...............................................................................
Filtration
Filtration
Definition and Processes (Chemistry)
What Filtration Is and How It's Done
by Anne Marie Helmenstine, Ph.D.
Filtration is a process used
to separate solids from liquids or gases using a filter
medium that allows the fluid to pass, but not the solid.
The term
"filtration" applies whether the filter is mechanical, biological, or
physical.
The fluid that passes through
the filter is called the filtrate.
The filter medium may be a
surface filter, which is a solid that traps solid particles, or a depth filter,
which is a bed of material that traps the solid.
Filtration is typically an imperfect process. Some fluid remains on the feed
side of the filter or embedded in the filter media and some small solid
particulates find their way through the filter.
As a chemistry and
engineering technique, there is always some lost product, whether it's the
liquid or solid being collected.
Examples of Filtration
While
filtration is an important separation technique in a laboratory, it's also
common in everyday life.
· Brewing coffee involves passing
hot water through the ground coffee and a filter. The liquid coffee is the
filtrate. Steeping tea is much the same, whether you use a tea bag (paper
filter) or tea ball (usually a metal filter).
· The kidneys are an example
of a biological filter. Blood is filtered by the glomerulus. Essential
molecules are reabsorbed back into the blood.
· Air conditioners and many
vacuum cleaners use HEPA filters to remove dust and pollen from the air.
· Many aquariums use filters that
contain fibers that capture particulates.
· Belt filters recover precious
metals during mining.
· Water in an aquifer is
relatively pure because it has been filtered through sand and permeable rock in
the ground.
Filtration Methods
There
are different types of filtration. Which method is used depends largely on
whether the solid is a particulate (suspended) or dissolved in the fluid.
· General Filtration: The most basic form of
filtration is using gravity to filter a mixture. The mixture is poured from
above onto a filter medium (e.g., filter paper) and gravity pulls the
liquid down. The solid is left on the filter, while the liquid flows below it.
· Vacuum Filtration: A Büchner flask and
hose are used to pull a vacuum to suck the fluid through the filter (usually
with the aid of gravity). This greatly speeds the separation and can be used to
dry the solid. A related technique uses a pump to form a pressure difference on
both sides of the filter. Pump filters do not need to be vertical because
gravity is not the source of the pressure difference on the sides of the
filter.
· Cold Filtration: Cold filtration is used
to quickly cool a solution, prompting the formation of small crystals. This is
a method used when the solid is initially dissolved. A common method is to
place the container with the solution in an ice bath prior to filtration.
· Hot Filtration: In hot filtration, the
solution, filter, and funnel are heated to minimize crystal formation during
filtration. Stemless funnels are useful because there is less surface area for
crystal growth. This method is used when crystals would clog the funnel or
prevent crystallization of the second component in a mixture. Sometimes filter aids are used to improve flow through a filter. Examples of
filter aids are silica, diatomaceous earth, perlite, and cellulose.
Filter aids may be placed on
the filter prior to filtration or mixed with the liquid. The aids can help
prevent clogging of the filter and can increase the porosity of the
"cake" or feed into the filter.
Filtration Versus Sieving
A
related separation technique is sieving. Sieving refers to use of a single mesh
or perforated layer to retain large particles while allowing the passage of
smaller ones.
In
filtration, in contrast, the filter is a lattice or has multiple layers.
Fluids follow channels in the medium to pass through a filter.
Alternatives to Filtration
In
some situations, there are better separation methods than filtration. For
example, for very small samples where it's important to collect the filtrate,
the filter medium may soak up too much of the fluid.
In other cases, too much of
the solid becomes trapped in the filter medium. Two other processes that can be
used to separate solids from fluids are decantation and centrifugation.
Centrifugation involves
spinning a sample, forcing the heavier solid to the bottom of a container.
Decantation can be used following centrifugation or on its own.
In decantation, the fluid is
siphoned or poured off of the solid after it has fallen out of solution.
Anne Marie Helmenstine,
Ph.D.
· Ph.D. in biomedical sciences from the University of Tennessee at
Knoxville - Oak Ridge National Laboratory.
· Science educator with experience teaching chemistry, biology,
astronomy, and physics at the high school, college, and graduate levels.
· ThoughtCo and About Education chemistry expert since 2001.
· Widely-published graphic artist, responsible for printable periodic
tables and other illustrations used in science.
Experience
Anne Helmenstine, Ph.D. has covered
chemistry for ThoughtCo and About Education since 2001, and other sciences
since 2013. She taught chemistry, biology, astronomy, and physics at the high
school, college, and graduate levels. She has worked as a research
scientist and also abstracting and indexing diverse scientific literature for
the Department of Energy.
In addition to her work as a science writer, Dr.
Helmenstine currently serves as a scientific consultant, specializing in
problems requiring an interdisciplinary approach. Previously, she worked
as a research scientist and college professor.
Education
Dr. Helmenstine holds a Ph.D. in biomedical
sciences from the University of Tennessee at Knoxville and a B.A.
in physics and mathematics with a minor in chemistry from Hastings
College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive
chemical detection and medical diagnostic tests.
Anne Marie Helmenstine, Ph.D.
ThoughtCo and Dotdash
ThoughtCo is a
premier reference site focusing on expert-created education content. We are one
of the top-10 information sites in the world as rated by comScore, a leading
Internet measurement company. Every month, more than 13 million readers seek
answers to their questions on ThoughtCo.
For more than 20 years, Dotdash brands
have been helping people find answers, solve problems, and get inspired. We are
one of the top-20 largest content publishers on the Internet according to
comScore, and reach more than 30% of the U.S. population monthly. Our brands
collectively have won more than 20 industry awards in the last year alone, and recently
Dotdash was named Publisher of the Year by Digiday, a leading industry
publication.
No comments:
Post a Comment