The invention relates to a digester particularly for the preparation of palm fruit from which palm oil is to be extracted by mechanical means.

The process of preparing palm oil is as follows : after having been detached from the cluster branches in a thresher, the palm fruits are further treated in a heated stirring vessel to detach the palm kernels from the flesh.

The thus treated pulp is then continuously fed into a press to extract the crude palm oil.

The currently known digesters or stirring vessels are generally cylindrical with a central, vertical rotation shaft to which a number of horizontally and radially extending stirring arms are fixed. In most cases, also a number of radial projections are fixed to the vessel inner wall between the stirring arms in order to stimulate the stirring and kneading action. The vessel wall is heated and a softening agent, e.g. steam and/or hot water, is fed into the vessel to stimulate the scalding of the flesh from around the kernels.

It is generally known to use radially extending stirring arms in digesters, the cross section (tangential to the direction of rotation) of said arms being V-shaped with the point of the V directed forward in the direction of rotation. Special ejection systems are then often provided for the treated products at the bottom of these vessels.

It is now one of the objects of the present invention to provide i.s.o. these systems ejection arms with the same shape and dimensions as the stirring arms, but which are fixed upside down to the vertical shaft with respect to said stirring arms. Moreover these ejection arms offer a proper ejection performance and have the advantage that only one type of arms is necessary which simplifies their replacement (less spare parts).

A preferred embodiment of the stirring arms and the digester will now be described with reference to the adjoined figures for illustration of the subject and the special advantages of the invention.

• Figure 1 is a view in elevation of the digester with the stirring arms and ejection arms.
• Figure 2 is a horizontal cross sectional view of the digester.
• Figure 3 is a detailed view of the stirring arm.
• Figure 4 is a cross sectional view of the stirring arm tangential to its direction of rotation.

A digester 1 according to the Figure 1 comprises a cylindrical double wall 2 in which a heating means, for example steam, can circulate. In the vessel, a vertical rotation' shaft 3 with e.g. square cross section is bearing-mounted. This shaft can be driven by a motor 4 and bears at different levels horizontal stirring arms 5 which extend radially in the vessel, as well as ejection arms 6 at the bottom of the Vessel. Preferably radial projections 7 are fixed to the inner wall of the vessel between the levels of the stirring arms, to stimulate the agitation. These radial projections are generally fixed to cylindrical wear rings 8 in the wall. Each ring then supports for example four radial projections. A supply funnel 9 and a discharge funnel 10 are provided respectively at the top and at the bottom of the vessel. A supply pipe 11 for the fruit softening agent (e.g. steam) is also connected to the vessel.

The main feature of the invention is to be found in the shape and the position of the stirring arms resp. the ejection arms in the vessel. These characteristics are further illustrated in Figures 2, 3 and 4. The stirring arms 5 and the ejection arms 6 are fixed to the rotation shaft by means of clips 12 and extend horizontally and radially in the digester. Each stirring arm 5 has a V-shaped vertical cross section tangential to the direction of rotation of the arm, and with the point 13 of the V directed forward in the direction of rotation of the stirring arm. The direction of rotation is indicated by means of arrow 14 in Figure 2. This figure also shows that the stirring arm is preferably slightly concavely bent at its front side 15. It has namely been found that the stirring arms used previously have a tendency to sling the pulp against the vessel wall during the agitation process which causes too much wear to the wall and also reduces the defibration efficiency of the stirring arms. The concavely bent stirring arms take up the pulp somewhat as a scoop during the agitation process so as to bring about a scraping of the wall. The ejection arms 6 however which are fixed upside down to the vertical shaft 3 (with respect to the stirring arms), swing through the digester with their convex side and with the V-grooves 24 as front sides. This stimulates the movement of the pulp at the bottom of the vessel towards the vessel wall 2 and towards the discharge funnel 10. The connection line with the end points A, resp. B of the front side as shown in Figure 3 may form an angle b to the radial direction 16 of the arm 15. This angle b may vary between 20° in advance or 10° behind, but preferably between 10° in advance and 0° in advance. Angle b according to the embodiment in Figure 3 is 6° in advance.

Further features of the shape of the stirring arms are illustrated in Figures 3 and 4. The inclined surfaces 17 and 18 of the stirring arm preferably form an angle c of at least 50°. It has also proved advantageous to provide these surfaces with oblique grooves 19, whereby a same groove near point 13 of the V (e.g. in spot 20), is closer to the wall 2 of the vessel than at the rear side 21 (e.g. in spot 22). The direction 23 of each groove thereby forms an angle a to the direction 16, which is preferably between 20° and 80°.

The successive groove crests and troughs strongly stimulate the defibration process by providing the grinding effect of the fruit on the stirring arm surfaces 17 and 18. Moreover, the oblique direction of the grooves slightly forces the pulp towards the center of the digester during the agitation. This efficiently counteracts the natural tendency of the pulp to slide against the vessel wall (with the resulting excessive wear load on the vessel wall and low defibration efficiency).

The grooves preferably have rounded crests and troughs and form a ribbed edge near the front side 15 of the stirring arm. This edge appears to have a very useful scouring effect on the fruit to be processed and so stimulates the detachment of the pulp fibres sticking to the kernels without damaging them.

The mutual length-to-width ratio of the stirring arm as shown in Figures 3 and 4, also appears to be optimal for the defibration efficiency (maximum defibration for a given flow and energy supply). The stirring arm may be made of cast iron and according to the kind and maturity of the fruit, a rotation speed of 15 to 30 revolutions per minute is applied for full capacity operation.

Although the digester was specifically described with a view to the defibration of palmfruits, its application is not limited to that. Similar defibration treatments on other kinds of fruit and with increased efficiency can also be carried out by using the stirring vessel ; particularly the stirring and ejection arms described above.