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.