Continuous and Conventional Sterilization

In order to maximize the production of CPO in a palm oil mill, various technologies have been developed by the people in the industry. Being the first process in the mill, sterilization is a crucial part of the whole processes since it influences the quantity and the quality of CPO produced later. FFBs are cooked using steam as the heating medium in a sterilization process. A lot of steam is used in this part of the mill that accounts thirty to sixty percent of the total process steam supplied from the boiler.

Therefore, the type of sterilizer technology chose greatly affects steam and power consumption of the sterilization process.

With the growing demand for energy efficiency at palm oil mills, the selection of sterilizer is based mainly on its relevance to steam and power consumption because this will affect the overall energy efficiency of the palm oil extraction process. It is vital to ensure that the sterilizer operates correctly so that it will produce minimum oil loss and generate proper oil extraction rate. Oil losses that correspond from sterilizing process are oil losses in condensate, empty fruit bunch, un-striped bunch and partly striped bunch.

Basically sterilizers are design to operate continuously or in batches. The conventional and continuous type processes are described to select most economical sterilizer technology.

In a conventional sterilization, the sterilization process of fresh fruit bunches is carried out in cylindrical pressure vessels that lies horizontally or vertically. The concept of conventional sterilization is shown in Figure 37. After that they are filled with steam under pressure as a batch process. The most common type of sterilizer used is the horizontal sterilizer fitted with two quick opening doors [11]. Fruit cages are used to transfer the bunches in and out of sterilizers, and various other equipment are needed for handling these cages, including overhead cranes, tippers, conveyors, transfer carriages and tractors.

Currently, there are many types of innovation and improvement of conventional type sterilizers such as cage material handling using indexing system, CMC Systems, Vertical Sterilizer and Tilting Sterilizer. To comply with the amount of FFB processed per day, palm oil mill owners usually choose one of these technologies to be implemented.

The introduction of a three peak cycle process in the sterilization process allows a synchronized integration. This involves the discharge of condensate and air in the pressure vessel, fasten pressure built and evenly distributed steam. From this batch type process, time and steam usage can be saved. With horizontal position, the cylindrical vessel sterilizer has fairly good disposition because the oil palm fresh fruit bunches placed in cages with a low stacking height are more uniformly spread out in this position across the length of the elongated vessel, as opposed to a vertical sterilizer. Thus, when pressurized steam is injected into the interior of the horizontally positioned cylindrical vessel, the steam can reach out to different directions and corners of the contents within

the cages thereby helping treatment of the fruit bunches. Due to the low stacking height of the fruit bunches in the cages, condensate drains out freely from the fruit bunch stack facilitating heat penetration. Batch process however arrests the oil quality deterioration due to enzymatic activity. Considerable space and a system of rails are required to facilitate the fitting of the cages and the charging and discharging of the sterilizers.

Generally, when the received crops are less in capacity, the fruit bunches will be bulked and processed the next day. This will result in the increment of FFA content in the fruits and leads to poor quality of CPO.

Figure 37: Concept of Conventional Sterilization Process.

Continuous type sterilization process is also famously adopted in palm oil mill due to its much beneficial factors if compared to batch sterilization. The concept of continuous sterilization process is shown in Figure 38. The continuous sterilizer was introduced as an alternative to pressure vessels and batch process of sterilization and offer advantage in terms of use of unpressurised heating cabin and steady steam demand for the sterilization process [12]. In current technology, the process is carried out in a heating cabin operating with steam at atmospheric pressure.

Initially, the fruit bunches are split using a mechanical splitter machine before it is transported by scraper conveyor within the heating cabin to expose the material to steam.

The splitting of the fruit bunches facilitates steam penetration into inner layers of the fruit bunch. A significant advantage of continuous sterilization over batch sterilization is that it renders the palm oil milling process a continuous operation from start to finish, making it cost-effective to automate the bunch handling operations. It also eliminates the use of

sterilizer cages, rail tracks, overhead cranes, tippers, transfer carriages and tractors and thereby facilitates the design and construction of mills having significantly smaller footprints than conventional mills The process leads to improvements in the design of mills, reduces the number of process operators, lowers the operating and maintenance costs, and simplifies mill operation. Mills using the process can be more easily supervised and automated.

By avoiding the use of pressure vessels for sterilization and cages and cranes for the handling of bunches, palm oil mills are made safer for operators. The use of conveyors in place of cages also minimizes spillage of fruits and oil making mills cleaner.

Although the new process is carried out using steam at low or atmospheric pressure, the process significantly improves strippability of bunches as researched by Sivasothy. A plus point is that avoiding fluctuations in the steam flow to the sterilization process provides a considerable advantage in maintaining overall process steam pressures and temperatures in the mill. However, the continuous sterilizer suffers a significant disadvantage in terms of high steam consumption.

Figure 38: Concept of Continuous Sterilization Process.