PALM OIL MILLING PROCESS
Palm oil is extracted from fresh fruit bunches (FFB) by mechanical process, where a mill commonly handles 60 to 100mt per hour of FFB. The modern palm oil mill of today is based predominantly on concepts developed in the early 50s (Mongana Report). An average size FFB weighs about 20-30kg and contains 1500-2000 fruits (Figure1). The FFBs are harvested according to harvesting cycles, and delivered to the mills on the same day. The quality of crude palm oil is dependent on the care taken after harvesting, particularly on the handling of the FFBs.
Figure 1. Fresh fruit bunches waiting for processing at palm oil mill A palm oil mill produces crude palm oil and kernels, as primary products and biomass as secondary product. The capacity of mills varies between 60- 100 tons FFB/h. A typical mill has many operation units as shown in Figure 2. This comprises of sterilization, stripping, digestion and pressing, clarification, purification, drying and storage. For the kernel line, there are steps such as nut/fibre separation, nut conditioning and cracking, cracked mixture separation, and kernel drying, storage. The dried kernels are often sold to palm kernel crushers for extraction of crude palm kernel oil. In some integrated plants, kernel crushing facilities exist side by side at the same complex.
Figure 2. Flow chart for the palm oil process (from Sivasothy, 2000).
This first step in the process is crucial to the final oil quality as well as the strippability of fruits. Sterilization inactivates the lipases in the fruits, and prevents build-up of free fatty acids (FFA). In addition, steam sterilization of the FFBs facilitates fruits being stripped from the bunches. It also softens the fruit mesocarp for digestion and release of oil, and conditioning of nuts to minimize kernel breakage. Air is removed from the sterilizer by sweeping in steam in single-peak, double-peak or triple-peak cycles. In general, bunches are cooked using steam at 40psig. in horizontal cylindrical autoclaves for 60-90 minutes. The length of the sterilizer is dependent on the number of cages required for operation of the mill. Each cage can hold 2.5 to 10tons of FFB. Steam consumption varies from 140 kg/ton FFB for a single-peak cycle to 224 kg/ton FFB for a triple – peak cycle (Sivasothy et al.1986). Inadequate sterilzation affects the subsequent milling processing stages adversely. In recent years, new technology on sterilization saw the introduction of continuous sterilizers. Sivasothy’s (2006) continuous sterilizer showed improved fruit strippability, even with usage of low pressure steam or atmospheric steam. The new system consists of conveyor belt taking crushed FFBs into the continuous sterilizer, where the fruits are sterilized and subsequently discharged. This reduces much of the machinery associated with conventional sterilizers. In addition, there are cost savings in terms of manpower requirements, and maintenance. Vertical sterilizers are also available, which are much cleaner and easier to operate than conventional sterilizers. Another type of sterilizer technology, the Tilting sterilizer (Loh, 2010) also eliminates much of the machinery associated with conventional sterilizers. The technology is the latest design that offers improved milling efficiency, and reduced labour and maintenance cost.
Figure 3. A conventional sterilizer
Stripping or threshing involves separating the sterilized fruits from the bunch stalks. Sterilized FFBs are fed into a drum stripper and the drum is rotated, causing the fruits to be detached from the bunch. The bunch stalks are removed as they do not contain any oil. It is important to ensure that oil loss in the bunch stalk is kept to a minimum. The stalks are often disposed by incineration, giving ash as potash fertilizer, and fuel for boilers. Others are transported to the plantations for use as fertilisers in mulching near the palms....
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