The sustained maximum chlorine withdrawal rate will
vary from plant to plant, depending primarily upon the
temperature surrounding the container, characteristics
of the chlorinator installed, and the size of the container.
Chlorine is packed in container as a liquefied gas under
pressure, resulting in the container having both the
liquid and gas phase. The liquid becomes gas as it absorbs
heat from the surrounding air. This heat absorption
has a cooling effect on the container, especially during
high feed rates. If the container cools to the dew point
temperature of the air, moisture in the air condenses
on the container. If cooling continuous, the moisture
freeze and insulates the container thereby inhibiting
heat transfer to the liquid chlorine, and reducing the
rate of vaporization. This frost does not mean the chlorine
is frozen. Therefore, circulating air has a significant
effect on increasing the withdrawal rate. Air circulating
is the single most important factor in achieving dependable
and continuous withdrawal of gaseous chlorine from cylinders
and ton containers. Other factors that govern withdrawal
rates include air temperature, container wall thickness,
humidity, and volume of liquid chlorine remaining, but
these factors are far less important the air circulation.
Chlorinators require a minimum
gas pressure for proper operation, again depending upon
the type of chlorinator. Most vacuum type units require
at least 10 psi but are more reliable at 15 psi inlet
pressure. The gas pressure at the point of withdrawal
is a function of the chlorine liquid temperature in
the container. This relationship is shown in below table
(Vapor Pressure vs. Container Liquid Temperature).
For a given chlorinator,
the temperature at which the minimum required gas pressure
is reached is termed the "threshold temperature."
For example, below table (Withdrawal Factors) shown
the pressure needed for a typical vacuum-operated chlorinator
occurs at about 0oF (14 psi). Therefore 0oF
is the threshold temperature for that type of chlorinator.
The liquid chlorine in the cylinder or container must
be kept at 0o F to maintain the gas pressure
required for proper operation.
When gas is withdrawn from
the container, the liquid in the container is chilled.
To maintain the liquid at the threshold temperature,
heat must pass from the air surrounding the container
through the walls of the container and into the liquid.
Therefore the maximum sustained rate of withdrawal is
directly dependent upon the rate at which this heat
transfer takes place. Several factors influence the
heat transfer rate, including ambient temperature, air
circulation, humidity of the air, amount of liquid remaining
in the container, and size and type of container. In
general the important factors are: (1) ambient air temperature
and (2) size and type of container. Ambient air temperature
is simply the air temperature surrounding the container.
Vapor Pressure vs Container
Liquid Temperature |
Temperature
(oF) |
Vapor
Pressure of Cl2 (psi) |
0 |
14 |
10 |
21 |
20 |
28 |
30 |
37 |
40 |
47 |
50 |
59 |
60 |
71 |
70 |
86 |
80 |
102 |
Withdrawal Factors |
Chlorine
Gas |
Withdrawal
Factor |
150
Lb Cylinder |
1.0 |
1
Ton Container |
8.0 |
[Sample
Calculation]
The equation used to calculate
the maximum withdrawal rate from a container is as follows:
(Temp. of Room - Threshold
Temp.) x Withdrawal Factor = Max. Withdrawal rate, lb/day
Assume room temperature or
ambient temperature surrounding the container is 70oF,
the withdrawal temperature is 10oF. The withdrawal
factor is 8 for a ton container. Then we have (70 -
10) x 8 = 480 lb/day. The rule of thumb for acceptable
vapor withdrawal from a one ton container is 300 to
350 lbs/day at room temperature.
Higher rates of withdrawal
will cool the liquid in the container to the point that
the vapor being withdrawn will contain a mist of liquid
chlorine. For a variety of reasons, this will require
more system maintenance and the liquid chlorine carryover
will severely attack the plastic parts in the chlorination
equipment.
When the operator wants to
implement the Pressure Control System described in this
Section, the withdrawal factor (8) of the ton container
can be used to calculate the withdrawal factor for any
size chlorine storage container including railcars,
provided that the surface area of the storage container
is known. It is a simple ratio and proportion exercise
in arithmetic. The surface area of a ton container is
approximately 63.5 sq. ft. Assuming a chlorine storage
tank has 450 sq. ft. The problem is solved as follows:
Let Y equal the storage tank withdrawal factor:
Y/450 = 8/63.5
then Y = 450 x 8/63.5 = 57
Now the threshold temperature
must be selected. If a value is too low then the chlorine
liquid misting problem will be certain to begin. This
should be avoided. In this hypothetical case a threshold
temperature of 40oF is suggested. Then the
withdrawal rate is:
(75o - 40o)
x 57 = 35 x 57 = 1984 lb/day
This will probably allow
vapor withdrawal without the liquid chlorine misting
phenomenon.
1) 150 lb Cylinders
Unlike ton containers, gas
only is withdrawn from cylinders. The withdrawal rate
is limited to 40 lb/day/cylinder. If gas is withdrawn
from a cylinder at too high a rate, the cylinder (or
chlorine) will freeze. As available heat is utilized
to convert liquid chlorine to gas, the liquid chlorine
cools and draws heat from the cylinder. When the cylinder
surface cools to the dew point of the air envelope,
moisture in the air will condensate on the cylinder
surface. A frost build-up and eventually ice occurs
from cooling of the condensed moisture. The ice insulates
the cylinder, inhibits heat absorption by the liquid
chlorine, and reduces vaporization.
Under the following conditions,
as stated by the Chlorine Institute, dependable, continuous
discharge rate of chlorine gas from a single 150 lb
cylinder is 42 pounds per 24 hours.
- Without the appearance
of moisture that will condense on the cylinder surface.
- At 70oF (21oC)
without induced circulation of air.
- Discharge against 35 psi
pressure.
The maximum chlorine gas
withdrawal rate for 150 pound cylinder using Capital
Controls equipment in a system designed by Capital Controls
Company and operated maintained in accordance with Capital's
published manual is as follows:
- Continuous Withdrawal:
2 kg/hr at minimum temperature 4oC.
- Shock Dosing Withdrawal:
10 kg/hr for 30 minutes with at least 6 hours rest
between shock dosing at a minimum temperature 4oC.
Air ventilation would enable to withdrawal gas for
shock dosing as follows: 10 kg/hr for 60 minutes with
at least 6 hours rest between shock dosing at a minimum
temperature of 4oC.
2) Ton-Containers
(1) Gas Withdrawal
Space requirements depend
upon whether the containers are for liquid or gas withdrawal,
the rate of withdrawal, the quantity price break for
the number of containers delivered at one time, and
the length of time containers can be used without incurring
demurrage charges.
At room temperature the maximum
gas withdrawal rate from a ton container is approximately
400 lb/day. If the maximum chlorinator capacity is 400
lb/day, then one container in service will suffice.
If it is 500 lb/day unit, two ton containers must be
in service simultaneously. Theoretically, gas withdrawal
can be used up to any capacity if enough containers
are connected to the supply header. Switching from gas
withdrawal to liquid withdrawal utilizing an evaporator
is necessary when a continuous rate of 1,500 lb/day
is reached. The one exception to this rule of thumb
is in intermittent operating installations, as employed
on cooling water circuits. At such installations it
is customary practice to withdraw rates up to 1,000
lb/day for 30 minutes if the temperature of the storage
area never goes below 50oF. In warmer areas
1,500 lb/day gas withdrawal is safe up to one hour from
a single ton container. This assumes a temperature-pressure
restoration period of no gas withdrawal of at least
twice the length of time as the withdrawal period.
For up to 1,500 lb/day maximum
continuous withdrawal, space should be provided for
four containers in simultaneous service, four standby
containers, and four empty spaces for the next delivery.
Beyond this rate, an evaporator should be installed.
However, there is one exception, as follows:
In hot climates where the
"in shade" summer temperatures exceed 95 to
100oF on a consistent basis it is desirable
to consider the use of an evaporator. Normally those
climates experiencing summer temperatures of 100oF
usually experience winter temperatures of 15oF.
Therefore, the evaporator concept takes care of both
extremes of climatic temperature.
Liquid withdrawal of chlorine
to an evaporator from a supply system is least affected
by ambient temperature. The only precaution is to prevent
direct sunlight on the containers. For winter operation
no special precautions are required (such as artificial
heating) because the gas temperature at the outlet of
the evaporator will usually exceed 100oF
regardless of room temperature, and the temperature
of the gas in the vacuum line to the injector will never
fall below the critical temperature(35oF)
where chlorine hydrate occurs.
Evaporators are available
in capacities of 4000 lb/day, 6000 lb/day, and 8000
lb/day. In a pinch, one container can discharge liquid
to an evaporator at a rate as high as 12,000 lb/day.
This means that an evaporator can be used to conserve
space for container storage if necessary. The optimum
storage requirements should be based on the quantity
discount price break that is offered by the local chlorine
supplier. This usually occurs at a quantity of five,
thus dictating space for five in service, for five empties,
and a vacant space for the incoming five, or a total
space for fifteen ton containers.
If the gas phase is used,
the same consideration must be given to the design of
ton container storage space as for 150-lb cylinders.
All ton container installations using gas withdrawal
should be equipped with special filter installed as
close as possible to the last ton container. If the
header between the last container and the chlorinator
is subject to temperature variations of 20oF
or more over a 24-hour period, then it is desirable
to install an external pressure-reducing valve. This
valve prevents liquefaction of the chlorine in the header
system and the chlorinator mechanism caused by wide
variations of the ambient temperature. This valve should
be installed immediately downstream from the filter.
This filter is combination sedimentation trap and filter.
The filter medium is spun fiber glass, which is held
in place by a stainless steel insert and screen assembly.
(2) Liquid Withdrawal
The chlorine header system
for liquid withdrawal is somewhat different from that
for gas withdrawal. The piping and support system are
the same, except that the flexible connections to the
auxiliary header valve are connected to the bottom container
outlet valve (the top valve is for gas withdrawal).
Valves at both ends of the flexible connection protect
the operator when changing cylinders. Although the method
makes it possible to trap liquid chlorine in the flexible
connection under adverse conditions such as a fire,
operators are well aware of this possibility and still
prefer the method because it eliminates their exposure
to chlorine when changing containers. Therefore they
adjust their habits to follow all the required steps
for maximum safety and minimum risk.
Because of the evaporator
in the system, the filter is located immediately downstream
from the evaporator outlet. The filter must always be
installed in the gas phase. It is not possible to filter
out chlorine impurities in the liquid phase, because
the impurities are in solution. This places the filter
just from the chlorine pressure-reducing valve, which
now becomes an automatic shut-off valve as well. (This
pressure-reducing and shut-off valve is always a part
of the evaporator system. It is electrically interlocked
with the evaporator water bath temperature, and automatically
shuts off the chlorine supply in the event the water
bath temperature falls below 150oF.
When contemplating the use
of liquid withdrawal versus gas withdrawal, the following
advantages of a liquid withdrawal system should be considered.
- The danger of reliquefaction
of chlorine between the containers and the chlorinator
is all but eliminated.
- Fewer containers need
to be connected at one time. Liquid withdrawal rates
of a ton container can be as high as 10,000 lb/day.
- The evaporators, although
insulated, to give off some heat in the equipment
room.
Liquid-withdrawal systems
do not have the same critical design problems with regard
to temperature considerations as the gas-withdrawal
systems, except for inadvertent trapping of liquid in
the header system, which constitutes a temperature pressure
hazard. If liquid chlorine is trapped between two shutoff
valves and ambient temperature rises a few degrees,
the liquid chlorine will try to expand.
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