Ammoniators

 

1. GENERAL

 

Gas ammoniators are used to feed gaseous ammonia at a controlled rate from the supply system to the point of application. Ammoniators are very similar in operation to chlorinators. Ammoniators are available in both vacuum feed and pressure feed systems.

 

1. Interchangeability

The materials of construction for ammoniators (all PVC) are different form materials used in chlorinators (PVC & ABS). Additionally the gas densities differ and ammoniators should not be used on other than ammonia gas.

 

2. Sizing

The procedure for sizing ammoniators is the same as for chlorinators. Because of the difference in density between chlorine and ammonia gases, only half (50%) of the equivalent chlorine capacity is availability in any ammoniators series.

 

2. APPLICATIONS

 

1. Industrial Waste Treatment: Used to create a more suitable environment for aerobic organisms.
2. Petroleum Industry: Sulfur removal, corrosion prevention.
3. Other Industries: The addition of ammonia will neutralize acid waste, increasing the pH.

 

3. THE HARD WATER CONSIDERATION

 

When ammonia gas is added to water as occurs in the ejector of an Ammoniators, the pH is raised. This pH change, at times, has an extreme effect on the operation of the system and on the system maintenance requirements. The extent of the problems involved depends mainly on the amount of hardness present in the water.

 

In water, increasing the pH decreases the solubility of the calcium and magnesium salts (hardness) present. If the concentration of these salts is near their maximum solubility, increasing pH will cause calcium and magnesium carbonates and hydroxides to precipitate. The harder the water, the more severe the problem becomes since there will more precipitation for a given change in pH. Deposits will appear in the ejector throat, the solution line between the ejector and the point of application, and the ammonia solution diffuser.

 

An example of the type of problem that can occur is a case in which the deposits in the ejector throat were sufficient to destroy the efficiency of the ejector and shut down the ammoniator due to lack of vacuum. In a second care, the amount of precipitate was sufficient to plug 250 feet of solution line. Obviously, the problem is one of chemistry and application, not one of equipment design.

 

What can be done to keep these problems from occurring on future ammoniator installations? Here are several suggestions that will help:

 

1. Keep the length of the solution line to an absolute minimum by location the ejector as close as possible to the point of application. Oversize the ejector by using a larger nozzle and throat, so that even if there are deposits in the throat, sufficient vacuum will be produced for operation. Also, the increased flow due to the over-sizing ejector decreases the ammonia concentration, decreasing the change in pH.

 

2. Oversize the solution line so the deposits that are formed don't have an immediate effect on operation. In this case, you are 'buying' time before the line has to be cleaned or replaced.

 

3. When the hardness of the ejector water supply exceeds about 40-50 ppm CaCO₃, the greater the potential for a problem. Where water hardness is excessive, reducing it to less than 40 ppm CaCO₃ by means of a water softener should reduce or eliminate possible problems.

 

4. The customer should have a spare nozzle and throat combination so that it can be installed when necessary while the originals are cleaned in a dilute acid solution.

 

Although the above suggestions may not eliminate all problems when feeding ammonia into hard water, the problems should be less serious and less frequent.

 

 

CHLORINATION FOR IRON & MANGANESE

 

Both of the elements, iron and manganese, are bound in the earth's crust, iron mainly as insoluble ferric oxide; manganese as manganese dioxide.

 

Since ground waters contain substantial amounts of carbon dioxide (35-50 mil per liter), appreciable amounts of ferrous carbonate, which is slightly soluble, may be dissolved to form soluble (up to 150 ppm) ferrous bicarbonate. Iron may also be present as ferric hydroxide, ferrous sulfate and colloidal or organic iron.

 

Manganese mostly occurs as manganese bicarbonate, which is very soluble. However, this element could also be present as manganous sulfate as well as ferrous sulfate, colloidal or organic manganese.

 

The insoluble ferric compound will only go into solution under reducing conditions; that is, in the absence of oxygen. This holds true for both iron and manganese.

 

The occurrence of iron and manganese in water supplies is usually limited to wells and impounded surface supplies. It is apparent that anaerobic conditions must develop in order for appreciable amount of iron and manganese to gain entrance to a water supply. Ground waters, which contain large amounts of these compounds are always devoid of dissolved oxygen and are high in carbon dioxide.

 

In an impounded water supply, decomposition of organic matter in the bottom of the source results in the elimination of dissolved oxygen which produces, biologically, carbon dioxide so that iron and manganese compounds in the flooded soil and rocks are converted to soluble compound. These soluble compounds rise to the surface in areas where fall overturns occur. At this point, they are oxidized and precipitated. They then settle to the bottom where resolution occurs due to absence of oxygen. As stated above, waters at the surface due to the available oxygen present are usually free of both iron and manganese. Impounded reservoirs of water usually present the greatest problem when the draw-off point is located at the bottom of the reservoir.

 

Generally speaking, there are three methods of removal:

 

1. Oxidation by aeration or chlorination, or a combination of both.
2. Precipitation with lime at a pH of 8 or above.
3. Ion exchange.

 

The major problems all of these methods create are the removal of the precipitate. Usually sedimentation and/or filtration are used depending upon the floc produced. Optimum pH is 7.0 for removal by chlorine. Whatever methods are used to prevent or control these elements, chlorination is also imperative in the removal process in order to prevent and control the growth of the creniform organisms which can cause severe problems in piping networks due to pluggages and fouling. If the water by proper chemical analysis shows more than 0.3 ppm iron, it should be chlorinated to prevent growths. If there is more than 0.03 and 0.05 ppm manganese present, it will probably precipitate out in a delayed reaction. When this happens, sometimes a sequestering agent such as a Tramfloc complex phosphate should be added.

 

 

CHLORINATORS

 

1.                   General

 

Gas feeders dispense gas at a controlled rate, from the supply system to the point of application. Feeders may be of the vacuum operated, solution feed type or the pressure operated, dry gas feed type. Both types are available constructed of materials capable of withstanding the corrosive action of chlorine, sulfur dioxide or ammonia (as specified). They should never be used for handling a gas different from that for which they have been purchased.

 

Although details concerning their operating is beyond the scope of this text, it is important to note that all gas dispensers contain: a flow control valve, to provide gas feed rate adjustment and a flowmeter, to provide visual indication of the gas feed rate. In addition, all dispensers are equipped with safety features, i.e., components which act automatically to protect both the equipment and operating personnel during abnormal operating situations. There is one safety feature common to all vacuum operated gas dispensers. By virtue of their fundamental principle, the gas flow meter is shut off automatically in the event that a leak develops down stream of the gas inlet valve. This feature cannot be incorporated into the design of pressure operated gas dispensers.  For this reason vacuum systems are the chlorinators of choice for gaseous fluids.

 

2.                   Vacuum Operated, Solution Feed Type

 

This type gas dispenser uses a vacuum-actuated, spring-loaded regulating valve to reduce the gas from a varying supply pressure to a constantly regulated vacuum, usually about 20 inches water column. The unit is called a vacuum regulator. Operating vacuum is created by a companion water jet aspirator, commonly called an ejector. The gas, then, is actually 'pulled' through the various components of the dispenser and into the ejector where it is thoroughly mixed with the water (or process liquid) to form a chemical solution. This solution is the delivered, using suitable rigid or flexible piping, to the main flow of process liquid at the desired point of chemical solution application.

 

Modern design concepts encourage mounting of the vacuum regulator on, or very close to, the gas supply system. By following this procedure, the pressurized portion of the gas system is reduced to one having few connections, thus increasing the safety of the installation. Should a leak develop, in a vacuum system, it will draw in air from the atmosphere rather than leak gas into the atmosphere. Vacuum operated automatic switchover systems fit this concept as well.

 

Vacuum operated, solution type gas dispensers are by far the most commonly used machines because of their intrinsically safe operating feature, e.g., gas flow from the supply system is automatically shut off by the closure of the vacuum actuated, spring-loaded regulating valve in the event of loss of operating vacuum for any reason. Therefore, almost without exception, they are preferred and selected for use throughout the field of water, sewage and industrial waste treatment. They are replaced by the pressure operated, dry gas feed type dispenser only in those instances where the main flow of process liquid being treated cannot be diluted with the water required to operate the ejector; or, the quality of the water (or process liquid) is such that it contains particulate matter which is too large or in too great a quantity to be filtered out and it cannot be used to operate the ejector.

 

From several points of view, it is more desirable to locate the ejector close to the point of chemical solution application. First from the safety viewpoint, it is better to have lines under negative pressure than positive pressure and this is true for both gas lines and chemicals solution lines. Should a chemical solution hose or pipe leak, the dispenser, not knowing the difference, will continue to operate. The result can be damage to equipment and danger to personnel. By location the ejector close to the point of application (remote from the cabinet and/or vacuum regulator) the pressurized chemical solution line is reduced or eliminated. A gas vacuum line connects the 'remote' ejector to other portions of the gas dispenser system. As a rule the economics of a typical installation will favor the use of a remote ejector. It is less expensive to run a single vacuum line from the dispenser to the ejector, than to run two lines, one for the ejector water supply and the other for chemical solution. Chlorine, for instance, dissolves rather poorly in water, and turbulence in long lines has a tendency to form gas bubbles at the top of the pipe or hose. This can be dangerous when the content exits from the diffuser, whether the diffuser be installed in a closed pipe line or in an open channel.

 

The last advantage of remote ejectors is that the response time for the dispenser to change its output (through either manual or automatic control) is longer when chlorine solution lines are used instead of vacuum gas lines. It must be remembered that the flow of water through an ejector remains nearly constant regardless of the output rate at which dispenser is operating. What changes (with varying dispenser output) is the strength of the chemical solution. Thus, there is a lag in dosage response equal to the time it takes for the different concentration chemical solution to travel from the ejector to the diffuser. In the case of vacuum gas lines, while there is some delay, the velocity increases in a more constant mass (the gas) and will allow a faster response.

 

Because of their popularity, many models covering a wide range of feed rates and many modes of operation are made available to the plant operator. The modes of operation available range from: the simple manual type, wherein gas feed rate is adjusted by positioning (opening and closing) a manual rate valve; through the semi-automatic type, wherein the manually set gas feed rate is automatically controlled by starting and stopping the flow of water (or process liquid) through the ejector; to the fully automatic type, wherein the gas feed rate is automatically adjusted in response to input signal(s) received from process liquid flow rate monitoring and/or chemical residual control devices.

 

3.                   Automatic Control

 

a.      General

There are numerous ways to control the output of a gas chlorinator and they are described in the following paragraph.

 

b.      Manual Control

The rate of chlorine feed is established by the plant operator and the chlorinator will continue to feed at that present rate. This method of control is fully adequate for installations where the main line flow rate and the chlorine demand of the water are both constant, or for manned installations where an operator is always present and can make adjustments for changing main line water flow. Should either or both of these conditions (flow rate and chlorine demand) vary to any degree or frequency, manual control will probably not be adequate as a control scheme.

 

c.      On-off control

Vacuum needed for operation is created by the flow of water through the ejector. By stopping and starting the vacuum, the gas flow through the chlorinator is started and stopped. The chlorinator used in this control mode is a simple manual control unit. Two methods can be employed to start and stop the vacuum. The first (more common) is to interrupt the water supply going to the ejector, through the use of an automatic on-off valve located in the water supply line. Alternatively, a water booster pump may be started and stopped resulting in a similar action. Another method is to use a valve placed in the chlorine gas vacuum line upstream of the ejector as inlet. By closing and opening this valve, the chlorinator is caused to stop and start. On/off control is used commonly to treat pumped flows. This approach is also widely used in the industrial area for the chlorination of cooling water using a 'shock' treatment cycle.

 

d.      Flow Pacing

The most common type of automatic control for a chlorinator is flow pacing. A mainline flowmeter is used in this system together with a flow transmitter having a

4-20 mA dc output, and this is directed to an automatic valve. The 4 mA signal represents zero flow while the 20 mA represents maximum flow. Thus the gas flow through the chlorinator is made proportional to the mainline flow, and the dosage rate of chlorine is constant over the range of the chlorinator even though there are variations in the mainline flow. In this control arrangement the dosage rate must be preset by the operating personnel to the desired level. It has a span of settings from 0.1 to 2.0. With a 1.1 setting on the dosage adjustment, at a 12 mA dc input signal, the chlorinator will feed 50% of its maximum capacity. The actual dosage adjustment setting is normally made on a trial and error basis using the measured chlorine residual in the treated water as the criteria. Additionally, it should be recognized that flow pacing systems cannot control the residual level as any change in the chlorine demand of the water is not automatically visible.

 

e.      Residual Only Control

In this approach a chlorine residual analyzer is utilized, taking a sample of the chlorinated water at a point downstream from where chlorine is applied. At first glance the Residual Only Control approach would seem to solve the problems of both varying mainline flow rate and chlorine demand changes in the water. Further study will reveal that this approach is an insensitive system in real time. Because of the application variables, it takes some period of time before the analyzer becomes aware of a change in the chlorinator feed rate. If the change is slow, the Residual Only Control concept can provide adequate control. However, should flows change quickly, the system will be without knowledge of the change for whatever system lag time exists, and during this time there will occur either under or over chlorination - both undesirable.

 

f.        Flow Pacing with Residual Control (also known as Compound Loop Control)

The last method is a combination of Flow Pacing and Residual Control as described in the two previous paragraphs. Flow pacing is intended to take care of volumetric flow changes in the mainline pipe or channel while residual control compensates for changes in quality (chlorine demand changes). This method is the ultimate in control strategy and when applied correctly produces a constant residual in the finished water or wastewater. The automatic valve (either directly or with the use of a controller) must have the ability to accept two 4-20 mA dc signals, one from the mainline flowmeter transmitter and the second from the residual analyzer. These signals are the multiplied together so that the stepping motor and chlorine valve can respond.

 

1.       A lower limit can be easily be placed on the output signal going to the chlorinator which can provide distinct operational advantages. Should either the water flow meter transmitter or the residual chlorine analyzer output fall to zero, the resulting output will also be zero, as any number multiplied by zero will equal zero. Thus, chlorination will cease under these circumstances. To prevent this from occurring, an electrical low limit is established. It is suggested that initially the incoming signal to the chlorinator be observed, or even recorded for a reasonable period of time. It is suggested that the minimum signal level be set at between 0.5 and 1 mA below that 'normal' minimum signal, thus protecting the system from operating without chlorine. In order to alert operating personnel that a problem exists, we can utilize a low level contact closure in the controller. The low level contact could be set between the normal minimum operating output signal and the pre-established minimum signal. Thus, operators are immediately informed through an alarm system that there is a low level signal problem from either the main line flow transmitter or the residual chlorine analyzer transmitter which requires attention. This precludes the confusion and concern that chlorination has not ceased, but is operating at the preprogrammed minimum level.

 

2.       In a similar manner, inadvertent over chlorination can be eliminated with a program to limit the maximum signal reaching the chlorinator as could occur with certain electrical component failures in the residual analyzer circuit. Identical logic could be employed using another alarm contact in the controller to inform operators about these situations.

 

3.       The combining of the flow and residual signals in the controller will usually result in a less expensive installation, as only a single pair of wires is required between the analyzer or control room, and the chlorinator.

 

4.       The flexibility of making changes to the chlorinator control configuration in the controller should be fully appreciated. These changes can be made through configuring quickly without the need to change and wiring and without using special equipment. They include ability to change proportional band and reset times, minimum or maximum limiters and alarm point levels.

 

When designing an automatic control system, the system time lag should be kept in the 3 to the 5 minute range (for raw water or wastewater) and 2 to 3 minutes for finished water if the best control is to be attained. Times longer that these amounts will decrease the precision of control. Times shorter can cause instability in the control system. The system lag time is made up of several small time elements, the major ones being:

 

a.      Time required for the chlorine solution to travel from the ejector to the point of application. This time is usually constant.

b.      Time required for the chlorinated water to travel from the chlorine solution diffuser to the sampling point (leading to the residual analyzer). The time is normally a variable - with mainline flow rate.

c.      Time required for the sample to travel from the sampling point in the main pipe line to the analyzer. The time element is a constant.

 

 

CHLORINE AND ITS PROPERTIES

 

Chlorine (chemical symbol Cl) belongs to the group of elements called halogens. Chlorine has the atomic number 17 and an atomic weight of 35.45. The chlorine molecule exists in the diatomic state and has the chemical symbol Cl₂. Although chlorine comprises from .015 to .03 percent of the earth's crust, it is not found in nature but is most commonly found in mineral compounds.

 

CHARACTERISTICS

 

At atmospheric pressure and normal temperatures chlorine is a yellow/green gas with a sharp, choking odor. At -35 degrees C chlorine liquefies to an oily, amber substance. The gas is 2.5 times as heavy as air and the liquid is 1.5 times as heavy as water. A quantity of gas will occupy 456.8 times as much volume as an equal weight of the liquid atmospheric pressure.

 

REACTIONS

 

Chlorine is a strong oxidizer and therefore reacts readily with many other materials. It is not flammable but because of its oxidizing properties, it will support combustion. As a result, chlorine is very corrosive to most metals, destructive to organic materials and will react dangerously with ammonia, turpentine, ether, hydrogen, powdered metals and other hydrocarbons and reducing agents. Chlorine is only slightly soluble in water, 0.64 percent by weight at 25 degrees C, but is it easily dissolved in alcohol.

 

MANUFACTURE AND STORAGE

 

Chlorine is the 8^th highest volume chemical produced in the United Sates. The most common method of manufacture is the electrolysis of salt brine. This results in chlorine gas with sodium hydroxide and hydrogen as by products. Commercial chlorine is approximately 99 percent pure with the remaining 1 percent consisting of bromine, benzene, chlorinated hydrocarbons and water. The finished product is packaged and transported in 120, 150, and 2000 lb. steel cylinders. Tank trucks carry up to 22 tons and railroad tank cars holding up to 90 tons are also used, as are 600 and 1120-ton barges. These containers are filled approximately 85 percent volume with pressurized liquid chlorine; and as a result, have an internal pressure of around 120 PSI at 25 degrees C.

 

HANDLING

 

The highly reactive nature of chlorine presents unique conditions for handling and exposure. Chlorine gas is readily detectable by smell at concentration of around 1 PPM. The gas is extremely irritating at low concentrations and inhalation of concentrations about 4 to 5 PPM will cause respiratory difficulty and eventual pulmonary edema. For these reasons strict adherence to safe handing practices, proper use of protective clothing and adequate ventilation of work spaces must be maintained at all times. Exposure to chlorine should be treated immediately with proper first aid measures. All personnel who handle or work around chlorine should be familiar with the correct response to chlorine emergencies.

 

HEALTH PRECAUTIONS

 

Use only in well ventilated areas. Eyewashes, showers, and oxygen should be available. Self-contained breathing apparatus or canister-type respirators should also be accessible.

 

PERSONAL PROTECTIVE CLOTHING SHOULD INCLUDE

 

1. Full face shield or non-ventilated chemical goggles.
2. Chemical resistant rubber gloves.
3. Apron or jacket.
4. Open shoes and sneakers should be prohibited.
5. Wear long sleeves and trousers.

 

EFFECTS

 

Low concentrations: burning in the eyes, nose, throat, redness in the face, sneezing, and coughing.

 

High concentrations: tightness in throat and chest, pulmonary edema. 1200 PPM is rapidly fatal.

 

FIRST AID

 

Inhalation:

 

1. Remove victim from contaminated area.
2. Keep victim warm in reclining position with head and shoulders elevated.
3. Give artificial respiration/CPR as necessary.
4. Administer oxygen as soon as possible.
5. Call physician immediately.

 

Skin contact:

 

1. Shower victim, removing all contaminated clothing.
2. Wash affected area with soap and water.

 

Eye contact:

 

1. Irrigate eyes with water for 15 minutes, holding eyelids wide apart.
2. Call physician immediately.
3. Irrigate for a second 15-minute period if physician is not immediately available.

 

CHLORINATION FOR ALGAE & SLIME CONTROL

 

GENERAL

 

One of the most important uses for chlorine, gas chlorine, and gas chlorinators is in the destruction or control of algae and slime. Algae and slime cause numerous difficulties in the various systems where they are found and although there are many different types of chemical agents used to combat them, chlorine is the most widely used because of its effectiveness and low cost.

 

Gas chlorine (liquid chlorine) and gas chlorinators have usually been used for treatment of the medium and large systems, but many of the smaller systems were not able to take advantage of the low cost of liquid chlorine in the past due to the high cost of equipment and installation. Direct cylinder mounted and ton container mounted gas chlorinators have provided the answer to low equipment and installation cost, simplicity of operation and minimum maintenance.

 

ALGAE AND SLIME

 

The general terms of algae and slime cover a multitude of microscopic organisms of both the plant and animal types. Algae are a group of plant organisms having many thousands of species in a wide variety of colors.  These organisms produce their own food through the use of light energy and water (photosynthesis). Since sunlight is required for their growth, they are found in almost all surface water supplies such as lakes, ponds, swimming pools, etc., where control methods have not been applied or have been ineffective. Long filaments or large clumps of algae colonies are not only unsightly, but can reduce flow in pipes, plug up valves and small orifices  and can induce corrosion. Although they do not live in enclosed systems, dead colonies still can create a considerable problem.

 

Slime is a general term widely used for clusters or long filaments caused by bacteria or fungi. These microorganisms do not produce their own food and therefore do not require sunlight.

Many strains grow best in dark areas and not only produce a problem due to their reproduction, but aggravate the problem by collecting inorganics, dead algae and other debris in their colonies. The most common types that cause difficulties are air-borne and, therefore, systems with open tanks, cooling towers and open water supplies are most susceptible. Slime not only presents the problem of system plugging, but also retards heat exchange in cooling water systems and promotes corrosion.

 

AFFECTED AREAS

 

Algae and slime affect many types of systems in municipal and industrial use and three of the most important ones are:

 

1. Water supply systems
2. Swimming pools
3. Cooling water systems

 

TREATMENT - WATER SUPPLIES

 

Chlorination of water supplies is a subject in itself since it includes treatment for disinfecting, taste, odor, color removal, etc.  The method and amount of chlorination may be dictated by one of these, rather than the need to eliminate algae and slime. In filtration plant systems for drinking water, algae can be carried into and through the filter system and into other sections of the treatment plant causing considerable difficulty. Strong chlorination of the water entering the plant is often recommended to kill the organisms, which can then more easily be filtered out. This also eliminates after affects of taste and odor caused by reactions with or decomposition of algae and slime.

Chlorination of deep well supplies usually required only a free chlorine residual of 0.5 to 1.0 PPM to destroy organisms as crenothrix.

 

Since this bacterium exists in iron and manganese-bearing waters, it may also be necessary to add a Tramfloc sequestering agent prior to the addition of chlorination in order to keep dissolved iron and manganese compounds in solution where chlorination alone would normal precipitate them. Chlorination of well systems usually is very effective in eliminating the source of troublesome bacteria, but in some cases periodic shock treatment of the system may be necessary to destroy bacteria growth that has already been established. Shock treatment may require residuals of 50 to 150 PPM, which are sealed into the section of the system being treated and allowed to stand for 12 to 24 hours, if possible.

 

SWIMMING POOLS

 

Swimming pool systems could receive shock treatment once a day, which would adequately eliminate algae and slime growths. Because of the need to eliminate disease-causing bacteria, it is recommended that continuous treatment be used and that a residual of 0.5 to 1.0 PPM exist in the pool at all times. This should adequately eliminate algae and slime problems, although in some systems in is also necessary to over-chlorinate to 5 to 12 PPM every week or two weeks and thoroughly brush the walls and bottom to bring the strong solution into contact with the organisms.

 

COOLING WATER

 

Cooling water treatment is most effective by continuous chlorination maintaining a free chlorine residual of about 0.5 to 1.0 PPM. This, however, is the most costly method of chlorination and is not always technically sound. Where cooling water is obtained from a river, stream or other surface supply and merely passed through the system once, it has often been found adequate to use shock treatment once every four, eight or twenty-four hours to eliminate organisms which have begun to collect in the system. The frequency and amount of both dosage and contact time will depend on the source. Many have found that shock treatment at a rate of 5 PPM every four or eight hours for duration of 15 to 30 minutes is adequate to give a residual of 0.5 to 1.0 PPM in the water leaving the system. Here again, it may be occasionally necessary to shock treat at higher values and for a longer contact period in order to keep the system clean. 

SULFUR DIOXIDE AND ITS PROPERTIES

 

GENERAL

 

Sulfur dioxide is one of the more important chemical compounds formed from the combination of sulfur and oxygen. The chemical formula for sulfur dioxide is "SO₂", and as the formula indicates, two atoms of oxygen are chemically combined with one atom of sulfur.

 

Although in many applications, sulfur dioxide is a bleach, reducing agent, solvent, or a raw material, it is also listed as one of the primary chemical air pollutions in the United States today.

 

In the areas of water and waste treatment, sulfur dioxide is one of the most popular compounds used for dechlorination of water and reduction of chromates in wastewater (see Table III for reactions).

 

Sulfur dioxide is commercially produced by heating of sulfur, sulfur-bearing ores, or by the recovering of stack gases to meet clean air requirements.

 

Sulfur dioxide is stored and transported in tank cars and cylinders as a liquid under pressure, and is classified by the Department of Transportation (DOT) as a non-flammable compressed gas that must be stored or shipped in DOT specification containers.

 

PROPERTIES

 

At atmospheric temperatures and pressure, sulfur dioxide is a colorless vapor with a characteristic, pungent odor. When compressed and cooled, sulfur dioxide forms a colorless liquid, which at atmospheric pressure boils at 14°F (-12°C) and freezer at -123.9°F (-75.5°C).

 

Liquid sulfur dioxide is heavier than water, having a specific gravity of 1.436 at 32°F (0°C). As a vapor, sulfur dioxide is heavier than air, with a relative density of 2.2636 when compared to air at atmospheric pressure and a temperature of 32°F.

 

When heated about its critical temperature, 314.82°F (157.12°C), sulfur dioxide can only exist as a vapor regardless of pressure.

 

Liquid sulfur dioxide exists in equilibrium with its vapor when stored in a closed container. The vapor pressure within the container is directly proportional to the temperature, which, when plotted, yields a smooth curve as shown in Figure 1.

 

Sulfur dioxide is somewhat soluble in water (18.59% by weight at 32°F/0°C) and forms a weak solution of sulfurous acid (H₂SO₃). The degree of solubility is directly dependent on temperature (see Figure 2).

 

Generally, undiluted (dry) sulfur dioxide is not corrosive to ordinary metals; however, when small amounts of moisture are present, sulfur dioxide will attack most metals.

 

VAPOR WITHDRAWAL

 

Although sulfur dioxide is normally shipped and stored in liquid form, many applications require sulfur dioxide to be supplied as a vapor. Due to the inherently low vapor pressure of sulfur dioxide, vaporization of the liquid requires heat, which must be supplied to the cylinders from an external source. Electric strip heaters or steam coils equipped with thermostatic control are generally used for this purpose.

 

Because fusible safety devices in the cylinders melt at 165°F (74°C), great care must be taken not to allow cylinders to reach exceedingly (125°F (51.7°C).

 

The withdrawal weight of sulfur dioxide vapor from 150 lb. (68 kg) and 1 ton (907 kg) cylinders is less than that of chlorine, 40-50 PPD (0.76-0.94 kg/hr) from a 150 lb. (68 kg) cylinder and 400-450 PPD (7.6-9.4 kg/hr) from a ton cylinder at 70°F (21.1°C). To maintain relatively high withdrawal rates without excessive frosting of cylinders or reliquefaction in the supply header, the ambient temperature in the vicinity of the cylinders should be set at 80-85°F (26.7-29.4°C).

 

LIQUID WITHDRAWAL

 

When the application requires large amounts of sulfur dioxide, vaporizer systems are usually specified. Generally, liquid sulfur dioxide and liquid chlorine are handled with identical vaporizer systems. Therefore, the same precautions apply for both, such as filters and drip legs in the supply and header systems.

 

Due to the fact that the latent heat of vaporization for sulfur dioxide is 150 Btu/lb. (83.3 g-cal/g) at 70°F (21.1°C) when compared to 123 Btu/lb. (68.3 g-cal/g) for chlorine; as would be expected, a standard vaporizer has slightly less capacity (approximately 80%) when used for sulfur dioxide.

 

In liquid systems, the sulfur dioxide is withdrawn from the bottom of the storage tank, either by a connection located at the bottom of the tank or by a connection at the top of the tank joined to a dip tube located within the tank.

 

In certain instances, it may be necessary to apply heat to the tanks to facilitate the flow of sulfur dioxide. Again, precautions should be taken not to allow containers to reach temperatures exceeding 125°F (51.7°C).

 

A major concern, which is inherent to liquid systems, is the possibility of excessive pressure developing between closed valves. Sulfur dioxide's large coefficient of expansion with temperature can develop pressures capable of causing piping to rupture. To remedy potential problems caused by excessive pressure, a liquid expansion system should always be provided.

 

MATERIALS OF CONSTRUCTION

 

Schedule 80 Steel pipe with socket welded fittings is normally used to transport undiluted (dry) sulfur dioxide. If the piping is to conduct diluted sulfur dioxide (containing over 1,000 PPM water), 316 stainless steel or Alloy 20 would be appropriate. Zinc-coated or galvanized pipe should never be used for service with sulfur dioxide.

 

Flanges can be slip-on and flat-face, and should be fitted carefully to prevent leaks. Flanges should allow for easy disassembly of pipe and provisions should also be made to protect pipe from the effects of expansion, contraction, jarring, vibration, and settling. After assembly, piping should be inspected for leaks or other unusual conditions. The majority of applications using sulfur dioxide will require flexible piping, tubing, or hose in the system. Pipes can be made flexible by providing a series of reverse bends in the line. Heavy-duty copper tubing may also be used to provide flexibility when transporting dry sulfur dioxide. Flexible metal hose for service with corrosive acids under pressure makes the best all purpose flexible line for carrying dry sulfur dioxide.

 

Normally, 316 stainless steel is satisfactory for valves, gauges, and pressure regulators in service with sulfur dioxide. Alloy 20 is better for 'wet' sulfur dioxide (see Table III).

 

Metallic and non-metallic gasket and packing material such as Teflon, graphite asbestos, and lead perform well in sulfur dioxide. If a packed valve is used for wet sulfur dioxide, Teflon packing is recommended.

 

STORAGE AND SHIPMENT

 

In the United States, sulfur dioxide is shipped in specially designed steel railroad tank cars and tank trucks that conform to DOT guidelines. Single unit railroad tank cars and generally range 15-55 ton capacity. The capacity for large tank trucks ranges between 15 and 20 tons, while capacity of smaller tank trucks may very depending on particular needs. Sulfur dioxide is also available in 150 lb. cylinders, 2000 lb. steel drums, and 1lb. containers for laboratory use.

 

Sulfur dioxide bulk storage tanks are normally constructed of carbon steel with provision made to prohibit process gases and solutions from entering the tank. Again, the size of the storage tanks will depend on particular requirements.

 

Sulfur dioxide storage tanks are designed and fabricated in accordance with the American Society of Mechanical Engineers code for certified pressure vessels.

 

SULFUR DIOXIDE LEAKS

 

When a sulfur dioxide leak occurs, it is easily detected by the sharp, pungent odor of the vapor. The location of the leak may be determined by means of ammonia vapor dispensed from a squeeze bottle, or by the use of an ammonia swab. When the ammonic comes in contact with the sulfur dioxide vapor, dense white fumes of ammonium sulfate form near the leak.

 

If a leak does occur, only authorized personnel should attempt to stop the leak. If there is any question as to the size of the leak, a suitable gas mask should be worn.

 

Normally, leaks that do develop are not serious and can be readily controlled. Where leaks do occur, the supply of sulfur dioxide should be shut off immediately by closing appropriate valves.

 

If the leak is large and continuous, all personnel in the immediate area should be removed and qualified help should be summoned. If possible, the leaking container should be moved to an open area where the danger of escaping sulfur dioxide is minimized.

 

While, at low concentrations, sulfur dioxide vapor is extremely irritating to the eyes and mucousal membranes of the upper respiratory tract, it is easily detectable at 3 to 5 PPM in the air. Exposure to high concentration produces a suffocating effect due to the closing of the glottis to shut out the gas.

For the physiological response to various concentration of sulfur dioxide see Table IV.

 

Exposure to escaping sulfur dioxide liquid will result in freezing action of the skin. This freezing action is a natural result of the escape of a liquefied refrigerant under pressure.

 

Persons having chronic lung diseases, heart disease or persons having shown evidence of hypersensitivity to sulfur dioxide should not be employed in areas where sulfur dioxide is being used. The odor makes it impossible for a person to voluntarily remain in a seriously contaminated area for a dangerously long period of time.

 

Any person, who has been burned or overcome by sulfur dioxide vapors, should be placed under a physician's care immediately.

 

Persons responsible for first aid services should be familiar with special procedures required in cases of sulfur dioxide exposure.