This paper mainly explores the powdering conditions for preparing collagen powder using an lab spray dryer. Firstly, the collagen hydrolyzate was prepared by alkaline dechromization, and then the influence of the concentration of the collagen hydrolyzate during powder spraying, the inlet and outlet temperature of the lab spray dryer, the speed of the peristaltic pump, the frequency of the fan and other experimental factors on the powder spraying efficiency were investigated. In order to explore the best process for powder spraying. The process route obtained after the experiment was as follows: the concentration of the collagen hydrolyzate during powder spraying was 30%, the inlet temperature was set to 220 °C, the outlet temperature was 110 °C, the peristaltic pump speed was 15 r/min, and the fan frequency was 40 Hz. The collagen powder sprayed under this condition was milky white, with a moisture content of 6.05% and a powder spraying amount of 2.749 g/min.
China is the world’s largest tannery country, with an annual leather output of about 600 million square meters, accounting for about 1/4 of the world’s total output. A large amount of chromium-containing solid waste will be generated during the production process. On August 1, 2016, the Ministry of Environmental Protection designated chromium-containing solid waste as hazardous waste, which is not allowed to be incinerated or buried, and can only be handed over to qualified hazardous waste plants for treatment. Therefore, the accumulation of chromium-containing leather scraps has seriously hindered the development of the tanning industry; some traditional disposal methods of waste leather scraps are mainly burial, incineration, manufacturing of recycled leather, vegetable tanning materials, etc.; even some illegal traders use chromium-containing leather The shavings produce poisonous capsules and leather milk, which are a serious threat to people’s health. Because the chrome-containing leather scraps contain a large amount of protein resources, it has high resource utilization value. Therefore, dechromating the chrome-containing leather scraps and preparing collagen powder can not only solve the problems of leather production enterprises, but also achieve the purpose of recycling hazardous wastes.
Collagen powder prepared by spray drying has the advantages of large evaporation area, short drying time (several seconds to tens of seconds), and less damage to active ingredients, and is widely used in chemical industry and food industry . After research, it was found that the alkali dechromization of chromium-containing leather scraps can effectively reduce the chromium content in the collagen hydrolyzate. Therefore, this paper mainly uses the alkaline method to prepare the collagen hydrolyzate, and uses the lab spray dryer to spray powder to prepare the collagen powder. The research on the powder spraying process is mainly carried out from the aspects of the optimum concentration of collagen hydrolyzate before spray drying, peristaltic pump speed, inlet and outlet temperature, and fan frequency. So as to determine the best powder spraying process route.
1.1Preparation of collagen hydrolyzate
Chromium in the chrome-containing leather scraps is mainly in the form of a complex complex with the carboxyl group of collagen, and the coordination capacity of hydroxide (—OH) and chromium is much greater than that of carboxyl (—COOH) and chromium. The ability to replace the carboxyl group of collagen from the chromium complex. Under the condition of hot alkali, the chrome-containing leather shavings also undergo a certain degree of degradation during dechromization, and then dissolve in water. The chromium in the chrome-containing leather scraps is combined with (—OH) in the alkali to form Cr(OH)3 precipitation, which is separated from the collagen hydrolyzate by suction filtration. There are many kinds of alkalis used in alkali treatment, among which there are mainly quicklime, sodium hydroxide, sodium carbonate, magnesium oxide, etc. In this experiment, the combination of quicklime and sodium hydroxide was used to hydrolyze the chrome-containing leather scraps, which not only ensured a certain degree of hydrolysis, but also achieved a good chromium removal effect .
2.1 Lab Spray drying of collagen hydrolyzate
The concentrated collagen hydrolyzate was investigated by using an lab spray dryer to spray powder. During the powder spraying process, the main influencing factors were the concentration of collagen hydrolyzate, the inlet temperature, the speed of the peristaltic pump and the frequency of the fan. Since the outlet temperature of the lab spray dryer cannot be set, it can only be adjusted automatically with the inlet temperature, so the outlet temperature is not explored. The setting of the needle beating of the lab spray dryer is mainly to let the material enter the drying bottle smoothly, so the fixed needle-passing time is set to 5 s, and the experiment is repeated three times under each influencing factor, and the average value is taken. Firstly, the concentration of collagen hydrolyzate was fixed at 20% to explore the influence of inlet and outlet temperature, peristaltic pump speed and fan frequency of the lab spray dryer on powder spraying.
- Results and discussion
- Exploration of inlet temperature
The preparation of the lab spray dryer at different inlet temperatures is shown in Table 1.
It can be seen from Table 1 that in the use of lab spray dryer powder spraying experiment, through the comparison of different inlet temperatures, it is found that when the inlet temperature reaches 220 °C, the water content of the collagen powder sprayed out is 6.45 %, in line with the industry standard that the moisture content of collagen powder is less than 8%, also the color of collagen powder is milky white, which is the best color. The price per ton of collagen powder produced by each factory varies greatly. The main influencing factors are the content of amino nitrogen, the content of chromium, and the color of the collagen powder. The color of the collagen powder sprayed from the hydrolyzed solution obtained by different hydrolysis processes is different. Although the moisture content of collagen powder gradually decreases with the continuous increase of the inlet temperature, the production cost will continue to increase with the increase of the temperature during the production process. Therefore, considering the overall consideration, the optimal inlet temperature should be set to 220 ℃, the outlet temperature is automatically set with the inlet temperature, and the outlet temperature at this time is 110 ℃.
- Exploration of peristaltic pump speed
The inlet temperature of the lab spray dryer was set to 220 °C, the rotational speed of the peristaltic pump was set to 10 r/min, 15 r/min, and 20 r/min, and the rest of the experimental conditions remained unchanged. The experimental data are shown in the Table 2.
From Table 2, it can be concluded that with the continuous increase of the peristaltic pump speed, the feed quantity per unit time increases continuously. Although it is beneficial to improve the powder spray efficiency of the lab spray dryer, when the peristaltic pump speed reaches 20 r/min, the moisture content of collagen powder is 9.75%, which exceeds the industry standard. Therefore, according to Table 2, it can be determined that the optimal speed of peristaltic pump is 15r/min.
- Exploration of blower frequency
Through the above experimental exploration, the optimal inlet temperature and the rotational speed of the peristaltic pump were determined. Therefore, the inlet temperature was set to 220 °C, and the rotate speed of the peristaltic pump was set to 15 r/min. Except for the blower frequency, the rest of the experimental conditions remained unchanged. The optimal blower frequency is determined by comparing the amount of powder spraying per unit time. The experimental data are shown in Table 3.
From Table 3, it can be concluded that with the gradual increase of the blower frequency, the amount of powder spraying per unit time shows a trend of first increasing and then decreasing, but the change range is relatively small. The main reason is: when the blower frequency is too low, a part of the collagen powder cannot be collected into the powder collecting bottle due to insufficient wind, resulting in a low powder spraying amount per unit time. With the gradual increase of the blower frequency, the unit time The collagen powder in the inner powder collection bottle gradually increases. However, when the blower frequency exceeds 40 Hz, due to the excessive wind force inside the powder collecting bottle, a part of the collagen powder flows into the air with the vent, resulting in a downward trend in the amount of powder spraying per unit time. Therefore, the optimum blower frequency was determined to be 40Hz.
- Exploration on the concentration of collagen hydrolyzate
The above experiments mainly explore the influence of the inlet temperature of the lab spray dryer, the peristaltic pump speed and the blower frequency on the powder spraying. Keeping the above optimal experimental conditions unchanged, the effects of different concentrations of collagen hydrolyzate on powder spraying were explored. The experimental data are shown in Table 4.
It can be seen from Table 4 that with the continuous increase of the concentration of the collagen hydrolyzate before powder spraying, the powder spraying amount per unit time continues to increase, and when the concentration reaches 30%, the powder spraying amount per unit time reaches 2.749g/min; When the concentration reaches 35%, because the concentration of collagen hydrolyzate is too high, the straw of the lab spray dryer cannot be inhaled, so that the powder spraying process cannot be performed. Therefore, the optimal concentration of collagen hydrolyzate before powder spraying is 30%, and the powder spraying amount can reach 2.749 g/min.
Tab.4 Preparation of collagen powder under different feed liquid solid content
Summarizing the above several sets of experiments, it can be found that the best process route for the preparation of collagen powder by the lab spray dryer is: the concentration of collagen hydrolyzate is 30%, the inlet temperature is 220 °C, the outlet temperature is 110 °C, and the peristaltic pump speed is 15 r/min, and the blower frequency is 40 Hz. Under this process condition, the powder spraying amount per unit time of the lab spray dryer is 2.749 g/min, and the water content is 6.05%.