Sugar Silo Systems Explained: Bulk Storage for Raw and Granulated Sugar

As a finished product, granulated sugar has traditionally been stored in bags. As is well known, this storage method has the following major drawbacks: moisture absorption, deterioration, and even lumps. It is also susceptible to contamination by microorganisms and other foreign matter. Consequently, sugar consumed directly does not meet food hygiene standards. Furthermore, the labor required for packaging, storage, stacking, and shipping is enormous. Furthermore, changes in packaging and sales methods have also contributed to the development of bulk sugar storage.

For this reason, starting in the 1950s, foreign countries first eliminated bagged storage for raw sugar and adopted bulk storage. This practice was subsequently adopted for granulated sugar, making bulk storage the sole storage method in modern sugar factories. In short, conditioning silos are a highly mature sugar production process in foreign countries. Based on their scale and influence, three types of aging silos and their corresponding finished product silos exist: DDS, Weibull, and A.B.R.

However, due to consumer habits and national conditions in China, sugar manufacturers haven’t paid enough attention to the mechanization of sugar agglomeration and storage. As a result, there are currently few fully operational sugar silos in China (COFCO Chongzuo’s silos are known as conditioning silos, also known as maturation silos). However, large raw sugar silos are in use at the 4,000-ton/day Lingyunhai Sugar Refinery in Rizhao, Shandong.

1. The Rise of Steel Silos: A Leap from Grain to Sugar

In my country, silo technology originated in the 1980s. However, since the 21st century, with the advancement of ventilation and drying technologies, silos have rapidly developed in the grain industry and are widely used in industrial production. Currently, grain storage is divided into three main types: room-type silos, concrete silos, and steel silos. Due to the rapid development of bulk grain transportation and the increasing maturity of technology in recent years, coupled with their distinct advantages such as large capacity, short construction period, small footprint, wide adaptability, and ease of mechanized production, steel silos are becoming increasingly popular in the grain and oil industry.

Steel silos, with their low investment, short construction period, and small footprint, have become the preferred storage type for grain. After more than 20 years of development in my country, the technology has matured and is gradually replacing concrete silos. In developed countries such as the United States and Canada, steel silos have become the mainstream storage type. The cost of a steel silo is generally one-third higher than that of a mechanized room-type silo. However, it shortens material loading and unloading processes, reduces operating and maintenance costs, eliminates laborious bagging operations, and facilitates mechanization and automation, making it one of the most prevalent forms of grain silo storage.

The packaging and storage of finished sugar is the final step in sugar production. With the emergence of various sophisticated packaging machines and changes in sales methods, small bags of 500-1000g will become more common. At the same time, the construction of suitable bulk sugar storage facilities is needed. Moreover, bulk sugar storage offers incomparable advantages over large 50kg or 100kg bags in terms of ensuring long-term sugar quality stability and food hygiene. Therefore, the proportion of existing large bags is gradually decreasing, and the application prospects of steel silos in sugar storage will continue to expand.

2 Steel Silo Process Description

The goal of bulk sugar process control is to ensure that sugar maintains stable quality standards during long-term storage, preventing moisture absorption, agglomeration, and contamination. After drying, the sugar flows through a sugar conveyor into the conditioning silo, where it is evenly distributed by a sugar distributor. The sugar moves from top to bottom within the silo. An air distribution device is installed at the bottom of the silo to ensure uniform contact between dehumidified cold air and the sugar, ensuring effective maturation. A sugar powder collection device is installed at the top.

In addition to the original receiving, drying, loading, and unloading functions, the silo process design should also include a temperature and humidity control system and ventilation system. Temperature and humidity control and ventilation and cooling systems are crucial for safe storage of materials in silos. Sensor cables contact the material, and a temperature and humidity detector displays the temperature and humidity of the stored material. Based on the temperature and humidity measurement system’s readings, fans are activated to exchange heat with the material, reducing temperature and humidity, thereby extending the storage life of the material.

The white sugar silo storage system is shown below.

To this end, before storage and packaging, white sugar should meet the following requirements: ① Moisture content below 0.05%; ② Temperature < 35°C; ③ Not too fine, with an average size of ≥ 0.4 mm; ④ Low fine crystal content, with less than 4% of crystals smaller than 0.15 mm. Because the water vapor partial pressure on the surface of the sugar tends to balance with that in the surrounding air, the key to bulk storage is maintaining a stable moisture content and preventing moisture migration between different parts of the sugar storage. If sugar in different parts of the storage have different moisture content and temperatures, this will result in different temperatures and humidity levels in the surrounding air. Diffusion and convection of air between different parts of the storage will transfer the water bubbles and heat it carries, leading to moisture migration and temperature changes in the sugar. In other words, the principle of process control should be: the temperature and moisture content of newly stored sugar should be as consistent as possible with the sugar currently in storage, the temperature and humidity of the air in the storage should be balanced with those of the sugar currently in storage, and the temperatures of different parts of the storage should be as close as possible. At different temperatures, there is a balance curve between the relative humidity of the air inside the silo and the moisture content of the stored sugar. Based on this relationship, the silo temperature should be maintained between 20-30°C and the relative humidity between 40-55% during operation. This ensures long-term stability in the quality of the stored sugar and prevents clumping.

3 Analysis of the Advantages and Disadvantages of White Sugar Silos

3.1 Comparison between Silos and Traditional Room-Type Silos

With the increasing demand for efficient transportation and storage of bulk white sugar, white sugar silos (especially cylindrical silos) are gaining popularity among companies due to their high degree of mechanization and small footprint, making them a promising form of sugar storage. Compared to traditional room-type warehouses, sugar silos offer significant advantages in land utilization and automated operation efficiency.

While traditional room-type warehouses retain their market share due to their cost advantages, sugar silos, with their smaller footprint and shorter investment payback period, are becoming increasingly mainstream amidst rising land prices.

Taking the storage capacity of 10,000 tons of white sugar as an example, the data comparison between cylindrical white sugar silos and traditional room-type silos is shown in the following table:

Table 1 | Comparison of Silo and Warehouse for 10,000 Tons (Sugar Storage)

Notes:

1.Warehouse① is equipped with a bridge stacker.

2.Warehouse② is equipped with a mobile conveyor belt.

In addition, bulk sugar storage silos primarily fall into two categories: cylindrical sugar silos and elongated parabolic silos. In terms of sugar stack height and floor space efficiency, cylindrical sugar silos offer superior performance. For example, a comparison of the main dimensions of different sugar storage silos for 10,000 and 40,000 ton storage capacities is shown below:

Table 2 | Dimensions for Two Types of Output Capacities

The above data shows that cylindrical sugar silos occupy only 18%-27% of the floor space of traditional sugar silos and 24%-35% of parabolic silos. Amidst the increasing land scarcity and rising land prices for factories and ports, space conservation has become a crucial factor in sugar silo design. This is one of the main reasons why countries have actively promoted the construction of cylindrical sugar silos in recent years. Therefore, the trend toward silo-based sugar storage has become irreversible.

3.2 Main Advantages of Using Sugar Silos

By comparison, the following key advantages of sugar silos can be summarized:

Large Single Silo Capacity and Small Footprint: A single sugar silo can hold up to 70,000 tons. For example, taking a uniform capacity of 10,000 tons, sugar silos save a significant amount of land compared to traditional warehouses.

Controllable Silo Environment Ensures Stable Sugar Quality: An intelligent air conditioning system regulates the temperature and humidity within the silo, ensuring consistent sugar quality over time.

Enclosed Storage Meets Food Safety Standards: Storing sugar in sealed silos effectively prevents contamination and meets food hygiene requirements. High automation and efficient warehousing and unloading: The high degree of mechanization significantly reduces labor costs. According to international data, traditional bagged sugar storage requires 1.25 man-hours per ton of sugar, while sugar silos only require 0.1185 man-hours.

Flexible year-round operation and high equipment utilization: Small packaging equipment can be configured based on sales needs, enabling year-round operation and reducing customs clearance and logistics costs. Although the initial investment is high, it can be quickly repaid in a short period of time, making it particularly suitable for large storage volumes or a single product category.

When storing a small number of material types or with a small storage volume, a standalone silo or single-row layout can be used. When storing a large variety of materials or with a large storage volume, a cluster layout can be adopted to further improve land utilization and economic benefits.

Although the initial investment for bulk sugar storage is higher than that for bagged sugar storage, the aforementioned advantages make it quickly payable. When storing a small number of material types or with a small storage volume, a standalone silo or single-row layout is used. When storing a large number of material types or with a large storage volume, a cluster layout can be used to further improve economic benefits.

4. Problems and Solutions Facing Silo Technology

4.1 Uneven Sugar Distribution and Sugar Crystal Damage

Currently, the most common method of feeding sugar silos is to drop sugar from the top of the silo. However, this method can cause the following problems when the sugar storage volume is low:

Severe dust generation: The large drop height and high-speed sugar drop easily raise sugar dust, increasing the burden of dust removal and explosion prevention;

Damaged sugar crystals: The strong impact force can easily damage the integrity of sugar crystals;

Particle grading: After the sugar drops to the ground, sugar particles of different sizes slide at different angles, resulting in uneven storage density at the center and periphery of the silo, which in turn affects temperature and humidity regulation within the silo and may even cause some sugar products to fail to meet quality standards.

To address this issue, some international sugar silos have incorporated liftable sugar distributors into their designs to reduce the drop height and ensure uniform sugar distribution. However, this also increases the construction and maintenance costs of the silo.

4.2 Dust Explosion Prevention and Electrical Safety Issues

Despite extensive research and prevention technologies for dust explosions, accidents still occur. According to the “Steel Silo Design Specifications,” all motors and electrical equipment must be dust-proof and explosion-proof. Furthermore, ventilation and dust removal systems should be properly configured during design to centrally collect and treat sugar dust to ensure explosion-proof safety in the storage system.

5. Promotion Prospects and Outlook

Large-scale sugar refineries internationally are striving for high recovery, low energy consumption, and automated sugar production processes. However, domestic development in this area has been relatively limited, with many process technologies practically stagnant. Currently, cultivated sugar still accounts for the majority of China’s sugar market consumption, but bagged storage and transportation are no longer suitable for the needs of a modern, large-scale sugar industry.

Although sugar silo technology has gained widespread adoption abroad, it is still relatively new in China, with few reports on its use. Its widespread application in China could improve the working environment and reduce labor intensity for workers. This would promote the advancement of my country’s sugar production technology and management to internationally advanced levels, ushering the sugar industry into a new era of modern management and enhancing the competitiveness of Chinese sugar companies in the international market. At the same time, its huge economic benefits also provide a strong driving force for the widespread application of cylindrical silos in domestic sugar factories in the future.