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 0^oF is the threshold temperature for that type of chlorinator. The liquid chlorine in the cylinder or container must be kept at 0^o 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When gas is withdrawn from the container, the liquid in the container is chilled.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.

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 70^oF, the withdrawal temperature is 10^oF. 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 40^oF is suggested. Then the withdrawal rate is:

(75^o - 40^o) 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 70^oF (21^oC) 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 4^oC.
• Shock Dosing Withdrawal: 10 kg/hr for 30 minutes with at least 6 hours rest between shock dosing at a minimum temperature 4^oC. 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 4^oC.

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 50^oF. 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 100^oF on a consistent basis it is desirable to consider the use of an evaporator. Normally those climates experiencing summer temperatures of 100^oF usually experience winter temperatures of 15^oF. 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 100^oF regardless of room temperature, and the temperature of the gas in the vacuum line to the injector will never fall below the critical temperature(35^oF) 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 20^oF 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 150^oF.

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.