WO1994013778A1

WO1994013778A1 - Improvements relating to a process for the manufacture of soap bars and apparatus for use in said process

Info

Publication number: WO1994013778A1
Application number: PCT/EP1993/003561
Authority: WO
Inventors: Dhanraj Kalyansundaram Chokappa; Vijay Mukund Naik; Jojo Philip
Original assignee: Unilever NV
Current assignee: Unilever NV
Priority date: 1992-12-17
Filing date: 1993-12-13
Publication date: 1994-06-23
Anticipated expiration: 1995-06-17
Also published as: AU5811894A; EP0675948B1; GB9226309D0; DE69303495D1; BR9307677A; EP0675948A1; DE69303495T2; ES2089914T3

Abstract

The invention provides a process for the manufacture of soap forms which includes the step of treating a soap feedstock by passage through a twin screw, intermeshing, counter-rotating extruder and apparatus for the manufacture of soap forms according to the process, said apparatus comprising a twin screw, intermeshing, counter-rotating extruder, wherein the extruder comprises two oppositely-threaded, closely intermeshing screws mounted for rotation within a barrel having a first end and a second end, said screws having a minimal screw-to-screw and screw to barrel clearance such that as the feedstock passes along at least a part of the barrel from said first end and towards said second end it is divided into a plurality of discrete, substantially C-shaped segments bounded by the screw and barrel surfaces and conveyed in a path whereby the bulk of the feedstock move substantially parallel to the rotational axis of the screws.

Description

IMPROVEMENTS RELATING TO A PROCESS FOR THE MANUFACTURE OF SOAP BARS AND APPARATUS FOR USE IN SAID PROCESS

Technical Field

The present invention relates to improvements relating to a process for the manufacture of soap forms and apparatus for use in said process. The invention is particularly concerned with improvements to the so-called soap

'plodder' and to a process which uses an improved plodder. While the invention is described with particular reference to the manufacture of soap bars it should be understood that the term 'soap' extends to materials comprising non- soap surfactants, including synthetic surfactants and the term 'forms' extends to other three-dimensional solid forms including soap bars, billets, tablets and so-called noodles.

Background to the Invention

The starting material for the production of a soap bar or billet is a mixture containing surfactants, other

functional ingredients and water at appropriate

proportions. Depending upon the composition of this mixture, its rheological and processing characteristics vary a great deal.

Generally, processing/finishing of such a mixture involves various process steps such as homogenisation, shear working, and forming into a required shape. One of the devices very commonly employed to carry out one or more of the above operations is a plodder. The function of a simple plodder is to form the mixture into bars or billets of required cross-sections which may subsequently be cut into smaller bars or stamped into tablets of required shape by suitable other means.

The function of a refiner plodder is to clean the mixture free of gritty particles or impurities and additionally homogenise/shear work the same to achieve the required degree of homogeneity or phase structure. The plodders may also be used to convert loose aggregates/chips/flakes into noodles for intermediate storage or for feeding subsequent process operations.

The heart of a plodder is a screw extruder. The simplest plodder has an extruder with a single worm. The feed stock, either in homogenised and worked form or in the form of noodles or crimpled chips, fed through the hopper enters the extruder barrel and fills the annular space between the extruder worm (screw) and the barrel. The barrel is stationary and the worm rotates inside the barrel. Frictional/viscous drag forces act on the

material, both at the barrel as well as at the worm surfaces. The resultant force is responsible for the forward transportation of the processed mass towards the discharge end. At the discharge end, the extruder may have a perforated plate, through which the processed mass is forced. This is generally known in the art as a

'noodle plate'. The processed mass emerges in the form of rods/ribbons/sheets from the perforated plate.

If the objective is to enhance homogenisation or shear working or to filter out gritty particles, then it is advantageous to fit a wire gauze in front of the

perforated plate but it tends to reduce throughput. If the objective is to form the mixed mass into billets or bars, then a cone and die/eye plate are provided at the discharge end of the extruder along with or without the noodle plate. The extruder forces the mixed mass through these end fittings to product the noodles, billets or bars. Designs of perforated plates, cones and die/eye plates vary considerably from application to application. It should be noted that the throughput rate is very sensitive to the resistance offered by wire mesh screens, perforated plates, cones and dies. Machines called duplex or twin worm plodders have two worms (or screws) which are parallel, non-intermeshing and mounted tangentially with respect to each other within a barrel. The worms may be co-rotating but usually they are counter-rotating. Intermeshed and co-rotating twin screw extruders are also known for processing of soap/detergent mass. In the case of both non-intermeshed duplex plodders as well as the intermeshed co-rotating twin screw

extruders, drag forces similar to those encountered in single worm plodders act on the processed mass and push the same in the forward direction.

Screw extrusion is apparently a simple operation, but the results in terms of quality of product, throughput rate, specific energy consumption, etc. can be influenced by a number of factors in a rather complex way. Generally, plodding is affected by soap factors and by machine factors. It is important to balance the various factors so as to achieve the best results. As the processed material moves forward, it is heated as a result of frictional and shear heat generation. Considerable structural breakdown may also take place in the case of some formulations. In certain instances, the heat

generated raises the temperature of the processed mass above desirable limits thereby adversely affecting certain properties of the soap. These may lead to deterioration of user properties as well as softer extruded forms which may cause complications in downstream processing. The excessive and wasteful shear dissipation may also lead to lower energy efficiency. From the above it can be seen that the functions of the screw extruder in the soap making process include mixing, working and conveying materials. Other apparatus exists for the performance of some or all of these process operations. For example, mixing can be performed in Z or Sigma-blade mixers: products can be worked by chilled rolls and transported using conveyors. As will be

appreciated these apparatus also have their disadvantages e.g. mixers generally only operate in batch mode, chilled rolls are expensive to manufacture and maintain and sticky products are difficult to transport on conveyors.

In recent years it has become commonplace to use synthetic surfactants in 'soap' bar formulations. Alkyl

isethionates being an example of such a surfactant. It has also become commonplace to include oily or fatty materials in the formulations so as to achieve skin benefits. Examples of such materials include fatty acids, free isethionate, mineral oils, paraffin oils and

silicones. The use of such materials complicates

processing as the feedstocks comprising such formulations can become particularly sensitive to variations in process temperature.

From the above it can be seen that the transport

characteristics of the conventional extruders are very sensitive to the rheology of the processed material, thereby necessitating several restrictions on formulations that can be processed satisfactorily. Brief Description of the Invention

We have now determined that some or all of the above- mentioned disadvantages can be overcome by the use of a twin screw, intermeshing, counter-rotating extruder, to convey and work the feedstock.

Detailed Description of the Invention

Accordingly, a first aspect of the present invention provides a process for the manufacture of soap forms which includes the step of treating a soap feedstock by passage through a twin screw, intermeshing, counter-rotating extruder.

A second aspect of the present invention provides

apparatus for the manufacture of soap forms which

comprises a twin screw, intermeshing, counter-rotating extruder.

Typically, the extruder comprises two oppositely-threaded, closely intermeshing screws mounted for rotation within a barrel having a first end and a second end, said screws having a minimal screw-to-screw and screw to barrel clearance such that as the feedstock passes along at least a part of the barrel from said first end and towards said second end it is divided into a plurality of discrete, substantially C-shaped segments bounded by the screw and barrel surfaces and conveyed in a path whereby the bulk of the feedstock move substantially parallel to the

rotational axis of the screws.

The operation of the plodders according to the present invention can be described with reference to the following equation: F = 2mNv

wherein,:

F is the theoretical flowrate of output,

m is the number of starts on the shaft,

N is the shaft speed, and,

v is the volume of the C-shaped segment mentioned above.

In practice, actual output is always less than theoretical output F due to the finite clearance between the shafts and the resistance to flow which is caused, by a noodle plate at the outlet. The ratio of the actual (measured) output to the theoretical figure derived above is known as the 'volumetric efficiency'. As will be appreciated by the comments made in the preamble, the volumetric

efficiency of a single worm plodder is strongly dependent on the frictional or other rheological properties of each product. It is believed that in a conventional plodder little of the energy supplied to the screw extruder is required to convey and work the feedstock, depending on the feedstock rheology. The excess energy expended during plodding is largely converted into heat which raises the temperature of the product. It is believed that in an extruder according to the present invention there is a reduction in the dependence on throughput rate on the rheology of the feedstock being treated and reduction in temperature rises in the feedstock during treatment, thereby overcoming some or all of the disadvantages mentioned above.

As will be discussed in more detail below it is believed that volumetric efficiencies of above 60% can be attained using the present invention whereas in single screw plodders according to the prior art the volumetric

efficiency was typically 9-19%. It is particularly convenient that the screws should be a matching pair, i.e. that they should have the same diameters and be substantially different only as regards the handedness of the screw.

The screws are preferably detachable from the extruder whereby whenever necessary the screws can be changed with another pair of screws having different screw layout. The screw may be either continuous or segmented. The

segmented screws have removable sections which can have varying pitches so that they can be manipulated depending upon the composition of the material being processed. The screws may be single-start or multi-start. Typically, apparatus according to embodiments of the present invention will further comprise means for rotating the screws in mutually opposite directions. It should be noted that the positive pumping characteristic of

extruders according to the present invention can be used either to increase the throughput of the extruder at a given rotational speed or to reduce the rotational speed required for a given throughput. Reduction in screw speed is particularly advantageous as it reduces the extent to which the product is heated during treatment, and enables the power input to be reduced.

Typically, apparatus according to embodiments of the present invention will further comprise means for

supplying the feedstock to the barrel of the extruder at the first end of the barrel. Generally, said means for supplying the feedstock comprise hopper means located above one end of the screws. An advantage associated with the use of counter-rotating screws is that a nipping action occurs and the feedstock is drawn from the hopper into the barrel of the extruder. Typically, apparatus according to embodiments of the present invention will further comprise means for forming the feedstock into soap forms at the second end of the barrel. Such means are well-known in the art. The forms can comprise noodles, bars and/or billets. The precise nature of the means for forming the feedstock will depend on the nature of the forms desired. Where noodles are desired said means comprise a perforated plate through which the feedstock is extruded. Where bars or billets are desired a conventional cone and eyeplate can be employed.

In certain embodiments of the present invention the pitch angle of the screws decreases in the direction of flow. Preferably the pitch angle is 20-30 degrees adjacent the first end of the barrel and 10-20 degrees adjacent the second end of the barrel. This is particularly useful where feedstock is being compacted and its overall density is increasing during progress down the screw from the first to the second end of the barrel.

In certain embodiments of the present invention it is preferable that the width of the flights increases in the direction of flow relative to the width of the channel between the flights. Preferably the width of the channel is at least three times the flight width adjacent the first end of the barrel and less than three times the flight width adjacent the second end of the barrel. This is particularly useful where feedstock is being compacted and its overall density is increasing during progress down the screw from the first to the second end of the barrel.

In particular embodiments of the present invention cooling means are provided to cool the feedstock within the barrel. The cooling means can include means for

circulating a cooling fluid through a cooling jacket around the barrel and/or through passages within one or both of the screws. While cooling means are generally employed in single screw extruders, the cooling means are optional in the present invention.

In conventional single screw extruders conveyance of the feedstock is influenced by the difference in friction between the feedstock and the barrel and screws. This difference is partly achieved by cooling the barrel but not the screws. The closely-intermeshing, twin screw extruder of the present invention has a positive pumping characteristic and consequently cooling is not required to maintain the pumping action. However some degree of cooling can be employed if it is desirable to cool the process stream during its passage through the extruder, thus enabling the apparatus

according to the invention to replace the chilled rolls used in conventional plant. In embodiments of the present invention the efficiency of cooling can be improved by cooling the screws in addition to the barrel.

It should be noted that with conventional plant, the provision of cooling means, whether to assist in pumping or remove heat generated by shear adds a considerable capital cost and increases operating costs per ton of capacity whereas in the present invention the capacity of the cooling means to remove heat from the feedstock need not be as great. In practice, this enables a higher temperature coolant to be used, e.g. cooled water rather than chilled water. Moreover, the relatively low

temperatures which can be attained in the product emerging from the apparatus according to the present invention greatly facilitate subsequent handling and processing of the product. A further mathematical relation which is of use in the understanding of the present invention is that concerning the heat flow: Q = MCdT

wherein:

Q is the overall heat transfer rate;

M is the mass flowrate of the coolant;

C is the specific heat capacity of the coolant, and; Dt is the temperature rise in the coolant.

Q is obviously the sum of at least two terms, one which relates to the heat removed from the product (whether sensible or latent or both) and one which relates to the heat added to the product by the rotation of the screw, the so-called viscous dissipation. As mentioned above, the cooling means in embodiments of the present invention can include means for circulating a cooling fuild through a cooling jacket around the barrel and through passages within one or both of the screws and this can

significantly improve the heat transfer rate, by

increasing the heat transfer area.

In use, the apparatus according to the present invention can apply the process of the invention to a wide range of feedstocks and use is not limited to those feedstocks which comprise synthetic, non-soap surfactants. In particular, the invention can be applied to the processing of transparent and/or translucent soap formulations which are known to be of high viscosity and difficult or

expensive to process, or to soap forms comprising

structuring and/or hardening aids. It is envisaged that all manner of aerated/solid and/or toilet/laundry soaps can be manufactured by a process according to the present invention. It should also be noted that the apparatus according to the present invention need not be manufactured to the same high engineering tolerances required in the manufacture of conventional counter-rotating extruders thereby further reducing set-up and manufacturing costs. A particular advantage as regards manufacturing is that the screws can be at least in part cast rather than machined from a blank. The invention will now be described, by way of

illustration only, with reference to the accompanying drawings in which:

Fig. 1 shows a schematic view of an extruder with an open sectional view of the screws for carrying out the method according to the invention;

Fig. 2 shows a cross-sectional view of the extruder

barrel;

Fig. 3 shows a transverse cross-sectional profile of the screw;

Fig. 4 shows a perspective view of two intermeshed

screws according to a preferred embodiment of the present invention;

Fig. 5 shows a comparison of actual and theoretical

volumetric efficiencies through an apparatus embodying the present invention for a range of soap blends as compared with a single screw plodder;

Fig. 6. shows (as 6a and 6b) details of two embodiments of the present invention; As illustrated n figure 1, the extruder comprises two closely intermeshed screws 1 and 2 mounted within a jacketed barrel 3 and driven by a motor 4. The two screws are oppositely threaded and closely intermeshed with each other. When one screw rotates clockwise, the other screw rotates anticlockwise. The screws shown are single-start screws, but they may even be multi-start ones. A cross section of a typical screw is shown in figure 3. The clearances in between the screws and between the screws and the barrel are kept to the minimum possible. This ensures a plurality of substantially closed C-shaped chambers which enable the soap/detergent material to be transported positively and relatively gently. This feature is shown in figure 2.

In an operating extruder there may be a need to compact discrete particulate or lumpy mass into a homogeneous extrudate. This may be achieved by varying the cross- sectional profile of the screws from the feed end to the output end in such a way that the flight width increases gradually/progressively from feed end to output end and at the same time the pitch, angle of one or both screws changes suitably to ensure that the trailing flank of the flight of one screw is in close proximity with the

adjacent leading flank of the flight of the other screw. This feature is seen in the embodiment shown in figure 4.

At feed end of the extruder a hopper 5 is typically provided, through which the soap/detergent material enters the extruder. At the other end of the extruder is mounted a perforated plate (not shown), a cone jacket (not shown) and an eye plate (not shown). In practice the eyeplate, jacket and plate would be mounted on the flange at the output end of the extruder as shown in figure 1. As the screws rotate, the material in each of said closed C-shaped cavities is transported linearly and pushed through the holes in the perforated plate in the form of rods which are welded together in the cone and a bar of the required cross-section emerges continuously from the eye plate.

A less preferred embodiment of the invention, with

partially intermeshing screws is shown in figure 6b, whereas figure 6a shows the more preferable arrangement having closely intermeshed screws. It should be noted that the screws of figure 6 do not have the particular features of the embodiment shown in figure 4 but rather represent the two extremes of the variation in flight width which occurs in figure 4 .

While the extruders described above all have single-stage extruder mechanism, it will be appreciated that the invention may equally be carried out using two- or other multi- stage extruders and in particular those which correspond to multi-stage vacuum plodders or refiners

A particular advantage of those embodiments of the present invention in which closely intermeshing screws are used is that the soap/detergent material to a considerable extent is prevented from rotating with the screws. This ensures that the bulk of the material moves substantially parallel to the extruder axis and thereby avoids any rheological or structural damages or rise in temperature in the material and results in soap/detergent mass with better downstream processing qualities. Such a process delivers a higher throughput at a giver rpm, i.e. a better volumetric efficiency and a lower specific energy consumption in comparison to plodding methods hitherto being used in the soap/detergent industry. These operational features of the invention can be seen by reference to Fig.5 which shows a comparison of actual and theoretical volumetric efficiency through an apparatus embodying the present invention for a range of soap blends.

In figure 5, results are given for a formulation based on sodium cocoyl isethionate and containing relatively low levels of soap, a formulation based on a mixture of sodium cocoyl isethionate and soap, and a formulation based on an acyl isethionate, sulphosuccinate, betaine surfactant system. These differences in formulation are not

distinguished between, but are in part responsible for the spread of results. Triangles are results obtained with closely intermeshing shafts similar to those shown in figure 6a. Crosses are results obtained with partially intermeshing shafts similar to those shown in figure 6b. Squares are results obtained with a conventional single screw extruder having the same screw size as the double screw apparatus. It can be seen very clearly from this graph that the volumetric efficiency of the apparatus employing closely intermeshed shafts is generally above 50%, whereas with partially intermeshing shafts the volumetric efficiency falls to 25-50% of the theoretical maximum. It should be noted that 25-50% volumetric efficiency still exhibits a general improvement over the results obtained in the conventional single screw plodder.

As the volumetric efficiency of the twin screw plodder is much higher than that of a single screw unit a lower tip speed is required in the twin screw plodder to achieve a particular heat transfer coefficient. Moreover,

experiment shows that the viscous dissipation in the product is both reduced in the twin screw unit (as opposed to the single screw unit) and reduced at lower shaft speeds. Even at small scale the reduction in viscous dissipation per unit throughput is around a factor of five as compared with the single screw unit.

Claims

1. A process for the manufacture of soap forms which

includes the step of treating a soap feedstock by passage through a twin screw, intermeshing, counter- rotating extruder.

2. Apparatus for the manufacture of soap forms according to the process of claim 1, said apparatus comprising a twin screw, intermeshing, counter-rotating

extruder .

3. Apparatus according to claim 2 wherein the extruder comprises two oppositely-threaded, closely

intermeshing screws mounted for rotation within a barrel having a first end and a second end, said screws having a minimal screw-to-screw and screw to barrel clearance such that as the feedstock passes along at least a part of the barrel from said first end and towards said second end it is divided into a plurality of discrete, substantially C-shaped

segments bounded by the screw and barrel surfaces and conveyed in a path whereby the bulk of the feedstock move substantially parallel to the rotational axis of the screws.

4. Apparatus according to claim 3 further comprising

means for supplying the feedstock to the barrel of the extruder at the first end of the barrel.

5. Apparatus according to claim 3 further comprise means for forming the feedstock into soap forms at the second end of the barrel.

6. Apparatus according to claim 3 wherein the pitch angle of each screw is 20-30 degrees adjacent the first end of the barrel and 10-20 degrees adjacent the second end of the barrel.

7. Apparatus according to claim 3 wherein the width of the channel of each screw is at least three times the flight width of the screw adjacent the first end of the barrel and less than three times the flight width adjacent the second end of the barrel.

8. Apparatus according to claim 3 further comprising

cooling means are provided to cool the feedstock within the barrel.

9. Apparatus according to claim 8 wherein the cooling means include means for circulating a cooling fluid through a cooling jacket around the barrel.

10. Apparatus according to claim 8 wherein the cooling means include means for circulating a cooling fluid through passages within one or both of the screws.