Transport Process And Unit Operations Solution Manual

[Adimar Nigerson]

Research and development work in many disciplines -biochemistry, chemical and mechanical engineering - and the establishment ofplantations, which provided the opportunity for large-scale fully mechanisedprocessing, resulted in the evolution of a sequence of processing steps designedto extract, from a harvested oil palm bunch, a high yield of a product ofacceptable quality for the international edible oil trade. The oil winningprocess, in summary, involves the reception of fresh fruit bunches from theplantations, sterilizing and threshing of the bunches to free the palm fruit,mashing the fruit and pressing out the crude palm oil. The crude oil is furthertreated to purify and dry it for storage and export.

Large-scale plants, featuring all stages required to producepalm oil to international standards, are generally handling from 3 to 60 tonnesof FFB/hr. The large installations have mechanical handling systems (bucket andscrew conveyers, pumps and pipelines) and operate continuously, depending on theavailability of FFB. Boilers, fuelled by fibre and shell, produce superheatedsteam, used to generate electricity through turbine generators. The lowerpressure steam from the turbine is used for heating purposes throughout thefactory. Most processing operations are automatically controlled and routinesampling and analysis by process control laboratories ensure smooth, efficientoperation. Although such large installations are capital intensive, extractionrates of 23 - 24 percent palm oil per bunch can be achieved from good qualityTenera.

Conversion of crude palm oil to refined oil involves removalof the products of hydrolysis and oxidation, colour and flavour. After refining,the oil may be separated (fractionated) into liquid and solid phases bythermo-mechanical means (controlled cooling, crystallization, and filtering),and the liquid fraction (olein) is used extensively as a liquid cooking oil intropical climates, competing successfully with the more expensive groundnut,corn, and sunflower oils.

Extraction of oil from the palm kernels is generally separatefrom palm oil extraction, and will often be carried out in mills that processother oilseeds (such as groundnuts, rapeseed, cottonseed, shea nuts or copra).The stages in this process comprise grinding the kernels into small particles,heating (cooking), and extracting the oil using an oilseed expeller orpetroleum-derived solvent. The oil then requires clarification in a filter pressor by sedimentation. Extraction is a well-established industry, with largenumbers of international manufacturers able to offer equipment that can processfrom 10 kg to several tonnes per hour.

Alongside the development of these large-scale fullymechanised oil palm mills and their installation in plantations supplying theinternational edible oil refining industry, small-scale village and artisanalprocessing has continued in Africa. Ventures range in throughput from a fewhundred kilograms up to 8 tonnes FFB per day and supply crude oil to thedomestic market.

Efforts to mechanise and improve traditional manual procedureshave been undertaken by research bodies, development agencies, and privatesector engineering companies, but these activities have been piecemeal anduncoordinated. They have generally concentrated on removing the tedium anddrudgery from the mashing or pounding stage (digestion), and improving theefficiency of oil extraction. Small mechanical, motorised digesters (mainlyscaled-down but unheated versions of the large-scale units described above),have been developed in most oil palm cultivating African countries.

Palm oil processors of all sizes go through these unitoperational stages. They differ in the level of mechanisation of each unitoperation and the interconnecting materials transfer mechanisms that make thesystem batch or continuous. The scale of operations differs at the level ofprocess and product quality control that may be achieved by the method ofmechanisation adopted. The technical terms referred to in the diagram above willbe described later.

In the early stages of fruit formation, the oil content of thefruit is very low. As the fruit approaches maturity the formation of oilincreases rapidly to about 50 percent of mesocarp weigh. In a fresh ripe,un-bruised fruit the free fatty acid (FFA) content of the oil is below 0.3percent. However, in the ripe fruit the exocarp becomes soft and is more easilyattacked by lipolytic enzymes, especially at the base when the fruit becomesdetached from the bunch. The enzymatic attack results in an increase in the FFAof the oil through hydrolysis. Research has shown that if the fruit is bruised,the FFA in the damaged part of the fruit increases rapidly to 60 percent in anhour. There is therefore great variation in the composition and quality withinthe bunch, depending on how much the bunch has been bruised.

Harvesting involves the cutting of the bunch from the tree andallowing it to fall to the ground by gravity. Fruits may be damaged in theprocess of pruning palm fronds to expose the bunch base to facilitate bunchcutting. As the bunch (weighing about 25 kg) falls to the ground the impactbruises the fruit. During loading and unloading of bunches into and out oftransport containers there are further opportunities for the fruit to bebruised.

In Africa most bunches are conveyed to the processing site inbaskets carried on the head. To dismount the load, the tendency is to dumpcontents of the basket onto the ground. This results in more bruises. Sometimestrucks and push carts, unable to set bunches down gently, convey the cargo fromthe villages to the processing site. Again, tumbling the fruit bunches from thecarriers is rough, resulting in bruising of the soft exocarp. In any case careshould be exercised in handling the fruit to avoid excessive bruising.

One answer to the many ways in which harvesting,transportation and handling of bunches can cause fruit to be damaged is toprocess the fruit as early as possible after harvest, say within 48 hours.However the author believes it is better to leave the fruit to ferment for a fewdays before processing. Connoisseurs of good edible palm oil know that theincreased FFA only adds ‘bite’ to the oil flavour. At worst, the highFFA content oil has good laxative effects. The free fatty acid content is not aquality issue for those who consume the crude oil directly, although it is foroil refiners, who have a problem with neutralization of high FFA content palmoil.

Fresh fruit arrives from the field as bunches or loose fruit.The fresh fruit is normally emptied into wooden boxes suitable for weighing on ascale so that quantities of fruit arriving at the processing site may bechecked. Large installations use weighbridges to weigh materials intrucks.

The field factors that affect the composition and finalquality of palm oil are genetic, age of the tree, agronomic, environmental,harvesting technique, handling and transport. Many of these factors are beyondthe control of a small-scale processor. Perhaps some control may be exercisedover harvesting technique as well as post-harvest transport andhandling.

The fresh fruit bunch consists of fruit embedded in spikeletsgrowing on a main stem. Manual threshing is achieved by cutting the fruit-ladenspikelets from the bunch stem with an axe or machete and then separating thefruit from the spikelets by hand. Children and the elderly in the village earnincome as casual labourers performing this activity at the factorysite.

Most small-scale processors do not have the capacity togenerate steam for sterilization. Therefore, the threshed fruits are cooked inwater. Whole bunches which include spikelets absorb a lot of water in thecooking process. High-pressure steam is more effective in heating buncheswithout losing much water. Therefore, most small-scale operations thresh bunchesbefore the fruits are cooked, while high-pressure sterilization systems threshbunches after heating to loosen the fruits.

Small-scale operators use the bunch waste (empty bunches) ascooking fuel. In larger mills the bunch waste is incinerated and the ash, a richsource of potassium, is returned to the plantation as fertilizer.

Sterilization or cooking means the use of high-temperaturewet-heat treatment of loose fruit. Cooking normally uses hot water;sterilization uses pressurized steam. The cooking action serves severalpurposes.

Heat helps to solidifyproteins in which the oil-bearing cells are microscopically dispersed. Theprotein solidification (coagulation) allows the oil-bearing cells to cometogether and flow more easily on application of pressure.

Fruit cooking weakens the pulpstructure, softening it and making it easier to detach the fibrous material andits contents during the digestion process. The high heat is enough to partiallydisrupt the oil-containing cells in the mesocarp and permits oil to be releasedmore readily.

The moisture introduced by thesteam acts chemically to break down gums and resins. The gums and resins causethe oil to foam during frying. Some of the gums and resins are soluble in water.Others can be made soluble in water, when broken down by wet steam (hydrolysis),so that they can be removed during oil clarification. Starches present in thefruit are hydrolyzed and removed in this way.

When high-pressure steam isused for sterilization, the heat causes the moisture in the nuts to expand. Whenthe pressure is reduced the contraction of the nut leads to the detachment ofthe kernel from the shell wall, thus loosening the kernels within their shells.The detachment of the kernel from the shell wall greatly facilitates later nutcracking operations. From the foregoing, it is obvious that sterilization(cooking) is one of the most important operations in oil processing, ensuringthe success of several other phases.

However, during sterilizationit is important to ensure evacuation of air from the sterilizer. Air not onlyacts as a barrier to heat transfer, but oil oxidation increases considerably athigh temperatures; hence oxidation risks are high during sterilization.Over-sterilization can also lead to poor bleach ability of the resultant oil.Sterilization is also the chief factor responsible for the discolouration ofpalm kernels, leading to poor bleach ability of the extracted oil and reductionof the protein value of the press cake.

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