A Brief Review on Sterilization of Oil Palm Fresh Fruit Bunches

Steam sterilization is the first and very critical step in the palm oil extraction process in a palm oil mill or factory.  The objective of using steam to sterilize the oil palm fresh fruit bunches (FFB) is to perform two major functions: 

(1) to soften the very hard FFB so that all the fruits can be readily detached from the bunches, and 

(2) to condition the mesocarp (pulp) of the fruits for efficient extraction of the oil contained in it during the subsequent steps.   

The following is a brief description of the various types of FFB sterilizer currently in commercial use and their operational characteristics.

Conventional Horizontal FFB Sterilizer

The conventional horizontal FFB sterilizer is known to be inherently inefficient in air removal from the stacks of fruit bunches loaded in cages inside the sterilizer.  Because of this technical weakness, horizontal FFB sterilizer would require high steam consumption of 360 to 400 kg per ton FFB at 4 barg pressure in triple-peak cycles over 90 minutes to achieve satisfactory FFB sterilization.  

Besides high steam (therefore energy) consumption, the conventional way in handling the FFB and steamed fruit bunches (SFB) using cages, capstans, winches and overhead lifting cranes is also the source of a host of operational inefficiencies such as high oil losses, more frequent down-times leading to higher maintenance costs and exposure of workers to higher risks of operational hazards.

 

Development of New FFB Sterilizers 

There are a number of second-generation FFB sterilizers developed over the past 1-2 decades which are now used commercially in new and upgraded palm oil mills.  These can be categorized into three types as listed below:

1.  Continuous FFB Sterilizer
2.  Vertical FFB Sterilizer
3.  Spherical FFB Sterilizer

The continuous FFB sterilizer is an unpressurised sterilizer system in which the fruit bunches are continuously moved through by conveyor inside a steaming cabin.  The vertical and spherical sterilizers are pressurized batch sterilizers but are relatively different in designs and mechanisms in FFB feeding, steaming control, condensate removal as well as SFB evacuation.   

Continuous FFB Sterilizer

In a continuous FFB sterilizer system, the fruit bunches are first mechanically split and transported through a horizontal sterilizer cabin via a scraper conveyor in a continuous process.  While inside the sterilizer cabin, the split fruit bunches are exposed to steam at atmospheric pressure.  After the stripping step, the stripped fruits will be exposed to another round of steaming to bring the fruits to the right condition for the digesting/pressing stages.  

In practice, the continuous FFB sterilizer requires 300 to 350kg of steam per ton of FFB and additional power consumption of 200kW to run the FFB bunch splitting machine and the scraper conveyer inside the sterilizing cabin in a 60mt/hr palm oil mill. 

Vertical FFB Sterilizer

Vertical FFB sterilizer basically resembles the FFB horizontal sterilizer in shape being positioned vertically.  It has a top inlet door for FFB feeding and a bottom outlet door for SFB evacuation.  By virtue of its vertical position, vertical FFB sterilizer occupies much smaller footprint and therefore requires smaller area for the sterilizer station.  

However, its stacking height of some 8-10m tall would invariably result in some degree of FFB compaction at the lower part of the sterilizing vessel.  This compaction would pose certain degree of restriction in steam penetration, air/condensate removal, and SFB evacuation.  Moreover, this lower-section FFB compaction phenomenon is also the main cause of frequent repairs and maintenance required in running vertical sterilizer.  

In practice, vertical FFB sterilizer consumes 250 to 300kg of steam per ton FFB over a steaming time of 60 minutes in a triple-peak cycle.

Spherical FFB Sterilizer

The configuration of the spherical FFB sterilizer station is about the same as for the vertical FFB sterilizer station except for the different geometric shapes and internal designs of the two types of sterilizing vessel.  

Spherical FFB sterilizer is a ball-shaped sterilizing vessel that can rotate 360 degree to facilitate both FFB feeding and SFB discharge through its single door.   The round geometry of the sterilizing vessel (4.5m in diameter) minimizes FFB compaction inside the vessel and hence optimum steam penetration, air and condensate removal resulting in great economy of steam. 

The simplicity in its design results in minimum repairs to both its internal and external parts and hence the lowest maintenance cost among all types FFB sterilizers in commercial use.

In practice, spherical FFB sterilizer consumes 220 to 250kg of steam per ton of FFB over a steaming time of 60 minutes in a two-peak cycle.

For further enquiries, please contact:

 
CIMG Resources Sdn Bhd, Malaysia

Email:  cimgrs@yahoo.com