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99 (2008) 205– 210
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Research Paper: PH—Postharvest Technology
Physical characteristics of compressed cotton stalks S.K. Jhaa,, Amar Singhb, Adarsh Kumarb a
Division of Post Harvest Technology, Indian Agricultural Research Institute, New Delhi 110012, India Division of Agricultural Engineering, Indian Agricultural Research Institute, New Delhi 110012, India
b
art i cle info
A study was carried out to evaluate the physical characteristics of chopped cotton stalks and to establish correlations of physical characteristics with moisture content and
Article history:
compression pressure. Chopped cotton stalks having a moisture content varying from
Received 12 October 2006
8.5% to 21.45% (w.b.) were densified into square blocks (80 mm by 80 mm) at compression
Accepted 25 September 2007
pressures ranging between 13.79 and 34.47 MPa and a dwell time of 1 min, using a vertical
Available online 26 November 2007
compaction machine. Physical characteristics of blocks, namely bulk density, compression ratio, resiliency and hardness, were evaluated. The bulk density of blocks varied from 542 to 794 kg m3, resiliency from 11% to 47%, hardness from 15 to 134 kg and compression ratio from 5.2 to 8.6. Analysis of variance indicated significant effects of moisture content and compression pressure on bulk density, resiliency and hardness of compressed cotton stalk blocks. A second-order polynomial was found to be adequate to correlate the physical characteristics of blocks with moisture content and compression pressure. A compression pressure of 34 MPa and a moisture content of 15% (w.b.) were found to be the most appropriate for high stability compressed blocks. Savings in transportation costs in block form could be up to 76% whereas maximum savings in storage cost of blocks could be as much as 88%. & 2007 IAgrE. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Cotton is one of the most important commercial crops and plays an important role in the national economy of India. The cultivation of cotton and its use in the textile industry has been practiced in India since pre-historic times. India has the largest area in the world under cotton cultivation, about 9.0 million ha, and the production of cotton is 15% of world total (Bhagirath & Gaurav, 2001). The production of cotton plant stalks is estimated to be 43.5 million tonnes per year (Prasad, 2002). Cotton stalks contain about 46% of alpha cellulose and about 26% lignin and can be used as a raw material for preparation of various products. Parlikar and Bhatawdekar (1987) reported its use for preparation of microcrystalline
cellulose. Production of edible mushrooms on cotton stalks was reported by Balasubramanya (1981). Shaikh and Sundaram (1988) reported a process for development of pulp and paper, and corrugated fibre board from cotton stalks. The use of cotton stalks as fuel was reported by Purohit et al. (2006). The production of boards from cotton stalks is reported to be cheaper than the conventional process as pulverisation consumes much less power compared to hard woods. Efforts have been made to produce lightweight, fire proof, corrugated roofing material, similar to asbestos cement sheets (Sundaram et al., 1989). However, the main problem with cotton stalks lies in its high transportation and storage cost due to their low bulk density. This could be solved through a densification process. The behaviour of the materials during densification is dependent on their physical and biochemical properties
Corresponding author.
E-mail addresses:
[email protected] (S.K. Jha),
[email protected] (A. Singh),
[email protected] (A. Kumar). 1537-5110/$ - see front matter & 2007 IAgrE. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biosystemseng.2007.09.020
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Nomenclature
B b0, b1, CR H L M
breadth of cotton stalk block, mm b2, b11, b12, b22 regression coefficients compression ratio hardness of cotton stalk block, kg length of cotton stalk block, mm
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P R T Ti W rb rraw
compression pressure, MPa resiliency, % thickness of stabilised cotton stalk block, mm initial thickness of cotton stalk block, mm weight of cotton stalk block, kg bulk density of cotton stalk block, kg m3 bulk density of loose cotton stalk, kg m3
moisture content, % (w.b.)
and the variables of the processing plant. The design of compression plants requires knowledge of the force and pressure needed to obtain a desired compressed density. A number of researchers have determined the pressure–density relationships for different agricultural materials. O’Dogherty and Wheeler (1984) studied the compression of straw and grass in the closed dies at pressures in the range of 12–31 MPa and established a pressure density relation for the straw and reported the optimum moisture content for the wafer formation as 10–20% (w.b.). Faborode and O’Callaghan (1986) proposed a model for analysing the compression behaviour of fibrous agricultural materials. Ferrero et al. (1990) reported the pressure–density behaviour of wheat, barley and rice straws of different moisture contents during compression in a cylindrical die at pressures of 20–100 MPa. It was also reported that up to 6 MPa pressure range, the relationship between density and pressure was linear, beyond which nonlinear relationship appeared. Wamukonya and Jenkins (1995) reported the densification of wheat straw, saw dust and shavings into fuel briquettes at 75 MPa pressure. Sawdust briquettes were found to be the most durable and exhibited the least degree of length expansion whereas the wheat straw briquettes were the least durable and expanded the most. The optimum moisture content was reported to be 12–20% (w.b.). Smith et al. (1997) reported that wheat straw could be compressed and stabilised to a density of 10 times that of normal bales by the application of pressure between 20 and 60 MPa after heating to a temperature between 80 and 140 1C. Ndiema et al. (2002) reported that there was considerable influence of the die pressure on the size and form of briquettes. For a given die size and storage condition, there was a maximum die pressure of 80 MPa beyond which no significant gain in the cohesion of briquette could be achieved. Singh et al. (2002) reported a minimum 4–5 times increase in bulk density of roughage-based feed materials, with an increase in compression pressure from 21 to 42 MPa during the densification process in the form of blocks. However, no studies are available on cotton residue. The objectives of the present study were (i) to evaluate the physical characteristics of chopped cotton stalks and (ii) to establish correlations of physical characteristics with moisture content and compression pressure.
A laboratory model vertical compression machine with a hydraulic cylinder as the compactor was used to study the compression characteristics of chopped cotton stalks. A sample of the chopped cotton stalks was taken and filled in a compression mould of cross-sectional area (80 mm by 80 mm). The hydraulic machine had its vertical crosshead fitted with a piston-punch to fit into the compaction mould for load application. A vertical load was applied on the sample until the desired pressure level was achieved. The pressure was recorded from a pressure gauge. The studied variables for block preparation were compression pressure (13.79, 20.68, 27.58, 34.47 MPa), and moisture content of cotton stalk (8.5%, 14.3%, 21.4% w.b.). The experiments were conducted using a factorial design and all treatments were replicated three times. All statistics were calculated using SYSTAT 8.0 software (Systat Software Inc., San Jose, CA, USA). The analyses of variance (ANOVA) were carried out using the linear model procedure. The effect of a dwell time of 1 min was also determined on the physical characteristics of blocks. To determine the effect of dwell time, vertical load was maintained for 1 min at desired pressure level. The blocks (cross-sectional area, 80 mm by 80 mm) were evaluated for their bulk density (rb ), compression ratio (CR), resiliency (R) and hardness (H).
2.
The compression ratio indicates volume reduction during compression. It was obtained from the ratio of bulk density of compact block to the initial density of the material being compressed
Methodology
Freshly harvested cotton stalks were chopped to a length of 10 mm using a commonly used fodder cutter in India. Chopped stalks were sun dried to a desired level of moisture content. Chopped and dried stalks were compressed at different compression pressures.
2.1.
Bulk density
Bulk density is an indicator of savings in storage and transportation space and cost of blocks. The bulk density of the compacted blocks was calculated using Eq. (1), with the sample weight and the measured volume. The volume was determined by the crosssectional area and variable thickness of the blocks. The thickness of blocks, which varies during post-compression recovery, after 24 h was used to calculate the stable density of blocks rb ¼
2.2.
CR ¼
W . LBT
(1)
Compression ratio
rb . rraw
(2)
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Resiliency
2.4.
2.5. Correlations of physical characteristics with moisture content and compression pressure Second-order models [Eq. (4)] were used to fit the observed data. The coefficients of the model were calculated through nonlinear regression also using SYSTAT 8 software: (4)
where Y is the dependent variable, M and P are the independent variables; b0, b1, b2, b11, b12 and b22 are regression coefficients.
24
29
res
sur
e, M
9
np
14
sio
21 19 b.) 17 (w. 15 % , ent 13 ont c 11 e r istu Mo
Fig. 1 – Response surface of the bulk density of cotton stalk blocks as a function of compression pressure and moisture content.
in the pressure range 6–40 MPa. A simple power law relationship in lower-density ranges (up to 400 kg m3) and logarithmic power law relationship in higher-density ranges were reported by O’Dogherty and Wheeler (1984) for barley straw. The density of chopped cotton stalk increased with increasing moisture content, reaching a maximum at about 15% and decreased thereafter. Table 1 shows that the linear as well as the cross-product term of compression pressure and moisture content significantly affect bulk density whereas only the quadratic term of moisture content is significant. Positive coefficients of M and P indicated that their increase results in increase of the bulk density of blocks. However, the negative coefficient of M2 indicated that above a certain limit (17%), the bulk density of block decreases with increased M.
Results
The effect of process parameters, namely moisture content, compression pressure on bulk density (rb), compression ratio (CR), resiliency (R) and hardness (H) of blocks, were determined. The bulk density ranged from 542 to 795 kg m3, compression ratio from 5.3 to 8.6, resiliency from 11% to 47% and hardness from 14.7 to 134 kg.
3.1.
res
Pa
Hardness reflects the degree of binding. It was measured as the maximum force recorded while a block was broken by a probe incorporated in a Texture Analyser (Model TA+HDis, Stable Micro Systems, UK).
3.
mp
(3)
Hardness
Y ¼ b0 ¼ b1 M þ b2 P þ b11 M2 þ b12 M P þ b22 P2
Co
19
T Ti 100. R¼ Ti
750 700 650 600 550 34
After the block was removed from the compaction mould, the resiliency (length recovery) was measured with time, varying from 5 min to 24 h. Resiliency indicates the elastic property of the material. It was determined as the ratio of increase in thickness to the initial thickness of the block [Eq. (3)]. The thickness of the blocks, which varied with time, was measured initially at 5 min intervals up to 30 min and then after 24 h:
Bulk density, kgm–3
2.3.
207
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Bulk density of chopped cotton stalk
The bulk density of the un-compacted samples of the chopped cotton stalks was evaluated at different levels of moisture content. It varied from 100 to 105 kg m3. The uncompacted density was used for comparison with the bulk density of the corresponding compact blocks.
3.2. Effect of compression pressure and moisture content on bulk density of blocks Fig. 1 shows the stable density of blocks of a typical sample as a function of compression pressure and moisture content. The density increased linearly with increasing compression pressure. A similar relationship by Ferrero et al. (1990) was reported for wheat, barley and rice straw up to a compression pressure of 6 MPa, but a nonlinear relationship was reported
3.3.
Compression ratio of blocks
The compression ratio of blocks varied from 5.2 to 8.6, and suggested that a maximum of 8.6 times could be saved on storage space and transportation costs of chopped cotton stalks by densification into blocks. This can be considered as an advantage of the compression process. Since compression ratio is a derived parameter from the bulk density of compacted and un-compacted cotton stalk, the effect of variables was similar (Fig. 2) as in the case of bulk density of compressed blocks.
3.4.
Resiliency of blocks
It was observed that the maximum resiliency (10–30%) occurred within 30 min, which was followed by slow decrease (3–11%) till 24 h. Wamukonya and Jenkins (1995) reported maximum expansion in wheat straw briquettes up to 2 h. O’Dogherty and Wheeler (1984) reported a total relaxation of 56–60% in 1 h in wheat and oil seed rape straws. Resiliency as a function of compression pressure and moisture content of a typical sample is shown in Fig. 3. As the moisture content of the feed and the compression pressure increased, the resiliency of the compressed block decreased. However,
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252.79 38.83*** 4.15** 1.25*** 0.003ns 0.13* 0.958
(nsnon-significant at 10% level, *Po0.1, **Po0.05,
***
sio
np
res
sur
e, M
Pa
9
15 2 ten 1 con e r u ist
Mo
Fig. 3 – Response surface of the resiliency of cotton stalk blocks as a function of compression pressure and moisture content.
37 27 17
18
sio
np
res
sur
e, M
Pa
14
res
19
mp
24
Co
29
139.2 19.47*** 3.36*** 0.57*** 0.01ns 0.12*** 0.986
res
14
Values of coefficients of Eq. (4) for hardness Constant b0 M b1 P b2 M2 b11 P2 b22 MP b12 R2 (observed vs. predicted)
mp
9
67.12 4.68*** 0.66*** 0.14*** 0.007ns 0.03** 0.968
21 18 .b.) (w t, %
18
Values of coefficients of Eq. (4) for resiliency Constant b0 60.71 3.39*** M b1 P b2 0.69*** M2 b11 0.11*** P2 b22 0.001ns MP b12 0.02** R2 (observed vs. 0.989 predicted)
Co
22
197.17 102.81*** 9.25*** 3.22*** 0.06ns 0.36** 0.964
26
Values of coefficients of Eq. (4) for bulk density 7.11 Constant b0 73.94*** M b1 P b2 8.63*** M2 b11 2.19*** P2 b22 0.06ns MP b12 0.49** R2 (observed vs. 0.972 predicted)
40 35 30 25 20 15 30
Dwell time
34
Compression
21 .) w.b
34
Coefficient
Hardness, kg
Variables
Resiliency, %
Table 1 – Coefficients and significance of variables of second-order regression equations
( 15 t, % n e t 12 on re c u t s i Mo
Po0.01).
7.5 7.0 6.5 6.0 5.5 34
Compression ratio
Fig. 4 – Response surface of the hardness of cotton stalk blocks as a function of compression pressure and moisture content.
res
sur
e, M
14
sio np
19
res
24
mp
29
Co
19
Pa
21
b.) 17 (w. 5 % 1 nt, 13 nte 11 ure co 9 ist Mo
Fig. 2 – Response surface of the compression ratio of cotton stalk blocks as a function of compression pressure and moisture content.
significantly affected resiliency whereas the quadratic term of only moisture content was found to have significant effect (Table 1). The negative coefficient of M and P indicates that with increasing moisture content and compression pressure, the resiliency of blocks decreases. However, the positive coefficient of M2 indicates that the decrease in resiliency of block is not linear throughout the whole range of moisture content, rather that above a certain limit (15%) of M, resiliency increases. The significance of the cross-product term indicates that it is not only moisture content or compression pressure alone, but the interaction of these two that is important in determining the resiliency of compressed blocks.
3.5. increases in moisture content beyond a certain limit (above 15%) resulted in an increase in resiliency, as also indicated by a positive coefficient of M2 (Table 1). The resiliency of the compressed block decreased almost linearly with increases in compression pressure (Fig. 3). The ANOVA revealed that the linear as well as the crossproduct term of compression pressure and moisture content
Hardness of blocks
Fig. 4 shows the effect of compression pressure and the moisture content on the hardness of blocks. It is evident from the response surface that as the compression pressure increased, the hardness of the compressed blocks increased. A similar relationship between die pressure and briquette strength for sawdust, rice husk, peanut shell, coconut shell,
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and palm fibre briquettes was reported by Chin and Siddiqui (2000). Increase in moisture content up to about 15% resulted in a linear increase in hardness but a decrease was observed beyond 17%. The ANOVA for hardness is shown in Table 1. The effects are similar to those in case of bulk density. Comparing Figs. 1 and 4, it is clear that the trend in increase/decrease of bulk density and hardness of compressed block with moisture content and compression pressure followed almost a similar trend. It can also be seen that the rate of increase in bulk density and hardness of blocks was more prominent in the compression pressure range of 29–34 MPa. Thus, the compression pressure of 34 MPa and moisture content of 15% (w.b.) were found to be appropriate for the increased stability of compressed block. At 34 MPa compression pressure and 15% (w.b.) moisture content, the bulk density and hardness of blocks were at a maximum whereas the resiliency was minimum, contributing to maximum stability to the compressed block.
3.6. Effect of dwell time on physical characteristics of cotton stalk blocks The bulk density, hardness and resiliency of compressed block with/without dwell time can be calculated by Eq. (2) using the coefficients given in Table 1. It was found that the dwell time of one min resulted in an increase in bulk density and hardness of the compressed blocks. This is in agreement with the work of Chin and Siddiqui (2000) on briquetting of some biomass such as sawdust, rice husk, peanut shell, coconut shell and palm fibre. Resiliency decreased with dwell time.
3.7. Correlations of physical characteristics of blocks with moisture content and pressure The second order model [Eq. (1)] was employed to establish correlations for bulk density, resiliency and hardness. It was found to predict the block physical parameters very well, as evident from the high correlation coefficients (0.936–0.993). The correlation coefficients between independent and dependent variables and the significance of parameters of independent variables on dependent variables are given in Table 1.
3.8. Cost of storage and transportation of cotton stalk blocks Normally, in India, transportation of crop residue materials to distant places is by trucks of various capacities, viz. (2.74 2.06 1.52) m, (4.27 2.06 1.83) m, (5.18 2.06 1.83) m, (5.48 2.06 2.44) m, (6.70 2.06 2.44) m having maximum permissible carrying weights of 2.5, 3.5, 6.0, 9.0 and 15 tonnes, respectively. Considering the average bulk density of chopped loose cotton stalk as 102.5 kg m3, these trucks can carry only 0.879, 1.370, 2.021, 2.823 and 3.451 tonnes of loose cotton stalk, i.e. less than 50% of their permitted weight. However, if the cotton stalks are transported in the block form, 7.559, 11.782, 17.38, 57.68 and 70.49 tonnes (considering maximum compression ratio of 8.6) of materials can be accommodated in the same space. However, this exceeds the maximum
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209
permissible carrying weight. Taking into consideration the permissible weight, savings in transportation cost in the block form should be in the range of 60–76%. However, costs in storage can be reduced in the proportion of maximum compression ratio. Thus, the maximum compression ratio of 8.6 results in approximately 88% savings in storage cost. However, if compression is achieved only to accommodate permissible weight in the truck, the compression ratio required will be 2.844, 2.554, 2.968, 3.188 and 4.346. This compression ratio can be achieved even at the lowest compression pressure and the highest moisture content. But the blocks obtained at the lowest compression pressure and the highest moisture content have poor stability. In terms of block preparation, considering the energy consumed in the process, stable blocks prepared at higher moisture content and at lower compression pressure are preferable. Lowering the moisture content of the raw material requires energy; it also requires a higher compression pressure. Taking these points into consideration, blocks prepared at 15% (w.b.) moisture content and at a lowest compression pressure of 13.79 MPa could be considered as the most appropriate for transportation. These blocks had stability, having a resiliency of less than 30% (Fig. 3).
4.
Conclusions
Bulk density and hardness of compressed blocks increased with increase in the compression pressure but the resiliency decreased. The increase in moisture content up to about 17% resulted in an increase in bulk density and hardness but decrease in resiliency of blocks, whereas a reverse trend was found at higher moisture contents. A compression pressure of 34 MPa and moisture content of 15% (w.b.) were found to be appropriate for the higher stability of the compressed block. Transportation costs of chopped cotton stalks can be reduced up to 76% and storage cost by 88% through densification into blocks. A second-order polynomial was found to adequately fit the data on physical characteristics such as bulk density, resiliency, compression pressure and hardness of blocks with moisture content and compression pressure.
Acknowledgements The authors gratefully acknowledge the funding and support of the National Agricultural Technology Project (NATP) of the Indian Council of Agricultural Research (ICAR) and the Indian Agricultural Research Institute, New Delhi. R E F E R E N C E S
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