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Rice Milling SystemRice Milling System
NitatNitat TangpinijkulTangpinijkulPostPost--harvest Engineering Research Groupharvest Engineering Research GroupAgricultural Engineering Research InstituteAgricultural Engineering Research Institute
Department of AgricultureDepartment of Agriculture
Rice MillingRice Milling
The processes of converting paddy into rice• Removing the husk from the paddy• Removing all or part of the bran layer
The basic objective of a rice milling system is to remove the husk and the bran layers, and produce an edible, white rice kernel that is sufficiently milled and free of impurities. Depending on the requirements of the customer, the rice should have a minimum of broken kernels.
Factors Affecting Milling Recovery• Paddy quality• Type of machinery, efficiency, system• Operator• Others
• milling degree• ambient condition
Milling Recovery = Weight produced milled rice x 100%Weight processed paddy
Course : Grain QualityModule 4 : Rice MillingLesson 1 : Introduction to the milling process and its outputs
Good Equipment
Skilled Miller
Good Paddy Rice
the use of good milling equipment and good quality paddy does not guarantee a high quality product
the miller is not skilled,
the mill will always produce poor quality milled rice, despite the skill or the miller or maintenance of the mill.
you use poor quality paddy,
the mill will produce high quality head rice.
you have good quality paddy in a well-maintained mill that is operated by a skilled miller,
ThenIf
Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy Rice Lesson 1 : The quality characteristics
Quality characteristics of paddyQuality characteristics of paddy
These characteristics are determined by the environmental weather conditions during production, crop production practices, soil conditions, harvesting, and post harvest practices
Moisture content has a significant influence on all aspects of paddy quality. To obtain high yields, it is essential that paddy be milled at the proper moisture content.
Paddy is at its optimum milling potential when its moisture content is 14%
Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy RiceLesson 1 : The quality characteristics
QUALITY CHARACTERISTIC: MOISTURE CONTENT
Immature rice kernels are very slender and chalky and result in the production of excessive bran, broken grains and brewer’s rice
Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy RiceLesson 1 : The quality characteristics
QUALITY CHARACTERISTIC: IMMATURE
A mixture of varieties in a sample of paddy causes difficulties in milling and usually results in reduced milling capacity, excessive breakage, lower milled rice recovery, and reduced head rice yields.
Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy RiceLesson 1 : The quality characteristics
QUALITY CHARACTERISTIC: VARIETAL PURITY
Dockage includes chaff, stones, weed seeds, soil, rice straw, stalks and other foreign matter. These impurities generally come from the field or from the drying floor.
Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy RiceLesson 1 : The quality characteristics
QUALITY CHARACTERISTIC: DOCKAGE
Water, insects and heat exposure can cause paddy to deteriorate through biochemical changes in the grain which may result in the development of off-doors and changes in physical appearance
Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy RiceLesson 1 : The quality characteristics
QUALITY CHARACTERISTIC: DISCOLORED
Overexposure of mature paddy to fluctuating temperature and moisture conditions leads to the development of crack in individual kernels.
Course : Grain QualityModule 3 : Determining the Physical Characteristics of Paddy RiceLesson 1 : The quality characteristics
QUALITY CHARACTERISTIC: CRACKED
• Weight per volume or density, gram/litreWeight per 100 or 1000 grains Thai rice variety: 100 grains weight 2.25 - 3.67 g
• Color of Husk Affecting color of parboil rice
• Color of Brown riceAffecting color of parboil riceMilling recovery and quality of milled rice grade
OTHERS QUALITY CHARACTERISTIC:
•• Grain DimensionGrain DimensionGrain size and shape (length-width ratio) is a varietal
property. Long slender grains normally have greater breakage than short, bold grains and consequently have a lower milled rice recovery
Type of Paddy : Length of brown riceExtra long ≥ 7.5Long ≥ 6.5 < 7.5Medium ≥ 5.5 < 6.5Short < 5.5
Sub-type of Paddy : Length/Width RatioSlender ≥ 3.0Bold ≥ 2.0 < 3.0Round < 2.0
Thai rice variety: Length 7.1 - 7.6 mmSlender 3.1 – 3.4
•• ChalkinessChalkiness• grain with opaque areas in the endosperm, caused by
the loose packing of the starch and protein particles, breaks more readily during milling than clear grain, greatly reducing its market value
• controlled by genetic and environment
white center white belly white back
Good Quality Paddy• uniformly mature kernels • uniform size and shape (vareital purity)• free of fissures and cracks• free of empty or half filled grains • less chalky, red kernel • free of contaminants such as stones and weed seeds • moisture content ~ 14%• high milling yield variety
Ø Good variety, good crop managementØ Harvest at optimum periodØ Drying immediately after harvest to safe level m.c.Ø Proper drying to prevent crackingØ Good aeration during storage to prevent grain deteriorationØ Prevent birds & rodents during storage
Purity of paddyMilling Recovery = Weight produced milled rice x 100%
Weight processed paddy
Purity % Impurity % Milled rice %100 0 68.0099 1 67.3298 2 66.5497 3 65.9696 4 65.2895 5 64.60
Embryo
Bran Ricegrain
BranEmbryo
+ =
Rice flourSplinters
Total bran produced when whitening cracked grains
Cracks
75.065.5 63.568.0Recovery
0.51.5 1.51.0Brewer Rice
75.567.0 65.069.0Milled Rice
7.510.0 10.08.0Bran
83.077.0 75.077.0Brown Rice
17.023.0 25.023.0Husk
100.0100.0 100.0100.0Paddy
JaponicaCrack ImmaturedGood
Paddy Quality
Size of Rice MillSize of Rice Mill
•• Small 12 ton/day• earning from bran, broken or money as milling wage• no grading of milled rice
• Medium 13-59 ton/day• commercial mill• for domestic market
• Large > 60 ton/day• for domestic market and export
43,2752,7313,2328,77928,5331997
44,4364,3865,11410,04924,8871996
44,4724,3905,12910,06524,8881995
44,5604,3955,15610,10424,9051994
45,0194,4005,22310,18325,2131993
KingdomPlainEastern
WholeSouthernCentralNorthernNorth-Year
Source : Factory Control Division, Ministry of Industry
Number of rice mill by region, 1993 - 1997
Number of village 962Number of Rice mill 194
< 5 ton/day 90 (46.4%)> 5 < 20 ton/day 42 (21.6%)> 20 ton/day 62 (32.0%)
Number of rice mill in Suphanburi province (2004)
Capacity (Paddy) Price (F.O.B)Ton/Hour Ton/Day (USD)
2-3 50-80 348,0004-5 100-120 624,0008-10 200-240 1,050,00018-20 420-500 1,824,000
PRICE LIST OF MODERN RICE MILLRICE ENGINEERING SUPPLY CO., LTD.
(BANGKOK, THAILAND)For Month January-April 2008
All price exclude installation cost, electric motor, cable, control etc.
The Aim of Rice Milling
• To attain the highest yield of white rice
• With the best quality– Remove least amount
of hull and bran– Minimum brokens– Little foreign matter
Rice milling system• 0ne step process • Two step process • Multi stage process
In a one step milling process, husk and bran removal are done in one pass and milled or white rice is produced directly out of paddy.
In a two step process, removing husk and removing bran are done separately, and brown rice is produced as an intermediate product.
In multistage milling, rice will undergo a number of different processing steps.
Rice Milling Process
Cleaning
Husking
Paddy Separation
Whitening & Polishing
Grading and Blending
Impurity
Husk
Bran
Head rice
Broken
Paddy
Paddy
Paddy, Brown rice
Brown rice
Milled rice
Paddy into rice mill
Paddy Intake• Quality input = quality output• Garbage in = garbage out!
Paddy intake chute
The quality of milled white rice will be dependant on the quality of the paddy or rough rice coming into the mill as well as the milling process. Good paddy into the mill means quality milled rice output.
Paddy Cleaning• After harvest, foreign matter can be as
high as 5-10%
Rice pre-cleaner
While most paddy has been cleaned after harvesting, some foreign material is still present. This can be as high as 5-10%.
• Remove foreign material such as sand, stones, straw, seeds etc. from paddy
• Prevent damage to the milling machinery• Prevent clogging which cause the reduction
in capacity and efficiency of rice mill• Remove materials that cause a reduction in
the grade of milled rice
Pre-cleaning of Paddy
Paddy Precleaner• Sieve: separate large/ small impurities• Aspirator: separate light impurities• De-stoner: separate same size impurities but
heavier • Trieur (Length Grader) : separate impurities
having same width but different in length• Magnet : separate irons
Magnetic separator
Magnet
Brass
A, B: Simple permanent magnetC: Rotating brass cylinder and
permanent magnet
Ratio of W/D ~ 1/6 or 1/7Composition:
• Emery grit 14, 16• Silicium carbide grit 16
Peripheral speed ~ 14 m/s
Particulars of the under-runner disc huller: V = peripheral speed (recommended 14 m/s); W = width of coating; D = stone diameter; and W/D = 1/6 or 1/7.
Under-runner disc huller peripheral speed-curve for V = 14 m/s.
D = stone diameter = 700 mm (0.7 m); and V peripheral speed = 14 m/s.
V = ¶ x D x n (m/s)60
n = 60 x V = 60 x 14¶ x D 3.14 x 0.7
= 380 rpm.
Diameter Capacity Motor(mm) (kg of paddy/hr) (hp)750 450-600 31000 700-1000 3.51250 1000-1400 41400 1600-2100 5.5
Capacity and power requirement of underrun disc huller
• Advantage§ Operational simplicity, low running cost§ The abrasive covering can be remade at the
site
• Disadvantage§ Given grain breakage § The abrasions to outer bran layers
Underrun disc huller
Dehusking principle of rubber roll huller:• The rubber roll huller consists of two rubber rolls.• Both rolls have the same diameter.• One has a fixed position, the other is adjustable to obtain
the desired clearance between the two rolls. • The rolls rotate in opposite directions.• The adjustable roll speed running about 25% slower than
the fixed one.
74010001025010250
90012003768.5220
90013202.5646150
(rpm)(rpm)(Inch)(mm)(Inch)(mm)
Lowspeed
HighspeedWidthDiameter
Size and speed of the rubber rollWhen the rolls are new, their peripheral speed is about 14 m/sso that a smaller roll runs faster than a larger one.
The relation between paddy variety and hulling capacity• The wear for long grain variety is higher than short grain.• The wear of adjustable roll is lower than the fixed one.
Durability of Rubber Rolls
15302.52035340754601106
140240817030010
Long grain(ton)
Short grain(ton)
Type (Inch)
Diameter
6.03.82.225425410
4.01.91.22201506
2.51.250.92201004
(hp)Short grain
Long grain(mm)(mm)(Inch)
MotorCapacity (ton/hr)WidthSize
Capacity and power requirement for different rubber roll hullers
Rubber Roll Huller
• Advantage§ Hulling efficiency is higher.§ Reduce grain breakage and loss of small
broken.§ The silver skin of brown rice is undamaged.§ Sieving the husked products is unnecessary.
• Disadvantage§ The cost of replacing rubber rolls.
Dehusker
Sieve
Paddy
Brown rice, Broken, Paddy, Bran, Husk
Coarsebran
Husk
Brewerrice
Husk Aspirator
Brown rice, Broken, Paddy
Paddy Separator
Husk Separation
Closed Circuit Husk Aspirator
1 เกลียวลําเลียงแกลบ2 เกลียวลําเลียงเมล็ดออน3 เกลียวลําเลียงขาวกลอง/
ขาวเปลือก4 แผนปรบัลม5 แผนปรบัลม
Paddy Separation
• Removal of paddy from brown rice
After husk separation, a mixture of 85-90% brown rice and 10-15% paddy is fed into the paddy separation stage. The paddy must be separated before the brown rice goes to the bran removal stage. The separated paddy is returned to the husker for dehusking.
• Weight per volume of paddy is less than that of brown rice,• Specific gravity of paddy is lower than that of brown rice,• Dimensions of paddy are longer, wider and thicker,• Coefficient of friction is different.
Paddy and brown rice have different characteristics
Compartment-type SeparatorThe main part of this paddy separator is the oscillating compartment-assembly where the actual separation of paddy and brown rice takes place. The compartment-assembly consists of a number of compartments in one, two, three, or sometimes four decks.The number of compartments depends on the capacity of the rice mill,
PaddyPaddy
low
high
Brown riceBrown rice
Paddy, brown ricePaddy, brown rice
Paddy, brown ricePaddy, brown rice
Brown riceBrown rice
PaddyPaddy
Brown riceBrown rice
PaddyPaddy
aa
HighHigh
LowerLower
AdjustmentAdjustment•• Speed Speed 90 90 -- 120 strokes/min120 strokes/min•• SlopeSlope•• StrokeStroke
HUSKERHUSKER
WHITENERWHITENER
Whitening Machine
• Removes bran layer to produce white rice. • Amount of bran removed (usually 8-12%)
Whitening Actions
• Abrasive typeHigh speed / Low pressure
• Griding action• Impact action
• Friction typeLow speed / High pressure
• Tearing action• Cutting action
Whitening
• Vertical abrasive whitening cone• Horizontal abrasive whitener• Horizontal friction-type whitener• Bottom-Up Vertical friction-type whitener
Whitening Machine
Vertical Abrasive Whitening Cone
Leveladjust
Wiremesh
Abrasivecone
Rubberbrake
Bransweeper
BranRice
Air
Feeding
Number of Brakes = Cone Diameter - 2100
Peripheral Speed 13 m/s
Eg. Cone Diameter = 600 mmNumber of Brakes = 600/100 – 2 = 4
Diameter Power
mm hp Long grain Short grain Long grain Short grain Long grain Short grain
500 4 350 420 570 680 680 800
600 7.5 550 650 890 1100 1050 1250
800 10 750 900 1230 1450 1460 1725
1000 15 1000 1200 1700 2000 1900 2300
1250 20 1350 1600 2200 2600 2600 3000
1500 25 1700 2000 2700 3200 3200 3800
1 Cone 2 Cones 3 Cones
Capacity of Cone-type Whitener(kg of brown rice / hr)
Composition
Whitening Cone No. 1 and No. 2Emery grit 16 25%Silicium carbide grit 16 50%Silicium carbide grit 18 25%
Whitening Cone No. 3Emery grit 18 25%Silicium carbide grit 18 75%
Shaping Action Factors
LowMediumHighPressure
LargeMediumSmallRoll Grit
HighMediumLowPeripheral Speed
Round ShapeSlim shapeFlat ShapeCondition
Polisher
• Humidifying Rice Milling Machine• Vertical Polishing Cone – Brush type• Vertical Polishing Cone – Leather type• Horizontal Polisher – Leather type
Whitening SystemSystem 1 2 3 4 5
Long grain (Indica)
1 Abrasive Abrasive Abrasive - -
2 Abrasive Abrasive Abrasive Vent Friction -
3 Abrasive Abrasive Abrasive Hu Friction -
4 Abrasive Abrasive Abrasive Vent Friction Hu Friction
Short grain (Japonica)
1 Abrasive Vent Friction Vent Friction Vent Friction Hu Friction
Milling System for Indica-type variety
Abrasive Friction
ABRAS ABRAS ABRAS ABRAS ABRAS ABRAS FRICT FRICT
A from B : By thicknessA from C : By thickness / lengthB from C : By length
A from B : By length
Bold grainA from B/C : By thickness / lengthB from C : By length
Slender grainA from B/C : By lengthB from C : By length
Grading of White Rice
• After polishing, white rice contains:– Head rice– Large brokens– Small brokens
• The white rice is separated using – Sifter – Length grader
Rotary Seiver
White Rice Grading
One double layers sifter and two trieurs in parallel.
One double layers sifter and three trieurs in series.
Rice Mixing• An efficient rice mill will produce:
– 50% head rice (whole kernels)– 5-15% large broken and – 5-15% small broken kernels
• Depending on the country’s standards, rice grades in the market will contain from 5-25% broken kernels.
Rice Mixing
Whole kernels
Large brokens
Small brokens
Variable speed motors allow the mill operator to mix the appropriate amount of whole kernels with large and small brokens.
Rice Weighing• Rice is normally sold in
50 kg sacks which must be accurately weighed and labeled.
Bag filler and weighing
scales
Conveying SystemConveying System
Duct Collecting SystemDuct Collecting System
Scaling and PackagingScaling and Packaging SystemSystem
• Sampling / dividing• Measure moisture content• Cleaning sample• Milling test• Grading
Assessment the milling quality of paddy
Moisture MeterResistance typeCapacitance type
Oven for m.c. determination130oC 1 hr103oC 72 hr105oC 5 hr
Using Abrasive-type Whitener•• Weigh clean paddy 250 g• De-husk paddy with the rubber roll husker• Weigh brown rice• Whiten brown rice w the abrasive type whitener for 90 s• Weigh milled rice• Separate broken from milled rice w the trieur for 90 s• Weigh head milled rice
Procedures for Milling Quality Assessment
Using Friction-type Whitener (De-husking method)•• Weigh clean paddy 100 g• De-husk paddy with the rubber roll husker• Weigh brown rice• Whiten brown rice w the friction-type whitener for 25 s• Weigh milled rice• Separate broken from milled rice w the length grader for 90 s• Weigh head milled rice
Procedures for Milling Quality Assessment
Using Friction-type Whitener (Un-husking method)• Weigh clean paddy 100 g• Whiten paddy with the friction-type whitener for 30 s• Weigh milled rice• Separate broken from milled rice with the trieur for 90 s• Weigh head milled rice
Procedures for Milling Quality Assessment
Brown Rice Recovery = Wt of Brown Rice x 100 %Wt of Paddy
Milled Rice Recovery = Wt of Milled Rice x 100 %Wt of Paddy
Head Rice Recovery = Wt of Head Rice x 100 %Wt of Paddy
Milling Degree = (Wt of WBR – Wt of WMR) x 100 %Wt of WBR
WBR = Weight of whole brown rice 1000 kernelsWMR = Weight of whole milled rice 1000 kernels
Moisture Content = Wt of Water x 100 Wt of Sample
After Drying Weight = (100 - Initial MC.) x Initial Weight (100 - Final MC.)
Eg. Paddy 500 t with initial MC 26% Final MC after drying is 14%Then After Drying
Weight = (100 - 26) x 50000 = 430,23 t (100 - 14)