Gm 100% 0.36 Gm 100%

Sodium 25.63 mg/kg 100% 25.63 mg/kg 100%

Cholesterol 0 Gm 100% 0 Gm 100%

Dietary fiber 12.48 Gm 100% 12.48 Gm 100%

http://www.cryofoods.com/what-is-cryogenic.asp, accessed on 28-11-14


All grinding properties viz. average particle size, volume surface mean diameter, mass mean diameter and volume mean diameter were found lower in cryogenic grinding as compared to ambient grinding of fenugreek, coriander, black pepper, turmeric and cinnamon.

Specific energy consumption and energy constants were also found lower in cryogenic grinding than ambient grinding.

Highest specific energy consumption was for ambient grinding of black pepper (202.17 (kWh/tonne) and that of lowest for cryogenic grinding of fenugreek (14.43 kWh/tonne).

Studies on cryogenic grinding for retention of flavour and medicinal properties of some important Indian spices,

CIPHET, Ludhiana


ADVANTAGES OF CRYOGENIC GRINDING

Smaller particles size

More uniform particle size distribution

Process cooling/temperature control

Increased throughput

Reduced power consumption

Minimal loss of volatile components Improves the aroma by minimizing the loss of essential oils when

compared with grinding at normal temperature


DISADVANTAGES OF CRYOGENIC

GRINDING

Cryo-grinding do have distinctive advantages over the conventional cooling processes, but… The application of cryogen in moist atmosphere may cause formation of ice around the delivery nozzle and the piping system carrying the cryogen. This may cause a possible blockage in the delivery system of liquid nitrogen.

Econonomic considerations should be solved.


Health hazards of cryogenic liquids1. Skin and eye hazard- Cryogens are extremely cold and can cause

instant, severe frostbite. The eye’s fluids will freeze in contact with a cryogen, causing permanent eye damage.

2. Cold Embrittlement- At cryogenic temperatures many materials, such as rubber, plastic and carbon steel can become so brittle that very little stress can break the material.

3. Oxygen Enrichment- When transferring liquid nitrogen through uninsulated metal pipes, the air surrounding a cryogen containment system can condense. Nitrogen, which has a lower boiling point than oxygen, will evaporate first. This can leave an oxygen-enriched condensate on the surface that can increase the flammability (combustibility) of materials near the system, creating potentially explosive conditions.


PRECAUTIONS

1) Be familiar with hazards associated with cryogen use.

2) Work in an open, well-ventilated location.

3) Always wear safety goggles and/or face shield and appropriate cryogen gloves.

4) Examine containers and pressure relief valves for signs of defect.

5) Ensure that all equipment and containers are free of oil, grease, dirt, or other materials which may lead to flammability hazard upon contact with liquid oxygen.


CONCLUSION

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.

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 systems are enabling Cryogenic systems are enabling food processors to improve both food processors to improve both

their product quality and their product quality and operational efficiencyoperational efficiency.


1. http://en.wikipedia.org/wiki/Cryogenics, accessed on 28 November 2014.2. http://www.cryofoods.com/what-is-cryogenic.asp, accessed on 28-11-143. http://www.nzifst.org.nz/unitoperations/sizereduction1.htm, accessed on 22-

12-144. http://www.angrau.ac.in/media/10829/fden223fpequii.pdf, accessed on 22-

12-14.5. ICAR. 2014. Studies on Cryogenic Grinding for Retention of Flavour and

Medicinal Properties of Some Important Indian Spices. Final Report of NAIP component-4 Sub-Project on Basic and Strategic Research in Frontier Areas of Agricultural Sciences. 2009-2014. CIPHET, Ludhiana. 60p.

6. http://en.wikipedia.org/wiki/Cryogenic_grinding, accessed on 27-12-14.

REFERENCES


7. M. Wilczek et al.( 2004) Optimised technologies for cryogenic, International Journal of Mineral Processing. 74S (2004) S425 – S434.

8. Murthy K.L.N. et al. A review on cryogenic grinding of species , International Journal of Latest Trends in Engineering, Science and Technology, 1(10).

9. A Technical Paper on Cryogenic grinding- by Dharmendra Kumar Madhukar.

10. K.K. Singh and T.K. Goswami (1999) Design of a cryogenic grinding system for spices, Journal of Food Engineering ,39 (359-368).