2.
• 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.
3.
• 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)
ᵤ
= the wall velocity (m/s)
wall
4.
• 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.
5.
• 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.
6.
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.
7.
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.
8.
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.
9.
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.
10.
• 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.
11.
Twin Screw Extruder
• Twin-screw extrusion systems incorporate two
parallel screws into the extruder barrel. The screws
may be co-rotating or counter-rotating
12.
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:
13.
• 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:
14.
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)
15.
• 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.
16.
• Example
A wheat flour dough is being extruded in a singlescrew 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.
A particular slide catching your eye?
Clipping is a handy way to collect important slides you want to go back to later.
Clipping is a handy way to collect and organize the most important slides from a presentation. You can keep your great finds in clipboards organized around topics.
Be the first to comment