WO2009086781A1

WO2009086781A1 - Method for producing ethanol from root and tuber crops

Info

Publication number: WO2009086781A1
Application number: PCT/CN2008/073879
Authority: WO
Inventors: Guojun Yue; Yong Jiang; Zhimin Liu; Likang Deng; Xiaofeng Liu; Yongxin Zhang; Xiaoming Hao
Original assignee: Cofco Limited
Priority date: 2008-01-04
Filing date: 2008-12-31
Publication date: 2009-07-16
Also published as: CN101289674A; CN100489108C

Abstract

A method for producing ethanol from root and tuber crops comprises pulverizing skin-removed raw material by subjecting skin-removed raw material to a first pulverization to give a first pulverization product, subjecting part of the first pulverization product to a second pulverization to give a second pulverization product,,mixing the rest part of the first pulverization product and the non-pulverized skin-removed raw material, and subjecting the resulting mixture to a first pulverization; mixing the pulverization product with enzyme, and subjecting the resulting mixture to enzymolysis; and fermenting the enzymolysis product. The inventive method has the advantage of low water consumption, low energy consumption, high facility utilization rate, and good pulverization effect. Moreover, the pulverization method in the present invention can well control particle size of the pulverized raw material, and achieve small and uniform particle size of the raw material.

Description

Method for producing ethanol from root and tuber crops

Technical Field

The present invention relates to a production method of ethanol, and particularly relates to a method for producing ethanol from root and tuber crops.

Technical Background

Root and tuber crops, such as sweet potato, potato, and cassava etc., are rich in starch, and thus are widely used in production of sugar by fermentation and production of starch.

Cassava is tropical and subtropical perennial, or temperate annual shrub belonging to Manihot Mill., originated from South America, and is suited to grow in low latitude area with average temperature of 25-29 ^°C and annual average precipitation of 1,000- 1,500mm. Before about 1820, cassava was introduced to southern China, and mainly planted in Guangdong, Guangxi, and Hainan. Nowadays it has been expanded to other provinces, such as Yunnan, Fujian, and Guizhou, etc. Cassava may be divided into two types: bitter cassava (toxic cassava) and sweet cassava (nontoxic cassava). Water is the primary chemical component of fresh root tuber of cassava, and carbohydrate is the second; additionally, cassava also contains a small amount of protein, fat, and pectin. The starch content of fresh cassava can reach 25-30wt%, and the starch content of dry cassava is about 70wt%.

As the root tuber of cassava is relatively large, before enzymolysis and fermentation for ethanol production, cassava raw material is usually subjected to pulverization to destroy texture thereof, so as to disintegrate and separate starch particles from the root tuber. The cassava texture has to be disintegrated fully and finely to realize more thorough separation of the starch particles to improve glucose extraction ratio. In the available method for producing ethanol from cassava, the pulverization process of cassava usually comprises mixing cassava and water, and then pulverizing to give pulverization product. This method requires high water consumption; and for ensuring that the pulverization product has satisfied particle size, the available pulverization method requires high energy consumption, and has low facility utilization rate.

Summary of the Invention

The object of the present invention is to overcome the disadvantages of the pulverization method in available methods for producing ethanol from root and tuber crops, such as high water consumption, high energy consumption, and low facility utilization rate, and provide a method of producing ethanol from root and tuber crops using a pulverization process with low water consumption, low energy consumption, and high facility utilization rate.

The inventor of the present invention has found that in the pulverization process of available methods for producing fuel ethanol from root and tuber crops, no matter whether the raw material is fresh cassava or dry cassava, the raw material has to be mixed with water (usually at mixing ratio of 1 :1) before pulverization, and the water consumption during pulverization is relatively high; therefore, the quantity of pulverization product is large, which leads to storage difficulty. Additionally, the available pulverization process usually comprises two-step pulverization, i.e. a first pulverization is carried out after cassava raw material is mixed with water, and then the first pulverization product is subjected to second pulverization to obtain the final pulverization product. The particle size of the final pulverization product obtained by the available pulverization method is not uniform and relatively large. In order to ensure that the pulverization product obtained by the available method satisfies certain requirement, the available pulverization method requires long pulverization time, which leads to high energy consumption and low facility utilization rate.

The present invention provides a method for producing ethanol from root and tuber crops, which comprises: pulverizing skin-removed raw material by subjecting skin-removed raw material to a first pulverization to give a first pulverization product, subjecting part of the first pulverization product to a second pulverization to give a second pulverization product, ,mixing the rest part of the first pulverization product and the non-pulverized skin-removed raw material, and subjecting the resulting mixture to a first pulverization; mixing the pulverization product with enzyme, and subjecting the resulting mixture to enzymolysis; and fermenting the enzymolysis product.

In the inventive method for producing ethanol from root and tuber crops, the pulverization process of skin-removed raw material comprises two-step pulverization, and is different from the available method in that it is not required to mix the raw material with water before pulverization, or only small amount of water is added, then part of the first pulverization product is returned back to be mixed with non-pulverized skin-removed raw material, and then subjected the resulting mixture to a first pulverization. The returned part of the first pulverization product is starch slurry of raw material particles, which can exert dilution action, and also can be pulverized again together with non-pulverized skin-removed raw material. The inventive method not only has low water consumption, low energy consumption, high facility utilization rate, and high pulverization efficiency, but also can well control particle size of raw material particles in the pulverization product, so as to obtain raw material particles with small and uniform particle size, and solve the storage problem of the pulverization product. Additionally, the inventor of the present invention has surprisingly found that the ethanol yield of the inventive method for producing ethanol from root and tuber crops is significantly increased; the possible explanation could be that as the inventive pulverization process can well control particle size of the pulverization product to obtain raw material particles with small and uniform particle size, starch in the pulverization product can be fully released, thus starch microparticles in the final pulverization product can be fully utilized, i.e. fully contacted with enzyme for enzymolysis, to increase starch utilization rate and ethanol yield. For example, under same conditions, the method in example 2 has ethanol yield based on starch of 52.5%, and residual glucose content of 1.5%; while the method in comparison example 1 has ethanol yield based on starch of 51.0%, and residual glucose content of 2.0%; therefore, the ethanol yield is increased by 2.9%, and the residual glucose content is decreased by 25%.

Description of Drawings

Fig.1 shows front view of the skin-removing device for root and tuber crops used in the method according to the present invention;

Fig. 2 shows longitudinal section view of the skin-removing device;

Fig. 3 shows cross section view representing connection relationship of chain wheel and rotary drum.

Embodiments

According to the present invention, the inventive method for producing ethanol from root and tuber crops comprises: pulverizing skin-removed raw material by subjecting skin-removed raw material to a first pulverization to give a first pulverization product, subjecting part of the first pulverization product to a second pulverization to give a second pulverization product, ,mixing the rest part of the first pulverization product and the non-pulverized skin-removed raw material, and subjecting the resulting mixture to a first pulverization; mixing the pulverization product with enzyme, and subjecting the resulting mixture to enzymolysis; and fermenting the enzymolysis product.

According to the present invention, the rest part of the first pulverization product is 5-20wt% of the total first pulverization product, and is 5-20wt% of the total weight of the rest part of the first pulverization product and the non-pulverized skin-removed raw material. Preferably, the rest part of the first pulverization product is 5-10wt% of the total first pulverization product, and is 5-10wt% of the total weight of the rest part of the first pulverization product and the non-pulverized skin-removed raw material. The final pulverization product obtained by the inventive method has small particle size and uniform particle size distribution, and the particle size can be less than 2.5mm but not less than 1.6mm, preferably 1.6-1.8mm.

According to the present invention, it is not required to mix the skin-removed raw material with water before the first pulverization, or only small amount of water is added, for example, the weight ratio of the added water to the dry weight of the raw material can be 0.5-2, preferably 1-1.5. Preferably, it is not required to mix the raw material with water before pulverization, while part of first pulverization product is returned back, and mixed with non-pulverized skin-removed raw material, and then the resulting mixture is subjected to first pulverization. The returned part of first pulverization product is starch slurry of raw material particles, which can exert dilution action, and thus there is no need for mixing raw material with water in initial pulverization. Additionally, the returned first pulverization product can be pulverized again with fresh raw material, which has the advantage of low water consumption, low energy consumption, high facility utilization rate, and good pulverization effect. Moreover, the pulverization method in the present invention can well control particle size of the pulverized raw material, and achieve small and uniform particle size. According to the present invention, for ensuring the raw material particle size of the final pulverization product is less than 2.5mm but not less than 1.6mm, preferably 1.6- 1.8mm, the raw material particle size of the first pulverization product is controlled within 2.5- 10mm, preferably 2.5-5mm.

According to one embodiment of the present invention, when the feeding manner is continuous, part of first pulverization product, resulted from first pulverization of mixture of part of first pulverization product and non-pulverized skin-removed raw material, can be taken out, further mixed with non-pulverized skin-removed raw material, and subjected to first pulverization. The pulverization can be continuously cycled according to the aforementioned method.

According to another embodiment of the present invention, when the feeding manner is intermittent, first pulverization product, resulted from first pulverization of mixture of part of the first pulverization product and non-pulverized skin-removed raw material, can be directly subjected to second pulverization to give second pulverization product, or the first pulverization product can be mixed with the initial first pulverization product, and then subjected to second pulverization.

Various routine pulverization equipments can be adopted, such as SFSP series hammer-type pulverizers.

The root and tuber crops in the present invention can be various, such as sweet potato, potato, and cassava, etc. In the embodiment of the present invention, cassava is adopted as root and tuber crops. Either fresh or dry cassava can be adopted. When dry cassava is adopted, mixing with water is required before first pulverization, and the amount of the added water can be determined according to the aforementioned method in the present invention. Mixture of fresh cassava and dry cassava can also be adopted. There is no special requirement on weight ratio of dry cassava to fresh cassava, such as 1 :1.5-2.5, preferably 1 :1.5-2.

Ripe cassava has diameter of about 4-8cm, and length of 20-30cm, which is podgy and in shape of short cylinder. Cassava has structure of flesh and skin from inner to outer, wherein the skin comprises inner skin and outer skin. The outer skin is in dark brown color, and has spaced white circular strips, and the inner skin and flesh are in white color. The skin of cassava, particularly inner skin, contains cyanide and cyanogenic glycoside, linamarin, which is capable of causing food poisoning. Linamarin can generate hydrocyanic acid after being hydrolyzed. Hydrocyanic acid and cyanide are both extremely toxic and can cause acute poisoning. They can enter into human body via various means, such as absorption via skin or wound, or intake via respiratory passage or oral administration by mistake; after entering into human body, they can cause paralysis of central nervous system, and intoxication of respiratory enzyme and hemoglobin in blood, and thus lead to respiratory difficulty, oxygen deficiency/suffocation of body cells, and death. Therefore, the inner skin of fresh cassava raw material has to be removed before pulverization.

There are many methods for removing inner skin of fresh cassava raw material; the available methods usually adopt manual skin-removing means to remove inner and outer skin of fresh cassava raw material, as well as sand/soil on the raw material surface. If the inner skin is not removed or not thoroughly removed, cyanide residue content in cassava raw material will be high, which can lead to inactivation of enzyme and intoxication and death of yeast during enzymo lysis and fermentation process, and thus lead to high residual glucose content and low alcohol degree of mash, i.e. low ethanol yield. And the manual skin-removing means has low production efficiency and high labor intensity.

Therefore according to the present invention, the inventive method preferably further comprises removing skin of fresh cassava raw material by using a skin-removing device. As illustrated in Figs. 1 and 2, the skin-removing device comprises: a pedestal 1; a rotary drum 2 rotatably fitted on the pedestal 1 and having an inlet 4 and a outlet 5; a helical screw feeder 3 provided in the rotary drum 2 and fixedly connected with the inner wall of the rotary drum 2; and a driving unit for driving the rotary drum 2 and the helical screw feeder 3 to rotate together.

The step of removing skin comprises feeding raw material of root and tuber crops into the rotary drum 2 via the inlet 4, and allowing the driving unit to drive the rotary drum 2 and the helical screw feeder 3 to rotate together. The rotation speed of the rotary drum 2 and the helical screw feeder 3 can be 2-50 rpm, preferably 5-25 rpm. Under push action of the helical screw feeder 3, the raw material is continuously moved forward, and at the same time the raw material rotates along with the rotary drum 2 and the helical screw feeder 3. During rotation, friction is generated not only among the raw material, but also between the raw material and the rotary drum wall and the helical screw feeder, so as to remove the skin of the raw material; and the skin-removed material is discharged from the outlet 5.

The rotary drum 2 can be made from various wear-resistant materials, such as steel, rubber, or rigid plastics. The rotary drum 2 can be further provided with spraying unit therein. The spraying unit can be fixedly fitted on the inner wall of the rotary drum, and located close to the inlet of the rotary drum. The spraying unit can be various common spraying units. According to the invention, the method can further remove dirt (such as soil or impurities) on the raw material by spraying water on the material via the spraying unit. There is no special limitation on the amount of sprayed water, as long as it is sufficient to remove dirt on the raw material.

Preferably, as shown in Fig. 1, the rotary drum 2 comprises, from inlet end to outlet end, a first section rotary drum 2a and a second section rotary drum 2b which are communicated with each other, and a spraying unit is provided in the second section rotary drum 2b. The spraying unit can be fixedly fitted on the inner wall of the second section rotary drum 2b, and located close to the inlet of the second section rotary drum.

According to the method in the present invention, a friction structure may be further provided on the inner wall of the rotary drum 2 in order to achieve better skin-removing effect. The friction structure can be various structures with rough surface, preferably one or more ribbed steel bars, more preferably multiple ribbed steel bars. The ribbed steel bar has transversal ribs, and can be various conventional hot rolled ribbed steel bar and cold rolled ribbed steel bar, such as ribbed steel bar complying with Chinese National Standard GB 1499- 1998. The nominal diameter of the ribbed steel bar can be 6-25 mm, preferably 8-20 mm. The interval between the transversal ribs can be 3-16 mm, preferably 4-12mm. The grade of the ribbed steel bar includes, but is not limited to, HRB335, HRB400 and HRB500. The ribbed steel bar is fixedly connected with the inner wall of the rotary drum 2, such that it can exert friction action to the raw material during rotation of the rotary drum. For easily fixedly connecting the ribbed steel bar onto the inner wall of the rotary drum 2, preferably the ribbed steel bar is parallel to the central axis of the rotary drum.

The rotary drum 2 can be horizontally or obliquely installed on the pedestal 1. When the rotary drum is horizontally installed, the raw material is moved forward under push action of the helical screw feeder 3, and finally discharged via the outlet of the rotary drum. When the rotary drum is obliquely installed, since the position of the inlet is higher than that of the outlet, the raw material can be moved downward under its own gravity action (i.e. moved forward) as well. The inclination angle of the rotary drum 2 can be 0-15 degrees, preferably 5-10 degrees. The length of the rotary drum 2 can be 2-10 m, preferably 3.5-7 m. When the rotary drum comprises the first section rotary drum and the second section rotary drum, the length refers to sum of lengths of the first section rotary drum and the second section rotary drum. The inclination angle refers to included angle between the central axis of the rotary drum and the horizontal line. There is no special limitation on inner diameter of the rotary drum, which can be selected according to the amount of the raw material to be processed. For example, generally, the inner diameter of the rotary drum is 1-2 m.

The helical screw feeder 3 can be various common helical screw feeders in the mechanical field. The helical screw feeder 3 can be connected on the inner wall of the rotary drum 2 via various fixed connection means. For example, as shown in Fig. 2, the helical screw feeder 3 is fixedly connected on the inner wall of the rotary drum 2 via a fastener 8. To achieve better skin-removing effect, the pitch of the helical screw feeder 3 is preferably 0.3-0.8m, and the height of the screw thread is preferably 0.1 -0.4m. The helical screw feeder can be made of various wear-resist materials, such as steel, rubber, or nylon, etc.

The present invention has no special limitation on the driving unit, as long as it can drive the rotary drum 2 and the helical screw feeder 3 to rotate together. For example, the driving unit may comprise a driving source, a transmission chain, and a chain wheel 6. As shown in Fig. 3, the chain wheel may be fixed on the rotary drum 2. When the rotary drum 2 comprises the first section rotary drum 2a and the second section rotary drum 2b, the chain wheel 6 is preferably fitted between the first section rotary drum 2a and the second section rotary drum 2b. As the rotary drum 2 is rotatably fitted on the pedestal 1 , the chain wheel can drive the rotary drum to rotate when the transmission chain transfers the driving power of the driving source to the chain wheel. The rotatable fitting manner can be realized by various common methods, for example, support roller or frame can be adopted to fit the rotary drum on the pedestal to allow the rotary drum to rotate around the central axis. The driving source can be various units capable of generating power, such as motor.

For the convenience of feeding, the skin-removing device may further comprise a windmill feeder 7. As shown in Fig. 1 or 2, the windmill feeder 7 is fitted at the inlet 4 of the rotary drum. The windmill feeder 7 can be various common windmill feeders in the mechanical field.

The skin-removing device in the present invention utilizes friction action among raw material and between the raw material and the rotary drum wall to remove skin of the raw material. By removing the skin of cassava material according to the method of the present invention, the cyanide removal rate can reach 75% or higher, and raw material loss rate can be kept below 5wt%; thus ethanol yield is significantly increased.

The root and tuber crops may be various, such as sweet potato, potato, and cassava, etc. In the embodiment of the present invention, cassava is adopted as root and tuber crops. As the raw material may contain soil, sand/stone, and iron impurities which may cause damage to the skin-removing device, the raw material is preferably subjected to pretreatment process before removing the skin. The pretreatment process may be conventional method, and usually comprises removing impurities and cleaning. For example, after harvest of fresh cassava, soil, root, whisker, xylem part and sand/stone on the cassava are removed; and cassava is cleaned by using the equipment and method well known to those skilled in the arts. The enzymolysis procedure can be carried out by common methods in the field, for example, the procedure may comprise adding enzyme-producing microbes and/or enzyme into the pulverized material, and incubating at the growth temperature of the enzyme-producing microbes or activation temperature of the enzyme. The enzyme-producing microbes are those capable of secreting amylase. The said enzyme comprises amylase.

As microbes will produce byproduct during growth, it is preferred to add enzyme directly. The effect of enzymolysis will be better if more amount of enzyme is used. In consideration of cost, the usage amount of amylase is preferably 4-50 enzyme activity units, more preferably 10-30 enzyme activity units per gram of pulverized material (on dry basis).

The enzyme activity unit in the present invention is defined as below: one enzyme activity unit is the amount of enzyme required for converting starch lmg into glucose within lmin at 70 ^°C and pH of 6.0.

The enzymolysis can be carried out at any temperature suitable for amylase to exert action, generally 50-90^°C , more preferably 60-70 ^°C. Theoretically, the effect of enzymolysis will be better if the enzymolysis is carried out longer. For the sake of equipment utilization efficiency, the enzymolysis time is 20-240min, more preferably 30-120min. The pH of enzymolysis can be any pH suitable for amylase to exert action, generally 3.0-7.0, more preferably 5.0-6.0. As little pH fluctuation occurs during enzymolysis, pH of enzymolysis can be adjusted before addition of enzyme by known methods in the field. For example, pulverized material is mixed with water or culture medium (usually enzyme is mixed with water, and enzyme-producing microbes are mixed with the culture medium for the microbes). Usually the resulting mixture has solid content of 20-40wt%, and pH of the mixture to be enzymolyzed is adjusted to 3.0-7.0, preferably 5.0-6.0 by using sulfuric acid or sodium hydroxide.

Amylase generally refers to a kind of enzymes capable of decomposing starch glycosidic linkage, and the may comprise α-amylase, β-amylase, glucoamylase, and isoamylase. α- Amylase, also called 1 ,4-α-D-glucan glucanohydrolase, can arbitrarily and irregularly cleave α-1, 4-glucosidic linkage in starch chain, and hydro lyze starch into maltose, oligosaccharide with 6 glucose units, and oligosaccharide with side chain. The microbes producing α-amylase mainly comprise Bacillus subtilis, Aspergillus niger, Aspergillus oryzae, and Rhizopus spp. β- Amylase, also called 1 ,4-α-D-glucan maltohydrolase, can cleave 1, 4-glucosidic linkage from non-reducing end of starch molecule to produce maltose. The product resulted from action of the enzyme on starch is maltose and limit dextrin. The enzyme is mainly produced by Aspergillus spp., Rhizopus spp., and Endomyces spp.

Glucoamylase, also called α-l,4-Glucan glucohydrolase, sequentially acts on α-1, 4-glucosidic linkage in starch molecule from non-reducing end with glucose as unit to yield glucose. The product resulted from action of the enzyme on branched starch is glucose and oligosaccharide with α-1, 6-glucosidic linkage, while the product resulted from action of the enzyme on linear starch is almost glucose. The microbe producing the enzyme mainly comprises Aspergillus niger (Aspergillus usamii, and Aspergillus awamori), Rhizopus spp. (Rhizopus niveue, and Rhizopus delemar), Endomycopsis spp., and Monascus spp. Isoamylase, also called Glucan 1,6-α-glucosidase or branching enzyme, acts on α-1, 6-glucosidic linkage of branching spot of branched starch molecule, and cleaves off the whole side chain of the branched starch to give linear starch. The microbes producing the enzyme mainly comprise anaerobic Bacillus spp., Bacillus spp., and certain Pseudomonas spp.

Preferably the enzymes adopted in the enzymolysis further comprise phosphatase. As phosphatase can hydrolyze phosphorylated dextrin, which is formed from the esterification of phosphoric acid and alcoholic hydroxyl, into glucose while releasing phosphoric acid, it has significant liquefying ability. Therefore, the enzyme adopted in the enzymolysis includes phosphatase in order to fully hydrolyze starch to increase ethanol yield.

All microbes capable of fermenting monosaccharide (such as glucose and/or fructose) and oligosaccharide (such as sucrose and/or galactose) can be used in the fermentation process of the present invention. Saccharomyces cerevisiae is preferably used in the present invention, because it is widely adopted microbe for hexose fermentation with ethanol resistance, less byproducts, and high ethanol yield.

Relative to per gram of enzymolysis product, the inoculation amount of the said yeast used in the fermentation is about 10³-10⁸ colony forming units, more preferably 10⁴-10⁶ colony forming units.

The said colony forming unit is defined as below: microbial unicells in a certain amount of diluted microbial liquid are dispersed on a culturing medium plate by casting or coating the liquid on the medium plate, and each unicell forms one colony after cultivation, i.e. unicell number contained in each millimeter of microbial liquid.

The yeast used in the fermentation of the present invention can be commercial yeast solid preparation (such as dry yeast powder) or yeast strain, such as Rasse II yeast (also called Germany II yeast), Rasse XII (also called Germany XII yeast), Yeast K, Nanyang V yeast (1300), and Nanyang mixed yeast (1308). The colony forming unit of the yeast can be determined according to known methods in the field, such as plate count using methylene blue dye, the detail of which comprises: dissolving dry yeast powder Ig in sterile water 10ml, or diluting strain activation liquid with sterile water to 10ml; adding 0.1 wt% methylene blue 0.5ml; holding at 35 ^°C for 30min; and counting viable bacteria (viable bacteria will not be dyed while dead bacteria will be dyed) in the solution by using blood cell counting plate and 10x optical microscope to give number of viable bacteria in Ig of the dry yeast or ImI of the strain activation liquid, i.e. the number of colony forming units.

The yeast can be inoculated by routine methods, such as adding seed liquid 5-15vol% into enzymolysis product. The seed liquid can be either aqueous solution or culturing medium solution of dry yeast, or activation seed liquid of dry yeast or commercial strain.

The fermentation temperature can be any suitable for yeast growth, preferably 30-36 ^°C , more preferably 30-33 ^°C . And the fermentation pH may be 4-6, preferably 4-4.5. The fermentation time is the time from inoculation to occurrence of decline phase of the yeast growth (i.e. the fermentation time is the sum of lag phase, log phase, and stable phase), which is preferably 55-70hr, more preferably 60-70hr. The ethanol as fermentation product can be subjected to separation and refinement, such as distillation, concentration, and dewatering, according to requirement of different industrial products (for example fuel ethanol is required to have a purity of 99% or higher).

The present invention will be described in further details through following examples.

Example 1

The present example is for illustrating the inventive method for producing ethanol from cassava material.

(1) Skin-removing and pulverization of cassava raw material The skin-removing device is shown in Fig. 1, 2, and 3.

A rotary drum 2 includes a first section rotary drum 2a and a second section rotary drum 2b from up to down, the first section rotary drum 2a and the second section rotary drum 2b are communicated, and the lengths of the first section rotary drum 2a and the second section rotary drum 2b are respectively 1.8m and 1.6m. The rotary drum 2 is made of steel, with inner diameter of 1.6m. 40 hot-rolled ribbed steel bars (Grade No. HRB335, and nominal diameter of 12mm) are fixed on the inner wall of the first section rotary drum 2a, parallel to the central axis of the rotary drum, and distributed at equal interval of 0.125m along circumference of the rotary drum inner wall. 50 hot-rolled ribbed steel bars (Grade No. HRB500, and nominal diameter of 16mm) are fixed on the inner wall of the second section rotary drum 2b, parallel to the central axis of the rotary drum, and distributed at equal interval of 0.1m along circumference of the rotary drum inner wall. The rotary drum 2 is obliquely fitted on a pedestal 1 at inclination angle of 5 degrees. A helical screw feeder 3, which is made of rubber, and has pitch of 0.5m, and screw thread height of 0.2m, is fixedly connected on the inner wall of the rotary drum 2 via a fastener 8. A driving device comprises a motor, a transmission chain, and a chain wheel 6. The chain wheel is fixed on the rotary drum 2, the transmission chain transfers the power of the motor to the chain wheel, and the motor has a power of 5.5kW.

The motor is started to drive the rotary drum 2 and the helical screw feeder 3 to rotate around the central axis of the rotary drum at 7rpm. The harvested cleaned fresh cassava 100kg with diameter of 4-8cm, length of 20-30cm, and water content of about 65wt% are continuously fed into the rotary drum 2 via the inlet 4, and the skin-removed cassava is discharged from the outlet 5 to obtain skin-removed cassava material 95kg. The average retention time for cassava in the rotary drum is 1.5 min.

The skin-removed cassava material 95kg is cleaned, cut into about pieces 1 cm thick, and pulverized by using SFSP series hammer pulverizer. The pulverization process comprises: subjecting 60kg of skin-removed cassava to first pulverization for 20min to obtain 60kg of first pulverization product with particle size of 4-5mm, mixing 10wt% of 60kg of the first pulverization product with 35kg of the rest non-pulverized skin-removed fresh cassava, and subjecting the mixture to first pulverization for 15min to obtain first pulverization product with particle size of 3 -4mm; and subjecting the total aforementioned first pulverization product to second pulverization for 5min to obtain 95kg of pulverization product with cassava particle size of about 2mm.

1Og of aforementioned pulverization product is filtered and dried at 45 ^°C to constant weight of 3.4g, and 300. Omg of the dried pulverization product is weighed, and placed in 100ml dry conical flask (with weight of 80g). 72wt% sulfuric acid 3.00ml is added into the conical flask, and stirred for lmin. Then the conical flask is disposed in 30^°C water bath for 60min, and the content is stirred once every 5min for ensuring uniform hydrolysis. After hydrolysis is completed, the sulfuric acid solution is diluted to 4wt% with deionized water, and filtered to give filtrate 84ml. 20ml of the filtrate is transferred to dry 50ml conical flask, pH of the filtrate is adjusted to 5.5 with 2.5g of calcium carbonate, then the filtrate is allowed to stand for 5hr, and the supernatant is collected. The collected supernatant is filtered with 0.2μm filter membrane, and the obtained filtrate is analyzed by HPLC (Biorad Aminex HPX-87P) at following conditions: sample size: 20 μl; mobile phase: HPLC ultrapure water filtered with 0.2μm filter membrane and degassed via ultrasonic oscillation; flow rate: 0.6ml/min; column temperature: 80-85 ^°C ; detector (refractive index detector) temperature: 80-85 ^°C; and run time: 35min.

D-(+) glucose with concentration of 0.1-4.0mg/ml is adopted as standard. The

HPLC result shows that the glucose concentration of acidic hydrolysis liquid of the pulverization product is 3.70mg/ml, which can be calculated to give that Ig of pulverization product can obtain 0.31 Ig of glucose via acidic hydrolysis. As 72wt% sulfuric acid solution can completely hydrolyze the starch in the pulverization product into glucose, the obtained glucose weight is 1.11 times of the weight of the starch in the pulverization product, i.e. starch contained in Ig of pulverization product is

0.28Og. Therefore 95kg of pulverization product contains starch 26.6kg.

(2) Enzymolysis

The pulverization product from step (1) is mixed with 21kg of water, heated to 70 ^°C after pH is adjusted to 5, added with α-amylase (purchased from Novozymes) 20 enzyme activity units per gram dry pulverized product, and held at 70 ^°C for enzymolysis for 60min to give enzymolysis product.

(3) Fermentation

The enzymolysis product is cooled to 33 ^°C, and inoculated with distillery yeast (Angel super alcohol active dry yeast, Hubei Angel Yeast Co., Ltd., China) at 10⁵ colony forming units per gram enzymolysis product. The resulting mixture is cultivated at 33 ^°C in fermentation tank for 65 hr with stirring. The fermentation product is distilled at 100^°C, and then the obtained distillation fraction is redistilled at 78.3 ^°C to obtain ethanol 14.05kg. The ethanol yield can be calculated according to the equation as below, and the result is shown in Table 1.

Ethanol yield = 100%^χ weight of ethanol /weight of starch in cassava raw material lOOg of fermented mash after ethanol distillation is filtered with Buchner's filter, 20ml of the filtrate is transferred to 50ml dry conical flask, and allowed to stand for 5hr to collect supernatant. The supernatant is filtered with 0.2μm filter membrane, and analyzed according to the HPLC condition in the step (1). The calculated glucose content in the fermented mash is 342g, the residual glucose content is calculated according to following equation, and the calculation result is shown in Table 1.

Residual glucose content = residual glucose content of fermented mash/weight of starch in cassava raw material Example 2

Ethanol is produced according to the same method as the example 1, except that manual skin-removing method is adopted in step (1) to remove inner skin of the cassava raw material to obtain skin-removed cassava material 95kg. The cassava material is pulverized according the method in example 1 to give 95kg of pulverization product with particle size of about 2mm. The pulverization product is analyzed according to the HPLC condition in step (1) of the example 1, and calculation result is that the 95kg of pulverization product contains starch 26.5kg. The rest pulverization product after testing is subjected to enzymo lysis and fermentation according to the conditions in the example 1 to give ethanol 13.91kg. lOOg of fermented mash after ethanol distillation is filtered with Buchner's filter, 20ml of the filtrate is transferred to 50ml dry conical flask, and allowed to stand for 5hr to collect supernatant. The supernatant is filtered with 0.2μm filter membrane, and analyzed according to the HPLC condition in the step (1) of the example 1. The calculated glucose content in fermented mash is 395g, the residual glucose content and ethanol yield are calculated according to the equations in the example 1 , and the calculation result is shown in Table 1.

Comparison example 1

This comparison example is for illustrating reference method for producing ethanol from cassava.

Ethanol is produced according to the same method as the example 2, except that skin-removed cassava material 95kg is mixed with water 95kg (i.e. weight ratio of fresh cassava material: water is 1 :1), and subjected to first pulverization for 35min; then the first pulverization product is directly subjected to second pulverization for 5min to obtain 190kg of pulverization product with cassava particle size of 4-6mm. The pulverization product is analyzed according to the HPLC conditions in step (1) of the example 1, and it is calculated that the 190kg of pulverization product contains starch 26.5kg.

The rest pulverization product after testing is subjected to enzymolysis and fermentation according to the conditions in the example 1 to give ethanol 13.51kg. lOOg of fermented mash after ethanol distillation is filtered with Buchner's filter, 20ml of the filtrate is transferred to 50ml dry conical flask, and allowed to stand for 5hr to collect supernatant. The supernatant is filtered with 0.2μm filter membrane, and analyzed according to the HPLC condition in the step (1). The calculated glucose content in fermented mash is 53Og, the residual glucose content and ethanol yield are calculated according to the equations in the example 1, and the calculation result is shown in Table 1.

Example 3

The present example is for illustrating the inventive method for producing ethanol from cassava material. Ethanol is produced according to the same method as the example 1 , except that the pulverization process uses SFSP series hammer mill and comprises the following steps:

(a) first pulverizing 30kg of skin-removed fresh cassava material for 12min to give 30kg of first pulverization product with particle size of 5 -6mm;

(b) mixing 5wt% (i.e. 1.5kg) of the first pulverization product obtained in step (a) with 25kg of rest non-pulverized skin-removed fresh cassava material and carrying out first pulverization for lOmin to give first pulverization product 55kg with particle size of 3 -4mm;

(c) mixing 5wt% (i.e. 2.75kg) of the first pulverization product obtained in step (b) and 40kg of the rest non-pulverized skin-removed cassava and carrying out first pulverization for 15min to give first pulverization product; and

(d) subjecting the total 95kg first pulverization product obtained in steps (a), (b) and (c) to second pulverization for 3min to give 95kg of pulverization product with cassava particle size of 1.8-2mm.

The pulverization product is analyzed according to the HPLC conditions in step (1) of the example 1 , and calculation result is that the 95kg of the pulverization product contains starch 26.8kg.

The rest pulverization product after testing is subjected to enzymolysis and fermentation according to the conditions in the example 1 to give ethanol 14.28kg. lOOg of fermented mash after ethanol distillation is filtered with Buchner's filter, 20ml of the filtrate is transferred to 50ml dry conical flask, and allowed to stand for 5hr to collect supernatant. The supernatant is filtered with 0.2μm filter membrane, and analyzed according to the HPLC condition in the step (1). The calculated glucose content in fermented mash is 292g, the residual glucose content and ethanol yield are calculated according to the equations in the example 1, and the calculation result is shown in Table 1.

Example 4

Ethanol is produced according to the same method as the example 1 , except that 64kg of fresh cassava is skin-removed according to the method in the example 1 to give 61kg of cassava material, then sliced, and mixed with 40kg of dry cassava slice (with water content of 13wt%) and 65kg of water to give 166kg of mixture. The mixture is pulverized by using SFSP series hammer mill and the following steps:

(a) subjecting 40kg of skin-removed cassava material mixture to first pulverization for 20min to obtain 40kg of first pulverization product with cassava particle size of 5-6mm;

(b) mixing 8wt% (3.2kg) of the pulverization product obtained in step (a) and 30kg of the rest cassava material mixture and carrying out first pulverization for lOmin to give first pulverization product with particle size of 4-5mm;

(c) mixing 10wt% (7kg) of the first pulverization product obtained in step (b) and 40kg of the rest cassava material mixture and carrying out first pulverization for lOmin to give first pulverization product with particle size of 3 -4mm;

(d) mixing 6wt% (6.6kg) of the first pulverization product obtained in step (c) and 56kg of the rest cassava material mixture and carrying out first pulverization for lOmin to obtain first pulverization product with particle size of 2-3mm; and (e) subjecting all the first pulverization product obtained in steps (a), (b), (c) and (d) to second pulverization for 5min to obtain 166kg of pulverization product with cassava particle size of 1.6-1.8mm.

The pulverization product is analyzed according to the HPLC conditions in step (1) of the example 1, and the calculation result is that the 166kg of the pulverization product contains starch 43.65kg.

The rest pulverization product after testing is subjected to enzymo lysis according to the same method as the example 1, except that the water addition amount is 30kg during enzymolysis, and the enzymolysis product is subjected to fermentation to give ethanol 23.48kg. lOOg of fermented mash after ethanol distillation is filtered with Buchner's filter, 20ml of the filtrate is transferred to 50ml dry conical flask, and allowed to stand for 5hr to collect supernatant. The supernatant is filtered with 0.2μm filter membrane, and analyzed according to the HPLC condition in the step (1). The calculated glucose content in fermented mash is 566g, the residual glucose content and ethanol yield are calculated according to the equations in the example 1, and the calculation result is shown in Table 1.

Table 1

The inventive method has low water consumption, can complete pulverization in short time, and has low energy consumption, high facility utilization ratio, and high pulverization efficiency; additionally the inventive pulverization method can well control cassava material particle size of pulverization product, and obtain raw material particles with small and uniform particle size.

It can be seen from the data in the Table 1 , the ethanol yield of the inventive method for producing ethanol from root and tuber crops is significantly higher than that of the reference method; compared with the available methods, the inventive method has dramatically reduced residual glucose content.

Claims

1. A method for producing ethanol from root and tuber crops, comprising: pulverizing skin-removed raw material by subjecting skin-removed raw material to a first pulverization to give a first pulverization product, subjecting part of the first pulverization product to a second pulverization to give a second pulverization product, mixing the rest part of the first pulverization product and the non-pulverized skin-removed raw material, and subjecting the resulting mixture to a first pulverization; mixing the pulverization product with enzyme, and subjecting the resulting mixture to enzymolysis; and fermenting the enzymolysis product.

2. The method according to claim 1, wherein the rest part of the first pulverization product is 5-20wt% of the total first pulverization product, and the rest part of the first pulverization product is 5-20wt% of the total weight of the rest first pulverization product and the non-pulverized skin-removed raw material.

3. The method according to claim 2, wherein the rest part of the first pulverization product is 5-10wt% of the total first pulverization product, and the rest part of the first pulverization product is 5-10wt% of the total weight of the rest first pulverization product and the non-pulverized skin-removed raw material.

4. The method according to claim 1, wherein in the first pulverization product, the particle size of the raw material is 2.5- 10mm, and in the second pulverization product, the particle size of the raw material is less than 2.5mm but not less than 1.6mm.

5. The method according to claim 1, wherein the raw material of root and tuber crops is a mixture of fresh cassava and dry cassava, and the weight ratio of the dry cassava to the fresh cassava is 1 :1.5-2.5.

6. The method according to claim 1, wherein the raw material of root and tuber crops is fresh cassava, and the fresh cassava is skin-removed by by using a skin-removing device which comprises a pedestal (1), a rotary drum (2) rotatably fitted on the pedestal (1) and having an inlet (4) and a outlet (5), a helical screw feeder (3) provided in the rotary drum (2) and fixedly connected with the inner wall of the rotary drum (2), and a driving unit for driving the rotary drum (2) and the helical screw feeder (3) to rotate together, wherein raw material of root and tuber crops is fed into the rotary drum (2) via the inlet (4), and the driving unit drives the rotary drum (2) and the helical screw feeder (3) to rotate together.

7. The method according to claim 6, wherein a friction structure is provided on the inner wall of the rotary drum (2).

8. The method according to claim 7, wherein the friction structure is one or more ribbed steel bars.

9. The method according to claim 8, wherein the ribbed steel bar is parallel to the central axis of the rotary drum (2).

10. The method according to claim 6, wherein the rotary drum (2) has inclination angle of 0-15 degrees, and length of 2-10 m; and the rotation speed of the rotary drum (2) and the helical screw feeder (3) is 2-50 rpm.

11. The method according to claim 6, wherein the helical screw feeder (3) has a pitch of 0.3-0.8 m, and a screw thread height of 0.1 -0.4m.

12. The method according to claim 6, wherein the driving unit comprises a driving source, a chain wheel (6) fixed on the rotary drum (2), and a driving chain for transferring the power of the driving source to the chain wheel.

13. The method according to claim 1, wherein the enzyme used in step (c) comprises amylase of which the usage amount is 4-50 enzyme activity units per gram of dry raw material of root and tuber crops; and the enzymolysis is carried out at temperature of 50-90 ^°C and pH of 5-6 for 20-240min.

14. The method according to claim 13, wherein the amylase is one or more of α-amylase, glucoamylase, transglucosidase, and phosphatase.

15. The method according to claim 1, wherein the inoculation amount of yeast used in the fermentation is 10³-10⁸ colony forming units per gram of enzymolysis product, and the fermentation is performed at temperature of 30-36 ^°C for 50-75hr.