Chemical Factors
Whenever a chemical substance is placed in a container or equipment the chemical is exposed to the material of construction of the container or equipment. Therefore, the material of construction may contaminate the product or the product may destroy the material of construction.
(a)
Contamination of product:
Iron contamination may change the color of the products (like gelatin capsule
shells), and catalyze some reactions that may enhance the rate of decomposition
of the product. Leaching of glass may make the aqueous product alkaline. This
alkaline medium may catalyze the decomposition of the product. Heavy metals
such as lead inactivate penicillin.
(b)
Destruction of material of construction: The products may be corrosive. They may react with
the material of construction and may destroy it. The life of the equipment is
reduced. Extreme pH, strong acids, strong alkalis, powerful oxidizing agents,
tannins, etc. reacts with the materials. It is important to know what chemicals
are to be processed. Selection should be done consciously and while selecting
appropriate materials that comply with the chemical composition of the process
it should be ensured that it will perform to the expectations.
(c)
Chemical inertness:
Aggressive reaction environments tend to dissolve metals from unlined mild
steel or alloy reactors. Extractable metals, such as chromium, nickel,
molybdenum, and copper, can leach into and contaminate products, producing
undesired catalytic effects that can cause harmful fluctuations in the process
reactions. These metals can compromise product quality, negatively affect
product yield, and in some cases even cause runaway reactions.
Physical Factors
(a) Strength: The material should have the sufficient physical strength to withstand the required pressure and stresses. Iron and steel possess these properties. Tablet punching machine, die, upper and lower punches are made of stainless steel to withstand very high pressure. Glass though has strength but is brittle. The aerosol container must withstand very high pressure, so tin plate containers coated with some polymers (lacquered) are used. Plastic materials are weak so they are used in some packaging materials, like blister packs.
(b)
Mass: For
transportation purposes light weight packaging materials are used. Plastic,
aluminum, and paper packaging materials are used for packing pharmaceutical
products.
(i)
Wear properties:
When there is a possibility of friction between two surfaces the softer surface
wears off and these materials contaminate the products. For example, during
milling and grinding the grinding surfaces may wear off and contaminate the
powder. When pharmaceutical products of very high purity are required ceramic
and iron grinding surfaces are not used.
(ii)
Thermal conductivity:
In evaporators, dryers, stills, and heat exchangers the materials employed have
very good thermal conductivity. In this case, iron, copper, or graphite tubes
are used for effective heat transfer.
(iii)
Thermal expansion:
If the material has a very high thermal expansion coefficient then as
temperature increases the shape of the equipment changes. This produces uneven
stresses and may cause fractures. In such cases, materials used should be able
to maintain the shape and dimensions of the equipment at the working
temperature conditions.
(iv)
Ease of fabrication:
During the fabrication of equipment, the materials undergo various processes
such as casting, welding, forging and mechanization, etc. For example, glass
and plastic may be easily molded into containers of different shapes and sizes.
Glass can be used as a lining material for reaction vessels.
(v)
Cleaning: Some
materials of construction can pose housekeeping issues when it comes to ease of
cleaning. A smooth and polished surface makes cleaning easier. Glass with an
anti-adhesive and nonporous surface resists the build-up of viscous or sticky
products. Borosilicate glass is a popular choice for processes where ease of
cleaning is critical. Upon completion of an operation, the equipment is cleaned
thoroughly to avoid contamination of the previous product into the next
product. Glass and stainless steel surfaces are generally smooth and polished and
are easy to clean.
(vi)
Sterilization: The
ideal feature of glass, transparency, allows us to see when equipment needs to
be cleaned without the need for interrupting the process and performing an
internal inspection. In the production of parenteral, ophthalmic, and bulk drug
products all the equipment is required to be sterilized. This is generally done
by introducing steam under high pressure. The materials must withstand this
high temperature (121°C) and pressure (15 lb/inch2 ). If rubber materials are
being used they should be vulcanized to withstand the high temperature.
(vii)
Transparency:
Unlike most plastic and metal materials, glass equipment provides transparency
to give an unobstructed view of what is going on inside the system, enhancing
the observation of the process. In reactors and fermenters, a visual port is
provided to observe the progress of the process going on inside the chamber.
For this purpose, borosilicate glass is often used. In parenteral and
ophthalmic containers the particles, if any, are observed from polarized light.
The walls of the containers must be transparent to see through them. In this
case, glass is the most preferred material of choice. For photosensitive
substances, brown-coated glass is also available to offer extra protection. If
there is concern over potential mechanical stresses inflicted on the glass,
Sectrans coating is applied. This coating covers the glass surface and provides
protection against scratches, blows, and splintering.
Economic Factors
The owner’s budget is very important. The initial cost of the equipment depends on the material used. Several materials may be suitable for construction from physical and chemical points of view, but from all the materials only the cheapest material is chosen for the construction of the equipment. Materials that require lower maintenance costs are used because in long run it is economical. Thus, capital expenditures need to be taken into consideration to ensure that the cost does not exceed the financial limits.
Expected Operating Life
Although operational life is less critical, they are still important to keep in mind as we establish the criteria for the plant. It is important to know how long we plan to keep the system in operation. Whether it’s a continuous or batch process, how frequently it is run, and how many years of service we hope to get out of it are all questions that need to be accounted for when determining the type of system components to employ.
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