Application and Development of Spray Drying Equipment in Dairy Production

In my country's dairy production, milk powder accounts for approximately 80% of the total output due to its ease of storage and transportation. The drying process is one of the most crucial steps in milk powder production; the amount of water evaporated during drying alone is almost equivalent to the quality of the produced milk powder. Generally, the production process determines the production equipment and control system, but the application of advanced equipment and control systems can also alter the production process. The application of spray drying equipment in milk powder production is a typical example.

The spray drying process involves pumping material into a spray dryer, where an atomizer disperses it into small droplets. Heated air is then introduced into the drying chamber to mix and contact with the material droplets. After drying, the dried product is separated and stored or used directly. Milk powder produced by spray drying is a uniform powder product, eliminating the need for further drying, cooling, and pulverizing processes. It can be directly packaged as a finished product, enabling mechanized, continuous, and large-scale production. Furthermore, the lower surface temperature of the material improves the quality of the milk powder product. Therefore, spray drying equipment plays a significant role in milk powder production, and there is a trend towards drying products with increasingly higher moisture content. However, spray drying is a high-energy-consuming process. Because of the high quality requirements after drying and the need for minimal fluctuations in moisture content, strict control of operating parameters such as drying temperature is essential. Therefore, the correct selection of spray drying equipment and the rational design of its operation and control system are crucial for ensuring product quality and energy conservation. This not only contributes to the technological progress and development of my country's dairy industry but also drives technological progress and development in other industries, making a greater contribution to the country's economic take-off.

1. Development of Spray Drying Equipment in Dairy Industry Production

Spray drying equipment was first used in the milk powder industry in 1901, but its practical application in milk powder production didn't begin until the 1920s, and it wasn't until the late 1940s that it started to be used in my country. The earliest structure was a pressure box type (horizontal), with material atomization using a two-fluid method, resulting in high power consumption. In 1958, the Ministry of Light Industry promoted animal-powered small-scale pressure spray drying for milk powder production in Heilongjiang Province. In 1955, the Harbin Songhua River Dairy Factory first used centrifugal spray drying to produce milk powder. Both of these types of spray drying equipment at that time had flat-bottomed structures, and powder output was intermittent, requiring manual output once per shift.

In the mid-1960s, a conical bottom structure with a spiral powder output device (auger) appeared in the box-type pressure drying equipment. The first vertical multi-nozzle pressure spray dryer was developed in the early 1970s. Its emergence reduced the effective volume of spray dryers by nearly half and eliminated the need for an auger, allowing for continuous powder output. In the 1980s, a single-nozzle vertical pressure spray dryer was produced. Its application in the milk powder industry was a key factor in promoting technological progress in my country's milk powder industry, laying the foundation for its rapid development.

2. Characteristics of Spray Drying Equipment

Spray drying equipment generally comes in three types: pressure, centrifugal, and airflow. Pressure spray dryers offer high drying intensity, smaller size, lower equipment and civil engineering investment, smaller product particles, and higher bulk density. For milk powder with a bulk density of approximately 0.165, the drying time is particularly short, only 10-30 seconds, making it suitable for the production of heat-sensitive materials. Therefore, in the milk powder and other industries, many centrifugal spray dryers have been converted to pressure spray dryers. Pressure spray dryers also consume less power. Centrifugal spray dryers generally have a larger diameter and smaller height, resulting in lower drying intensity compared to pressure spray dryers. They produce larger particle sizes and are adaptable to a wider range of materials; generally, materials suitable for pressure spray dryers are also suitable for centrifugal spray dryers. Centrifugal spray dryers are suitable for materials with higher viscosity. Airflow spraying, also known as two-fluid spraying, utilizes the high-speed movement of steam or compressed air (typically 200–300 m/s) to create a high relative velocity between the gas and liquid. The liquid film is drawn into filaments and then breaks into fine droplets. The sudden increase in surface area allows for rapid heat and mass exchange with the hot air, vaporizing the moisture and producing a powdery dried material. It is suitable for a particularly wide range of materials, but consumes a very large amount of power. Generally, this method is avoided whenever other spraying methods can be used. For airflow atomization, the material should be heated to an appropriate temperature to minimize its viscosity; therefore, airflow spraying is suitable for materials with higher viscosity.

3. Development Trends of Spray Drying Equipment

From the perspective of large-scale industrial production, energy conservation, and consumption reduction, spray drying equipment should be vertical, large-scale, continuous, and automated. This type of operation is the most reasonable. Continuous operation means that the spraying is continuous, i.e., both feeding and discharging are continuous. In terms of scale, appropriate equipment is adopted according to different production types. Generally, in the food industry, the trend is towards large-scale, multi-nozzle systems, while in the pharmaceutical industry, the scale is generally smaller. Like other equipment, the selection of spray drying equipment and the matching of its internal components must be adapted to the materials being processed. The drying intensity of the equipment should be maximized, i.e., the amount of moisture dried per unit volume per unit time, thereby minimizing the size of the spray drying equipment and minimizing equipment and civil engineering investment.

311. Hot Air Distribution Method

Hot air distribution in spray drying equipment is a crucial aspect. The method of hot air introduction significantly impacts the overall performance of the drying equipment. A proper hot air introduction method not only ensures more thorough and effective mixing of droplets and hot air, thereby improving heat exchange efficiency, but also effectively addresses the drying of certain special materials, preventing material sticking to the walls and ceiling. Traditional pressure spray dryers often employed a sleeve-type air regulating structure. Hot air was regulated through the sleeve and then distributed into the inner cylinder of the spray dryer. This type of equipment was relatively large. Now, multi-layer sieve plate air distribution is more commonly used, as it is smaller and therefore more widely applied. In pressure spray dryers operating in a co-current flow, the air velocity of the hot air entering the duct is approximately 9 m/s. In counter-current or top-exhaust spray dryers, the air velocity entering the duct should be appropriately higher. Centrifugal spray dryers generally employ hot air distributors. Hot air enters the main body of the spray dryer through a volute air distributor and a guide plate, with the hot air swirling in the same direction as the centrifugal atomizer. The main functions of the hot air distributor in centrifugal spray dryers are threefold: to introduce the drying medium (hot air) into the drying tower so that it contacts the material being dried, completing the drying task; to select an ideal structure to ensure thorough mixing of the introduced hot medium and the material being dried, achieving high heat exchange efficiency; and to reduce or avoid the phenomenon of certain materials sticking to the walls or top. With the development of spray drying technology, hot air distributors are constantly being improved.

312. Tower Exhaust Form

In co-current spray dryers, bottom exhaust is generally used. In pressure spray dryers, the diameter of the equipment is increased at the top of the tower's conical section, and four exhaust ducts are evenly arranged in the annular zone, then converged onto a powder collector. Alternatively, four bag filters can be directly installed in the upper part of the annular zone, and then the exhaust ducts converge at the exhaust fan inlet. Currently, pressure spray dryers commonly use top-exhaust systems. Hot air enters the tower from the top, and the material is also sprayed into the tower from the top. Four exhaust pipes are evenly distributed around the circular section of the tower, and the exhaust gas is then collected and discharged through a dust collector. This type of spray dryer has a relatively high hot air heating temperature, high drying intensity, a smaller tower volume, and lower equipment and civil engineering costs.

Centrifugal spray dryers typically have their exhaust pipes led out from the conical section of the tower and then into the dust collector. This maximizes the effective volume of the spray dryer tower without affecting normal powder output, minimizing the overall volume.

313. Equipment Cleaning

Imported spray dryers generally do not have this device and instead use cleaning methods. Domestic spray dryers generally consider this issue. Pressure spray dryer tower walls are generally cleaned using a hanging plate-type dust collector, which is safe, reliable, and provides thorough cleaning. However, the conical section of the tower should have a cleaning door for external cleaning. The tower of a centrifugal spray dryer is typically cleaned using a push-type lifting dust sweeper, which provides a thorough cleaning. After sweeping, the dust sweeper can be moved outside the tower through the tower door.

The tower of a spray dryer is generally equipped with a vibrating tower device to prevent the dried material from adhering to the tower wall. This typically uses several air-operated or electric hammers, programmed with a specific vibration sequence. Air-operated hammers are better than electric hammers, but require compressed air.

314. Dust Removal from the Equipment

Spray dryers generally use a two-stage dust removal process. The first stage uses cyclone separation for dust removal, and the second stage uses a bag filter. In milk powder production, the filter cloth of the bag filter is generally 130 mesh, while in protein powder production, 200-240 mesh is generally required. Single-stage dust removal is also sometimes used in milk powder production. In the spray drying production of soybean protein and isolated protein, hot air is generally sent into the drying tower from the top, and the material is also sprayed in from the top. The powder discharge and exhaust air both enter the powder collector-cyclone separator-from the powder outlet. After the material is discharged from the cyclone separator, it is transported to the powder silo by a Roots blower. The exhaust gas enters a bag filter for powder collection before being discharged into the atmosphere. The fine powder is sent into the powder silo together with the powder discharged from the cyclone. The air pressure of the inlet and exhaust fans in spray drying equipment used for protein powder production is generally higher than that of the inlet and exhaust fans in spray drying equipment used for milk powder production. The air volume matching is the same as that of spray drying equipment used for milk powder production, with the exhaust fan air volume being 30% larger than the inlet fan air volume. There are three reasons for this: First, the inlet fan blows in room temperature air, while the exhaust fan discharges hot air, resulting in a larger air volume strain. Second, during spray drying, a vacuum of 0.1147 kPa is used in the drying tower, which facilitates smooth material descent and reduces splashing. Third, moisture vaporizes during spray drying, which also occupies a certain volume.

The fine powder from the cyclone separator and bag filter chamber is sent back into the tower by a Roots blower for fine powder agglomeration, increasing particle size and solubility.

The filter bags in the bag filter are generally vibrated using a pulse backflushing method, which is more expensive and consumes more compressed air. Currently, a reciprocating vibration mechanism using a cylinder is generally employed, with the vibration interlocked with the closing of the air regulating butterfly valve. This allows for the instantaneous vibration of the filter bag without exhaust, resulting in a low cost and good performance.

315. Powder Discharge from the Equipment

The powder discharge from the spray drying equipment is continuous, connected to a vibrating fluidized bed at the outlet of the drying tower. This allows for secondary drying of the powdered material, followed by the application of lecithin or other flavorings. This expands the capacity of the spray drying equipment, improves product quality, and enables timely cooling of the material after secondary drying, bringing the product to a state ready for direct packaging, thus significantly reducing the risk of contamination before packaging.

4. Drying Process Control System

The rapid nature of the spray drying process is crucial for effective control. Currently, research on drying control theory and practice both domestically and internationally is still in its early stages, lacking comprehensive and systematic studies. Based on the analysis of the production process, the purpose of applying automated control technology in the milk powder drying process is to:

(1) Avoid the instability of equipment operation caused by manual intermittent operation, thereby achieving continuous production and stabilizing the multivariable and highly nonlinear system operation process of spray drying;

(2) By introducing an automated control system, while ensuring product quality and output, minimize the number of online signal acquisition points of the control system and reduce unnecessary influence between variables;

(3) Save manpower, reduce downtime caused by human factors, and shorten the work cycle;

(4) Through the correct preset of the parameters of each variable in the automated control system, enable the entire system to work in the optimal state, maximize the efficiency of the equipment itself, and achieve the purpose of energy saving and consumption reduction;

(5) Improve product quality and minimize operation and management costs.

The main indicators for evaluating the success of drying equipment application under specific process conditions are the output and quality of the dried product, as well as the particle size distribution, appearance, and final moisture content of the product. Product output is determined by the economic performance of the production load, the heat provided by the hot air generator, the drying intensity of the dryer, the hot air flow rate, and the solid content in the slurry. Many factors influence product quality: the particle size and appearance are determined by the process performance of the drying equipment and the slurry preparation parameters, while the final moisture content is directly related to the selection of parameters such as the hot air inlet temperature, exhaust gas outlet temperature, and negative pressure within the drying tower.

Spray drying equipment has revolutionized milk powder production. However, spray drying equipment is not a universal solution. Its atomization structure and form, tower structure, air distribution, exhaust, powder coating, powder discharge method, combination with other process equipment, and automatic control system design must all be suitable for the materials being processed. Only in this way, with the development of the times and the continuous progress of science and technology, can spray drying equipment adapt to a wider range of material drying requirements and make an increasingly greater contribution to the development and construction of the national economy.