solid and liquid storage class lecture
DESCRIPTION
Solid Liquid StorageTRANSCRIPT
Storage of Bulk solids and Liquids
A bulk solid is any material composed of many individual solid
particles. Examples of bulk solids include rice, coffee, cereals, fertilizers,
detergents, catalysts, sand, gravel, cement, pigments, coal, and many
others. They are commercially important in applications as diverse as the
chemical, food and pharmaceutical industries, agriculture and
energy production. Bulk solids are usually stored in silos, bins, bunkers or hoppers. A bunker (or bin) is a container whose walls are parallel and a
hopper is a container whose walls converge towards a relatively
small opening at the base. A silo is the combination of a bin and
hopper.
Storage of Bulk Solid
The most common silo shape is vertical cylindrical shell fitted
with a conical outlet. Rectangular section silos with symmetrical
or non-symmetrical, pyramid-like outlets, are also common.
Pyramid hopperSquare opening
Conical hopperCircular opening
Wedge hopper
According to the way of unloading bulk solids from silos, two flow
patterns can be distinguished: Mass flow and Funnel (Core) flow In mass flow (figure I.1a) The bulk solid is in motion in every point within the silo when the
material is unloaded from the outlet. Mass flow is only possible if the hopper walls are sufficiently
steep and/or smooth. In funnel flow (figure I.1b-c) The slope of the hopper walls is too flat and/or the surface
roughness of the hopper wall is too great to achieve mass flow. In this case only the bulk solid above the outlet flows forming a
flow channel going from the outlet up to the free surface of the stored
bulk solid.
Figure 1 . Flow profiles: a: mass flow, b and c: funnel flow
Fig : Mass flow (on the left), funnel flow (on the right)
The bulk solid near the silo wall remains forming the so called
“dead zones” In some cases, only after the bulk solid in the funnel has been
partially discharged, can the bulk solid of the upper part of the dead
zone move slipping in layers towards the funnel outlet.
Most frequent problems emerging during handling or storage of
bulk solids are Arching Piping or Rateholing Flooding Segregation
When the bulk solid is cohesive and the size of the silo outlet is
too small, arches or pipes (rateholing) can form.
Arching occurs when a stable dome is formed above the outlet so
that the flow of the bulk solid is stopped. Occurs in mass flow hopper (generally) It can be prevented by designing sufficiently large outlet sizes. In case of fine grained, cohesive bulk solid, the reason of arching
is the strength of the bulk solid which is caused by the adhesion
forces acting between the particles. [Bulk solid has gained sufficient strength to support itself;
therefore impossible in free-flowing (non-cohesive)] In case of coarse grained bulk solid, arching is caused by blocking
of single particles.
Piping or rateholing occurs in the case of funnel flow when,
because of its high strength, the bulk solid in the dead zones stays in
place even after the funnel is emptied. Occurring in funnel flow hopper A vertical pipe going from the outlet of the silo up to the upper free
surface of the stored bulk solid forms. Generally the walls of the pipe are steeply convergent towards the
silo outlet, but in some cases they may even be vertical. Flooding Bulk solids made of fine powders can be suspended in the
surrounding air when falling downwards to the silo outlet. In these
conditions the solids’ motion is lubricated by the entrained air and so
they can flow out of the silo like a fluid.
This behavior is called flooding and is characterized by high flow
velocities. Flooding can, therefore, cause a lot of dust, and
dispersion in the discharge area. Segregation (separation of particles on the basis of size) Segregation due to particle size, density or shape, which leads to
an unsteady product composition at the outlet in funnel flow silos. In fact, the larger particles, because of their greater mass and so
their greater inertia, can reach the silo walls, while the smaller ones
accumulate in the centre. In the case of funnel flow, the finer
particles, which collect close to the centre, are discharged first
while the coarser particles are discharged at the end. In the case of mass flow, the bulk solid will segregate during
filling in the same manner, but it will become remixed, when
flowing downwards into the hopper.
Therefore in case of mass flow hopper batches of discharged
solids will have a uniform size distribution in time. On the other hand, the advantage of funnel flow silos is their
lower construction cost.
In industry, there are many different types of equipment used for the
storage of liquids and gases. The method of storage chosen depends on the following :The quantity of fluid being stored. The nature of the fluid - is it toxic, flammable, corrosive.The physical state of the fluid - gas or liquid, the temperature and
pressure.
Storing Liquids Small quantities of liquids are often stored in 'CARBOYS’ Larger quantities in 'DRUMS' and Bulk quantities in 'TANKS' or 'RECEIVERS'
Storage of Liquids
1. CARBOYS A carboy is a large, fat, glass, bottle-type container with a flat
base for stability. They are used mainly for storing corrosive chemicals. Modern carboys may be made of a plastic material and may be
cylindrical or rectangular in shape. Carboys generally contain 20 litres or more of liquid. They should NOT be stored near heat sources, should NOT be
rolled NOR contain pressure liquids.
2. DRUMS (OR BARRELS) Drums (barrels), are used to store larger volumes of liquid. They are cylindrical in shape and can be made from a variety of
materials depending on the liquid (or solid) to be stored.
Stainless steel, Aluminum or Mild steel with rubber or plastic
lining are examples of materials used. Like carboys, drums must be handled with care. They CAN be rolled but should NOT be pressurized and NOT
stored near heat sources. All containers should carry labels showing their contents and any
hazards associated with them.
3. STORAGE TANKS Large volumes of harmless liquids involving little or no pressure
are usually stored in simple, mild steel, rectangular or cylindrical
tanks fitted with a flat or conical roof - known as a 'Fixed Roof
Tank'.
Flammable or toxic liquids can also be stored in similar vessels
but generally are located well away from residential and process
areas due to the safety hazards involved. When a number of tanks are installed, they are usually referred to
as 'Tank Farms' and each tank may contain many thousands of
tons (or tonnes) of liquid. The larger the tank, the lower the capital
cost per ton (tonne), of liquid stored. Figure shown such a tank and the possible combination of
features built in. Note that not all tanks will have all of the features shown - some
types of fittings depend upon the type of liquids the tanks contain. The diagram shows all the main features that MAY be found on a
storage tank. For example, some tanks may be open and will not
need a relief valve.
Roof Access Ladder - As its name implies, is used for safe access
to the tank roof.Access Manholes -Usually fitted at ground level or on the roof for
access to the inside of the tank for cleaning, maintenance and
repair. Water Drain - For use where water separation occurs in the tank
and is to be drained off. Also useful during internal cleaning
operations. Transfer Pumps - These are used to transfer some or all of the
tank contents to a process or to other storage tanks, ships, vehicles ..
etc. Depending on their piping arrangement, the pumps may be used
for re-circulation (mixing) of the tank contents or pumping liquid
into the tank from another source.
Bund Walls (or Firewalls) - These can be of two types : A wall surrounding the tank, high enough to contain the entire
tank contents in the event of a burst or severe leak. In this way,
flammable, toxic or corrosive substances can be contained safely
until recovered. An alternative to the above, is having the tank situated in a
cylindrical hole in the ground, again large enough to contain all of
the tank contents in the event of burst or leakage. Relief Valves - Installed where a tank contains pressure and set
to relieve excess pressure if it rises to the safe operating limit. Inert Gas Blanket - This facility is installed when a tank contains
volatile liquid which may produce vapors that become flammable or
explosive when mixed with air.
The inert gas injection (often Nitrogen), replaces the vapor above the
liquid and will not react chemically nor produce an explosive mixture with
the tank's contents. Foam Injection -In emergency, foam can be sprayed into the tank and
over the surface of the liquid. Hazards arising from toxic or flammable
vapors can be minimized. Vapor Vent - (Often called the 'Breather Valve’ or pressure-vacuum
valve) – This allows the tank to 'breathe' when emptying or filling. If a vent is not fitted, the tank could over-pressure when filling and
cause a rupture in the tank and, when emptying, a vacuum would be pulled
possibly causing the tank to collapse (implode). The vapour vent can consist of simply an open hatch where the liquid
being stored has no hazards. For hazardous materials, the vapor vent may consist of a two-way
safety valve.
This will operate at a rising, pre-set pressure to vent excess gas
from the tank to atmosphere or flare system to prevent rupture of
the tank. At a falling pre-set pressure, the valve will operate to admit air,
gas or inert gas (called 'Blanket Gas'), into the tank to prevent
collapse of the tank if the pressure falls to a vacuum. (Gas or inert
gas would be used where ingress of air is undesirable). Sampling & Gauging Hatch - As the name implies, this is
installed in order to obtain samples of the tank's contents for analysis used for dropping a ' dip-tape ' or ' dip-stick ' into the tank to
check the liquid level also check the automatic level measuring instruments.
Earth Connections -An ' Earthing-strip ' is connected to tanks
(and other equipment) in order to carry away and prevent build up of
'Static Electricity' which tends to form during filling and emptying
operations. If static was allowed to build up where flammable liquids are
being stored, then we would have a potential fire or explosion
hazard. The storage of large volumes of liquid can lead to problems
arising from the daily and seasonal variations in the weather. The
most important variable, is that of the ambient temperature.
Floating Roof Storage Tanks
A 'Floating Roof', as its name implies, actually floats on the
surface of the liquid in the tank. As the liquid level changes the
roof is designed to move up and down with the liquid level - i.e.
Filling, Emptying, Expansion and Contraction due to temperature
changes. This type of tank roof minimizes the vapor space between it and
the liquid surface. Since there is no large vapor space for the liquid
to evaporate into, vapor losses are also minimized. There are 2 types of floating roof: Internal floating roof is where the roof floats on the product in a
fixed roof tank. External Floating roof is where the roof floats on the product in
an open tank and the roof is open to atmosphere.
Between the edge of the floating roof and the inside of the tank
shell, special seals are fitted to minimise leakage of vapor. The seal used is generally of a continuous strip of flexible, special
rubber material which is attached to the roof and to the seal ring
around the inside circumference of the tank shell. The underside of the roof is fitted with support legs in order to
leave space between the roof and the tank bottom when the tank is
empty. These legs allow access to the tank for cleaning and / or
maintenance. A ladder is installed up the side of the tank and over on to the
rooftop. This is used for access for sampling, gauging or
maintenance ..etc. As the roof moves up and down, the internal
section of the ladder will slide back and forth on wheels running
along a track.