CREATE SYSTEM FOR DRY DEPITHING

SUBMITTED BY : SURJEET PANDEYB.TECH 2ND YEAR

NIET GREATER NOIDA

Paper industry is always on the look out for

alternative materials that can be used as a source of fibre.

Many countries have been using bagasse, which is a waste material left after the crushing operations in the sugar mills producing sugar from sugar cane

Though bagasse is a potential raw material for the paper industry, has one of the serious drawback of pith associated with the fibrous portion.

INTRODUCTION :

Along with the fibrous portions of the bagasse, there is

pith, which is undesirable from the point of view of paper making.

The presence of pith in the bagasse has adverse effect in respect of lower pulp yield, high chemical consumption and quality related problems besides exhibiting runnability problems on the paper machine.

Thus, there has always been interest in the efficient removal of this pith from the fibrous portion of bagasse so that the same can be used for pulp and paper making in an efficient and better manner.

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DRY DEPITHING

MOIST DEPITHING

COMBINATION OF BOTH

TYPES OF DEPITHING

Depithing process have been mainly on methods which

have used the DRY/MOIST depithing or a combination of moist and wet depithing.

In both the processes the bagasse is mechanically abraded to break the clusters of pith away from the fibrous portion of bagasse

Dry depithing has been accomplished by using a hammer mill followed by dry screening.

In the wet depithing, a suspension of previously moist depithed bagasse is made in water, wherein after the pith is separated by utilizing the difference in the densities of the fiber and pith.

COMPARISON

Moist depithing involves direct depithing of

bagasse after crushing of sugar cane at a moisture of around 50%.

However, even by the best available methods, there is still some residual pith left in the bagasse (of the order of more than 15%).

Moreover, in the wet depithing operations there is the associated problem of a negative environmental impact as the pith is obtained in a slurry form which poses disposal problems besides huge capital and operational expenses in the process.

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Thus there is a demand from the industry to

develop an efficient, depithing process which does not have the problem from an environmental standpoint.

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Depithing operations in the Indian pulp and paper

industry are being carried out by the conventionally available methods of the dry, wet or a combination of the two methods. In the Indian mills, the average depithing efficiency is only of the order of 50%, thereby generation at best a bagasse having a residual pith of about 15-27%. Even in case of the most efficient depithing operations being practiced in India, the residual pith content in the depithed bagasse has never been less than 15%.

What is PITH ??

The pith is composed mainly of parenchyma cells. Pith can’t be

converted into a satisfactory pulp despite its resemblance to fiber in

chemical composition because of its small dimensions non fibered

physical nature and close association with dust.

What is PITH ??

Lower yield and high cooking chemical consumption as

pith consumes the chemical reducing their availability for pulping and bleaching operations thereby necessitating use of high chemical doses.

Slower drainage rate at all dewatering stages. Typically higher soda loss during brown stock washing operation.

Requirement of more bleaching chemicals and other effects on paper machine.

NOTE: (The residual pith content in the depithed bagasse has never been less than 15%).

Drawbacks of pulping bagasse with PITH

Bagasse which is a potential raw material for the paper industry,has serious problem of PITH associated with the cellulosic fibrous material which is undesirable from the point of view of papermaking.

DEPITHING OF BAGASSE

Whole bagasse

Dissolved Matter(10%)

Useful Fibre(57%)PITH(33%)

The presence of pith in the bagasse results in lower pulp yield,high KAPPA NO. and high chemical consumption during pulping.There has always been interest in the removal of this pith from the fibrous portion of bagasse so that the same can be effectively for high quality pulp production.

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Pith separation from the fibre is essential to

upgrade the quality of raw material. Pith constitutes nearly 30-35% of bagasse the

rest being useful fibre (60-65)and soluble (5%). The chemical properties of fibre and pith are

more or less similar but they differ vastly in physical and morphological properties.

Pith contains a lot of soft , thin walled irregular shaped parenchymatous cell with higher quantity of inorganic ASH and high absorbency.

BAGASSE PREPARATION

A. DRY DEPITHING PROCESS : It is carried out on

stored bagasse having a moisture content of less than 35%.

Hammers and shredder are used in the separation of pith from bagasse .

This method has several disadvantage like heavy wear and tear of the process equipment, loss of valuable fiber along with the pith and production of lots of dust etc.

The depithing efficiency of the process is quite low and not more than 40%.

DEPITHING OPERATIONS

DRY DEPITHER

The depithed bagasse obtained by this process still

contain 20%residual pith as against the original pith content of 30-35% in the original bagasse.

B. MOIST DEPITHING: This type of depithing is generally done at the sugar mill when the wet bagasse has about 50% moisture. Several type of depithers such as Horkel, Rieth,

Gunne, Peadco and others are commerciallly used for moist depithing.

These depithers are designed to break open the fiber

DEPITHING OPERATIONS

Bundles and to dislodge the pith by mechanical rubbing and mild disintegrating action. The unit consist of a rotor with sewing or rigid

hammer attached to it. The hammers are enclosed fully or partially by

perforated screen plates through which pith fraction is discharged.

Upto 50% of the original pith content in bagasse is removed by this method.

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MOIST DEPITHED BAGASSE

USEFUL FIBER (68.5%) PITH (25%) DISSOLVED MATTER

(6.5%)

The process is suitable for entire baled bagasse

delivered from bulk storage. This method is more applicable at the pulp mill for

the final cleaning and depithing just before bagasse enters the digester.

The bagasse is fed to the hydropulper where it is thoroughly wetted and broken up at a consistency of around 2 ti2.5% which maintained by continuous recirculation of process water.

Residual pith content not less than 14-15%and depithing efficiency is around 70%.

C. WET DEPITHING PROCESS

WET DEPITHED BAGASSE

USEFUL FIBER(85.5%) PITH (12.5%) DISSOLVED MATTER(2%)

DISC MILL DEPITHING

The use of disk mill method for depithing

results in a clean raw material having a residual pith content of nearly 5%.

The procedure can also work with most bagasse having moisture content of 5o%

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DISC MILL DEPITHED BAGASSE

USEFUL FIBER(91.5%) PITH(<5%) DISSOLVED MATTER(2.5%)

RAW MATERI

AL

MAGNETIC

SEPARATOR

CONVEYER BELT SCREW FEEDER

TREATMENT CHAMBER WITH DISC AND MILLING ELEMENTS

PITH/FIBER SEPARATING SYSTEM WITH ENDLESS ROTATING BRUSH AND SCREENS

O/P SEPARATED PITH AND FIBER PORTION OF BAGASSE IN TWO OUTLETS

FLOW CHART

The use of disk mill method for depithing results in a clean raw material having a

residual pith content of nearly 5%. The procedure can also work with most bagasse having a higher moisture content of 50%.

The balanced rotating disk is mounted on a shaft with a friction brush assembly. The shaft is mounted in a castiron housing supported by double ball bearing.

There are specially designed milling component fixed along the periphery of both the disks. Each of the milling component is fixed on to the disk using a groove-screw method.

The diameter of the disks and the size of the milling component fixed on the disk depends on the size and throughput desiredfrom the equipment/set up.

In any case one milling component is placed at every 15 degrees on the disk. The length and width of the milling component is therefore fixed accordingly, depending on the diameter of the disks. The gap between the rotating and the stationary disk is adjusted by means of a device mounted on the basic housing of the shaft.

The whole assembly of the disks and the gap adjustment mechanism are mounted on a welded steel profile frame. The incoming raw material is distributed evenly with the help of special structures protruding over the surface of the rotary disk.

The separated pith and fibrous material falls though the sieves by gravity and is collected separately with the help of hoppers at the bottom of the sieves. The positioning of the sieves and the opening per linear inch is optimized to separate out the pith from the fibrous portion of bagasse.

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WHOLE BA

GASSE

DRY DEP

ITHED

WET DEP

ITHED

DISC M

ILL

05

101520253035

Residual Pith Content in Bagasse After Depithing

PITH

CONTENT%

AssumptionsPith in Original Bagasse :30%Pith in Conventional Depith Bagasse :18%Pith in Disc Mill Depith Bagasse :5%Accept after disc milling :70%Pulp Mill Capacity :100TPDPulp Yield from Conventionally Depith Bagasse :48%Pulp Yield from Disc Mill Depithed Bagasse :50%

Raw Material required for 100 Ton Pulp ProductionDisc Mill depith Bagasse (yield 50%) :100TPDOriginal Bagasse (accept 70%) :(l00x200)/70 = 286 TPD Raw Material to be processed will be 15 tonlhr for 20 working hrs.

Three-disc mill of 5 tonlhr capacity each are required. Drive Motor Capacity (3 nos.) : 250 kw

COST ECONOMICS OF DISC MILL DEPITHING PROCESS FOR SETTING UP

100 TON PER DAY PULP PLANT

Cost of three disc mills (3 x 8lacs) :24lacsCost of three motors (3 x 4lacs) :12lacsCost of Conveyers and other accessories :14lacsTotal Equipment Cost :50lacsCost of Disc Mill Depithing per Ton of PulpEquipment cost :50lacsDepreciation per year (10%) :5lacsInterest per year (10%) :5lacsInterest & depreciation per day :Rs3000Electricity consumption per day (100 x20 x3) :6000kwhCost of electricity per rate @ Rs. 4.5 per unit :Rs27000Maintenance (2% of equipment cost) per day :Rs300Manpower cost per day :Rs900(Three shifts two persons per shift)Rate 150 per day per person (6 x 150) Total operating cost for producing 100TPD bleached bagasse pulp =3000+27000+300+900 =31200Rs

COST OF PLANT

Cleaner Production Principle :The chemical properties are of obvious importance in manufacturing of cellulose derivatives. The chemical properties of cellulose cannot be regarded as fixed since cellulose undergoes appreciable chemical change upon aging as evidenced by the deterioration, which occurs even during storage of pulp and aging of paper. Cellulose is influenced a lot by heat, air, light, alkalinity, acidity etc.COMPARISONBase & present Process: In practice the fibers suffer considerable damages during sugar extraction and hammering during moist/dry Depithing. The mechanical depithing process contributes a lot of environmental disposal problem . In this mechanical  process the 8-10 % pith cell removed and 20-25 % useful fiber lost due to hammering and subsequently in wet depithing or washing  further generate heavy amount of wet pith with 70 -80% moisture contents, which create heavy disposal problem. Whereas this wet depithing process removed 3-5 % of pith and loss of useful fiber is up to 12-15%

DEPITHING OF BAGASSE BY CHEMICAL TREATMENT PROCESS

Process Changes as adopted: These alternatives for depithing of

bagasse, In place of putting bagasse in mechanical depithers or hammerers, the chemical treatment by soaking process was adopted, to avoid any type of physical losses of fiber. Chemical treatment by pre-treatment solution at ambient temperature is just a pre requisite for opening up the crystalline structure of bagasse, which is made up of lignin, hemicelluloses - xylan, mannose, arabidoglactan and cellulose (more than 50%). Chemical treatment causes hydrolysis, which breaks open the structure, releasing these components and larger amount of the pith cell removed easily during the subsequent washing process. This is as simple as soaking the cloths in detergents and rinsing with water under agitation. Moreover the spent liquor can be recycled at any number of time with little make of chemical by way of on line pH control.

Screw types de-watering device is further helpful to control the moisture contents in pith and reduce the water usage. Pith with lower moisture contents can be readily fire in boiler.

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ANALYSIS OF BAGASSE

ANALYSIS OF WHOLE BAGASS

MOISTURE

FIBER

PITH AND FINES

SOLUBLE

%

%

%

%

52

57

35

8

Blank With water

Chemically treated

Chemically treated

Chemically treated

Chemically treated

15 DAYS 15DAYS 30DAYS 45DAYS 60DAYS

FIBER 59.64 75.02 77.28 78.98 80PITH FINES

32.16 15.08 14.36 16.26 18.02

SOLUBLE

08.20 09.9 08.36 04.76 01.98

SUGAR Positive NILL NILL NILL NILL

RESULT

Net removal of pith is at par with mechanical

process. Saving of useful fiber up to 25-30% There is increase in yield of fibres as time goes

on. No damages of fibers Swelling of fiber makes wet depithing easier. Wet pith disposal is not a problem, by

adopting dewatering system

NOTE

An alternative method of chemical pre-treatment for the use of sugar-cane bagasse depithing permits to recover more useful fiber in the regular process, instead of mechanical hammering depithing process. This kind of process is of great importance saving of useful fiber and reduction of solid waste generation. Hence following advantages can be drive : Net removal of pith is at par with mechanical process. Saving of useful fiber up to 25-30% No damages of fibers under the changed process of wet condition Swelling of fiber makes wet depithing easier. Chemical treatment losses polysaccharides-lignin bond which can be

digested with low alkali, hence saving of alkali up to 20-25%. Wet pith disposal is not a problem, by adopting dewatering system. Cost of chemical treatment can be off set by saving of heavy power

consumption in mechanical depithers. Saving of heavy consumption of power used in hammering

Environmental and Economic Benefits

THANK YOU