Application and modification of centrifugal spray drying in dairy production
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1 The birth of centrifugal spray drying
The spray drying technology first applied in China's dairy industry is "pressure" and "two-fluid" technology. At that time, due to factors such as technical level and production scale, it was only in the form of horizontal, primary and multi-nozzle, and its evaporation. Most of the capacity is below 250kg/h. In the mid-1960s, Anda Dairy Factory installed a horizontal pressure spray drying equipment with 350kg/h evaporation, and 7 nozzles. In 1967, it assisted Albania. The spray drying equipment provided by the Beijing Nanjiao Dairy Factory was introduced by Japan in 1965. Evaporation capacity of 350kg / h double nozzle, horizontal, pressure spray dryer, requires evaporation of 150kg / h, also uses dual nozzles; in the late 1960s, Shanghai Milk Crystal No. 3 plant installed a vertical pressure spray dryer for the first time, its evaporation capacity is 250kg /h, there are still no less than 3 nozzles. At the time, the emergence of this multi-nozzle heat was mainly due to a wrong theory. Many people think that the nozzle aperture cannot be larger than a certain value, otherwise the material can not be atomized. When the equipment capacity was small at that time, it was inevitably caused by the use of multi-nozzle and small-aperture nozzles. The small nozzle aperture of Zui was only 0.5mm, which caused many problems in production operation and management, and more importantly, the particle size of the milk powder was small. The tone is very poor, and the color and organization status are not welcome to users.
In order to solve this contradiction, many manufacturers, research institutes, and colleges and universities have been looking for production of instant milk powder that has been prepared in a variety of ways. There have been trial productions such as "two-stage drying method" and "addition of lactose recrystallization method". However, due to the high cost and the inability to adapt to large-scale continuous production, it is impossible to promote. At the same time, centrifugal spray drying technology has emerged and has achieved certain development. The main reason is that the centrifugal sprayer produced by the local method has a low rotational speed of only 5000r. /mim), because of the low line speed, the sprayed milk powder particles are large, and the punching property is better than that of the pressure spray. It is relatively popular among users in the market. Until the end of the 1970s, the milk powder produced by centrifugal spray drying dominated the market. At that time, several representative milk powder factories were Songhuajiang Dairy Factory, Chahayang Dairy Factory, Beijing Xijiao Dairy Factory and Muir Dairy Factory.
2 Centrifugal spray drying decline
In 1980, Heilongjiang Dairy Machinery General Factory designed a single-head vertical pressure spray ten-dryer with evaporation capacity of 250kg/h. In 1982, it was put into normal operation in Tailai County Dairy Factory. After that, the newly built and expanded dairy factory adopts centrifugal drying technology. Reduced, some old factories that have used centrifugal spray technology, expansion also began to use pressure spray technology, manufacturers do not consider the inconvenience of the two technologies at the same time, such as Fuyu County Milk Crystal Factory, Jidong Dairy Factory, Beijing Xijiao Dairy Factory Hulan Dairy Factory, Songzhan Dairy Factory, etc. Some centrifugal spray technology has been used for many years in the old factory. After the new pressure spray drying equipment is installed, the original centrifugal spray drying equipment is deactivated or simply removed, such as Jidong Dairy Factory, Beijing Xijiao Dairy Factory; since the late 1980s, The phenomenon of "centrifugation" and "pressure" has been successfully transformed by the authors, including Fuyu Dairy Factory, Songhua Lake Dairy Factory, Songzhan Dairy Factory and Julang Ranch Dairy Factory. The emergence of these phenomena indicates that people have gradually recognized the "centrifugal spray"
And "pressure spray", centrifugal spray drying technology has been shaken in the market of China's dairy industry, pressure spray ten dry technology has occupied an absolute advantage, it can be said that "centrifugal spray" has begun to decline in China's dairy industry. The debate on the advantages and disadvantages of the two methods has rarely been heard, and there is no longer a certainty that the "centrifugal method" milk powder is better than the "pressure method".
There are many factors in the reasons for this change. The main points are as follows:
(1) Theoretical misunderstanding
The Danish K. Mastos' Spray Drying Manual clearly states that centrifugal spray drying is suitable for the production of small particles. Some people mistakenly think that the larger powdered milk powder produced by the atomization is not considered to be the advantage of centrifugal spray because we can't make a high-speed centrifugal sprayer.
(2) Centrifugal spray drying equipment design is not standardized
From the 1950s to the 1980s, the centrifugal spray drying equipment used in China's dairy industry was almost entirely out of the hands of "earth experts" and there was no standardized design. In order to invest less, it is quick to launch. Centrifugal sprayers are mostly made of "earth". The hot air distributors are varied, the wind is uneven, and they cannot be adjusted. There are "coke powder" problems, and no one has reached the design ability, causing and The previous process is not matched.
(3) Characteristics of raw material properties of milk powder production
The main raw material of milk powder is fresh milk. The additives such as sugar, milk powder, soy milk and other substances are soluble. When mixed with fresh milk, it will not form a precipitate. After concentration, the viscosity is not high. This material is also very suitable. Pressure spray drying is not only a dry spray with a centrifugal spray.
(4) The emergence of single-nozzle vertical pressure spray equipment
After the emergence of single-nozzle vertical pressure spray drying equipment in the early 1980s, it was quickly promoted and applied. Due to the large pore size of the nozzle, the particles of the powder reach or exceed the particles of the centrifugal spray method, and the brewing properties of the two products are almost the same. The advantage of the centrifugal spray method is no longer unique. The pressure spray drying method is no longer exclusive. The statement of poor tonality got a proper name.
(5) Insufficient centrifugal spray drying method
Compared with the vertical pressure spray drying method, the centrifugal spray drying method has the following disadvantages:
I. The evaporation intensity is low. Centrifugal spray, the difference between the movement trajectory of the material in the tower and the movement trajectory of the hot air in the tower, resulting in shorter heat exchange time between the material and the hot air than the vertical pressure spray drying. As a result, the evaporation capacity of the unit effective space is less than the vertical pressure. Spray drying also reduces the evaporation intensity. According to the actual measurement, the evaporation intensity is generally ≤3kg/h.M3, including the centrifugal spray drying equipment introduced in recent years. The evaporation intensity of the vertical pressure spray drying equipment is generally ≥5kg/h.M3, which makes the effective volume of the centrifugal spray drying tower of the same capacity larger than the vertical pressure spray drying tower by about 80%.
Due to the low evaporation intensity, the volume of the device is increased, which brings a series of problems:
a. The material consumption of the manufacturing equipment is high, the cost of the equipment is increased, and the centrifugal spray drying equipment with the same capacity is about 20% higher than the vertical pressure spray drying equipment.
b. Due to the relatively large equipment, it is difficult to manufacture, transport and install.
c. The space for installing equipment is large, and the plant for installing equipment needs to be increased, which increases the investment in the construction of the plant.
d. It is not convenient to use the equipment and clean it after the shift. It is necessary to add some auxiliary devices, such as the lifting powder sweeping device and the centrifuge lifting device.
II. There are many powders hanging on the inner wall of the tower. Centrifugal spray drying is parabolic due to the trajectory of the atomized material, and the hot air entering the tower has a certain rotational force. The tower is filled with powder (including the top of the tower), especially when producing products such as sugared milk powder and infant formula. To be serious. Heilongjiang Dairy Factory introduced the Danish "Nile" centrifugal spray drying equipment, the inner diameter of the tower is 7.4m, and the infant formula milk powder No. II is continuously produced for one month. The thickness of the sticky wall powder is more than 80mm. The factory thoroughly cleans it once a month. Some powders can no longer be used as grade products.
III. The product has the possibility of being contaminated by oil. Centrifugal sprayers for atomizing materials are required to be installed in the hot air distribution unit in the center of the tower. Due to the high ambient temperature and high-speed rotation of the centrifuge, lubricating oil is required to be lubricated and cooled. The lower end is sealed by a seal. Once the seal is damaged during operation, the oil will leak into the product inside the tower, contaminating the product, and in operation. It is difficult to find oil spills.
IV. Milk powder has a short storage period and fat is easily oxidized. The milk powder produced by the centrifugal spray drying method contains more air. During storage, the fat in the milk powder is easily oxidized by the air in the powder, so that the storage period is shortened.
3 Transformation of centrifugal spray drying equipment
From the structural characteristics of China's milk powder products, the drying equipment for the production of milk powder in China's dairy industry should be said to be a vertical pressure spray drying equipment. Practice has been proven that if the leader of the centrifugal spray drying equipment use unit recognizes this, it will be easier. Because, "centrifugation", change "pressure" is feasible, easy, and has practical benefits.
3.1 Feasibility
In recent years, there have been four dairy factories, which successfully changed the centrifugal spray drying to pressure spray drying and put them into normal operation for the following reasons:
(1), the same point, versatility
The basic principle of "spraying" and "pressure" spray drying is the same, that is, the material is atomized, the heat exchange and mass exchange are completed in an instant with hot air, and the dried material is separated from the air by a separating device to obtain a product.
Both of them belong to the cocurrent drying, that is, the atomized material and the hot air are dried in the same direction from top to bottom in the tower. The ancillary equipment is mostly the same and can be used universally. Such as air filter, air heater, inlet fan, exhaust fan, filter device and thick milk tank are the same, can be used universally.
These same parts, such as the original design, the selection is correct and reasonable, the transformation can be done without any changes, and the original shape can be used universally.
(2) Less difference, easy to transform
The nebulizer is different. The fundamental difference between the centrifugal and pressure spray dryers is that the material is atomized in different ways. The former uses a high-speed rotating centrifuge to atomize the material, while the latter uses a high-pressure pump pressure to atomize the material through the nozzle.
The hot air distributor is different, because the atomizers are different, the distributor of the hot air into the tower is also different. The centrifugal spray method adopts the volute type, and the pressure spray method adopts the baffle type. The latter has a simple structure, and the uniformity of manufacturing, installation and adjustment of the incoming air is easier and simpler than the former.
The diameter of the tower is different from the effective height. Because the fog distances of the two towers are different in the tower, the diameter of the centrifugal tower is more than 35% larger than the diameter of the same pressure tower; and the effective height is 40% lower than the latter.
The effective volume is different. Because of the different evaporation strengths of the two, the effective volume of the centrifugal tower with the same capacity is more than 70% larger than the effective volume of the pressure tower.
The effective height of the tower top to the roof is different. Because the centrifugal tower needs to hoist the centrifugal sprayer, the effective height from the top of the tower to the roof is more than 3m, and the pressure tower generally does not exceed 2m.
(3) Different point analysis
The atomizer and the hot air distribution tire are two different points, and can be solved only by replacing them during the transformation.
For the difference between the diameter of the tower and the effective height, the diameter of the centrifugal tower is larger than that of the pressure tower. After the transformation, the viscosity of the tower wall is about 50% less than that of the pressure tower. The top of the tower is almost non-sticky; if the effective height of the centrifugal tower is lower than the pressure tower For the required effective height lower limit, the tower should be properly connected with the effective height from the top of the tower to the roof.
Different effective volumes are very beneficial to the transformation, and the production capacity can be improved after the transformation.
3.2 Benefits after the transformation
After the centrifugal tower is transformed into a pressure tower, in addition to solving the above four shortcomings, the following benefits can be received:
(1) Dry matter utilization rate
Since the top of the tower is almost non-sticky, the sticky powder on the tower wall is reduced by 50%, the sticky wall powder is easily removed, and the problem of coke powder is solved, and the dry matter utilization rate of fresh milk is bound to increase accordingly.
(2) Improve the rating rate of products
Since the coke powder problem is solved, a factor related to the product impurity degree and solubility index is eliminated, and the product grade rate is improved.
(3) Increase production capacity
Since the effective volume of the centrifugal tower is more than 70% larger than that of the pressure tower of the same capacity, the evaporation intensity will increase after the transformation. After replacing the atomizer and the hot air distribution, the capacity can be increased by about 20%; The ability to make appropriate adjustments to the equipment can be increased by 50%-100%, and practice has been proven. Heilongjiang Province Fuyu Dairy Factory and Songzhan Dairy Factory's 10t fresh milk/class centrifuge tower has been transformed into 20t fresh milk/class; Liteng County Julang Ranch Dairy Factory's 5t fresh milk/class centrifugal tower, after evaporation, the evaporation amount reached 3l0kg /h, equivalent to processing fresh milk 12.4t / class.
(4) Improve the operating environment
The operation of the centrifugal tower is generally at the top of the tower with high temperature. After being transformed into a pressure tower, the operation can be completely operated on the second floor. The temperature of the second floor is 20-30 °C lower than that of the top of the tower, or Shiduo.
From the above, it can be seen that the centrifugal tower transforms the two ways of bamboo, which is a small transformation, that is, under the premise that the capacity is not increased or slightly improved after the transformation, only the atomizer and the hot air distributor are replaced, and the investment will not exceed 5 Ten thousand yuan, the production stoppage time can be completed in a week or so. If it is a major transformation, the required capacity will be increased by 50% or doubled, and the investment will not exceed 150,000 yuan; the production stoppage can be completed in about 4 weeks.
references
1, Huang Zhaobai, etc., spray drying manual. Beijing: China Building Industry Press, 1983
2, Tao Yunzhang translation, dairy manufacturing engineering. Beijing: Light Industry Press, 1987
3. Edited by Lin Qingwen. Dairy manufacturing. Huaxiangyuan Mountain Press
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