EXTRUSION

• Extrusion is a process that converts raw material into a product with desired shape and form by forcing the material through a small opening using pressure

• The process involves a series of unit operations such as mixing, kneading, shearing, heating, cooling, shaping and forming.

• Many food products are manufactured by extrusion cooking—a process that uses both thermal energy and pressure to convert raw food ingredients into popular products such as breakfast cereals, pastas, pet foods, snacks and meat products.

• Expression for volumetric flow rate of the Newtonian fluid in the channel cross-section

V = volumetric flow rate (m3 / s)

ΔP = pressure difference or change (Pa)

W = width of channel (m)

H = height of channel (m)

µ = viskositas (Pa.s)

L = length of channel or screw (m)

ᵤwall = the wall velocity (m/s)

• The mean velocity through the channel may be calculated from the following equation

• Example :

Corn meal with a moisture content of 18% (wb) is being extruded through a metering zone of an extruder with the following dimensions of the channel: width 5 cm, height 2 cm, length 50 cm. The wall velocity is estimated to be 0.3 m/s. The rheological properties of the extrudate can be estimated by a viscosity of 66,700 Pa s and a density of 1200 kg/m3 . If the pressure drop is maintained at 3000 kPa, estimate the mass flow rate of extrudate through the die.

• Rauwendaal (1986) approximated the volumetric flow rate at the extruder outlet for non-Newtonian fluids by using the following equation

• Example :A non-Newtonian (power-law) soy fl our extrudate with 25% moisture content (wb) is being pumped through an extruder. The channel in the metering section has the following dimensions: width 5 cm, height 2 cm, length 50 cm. The properties of the extrudate are described by a consistency coefficient of 1210 Pa sn , flow behavior index of 0.49, and density of 1100 kg/m3 . The wall velocity is 0,3 m/s. Estimate the pressure drop if the mass flow rate of 600 kg/hr is to be maintained.

Extrusion System

• Extrusion systems can be divided into four different categories. These four categories include two different methods of operations — cold extrusion or extrusion cooking — and two different barrel configurations — single or twin screw.

• Both barrel configurations may be used for either method of operation.

Cold Extrusion

• Cold extrusion is used most often to form specific shapes of extrudate at locations downstream from the die. In this process, the extrudate is pumped through a die without the addition of external thermal energy.

• As illustrated, one component of the final product is pumped through an opening of defined shape to create a continuous tube of the first component. At the same time, the second component of the final product is introduced just before the die and becomes a filler for the interior space within the outer tube, as illustrated in Figure 14.2 .

• During a final step in the process, the tubular product is cut into appropriate lengths.

Cooking Extrusion

• When thermal energy becomes a part of the extrusion process, the process is referred to as extrusion cooking.

• Thermal energy may be added to the extrudate during the process from an external source or may be generated by friction at internal surfaces of the extruder in contact with the extrudate.

• The addition of thermal energy occurs at the surface of the barrel of the extrusion system. Thermal energy may be transferred through the walls and surfaces of the barrel to ingredients used to create the extrudate. In addition, mechanical energy created by friction between surfaces and ingredients within the barrel is dissipated as thermal energy in the extrudate.

Single Screw Extruder

• In a single-screw extrusion system, the barrel of the extruder contains a single screw or auger that moves the extrudate through the barrel.

• A single-screw extrusion system has three components or sections Feed section, where the various ingredients are introduced

and initial mixing occurs. The rotating action of the screw moves the ingredients to the transition or compression section.

Compression or transition section, where the ingredients begin the transition to the extrudate as pressure and temperature begin to increase. As the dimensions of the flow channel decrease, the material is compressed and mechanical energy is dissipated as temperature increases. This section may be referred to as a kneading section, with significant changes in the physical and chemical characteristics of the ingredients occurring.

Metering (or cooking) section, where additional compression of the extrudate occurs as a result of additional reductions in the dimensions of the flow channel and increased shearing action. In some designs, the overall dimensions of the barrel are reduced as well.

Twin Screw Extruder

• Twin-screw extrusion systems incorporate two parallel screws into the extruder barrel. The screws may be co-rotating or counter-rotating

Extrusion System Design

• The power requirement for operating an extrusion system is a key design factor.

• The power needed for viscous dissipation has been expressed in terms of the Screw Power number (Np), as follows:

• For extrudate with rheological properties described by the power-law model, the Screw Rotational Reynolds number is defined as follows:

• The drag flow rate ( Vd) for the extruder screw can be estimated as follows:

pt = extruder power consumption (W)

ps = power consumption for viscous dissipation (W)

Vd = drag flow rate (m3 / s)

ΔP = pressure drop (Pa)

NP = screw power number

ρ = density (kg/m3)

N = screw speed (rps)

D = screw diameter (m)

L = length of channel or screw (m)

NRes= screw rotation Reynold number

H = height of channel (m)

K = consistency coefficient (Pa. Sn)

n = flow behavior index

W = width of channel (m)

• Example

Estimate the mechanical power requirements for a single-screw extrusion system during extrusion of 30% moisture content (wb) corn meal. The apparent viscosity of the extrudate is 1765 Pa.s at 135°C, and the density is 1200 kg/m3 . The diameter of the screw is 7.5 cm, the length of the screw is 50 cm. The channel depth within the barrel is 2 cm and width is 2.5 cm. The system is operating at a flow rate of 300 kg/hr with a screw speed of 75 rpm. The estimated wall velocity is 0.3 m/s.

• Example

A wheat flour dough is being extruded in a single-screw extruder. The screw length is 50 cm and its diameter is 6 cm. The channel dimensions are as follows, width 2 cm and height 1cm. The properties of the dough include a consistency coefficient of 4450 Pasn , flow behavior index of 0.35 and density of 1200 kg/m3 . Estimate the power requirements for the system when the flow rate is 270 kg/hr and the screw rotation speed is 200 rpm with an estimated wall velocity of 0.6 m/s.