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Processing techniques for ethanol production from sweet sorghum
Biomass 6 (1984) 111-117
Processing Techniques for Ethanol Production from Sweet Sorghum* C h a r l i e G. C o b l e a n d R i c h a r d P. E g g Agricultural Engineering Department, Texas A&M University, College Station, TX 77843, USA and Itzhak Shmulevich Agricultural Engineering Department, Technion, Haifa, Israel (Received: 27 May, 1984) ABSTRACT Several processing techniques for producing ethanol from sweet sorghum were investigated. Fermenting chopped stalks yieMed more ethanol than shredded sorghum or juice. Leaf removal prior to fermentation yieMed slightly more ethanol than solids removal before fermentation. Key words: sweet sorghum, processing, alcohol, ethanol, fermentation.
INTRODUCTION Sweet sorghum has been noted for its potential as an energy crop. If all the sugar it produces were converted to ethanol, the yield could be higher than 4000 liters ha -1. Unlike sugarcane, which is a tropical plant, sweet sorghum can be cultivated in nearly all temperate and tropical climatic areas. Most sorghums are more drought-tolerant than corn, the leading alcohol feedstock in the US. However, two serious drawbacks have prevented sweet sorghum from being widely utilized as an ethanol feedstock. The first is storability. While grain, the predominant ethanol * Approved by the Texas Agricultural Experiment Station as Manuscript No. TA 19330. 111 Biomass 0144-4565/84/$03.30- © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain
112
c. G. Coble, R. P. Egg, L Shmulevich
feedstock, can be stored indefinitely, sweet sorghum tends to deteriorate after harvest. Researchers have been working on I this problem with varying degrees of success, but no practical solution has been found. A second problem is sugar extraction with currently available technology. Over 90% of the sugar could be extracted using sugar mill equipment, but these mills are extremely capital and labor intensive, and would only be feasible for extremely large alcohol producing systems. Using simple three roll mills, only about 40% of the sugar can be extracted. 1 Reidenbach and Coble 2 explored alternative methods of processing sweet sorghum for ethanol production. The maximum sugar recovery reported was 74.2%, obtained by fermenting the whole stalk chopped into 1 cm lengths. However, water added to the chopped sorghum to enable fermentation resulted in low ethanol concentrations in the fermented beer. Stephenson 3 investigated several methods for sweet sorghum juice removal. Highest yields were obtained by finely chopping the sorghum and separating the pith which was then squeezed by a hydraulic press to remove the juice. Results obtained by these researchers indicate that sugar recovery can be substantially improved over the recovery of a simple roller mill which is currently about the only commercially available technology. The objective of the work reported here was to evaluate a number of alternative techniques for producing ethanol from sweet sorghum.
EQUIPMENT AND PROCEDURES The sorghum used in this study was a hybrid grain-sweet sorghum ATX 623X Rio developed at Texas A&M University.4 The sorghum was hand-harvested by cutting just above ground level and brought to the laboratory where the tops were removed and the leaves stripped. It was then processed, fermented and distilled. For all tests, except the study to determine the effect of the leaves on ethanol yield, the leaves were stripped, but the leaf sheath was left on the stalk. Three separate studies were conducted. The first study was used to evaluate processing techniques. Three methods were employed. Sorghum was either chopped into 1-2 cm lengths and shredded with a hammermill, or shredded with a hammermill followed by extracting juice with a
Ethanol production from sweet sorghum
1 13
cage press with a pressure on the shredded material of 10000 kPa. Sufficient water was added to cover the solid material (with the exception of the juice), heated to 95°C for 5 min and cooled. Yeast was then added and the sorghum was fermented at 32°C for approximately 72 h. The material was then distilled and the alcohol yield measured. The cooking and distillation was done in an 18-liter bench-top pot still. A second study evaluated the effect of sorghum leaves on ethanol yield. The treatments were stalks with leaves and leaf sheaths, leaves removed but leaf sheaths remaining on the stalks, and stalks only with the leaves and leaf sheaths removed. After removal of the leaves and/or leaf sheaths, the stalks were shredded in a hammermill, covered with water and fermented, as previously described. A third study involved solids removal from the mash or beer before and after fermentation. In this study sorghum was shredded in the hammermill. Water was added to cover the solids. The mixture was then heated to 95°C for 5 min and cooled. In one treatment, the liquid was then removed with the hydraulic press. Yeast was added to the liquid, which was then fermented and distilled. In the second treatment, the yeast was added to the mixture and allowed to ferment. After fermentation was complete, the liquid was extracted with the hydraulic press and distilled. In the control, the liquid was not separated from the solids, and the entire mixture was fermented and distilled.
RESULTS Results are reported in liters of 200 proof ethanol per tonne of unprocessed sorghum. In the case of the leaf study, the feedstock was after the leaves and/or leaf sheaths had been removed. Results of the study are shown in Table 1.
Processing Results obtained from the processing study showed no significant differences at the 0-05 level. However, yields were highest from the chopped sorghum. It was anticipated that shredding the stalks would produce the highest yield since more surface area is exposed to allow sugar diffusion into the fermentation media. Possibly the increased
114
C. G. Coble, R. P. Egg, L Shmulevich TABLE 1
Ethanol Yield from Sweet Sorghum Study
Treatment
Number of replicates
Ethanol yieM (liters t-1)
Processing
Chopped Shredded Juice
4 12 9
45.4 Aa 42.1 A 39.6 A
Leaf content
Stalk only Stalk and leaf sheaths Stalk, leaves and sheaths
3 17 10
37-2 A 38.5 A 29.3 B
Solids removal
Not removed Removed after fermentation Removed before fermentation
18 8 4
38-3 A 31.5 B 29-3 B
a Same letters for each study indicate no significant difference at the 0.05 level.
surface area allowed more microbial contamination and some deterioration occurred prior to sterilization, thereby slightly reducing ethanol yields. The time between shredding and sterilizing normally ranged from about 0 to 3 h. Compared with the whole stalk fermentations, the hydraulic cage press performed remarkably well. While a cage press is essentially a laboratory tool and not well suited for adaptation to continuous processing, the implication of these results is that high pressures can extract substantial amounts of sugars. The overall juice extraction using the cage press was 52% of the total stalk weight. Leaf c o n t e n t Removing the leaves prior to processing results in higher ethanol yields (Table 1). Leaving the leaf sheaths on the stalk, however, did not significantly affect ethanol yield. Stripping the leaves in the field prior to or during harvest would appear to be advantageous. The relative amount of leaves on ethanol yield per unit feedstock will vary somewhat depending on stage of maturity. Figure 1 shows the relative percent of total plant weight of the sorghum variety used throughout the harvest
115
Ethanol production from sweet sorghum
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period. Peak maturity occurred during the first week of August. Leaf weight ranged from 7-3 to 31.6% of total plant weight during the period 22 July-1 September. Solids removal
If whole-stalk fermentation using chopped or shredded sorghum is employed, the solid material must first be removed if the liquid phase is to be distilled in a conventional sieve tray distillation column. Removing the solids after fermentation yielded about 82% and before fermentation about 77% as much ethanol as obtained by not removing the solids (Table 1). Although the results were not significant at the 0.05 level, ethanol yield was slightly higher when the solids and liquids were separated after fermentation. This advantage could possibly be exploited in a type of semicontinuous whole-stalk fermentation system employing several fermentation stages.
CONCLUSIONS Fermenting whole-stalk sorghum results in higher ethanol yields than does juice extraction. A type of diffusion or continuous fermentation process fermenting whole-stalk material may result in higher ethanol yields from sweet sorghum than using juice extraction techniques for sugar extraction. Removal of leaves prior to processing results in higher yields per unit feedstock. While sweet sorghum has a high potential as an ethanol feedstock, much work is still needed before it is practical as an alternative to grain in existing ethanol production systems.
ACKNOWLEDGMENT This research was conducted by the Texas Agricultural Experiment Station with partial support from the Center for Energy and Mineral Resources, Texas A&M University.
Ethanol production from sweet sorghum
117
REFERENCES 1. Bryan, W. L., Monroe, G. E., Nichols, R. L. & Gascho, G. J. (1981).Evaluation of sweet sorghum for fuel alcohol, ASAE Paper No. 81-3571, St Joseph, American Society of Agricultural Engineers. 2. Reidenbach, V. G. & Coble, C. G. (1982). Sugarcane or sweet sorghum processing techniques, ASAE Paper No. 82-3562, St Joseph, American Society of Agricultural Engineers. 3. Stephenson, K. Q. (1983). Sweet sorghum biomass production and processing, ASAE Paper No. 83-2064, St Joseph, American Society of Agricultural Engineers. 4. Miller, F. R. & Creelman, R. A. (1980). Sorghum - a new fuel. In: Proceedings of the 35th Annual Corn and Sorghum Research Conference, Chicago, 9-11 December, pp. 219-32.
Processing Techniques for Ethanol Production from Sweet Sorghum* C h a r l i e G. C o b l e a n d R i c h a r d P. E g g Agricultural Engineering Department, Texas A&M University, College Station, TX 77843, USA and Itzhak Shmulevich Agricultural Engineering Department, Technion, Haifa, Israel (Received: 27 May, 1984) ABSTRACT Several processing techniques for producing ethanol from sweet sorghum were investigated. Fermenting chopped stalks yieMed more ethanol than shredded sorghum or juice. Leaf removal prior to fermentation yieMed slightly more ethanol than solids removal before fermentation. Key words: sweet sorghum, processing, alcohol, ethanol, fermentation.
INTRODUCTION Sweet sorghum has been noted for its potential as an energy crop. If all the sugar it produces were converted to ethanol, the yield could be higher than 4000 liters ha -1. Unlike sugarcane, which is a tropical plant, sweet sorghum can be cultivated in nearly all temperate and tropical climatic areas. Most sorghums are more drought-tolerant than corn, the leading alcohol feedstock in the US. However, two serious drawbacks have prevented sweet sorghum from being widely utilized as an ethanol feedstock. The first is storability. While grain, the predominant ethanol * Approved by the Texas Agricultural Experiment Station as Manuscript No. TA 19330. 111 Biomass 0144-4565/84/$03.30- © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain
112
c. G. Coble, R. P. Egg, L Shmulevich
feedstock, can be stored indefinitely, sweet sorghum tends to deteriorate after harvest. Researchers have been working on I this problem with varying degrees of success, but no practical solution has been found. A second problem is sugar extraction with currently available technology. Over 90% of the sugar could be extracted using sugar mill equipment, but these mills are extremely capital and labor intensive, and would only be feasible for extremely large alcohol producing systems. Using simple three roll mills, only about 40% of the sugar can be extracted. 1 Reidenbach and Coble 2 explored alternative methods of processing sweet sorghum for ethanol production. The maximum sugar recovery reported was 74.2%, obtained by fermenting the whole stalk chopped into 1 cm lengths. However, water added to the chopped sorghum to enable fermentation resulted in low ethanol concentrations in the fermented beer. Stephenson 3 investigated several methods for sweet sorghum juice removal. Highest yields were obtained by finely chopping the sorghum and separating the pith which was then squeezed by a hydraulic press to remove the juice. Results obtained by these researchers indicate that sugar recovery can be substantially improved over the recovery of a simple roller mill which is currently about the only commercially available technology. The objective of the work reported here was to evaluate a number of alternative techniques for producing ethanol from sweet sorghum.
EQUIPMENT AND PROCEDURES The sorghum used in this study was a hybrid grain-sweet sorghum ATX 623X Rio developed at Texas A&M University.4 The sorghum was hand-harvested by cutting just above ground level and brought to the laboratory where the tops were removed and the leaves stripped. It was then processed, fermented and distilled. For all tests, except the study to determine the effect of the leaves on ethanol yield, the leaves were stripped, but the leaf sheath was left on the stalk. Three separate studies were conducted. The first study was used to evaluate processing techniques. Three methods were employed. Sorghum was either chopped into 1-2 cm lengths and shredded with a hammermill, or shredded with a hammermill followed by extracting juice with a
Ethanol production from sweet sorghum
1 13
cage press with a pressure on the shredded material of 10000 kPa. Sufficient water was added to cover the solid material (with the exception of the juice), heated to 95°C for 5 min and cooled. Yeast was then added and the sorghum was fermented at 32°C for approximately 72 h. The material was then distilled and the alcohol yield measured. The cooking and distillation was done in an 18-liter bench-top pot still. A second study evaluated the effect of sorghum leaves on ethanol yield. The treatments were stalks with leaves and leaf sheaths, leaves removed but leaf sheaths remaining on the stalks, and stalks only with the leaves and leaf sheaths removed. After removal of the leaves and/or leaf sheaths, the stalks were shredded in a hammermill, covered with water and fermented, as previously described. A third study involved solids removal from the mash or beer before and after fermentation. In this study sorghum was shredded in the hammermill. Water was added to cover the solids. The mixture was then heated to 95°C for 5 min and cooled. In one treatment, the liquid was then removed with the hydraulic press. Yeast was added to the liquid, which was then fermented and distilled. In the second treatment, the yeast was added to the mixture and allowed to ferment. After fermentation was complete, the liquid was extracted with the hydraulic press and distilled. In the control, the liquid was not separated from the solids, and the entire mixture was fermented and distilled.
RESULTS Results are reported in liters of 200 proof ethanol per tonne of unprocessed sorghum. In the case of the leaf study, the feedstock was after the leaves and/or leaf sheaths had been removed. Results of the study are shown in Table 1.
Processing Results obtained from the processing study showed no significant differences at the 0-05 level. However, yields were highest from the chopped sorghum. It was anticipated that shredding the stalks would produce the highest yield since more surface area is exposed to allow sugar diffusion into the fermentation media. Possibly the increased
114
C. G. Coble, R. P. Egg, L Shmulevich TABLE 1
Ethanol Yield from Sweet Sorghum Study
Treatment
Number of replicates
Ethanol yieM (liters t-1)
Processing
Chopped Shredded Juice
4 12 9
45.4 Aa 42.1 A 39.6 A
Leaf content
Stalk only Stalk and leaf sheaths Stalk, leaves and sheaths
3 17 10
37-2 A 38.5 A 29.3 B
Solids removal
Not removed Removed after fermentation Removed before fermentation
18 8 4
38-3 A 31.5 B 29-3 B
a Same letters for each study indicate no significant difference at the 0.05 level.
surface area allowed more microbial contamination and some deterioration occurred prior to sterilization, thereby slightly reducing ethanol yields. The time between shredding and sterilizing normally ranged from about 0 to 3 h. Compared with the whole stalk fermentations, the hydraulic cage press performed remarkably well. While a cage press is essentially a laboratory tool and not well suited for adaptation to continuous processing, the implication of these results is that high pressures can extract substantial amounts of sugars. The overall juice extraction using the cage press was 52% of the total stalk weight. Leaf c o n t e n t Removing the leaves prior to processing results in higher ethanol yields (Table 1). Leaving the leaf sheaths on the stalk, however, did not significantly affect ethanol yield. Stripping the leaves in the field prior to or during harvest would appear to be advantageous. The relative amount of leaves on ethanol yield per unit feedstock will vary somewhat depending on stage of maturity. Figure 1 shows the relative percent of total plant weight of the sorghum variety used throughout the harvest
115
Ethanol production from sweet sorghum
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C. G. Coble, R. P. Egg, L Shmulevich
period. Peak maturity occurred during the first week of August. Leaf weight ranged from 7-3 to 31.6% of total plant weight during the period 22 July-1 September. Solids removal
If whole-stalk fermentation using chopped or shredded sorghum is employed, the solid material must first be removed if the liquid phase is to be distilled in a conventional sieve tray distillation column. Removing the solids after fermentation yielded about 82% and before fermentation about 77% as much ethanol as obtained by not removing the solids (Table 1). Although the results were not significant at the 0.05 level, ethanol yield was slightly higher when the solids and liquids were separated after fermentation. This advantage could possibly be exploited in a type of semicontinuous whole-stalk fermentation system employing several fermentation stages.
CONCLUSIONS Fermenting whole-stalk sorghum results in higher ethanol yields than does juice extraction. A type of diffusion or continuous fermentation process fermenting whole-stalk material may result in higher ethanol yields from sweet sorghum than using juice extraction techniques for sugar extraction. Removal of leaves prior to processing results in higher yields per unit feedstock. While sweet sorghum has a high potential as an ethanol feedstock, much work is still needed before it is practical as an alternative to grain in existing ethanol production systems.
ACKNOWLEDGMENT This research was conducted by the Texas Agricultural Experiment Station with partial support from the Center for Energy and Mineral Resources, Texas A&M University.
Ethanol production from sweet sorghum
117
REFERENCES 1. Bryan, W. L., Monroe, G. E., Nichols, R. L. & Gascho, G. J. (1981).Evaluation of sweet sorghum for fuel alcohol, ASAE Paper No. 81-3571, St Joseph, American Society of Agricultural Engineers. 2. Reidenbach, V. G. & Coble, C. G. (1982). Sugarcane or sweet sorghum processing techniques, ASAE Paper No. 82-3562, St Joseph, American Society of Agricultural Engineers. 3. Stephenson, K. Q. (1983). Sweet sorghum biomass production and processing, ASAE Paper No. 83-2064, St Joseph, American Society of Agricultural Engineers. 4. Miller, F. R. & Creelman, R. A. (1980). Sorghum - a new fuel. In: Proceedings of the 35th Annual Corn and Sorghum Research Conference, Chicago, 9-11 December, pp. 219-32.