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Experiments for Designing a Screen of a Screw Press for Dewatering of Cattle Dung Slurry

Kataria Mahendra B. #1, P.M.George #2, Himali Mehta#3 ,R.G.Jivani#4

#Mechanical Department, Gujarat technological UniversityBirla Vishvakarma Mahavidhyalaya, V.V.Nagar, Gujarat, India

#Associate Professor, Mechanical Department, Gujarat technological UniversityBirla Vishvakarma Mahavidhyalaya, V.V.Nagar, Gujarat, India

[email protected]@yahoo.com

[email protected]@yahoo.com

Abstract— Dewatering of digested cattle dung slurry from a biogas plant is important in today’s context as it offers conservation of important resource like water and also makes it possible to produce either compost or solid fuel. Recently, screw presses have found application for digested slurry dewatering in western countries. In order to develop an indigenous design of a screw press, some experiments were performed to assess the behavior of slurry under pressure and to find the particle size distribution. The data would essentially help in designing the screen of the screw press.

Keywords— Cattle dung slurry, Screw press, Screen, Total Solids

I. INTRODUCTION

Screw press conveyers are being used for the extrusion of solid from sludge from the bio waste the biogas plants is produced briskets which can be used as a substitute for fine wood or coal and is more environmental friendly. A helical blade is attached to a drive shaft which is coupled to a drive unit. The shaft is supported by end bearings, and intermediate bearing. The U-shaped trough has a cover plate with an opening for loading the conveyor. A discharge opening is provided at bottom of the rough.

The basic principle of flow of material along the screw is similar to the sliding motion of a nut along a rotating screw when the nut is not allowed to rotate. The weight of material and the friction of the material against the wall present the load from rotating with the screw.

II. SCREEN DESIGN

The biogas plants based on cattle dung operate at 8-10 % Total Solids (TS) content. TS of raw cattle dung is 16-20% and hence water is used to prepare slurry of desired consistency. In case of large biogas plants handling tons of cattle dung on daily basis, the volume of water used and slurry to be handled is huge. Digested slurry coming out of the biogas plant contains 3-4% TS. Recycling and reusing of digested cattle dung slurry is gaining importance in the light of nutrient recovery, better resource management and alternative energy generation. Integral to a digested slurry management system is a unit to separate the liquid from the solids.

Liquid thus separated could be recycled and solids could be used raw material for composting or for preparing solid fuel.

Equipment and processes for separating liquids and solids have been widely used in many industries. The unique characteristics of cattle dung slurry like no homogeneous, slimy, corrosive, abrasive, etc. require changes in equipment developed for other processes before they function acceptably in separating slurry solids. Several types of equipment or processes are used for manure liquid-solid separation. One such equipment is a screw press. Screw presses are composed of a screw-type conveyor in the center. The screw-type conveyor forces the manure slurry through a tube and past a cylindrical screen. Solids retained on the screen are pressed to the end and discharged. Rate of flow determines the solid removal from liquid manure. With most screw presses, the rate can be adjusted to control moisture content of the solids discharged. Conceptual view of a screw press is shown below fig-1

Fig -1 screw press

Sardar Patel Renewable Energy Research Institute (SPRERI) conducted some experiments on dewatering of digested cattle dung slurry from a biogas plant using a screw press. It was observed that even when effective separation of solid and liquid fraction of slurry was taking place, majority of solids (by weight) were going to the liquid fraction. This needed change in design of the screen. This paper presents the laboratory experiments conducted to collect data

13-14 May 2011 B.V.M. Engineering College, V.V.Nagar,Gujarat,India

National Conference on Recent Trends in Engineering & Technology

on dewatering of slurry for design of another screen for the existing screw.

III. MATERIALS AND METHODS

Screens are mainly of two types: bar type and perforated type. Profile bar and wedge wire screens are made by resistance welding wedge shaped bars of stainless steel parallel to each other. These bars have a truncated triangle cross-section. The wires are generally 0.219" and 0.247" high, and bars are either 3/8" or 1/2" in height. Bar screens are produced in flat panels that are subsequently rolled to the specified diameter. With bar screens the wide base of the triangle faces the material being filtered, thus providing relief on the discharge side. An important advantage of profile bar is that it can be self-cleaning. In some applications thereis less chance of a profile bar screen blinding than there is with perforated metal. Another advantage lies in the greater thickness of the bars as compared to perforated sheet. It means that the useful life is extended when pressing abrasive materials (paper, plastic, sand, glass) or materials containing tramp metal. This thickness also results in greater burst strength. Profile bar and wedge wire screens typically have 12% open area. Surprisingly, they generally have the same screw press capacity as perforated screens with double the open area. Perforated screens generally have 20% to 25% open area. Surprisingly, in most applications the smaller holes have significantly more press capacity than larger holes. Of course, the smaller hole screens are thinner and more susceptible to damage. However, perforated screens have the advantage of being an inexpensive material that can be changed out with a minimum of labor. Since the existing screw press at SPRERI had bar screen and data were already collected on a bar screen, it was decided to perform experiments on perforated screen. A PVC pipe of 90 mm diameter and 2mm thickness was taken. Since perforated screens are generally designed to provide 25% of openings, perforations of different sizes were drilled to cover 25% of the surface. Experiments were conducted with 3 perforation sizes i.e. 3 mm, 3.5 mm and 4 mm. A suitable wooden base was fitted at one end of the pipe to retain cattle dung slurry inside the pipe. Measured quantity of slurry (1 liter, 1003 grams) was filled and load was applied on this slurry through a piston using hydraulic press machine. Liquid fraction and solid fraction were collected separately and weighed. These fractions and raw slurry were analyzed for their TS content also. An experiment was also conducted on particle size distribution of the cattle dung slurry using standard sieves. Three sieve sizes i.e. 3.35 mm, 0.77 mm and 0.45 mm were taken which means data were available for solids of four sizes. Based on the data obtained through particle size distribution, an experimental screen was fitted in the existing screw press to replicate wedge wire screen using wire mesh. Photograph below in fig-2 shows the test screen prepared using wire mesh.

Fig-2 wire mesh screen for screw press

A test run was performed using this screen and data on TS were collected to assess the separation in cattle dung slurry.

A. Results and Discussion

Average data on experiments with perforated PVC pipe are given in the table-1 below:

Sr.

No.

Perforation Size, mm

Sample Total Solids,

%Wet Wt.

%TS of Initial TS

1. 3.0 Raw cattle dung slurry

5.6 --

S Liquid filtrate 2.2 36

Solid residues 16.0 64

2. 3.5 Raw cattle dung slurry

6.1 --

Liquid filtrate 4.0 56


3. 4.0 Raw cattle dungslurry

5.6 --

Liquid filtrate 3.2 55


Table-1In all the three experiments, separation was completed without any trouble. It could be seen from the above data that 4 mm perforation gave maximum TS in the solid residues but the mass balance shows maximum solids in solid residue fraction of 3 mm perforations there by giving minimum TS in liquid fractions.



B. Data on experiments on particle size distribution are presented in the table-2 below:

Sr. No.

Sample Experiment 1

Experiment 2

Experiment 3

1. TS of raw slurry, %

9.4 6.0 6.2

2. TS after filtration3.35 mm0.77 mm 0.45 mmLess than 0.45 mm

24.8(35%)*12.9(18%)1.4(2%)30.0(43%)

14.1(24%)16.5(27%)1.0(2%)25.0(42%)

9.7(21%)15.8(34%)1.2(2%)17.4(37%)

Table-2* Figures in the bracket give % distribution of TS in each fraction. There was some loss of material as it was not possible to collect each particle from each sieve.

From the above data it could be observed that majority of the solids were bigger than 0.77 mm and hence were retained on 3.35 mm and 0.77 mm sieves. However, a large amount of solids were also present in the fraction less than 0.45 mm. The data helped in concluding that even if the sieve size was kept as fine as 0.45 mm, around 40% of the solids, which would be in dissolved form, would always escape to liquid fraction and could never be brought to solid residues fraction. When actual experiment using screen made up of 0.7 mm wire mesh was performed, solid residues with 26% TS content were produced but the filtration rate was so low that it was totally impractical to design a screw press with that type of screen.

VI. CONCLUSION

From the experiments conducted, it could be concluded that for a perforated screen for cattle dung slurry, 3 mm perforation would give minimum TS in liquid fraction. Secondly, it would not be possible to reduce TS content in liquid fraction below 40% of the total TS in the system as these solids would be in dissolved form and hence can not be removed even by pressure filtration.

REFRENCE

1) Marcy Ford and Ron Fleming, Mechanical Solid-Liquid Separation of Livestock Manure - Literature Review, Ridgetown College - University of Guelph, Canada

2) M. Hjorth, K.V. Christensen, M.L. Christensen, S.G. Sommer, Solid–liquid separation of animal slurry in theory and practice - A review, Journal of Agron. Sustain. Dev. 30 (2010) 153–180.

3) Richard K. White, The role of liquid - solid separation in today's livestock waste management systems, Journal of Animal Science (1980) 50:356-359.

4) Thomas H. Manley and Robert B. Johnston, Screw presses in waste dewatering, Tappi Journal, Vol. 78, No. 12, 233-235.

5) www.vincentcorp.com.



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