Field of the invention

The present invention relates to a method for determining the amount of volatile surface liquid on a solid extrudate, to use of such method in a process for manufacturing a solid extrudate, and to a process for manufacturing a solid extrudate comprising such method.

Background of the invention

Extrusion processes wherein a mixture comprising a solid and a liquid is forced through an opening in order to manufacture a shaped solid extrudate are well known. Examples of such processes are: extrusion processes for the manufacture of plastic products, ceramic products such as cement-like products including calcium phosphate- base medical or orthopaedic products, feed products such as animal feed pellets, and food products such as snacks, noodles, meat analogues; various extrusion processes for forming pellets or tablets; and filament forming processes such as 3D printing. In these processes, a feed, typically a mixture of a solid and a liquid, is forced through a relatively small opening or a plurality of relatively small openings such as die holes or a nozzle. By forcing the solid/liquid mixture through a relatively small opening, the solid and liquid consolidate and a solid extrudate is formed with a cross-sectional profile similar to the cross-section of the opening(s). During forcing the solid/liquid mixture through the opening, part of the liquid may be pushed to the outside of the material and part of the liquid may evaporate.

An example of such extrusion process is extrusion-pelletizing of animal feed. In such process, a stream of comminuted solid animal feed ingredients is heated and mixed with water, typically by controlled steam injection, and fed to a die chamber wherein rollers force the heated water/solid mixture through the holes of a rotating circular die. Stationary knives, located outside the rotating die, cut off the compacted animal feed into pellets of desired length. Alternatively, a stationary circular die and rotating knives may be used. During the forcing of the feed mixture through the opening(s) of the die, friction occurs, typically resulting in a temperature rise of the mixture forced through it.

Control of extrusion-pelletizing of animal feed and other processes wherein a solid/liquid mixture is forced through an opening are generally based on empirical rules. Empirical rules have for example been established for the relationship between moisture addition to the solid or semi-solid feed mixture and the extent of die friction (often empirically determined as temperature rise over the die). Extensive die friction is undesired, since it results in high energy costs and possibly in quality loss due to undesired temperature rise. Moreover, the higher the friction in the die, the higher the shrinkage of the pellets.

There is a need in the art to improve the way friction in openings through which a solid/liquid mixture is forced is controlled.

Summary of the invention

The present inventors have found a method for accurately determining the amount of volatile liquid present at the surface of a solid extrudate obtainable by forcing a mixture comprising a solid and a volatile liquid through an opening, such as for example animal feed pellets obtained by extrusion-pelletizing. By flowing a gas stream through a chamber filled with a packed bed of the solid extrudate shortly after it has been obtained, and monitoring as a function of time the partial vapour pressure of the volatile liquid in the gas downstream of the packed bed and monitoring at least one of the weight of the packed bed and the temperature of the gas downstream of the packed bed, the volume of volatile liquid that is initially flashed off the solid extrudate can be calculated. The initially flashed off liquid was present at or near the surface of the solid extrudate and is referred to herein as surface liquid.

Accordingly, the invention provides in a first aspect a method for determining the amount of volatile surface liquid on a solid extrudate obtainable by forcing a mixture comprising a solid and a volatile liquid through an opening, the method comprising: a) providing a chamber with a gas inlet and a gas outlet; b) filling the chamber with a packed bed of the solid extrudate, wherein the packed bed has a weight ml ; c) continuously flowing a gas at a gas flux Q through the packed bed from the gas inlet to the gas outlet, wherein the gas has a temperature T1 at the gas inlet and a temperature T2 at the gas outlet, and wherein the volatile liquid has a partial vapour pressure p1 in the gas at the gas inlet and a partial vapour pressure p2 in the gas at the gas outlet, and wherein the packed bed has a weight m2; d) determining p2 as a function of time during c); e) measuring m2 and/or T2 as a function of time during c); f) calculating the amount of the volatile liquid evaporated during the time the partial vapour pressure p2 is equal to the saturation vapour pressure of the volatile liquid in the gas. Liquid distribution in a material that is forced through a small opening such as a die hole, a nozzle, or the like, is dynamic. It is strongly affected by and changing during the forcing through such opening. Due to differences between the viscosity of the solid and the viscosity of the liquid in the mixture forced through the opening, liquid may be pushed to the outside of the material and a surface liquid layer is typically formed. There is not a straightforward and predictable correlation between overall liquid content of a solid extrudate and the amount of surface liquid. Geometry of the opening (e.g., cross-sectional shape, size, and/or number of the opening(s)), process parameters such as extrusion speed, temperature of the solid/liquid feed, and the composition of the feed mixture (solid/liquid ratio, type of solid(s) and liquid, particle size of the solid(s)) will affect the amount of surface liquid. The amount of surface liquid is believed to strongly correlate to friction in the opening, since such surface liquid acts as a lubricating layer. In particular the thickness of the layer of volatile liquid at the outer surface of the extrudate that has been in direct contact with the opening(s) through which it has been forced is believed to strongly correlate to friction in the opening. Such thickness can be calculated as the quotient of the volumetric amount of initially flashed off liquid and the outer surface area of the solid extrudate in the packed bed that was in direct contact with the opening(s).

An advantage of the method according to the invention is that the amount of surface liquid can be accurately measured by a simple method that can be carried out in a relatively short time. Thus, the method can suitably be used to control an extrusion process by relatively quickly adapting the geometry of the opening (often referred to as die configuration), feed composition, and/or process parameters to achieve a desired amount of volatile surface liquid or a desired thickness of a volatile surface liquid layer. The method can also be used to determine a desired amount of volatile surface liquid or a desired volatile surface liquid layer thickness by correlating the amount or thickness as determined by the method to product (extrudate) properties and/or energy consumption of the extrusion process. Further, the method can be used to determine the effect of geometry of the opening, process parameters, and feed mixture composition on the resulting amount of volatile surface liquid or the resulting thickness of the volatile surface liquid layer.

Accordingly, in a second aspect, the invention provides use of a method according to the first aspect of the invention in a process for manufacturing a solid extrudate by forcing a mixture comprising a solid and a volatile liquid through an opening, for controlling the amount of volatile surface liquid on the solid extrudate. In a third aspect, the invention provides a process for manufacturing a solid extrudate, comprising determining the amount of volatile surface liquid on the solid extrudate, comprising: i) providing a mixture comprising a solid and a volatile liquid having a weight ratio of volatile liquid to solid and a temperature; ii) forcing the mixture comprising a solid and a volatile liquid through an opening, at an extrusion speed, to obtain the solid extrudate; iii) determining the amount of volatile surface liquid on the solid extrudate according to the method according to any one of claims 1 to 7; iv) adapting one or more parameters in order to adapt the amount of the volatile surface liquid on the solid extrudate obtained in ii), wherein the one or more parameters are selected from the group consisting of:

- the weight ratio of volatile liquid to solid in the mixture provided in i);

- particle size of the solid in the mixture provided in i);

- the extrusion speed at which the mixture comprising a solid and a volatile liquid is forced though the opening;

- the temperature of the mixture provided in i); and

- geometry of the opening.

Detailed description of the invention

The method according to the invention is a method for determining the amount of volatile surface liquid on a solid extrudate obtainable by forcing a mixture comprising a solid and a volatile liquid through an opening. Reference herein to a solid extrudate is to a shaped solid material obtained by an extrusion process. Reference herein to an extrusion process is to any process wherein material is forced through an opening with a desired cross-section to obtain a shaped solid material. Examples of extrusion processes wherein a mixture comprising a solid and a volatile liquid is forced through an opening are animal feed pelletizing, briquette extrusion, extrusion of ceramic products (e.g. artificial bones or dents), 3D printing, and food extrusion. Food extrusion processes include for example expansion extrusion to manufacture expanded food products such as snacks or breakfast cereals, extrusion of non-expanded products such as dry pasta, and extrusion of compositions comprising vegetable proteins to manufacture texturized fibrous products that can be used as meat analogues.

The mixture comprising a solid and a volatile liquid is forced through one or more openings in an extrusion die or through a nozzle for 3D printing. Due to compressive and shear stresses on the mixture when it is forced through the opening(s) and a difference in viscosity of the liquid and the solid in the mixture, part of the liquid may be forced to the outside of the material and redistribution of liquid in the material may take place. The solid extrudate obtained typically still contains liquid.

The mixture comprising a solid and a volatile liquid may be any suitable mixture that can be used as feed mixture for an extrusion process. Its composition will depend on the type of extrusion process and the desired solid extrudate to be obtained as end product.

Reference herein to a volatile liquid is to a liquid that has a measurable vapour pressure at a temperature of 20 °C and at a pressure of 1 bar (absolute), preferably has a vapour pressure of at least 0.1 kPa at a temperature of 20 °C and at a pressure of 1 bar (absolute), more preferably at least 0.5 kPa, even more preferably at least 1.0 kPa, still more preferably at least 2.0 kPa. The volatile liquid preferably is selected from the group consisting of water, an alcohol, an ester, a carboxylic acid, an ether, a (meth)acrylic monomer, and a combination of two or more thereof. Water, methanol, ethanol, n- propanol, isopropanol, n-butanol, sec. butyl alcohol, ethyl acetate, butyl acrylate, methyl methacrylate, and oxalic acid are suitable volatile liquids. Water is a particularly preferred volatile liquid.

For an extrusion process for manufacturing food products or animal feed, the volatile liquid preferably is water. The consistency, solid/liquid ratio, and viscosity of the mixture comprising a solid and a volatile liquid may vary widely, depending on the type of extrusion process. The mixture may for example be in the form of a paste, a dough, a slurry, or a suspension.

The solid preferably is a comminuted solid. It may be a fibrous or particulate solid. The mixture may comprise more than one solid. The mixture may comprise a non-volatile liquid in addition to the volatile liquid (such as for example an oil). Preferably, the mixture is free of any liquid other than the volatile liquid.

The solid extrudate preferably is an animal feed pellet.

The method for determining the amount of volatile surface liquid on the solid extrudate comprises: a) providing a chamber with a gas inlet and a gas outlet; b) filling the chamber with a packed bed of the solid extrudate, wherein the packed bed has a weight ml ; c) continuously flowing a gas at a gas flux Q through the packed bed from the gas inlet to the gas outlet, wherein the gas has a temperature T1 at the gas inlet and a temperature T2 at the gas outlet, and wherein the volatile liquid has a partial vapour pressure p1 in the gas at the gas inlet and a partial vapour pressure p2 in the gas at the gas outlet, and wherein the packed bed has a weight m2; d) determining p2 as a function of time during c); e) measuring m2 and/or T2 as a function of time during c); and f) calculating the amount of the volatile liquid evaporated during the time the partial vapour pressure p2 is equal to the saturation vapour pressure of the volatile liquid in the gas.

The chamber provided in a) may be any chamber with a gas inlet and a gas outlet suitable for continuously flowing a stream of gas through a packed bed of solid extrudate, such as for example animal feed pellets, contained in it. The chamber is preferably configured such that the gas is evenly flowing through the entire packed bed. The chamber may for example be a square or rectangular chamber defined by six walls with the gas inlet and the gas outlet in opposite walls, such as for example the top and bottom wall or two opposite side walls. Preferably, the chamber is configured such that, during operation of the method, the chamber is closed with no openings other than the gas inlet and the gas outlet. The chamber may be defined by walls of any suitable material. Suitable materials are those materials that do not absorb or adsorb the volatile liquid.

In b), the chamber is filled with a packed bed of the solid extrudate. The packed bed has a weight ml . The weight ml is the weight of the packed bed prior to flowing the gas through it in c). The extent to which the chamber is filled with the packed bed of extrudate is not critical. Preferably the packed bed is occupying at least 50% of the chamber volume, more preferably at least 70%, even more preferably at least 80%. Most preferably, the chamber extrudate is substantially completely filled, i.e. occupying at least 90%, preferably at least 95% of the chamber volume, with the packed bed. It will be appreciated that the packed bed has a porosity, mainly depending on the size and shape of the solid extrudate particles. The bed porosity (also referred to as macro porosity) is to be distinguished from the micro porosity, i.e. void volume inside the solid extrudate due to pores in the solid extrudate. The method according to the invention is particularly suitable for solid extrudates with low micro porosity, i.e. with less than 30 vol% internal voids, preferably less than 20 vol% internal voids, even more preferably less than 10 vol% internal voids. The volume% of internal voids can be determined by X-ray tomographic microscopy.

To minimize inaccuracy of the method due to evaporation of volatile surface liquid from the solid extrudate before the extrudate has been packed in the chamber, the chamber is preferably filled with a packed bed of the solid extrudate within one minute, more preferably within 30 seconds, even preferably within 10 seconds after the solid extrudate has been obtained by forcing the solid/liquid mixture through the opening. Such prompt filling can for example be achieved by positioning the chamber immediately after the opening, such as the extrusion die or the 3D printing nozzle. Preferably, the chamber is configured as an autosampler, for example by positioning the chamber just downstream of the extrusion die with the side of the chamber that is facing the extrusion die opened, such that part of the extrudate will enter the chamber, and then closing the chamber.

In c), a gas is continuously flowed at a gas flux Q through the packed bed of the solid extrudate from the gas inlet of the chamber to the gas outlet of the chamber. The gas may be any gas wherein the volatile liquid in the solid extrudate has a vapour pressure, such as for example air, nitrogen, or argon. Preferably the gas is air.

The gas has a temperature T1 at the gas inlet and a temperature T2 at the gas outlet. The volatile liquid has a partial vapour pressure p1 in the gas at the gas inlet and a partial vapour pressure p2 in the gas at the gas outlet. Reference herein to partial vapour pressure of the volatile liquid in the gas is to the pressure that molecules of the liquid in gaseous form exert in the gas. Thus, if the volatile liquid is water and the gas is air, p1 and p2 are the water vapour pressure in the air at the gas inlet and at the gas outlet of the chamber, respectively. Partial vapour pressure p2 fluctuates as a function of time during the time gas is flowing through the packed bed. It has been found that due to rapid evaporation of the volatile liquid present at or near the outer surface of the solid extrudate (referred to herein as volatile surface liquid), p2 initially steeply increases to the saturation vapour pressure of the volatile liquid in the gas. Once the surface liquid has been evaporated, p2 decreases below the saturation vapour pressure of the volatile liquid in the gas.

During c), i.e. during the time the gas is flowing through the packed bed, the packed bed has a weight m2. The weight m2 decreases in time due to evaporation of the volatile liquid during c).

In d) of the method, the partial vapour pressure p2 is determined as a function of time during c), i.e. during the time the gas is flowed through the bed. The partial vapour pressure p2 may be determined in any suitable manner. In case of water as the volatile liquid, determination of the partial vapour pressure p2 preferably comprises measuring the relative humidity (RH) of the gas at the gas outlet. This may be done by a suitable sensor. By multiplying RH with the saturation pressure of water in the gas at T2, the partial vapour pressure p2 is obtained. The saturation pressure of water in the gas may be determined using well-known empirical equations, such as for example the Tetens equation, that calculate the saturation pressure of water as a function of temperature.

The gas temperature T2 at the gas outlet may fluctuate as a function of time. The gas flux Q is chosen such that the volatile liquid can reach its saturation vapour pressure in the gas during initial flash off of the volatile liquid from the solid extrudate. Moreover, the gas flux Q is such that given the content of the volatile liquid in the extrudate and given the measuring speed (frequency) of any sensor determining p2, a saturation vapour pressure of the volatile liquid can be observed. The gas flux Q may fluctuate in time during c). Preferably the gas flux Q is constant during c). Typically, the gas flux Q will be such that the gas has a linear velocity through the packed bed of at least 0.01 m/s, preferably at least 0.05 m/s, more preferably at least 0.1 m/s. A practical upper limit for the gas flux Q is a gas flux Q such that the gas has a linear velocity through the packed bed of at most 10 m/s, preferably at most 5 m/s, more preferably at most 1 m/s. It will be appreciated that the linear velocity of the gas through the packed bed depends on the size and porosity of the packed bed.

The amount of volatile surface liquid on the solid extrudate can be determined by determining the amount of volatile liquid evaporated during the time the partial vapour pressure p2 is equal to the saturation vapour pressure of the volatile liquid in the gas. In the method according to the invention, the time the partial vapour pressure p2 is equal to the saturation vapour pressure of the volatile liquid in the gas is defined as the time elapsed between the start of the flowing of the gas through the packed bed and the moment that p2 drops below the saturation vapour pressure of the volatile liquid in the gas.

The amount of volatile surface liquid can be calculated from the weight loss of the packed bed during the time the partial vapour pressure p2 is equal to the saturation vapour pressure of the volatile liquid in the gas. Alternatively, or additionally, the amount of volatile surface liquid can be calculated by calculating the difference in the amount of the volatile liquid in the gas at the gas outlet and the amount of the volatile liquid in the gas at the gas inlet during that time, using the ideal gas law. This can be calculated by using p1 , T1 , p2, T2 and the gas flux Q as parameters.

The method thus comprises measuring m2 and/or measuring T2 as a function of time during c), i.e. during the time the gas is flowed through the bed.

Preferably, the method comprises measuring m2 as a function of time during c). The weight of the packed bed m2 may be measured in any suitable manner, for example by positioning the chamber or the packed bed on a mass sensor. The amount of the volatile liquid evaporated during initial flash off is then calculated from the weight loss of the packed bed during the time the partial vapour pressure p2 is equal to the saturation vapour pressure of the volatile liquid in the gas (m2 at the moment that p2 drops below the saturation vapour pressure of the volatile liquid in the gas minus ml). Once the volumetric amount of the volatile liquid evaporated during such initial flash off is calculated in f), the thickness of the volatile surface liquid layer can be calculated in g) as the quotient of such volumetric amount calculated in f) and the outer surface area of the solid extrudate in the packed bed that has been in direct contact with the opening. Only outer surface area of the solid extrudate that has been in direct contact with the opening(s), such as the die holes in an extrusion die, are of relevance since that is where any die friction occurs and surface liquid may act as lubricant. In the case of cylindrical animal feed pellets for example, any surface area at the opposite ends of the cylinder is to be ignored since that is surface area created by cutting the extrudate after it has exited the openings.

Reference herein to the outer surface area of the solid extrudate is to the surface area excluding any surface area inside the extrudate (such as any inner surface of any pores in the extrudate) is ignored. For animal pellets with a consistent geometry, the outer surface area can be calculated from the geometry of the extrudate and the amount of extrudate in the packed bed.

The method according to the invention can suitably be used to determine a desired amount of volatile surface liquid of a solid extrudate, for example by providing a set of otherwise comparable solid extrudates varying in volatile surface liquid content. The volatile surface liquid content can for example be varied by varying the volatile liquid content in the solid/liquid mixture that is forced through the opening for otherwise comparable feed and process conditions. By determining the amount of volatile surface liquid of the solid extrudates and by determining relevant product properties of the solid extrudates, correlations between product properties and the amount of the volatile surface liquid can be established. Thus, a desired amount of volatile surface liquid can be determined for a given product and process.

The method can also be used to determine the effect of geometry of the opening, process parameters, and feed mixture composition on the resulting amount of volatile surface liquid.

Once the desired amount of volatile surface liquid is known and the correlations between the amount of volatile surface liquid and parameters like geometry of the opening, process parameters, and feed mixture composition are known, the method can be used to control the amount of volatile surface liquid on the solid extrudate in a process for manufacturing a solid extrudate.

Accordingly, in a second aspect, the invention provides use of the method according to the first aspect of the invention, in a process for manufacturing a solid extrudate by forcing a mixture comprising a solid and a volatile liquid through an opening, for controlling the amount of volatile surface liquid on the solid extrudate.

In a final aspect, the invention provides a process for manufacturing a solid extrudate, comprising determining the thickness of a volatile surface liquid layer on a solid extrudate according to the method the first aspect of the invention. The process comprises: i) providing a mixture comprising a solid and a volatile liquid having a weight ratio of volatile liquid to solid and a temperature; ii) forcing the mixture comprising a solid and a volatile liquid through an opening, at an extrusion speed, to obtain a solid extrudate; iii) determining the amount of volatile surface liquid on the solid extrudate according to the method according to the first aspect of the invention; iv) adapting one or more parameters in order to adapt the amount of the surface liquid on the solid extrudate obtained in ii), wherein the one or more parameters are selected from the group consisting of:

- the weight ratio of volatile liquid to solid of the mixture provided in i);

- particle size of the solid in the mixture provided in i);

- the extrusion speed at which the mixture comprising a solid and a volatile liquid is forced though the opening;

- the temperature of the mixture provided in i); and

- geometry of the opening.

The process is an extrusion process as has been described in relation to the method according to the invention. Any features, preferences, and embodiments as has been described in relation to the method according to the invention also apply to the process according to the invention.

Reference herein to an opening is to one or more openings. The one or more openings may be die holes in an extrusion die or may be a nozzle for 3D printing. Preferably, the opening is a plurality of openings in an extrusion die.

The mixture comprising a solid and a volatile liquid provided in i) has a weight ratio of volatile liquid to solid and a temperature. Reference herein to the weight ratio and to the temperature of the mixture provided in i) is to the weight ratio and the temperature of the mixture just prior to it being forced through the opening, e.g., when entering the extrusion die.

In one embodiment, the process is an extrusion process for the manufacture of animal feed pellets. In such process the volatile liquid is water. The solid typically is a ground mash (e.g. hammer-milled) of one or more typical animal feed raw materials such as cereals, oil-containing seeds, agricultural residues, and nutrients. For the manufacture of animal feed pellets, the mixture that is forced through the die openings preferably has a weight ratio of water to solids in the range of from 0.10 to 0.20, preferably of from 0.12 to 0.18, more preferably of from 0.14 to 0.17. The temperature of the mixture provided in i) may be any suitable temperature, preferably in the range of from 40 to 95 °C, more preferably of from 50 to 90 °C, even more preferably of from 65 to 85 °C. Water is typically added as steam just prior to the feed mixture being forced through the extrusion die. The steam typically condenses to provide a heated solid/water mixture that is forced through the die holes.

In ii), the mixture provided in i) is forced at an extrusion speed through an opening to obtain the solid extrudate. Reference herein to an extrusion speed is to the linear velocity at which the mixture is forced through the opening.

In iii), the amount of volatile surface liquid on the solid extrudate is determined according to the method according to the first aspect of the invention and as described hereinabove.

In iv), the amount of volatile surface liquid on the solid extrudate obtained in ii) is adapted by adapting one or more parameters selected from the group consisting of:

- the weight ratio of volatile liquid to solid in the mixture provided in i);

- particle size of the solid in the mixture provided in i);

- the extrusion speed at which the mixture comprising a solid and a volatile liquid is forced though the opening;

- the temperature of the mixture provided in i); and

- geometry of the opening.

Reference herein to geometry of the opening is to the number of openings, total cross-sectional area of the opening(s), cross-sectional shape of the opening(s), and/or distribution of the openings.

Preferably, the weight ratio of volatile liquid to solid in the mixture provided in i) is adapted in iv).

The desired amount of volatile surface liquid may be determined by the method according to the invention as described hereinabove.

If the desired amount of volatile surface liquid is not achieved by adapting the one of more composition or process parameters, steps i) to iv) are repeated until the desired amount has been achieved.

Brief of the

Figure 1 : method set up Figure 2: graph of p2 plotted against time.

The invention will be illustrated by means of the following non-limiting example.

Example

A mixture of 50 wt% sugar beet pulp and 50 wt% corn kernels was ground using a hammermill to pass a 3-mm aperture screen. The ground feed mash was mixed for 15 minutes in a NautaOconical screw mixer before it was transferred into the feed bunker of a pellet mill and supplied to the conditioning chamber. The feed mash was transported by a single feed screw through a conditioning chamber to a rotary die (ring die). Steam was injected into the feed mash (in the conditioning chamber) to obtain a conditioned feed mixture of solid feed mash and water having a water to solid weight ratio of 0.163 and a temperature of 65 °C. Two rollers were then pressing the conditioned feed against a rotary circular die (ring die) having a thickness of 72 mm and die holes with an hole diameter of 6 mm. The roller-die gap was set at <0.2 mm and the rotation speed of the die was set at 300 rpm. Stationary knifes were adjusted to a gap of 24 mm relative to the ring die to obtain cylindrical pellets with a diameter of 6 mm diameter and a length of 24 mm. The pellet production rate was 240 kg/h.

Freshly cut pellets were directly sampled and 90.4 grams were packed in a square gas flow chamber of 5 cm x 5 cm x 5 cm in a measurement setup as shown in Figure 1 . Air was supplied at a gas flux Q of 30 L/min via supply conduit 1 and gas inlet 2 to gas flow chamber 3, and flowed through the packed bed of pellets (extrudate) to gas outlet 4 and was discharged from the chamber via discharge conduit 5. The air flux was controlled by valve 6. The relative humidity of the air at the gas inlet 2 was measured with sensor 7 and the temperature of the air at gas inlet 2 (T1) was measured with temperature sensor 8. The relative humidity of the air at the gas outlet 4 was monitored with sensor 9 and the temperature of the air at gas outlet 4 (T2) was monitored with temperature sensor 10. The chamber 3 was positioned on a weighting device 11 to monitor the mass of the packed bed of pellets in chamber 3.

The partial vapour pressure p1 of water at gas inlet 2 was determined by multiplying the relative humidity as measured with sensor 7 with the saturation vapour pressure for water at T1 as calculated using the Tetens equation. The partial vapour pressure p2 of water at gas outlet 4 as a function of time was determined by multiplying the relative humidity as monitored with sensor 9 with the saturation vapour pressure for water at T2 (Psat) as calculated using the Tetens equation. In Figure 2, the relative vapour pressure (p (p2/P _sa t) of water in the air at gas outlet 4 is plotted against time. It can be seen that the vapour pressure of water reaches its saturation pressure promptly after the start of the measurement (the relative vapour pressure (p of water is 100). The time from the start of the measurement until the vapour pressure of water drops below its saturation pressure (relative vapour pressure (p decreases below 100), is the time during which initial flash off of water occurs (indicated in Figure 2 as tf).

The time during which flash-off occurred (tf) was 45 seconds. During this 45 seconds, the weight loss of the pellets was 0.672 grams. The amount of surface water was 0.672 grams water per 90.4 grams extrudate, i.e. 0.0074 grams surface water/g extrudate. The thickness of the water surface layer was calculated to be 9 pm, based on a total surface area of 90.4 grams of pellets of 0.051 m ² .