Liquid ammonia tanks

Fig. 9.4 Process flow of Braun synthesis ioop 1-Synthesis gas compressor 2-4-Converter 5,9-Heat exchangers 6-8-Waste heat boilers 10-Ammonia cooler 11-Separator 12-Liquid ammonia tank 13-Set of recovery ammonia from vent gas.

A bursting or rupture disk is a pressure relief device that protects a vessel or system from excess pressures. They have been commonly used in aerospace, aviation, defense, nuclear, and oilfield applications often as a backup device for a conventional safety valve. In this instance, if the pressure increases and the fitted safety valve fails to operate, the rupture disk will burst as required. The discs are usually made from thin metal foil, and gold has been used in some instances because of its ductility and resistance to corrosion. Gold discs fitted to liquid ammonia tanks, for example, have shown good durability in this application compared to other metals. The use of gold in this application was reviewed in the 1970s when this industrial application for gold was more common [10]. 

Figure 16.1 shows part of a steel tank which came from a road tank vehicle. The tank consisted of a cylindrical shell about 6 m long. A hemispherical cap was welded to each end of the shell with a circumferential weld. The tank was used to transport liquid ammonia. In order to contain the liquid ammonia the pressure had to be equal to the saturation pressure (the pressure at which a mixture of liquid and vapour is in equilibrium). The saturation pressure increases rapidly with temperature at 20°C the absolute pressure is 8.57 bar at 50°C it is 20.33 bar. The gauge pressure at 50°C is 19.33 bar, or 1.9MN m . Because of this the tank had to function as a pressure vessel. The maximum operating pressure was 2.07 MN m" gauge. This allowed the tank to be used safely to 50°C, above the maximum temperature expected in even a hot climate. 

Figure 3.2. Liquid-level gauge for an ammonia tank.

Ammonia, when released is a toxic gas with little flammability. It is imported by sea into the 14,(XX) tonnes capacity tank at Shell UK Oil where the refrigeration maintains the temperature below the boiling point of the gas (33° C). Three ways were identified whereby several hundred tonnes of liquid ammonia could be released into the river to vaporize and disperse. The worst accident would have an accompanying explosion or fire on an ammonia carrier berthed at the unloading jetty. Next in order of severity is a ship collision and spillage into the river near the unloading jetty. The consequences of a collision between ships occurring within the area but not near the jetty were also calculated. 

Transfer of liquid ammonia from the ship using a special pipeline to the thermally insulated storage tank. The transfer, requiring 20 hours, uses the ship s pumps there are five transfers each year. 

A liquid chlorine tank was kept cool by a refrigeration system that used CFCs. In 1976 the local management decided to use ammonia instead. Management w as unaware that ammonia and chlorine react to form explosive nitrogen trichloride. Some of the armnonia leaked into the chlorine, and the nitrogen trichloride that was formed exploded in a pipeline... 

A report is given of explosion of a tank containing 7000 t of liquid ammonia, in Lithuania, consequent upon thermal disequilibrium and bumping of the liquid [10,11], The liberated ammonia caught fire in the open air, the first time this is thought to have been observed. 

Liquid ammonia is stored in a tank at 24°C and a pressure of 1.4 X 106 Pa. A pipe of diameter 0.0945 m breaks off a short distance from the vessel (the tank), allowing the flashing ammonia to escape. The saturation vapor pressure of liquid ammonia at this temperature is 0.968 X 106 Pa, and its density is 603 kg/m3. Determine the mass flow rate through the leak. Equilibrium flashing conditions can be assumed. 

The first example was formulated by Stoecker to illustrate the steepest descent (gradient) direct search method. It is proposed to attach a vapor recondensation refrigeration system to lower the temperature, and consequently vapor pressure, of liquid ammonia stored in a steel pressure vessel, for this would permit thinner vessel walls. The tank cost saving must be traded off against the refrigeration and thermal insulation cost to find the temperature and insulation thickness minimizing the total annual cost. Stoecker showed the total cost to be the sum of insulation cost i = 400jc° 9 (x is the insulation thickness, in.), the vessel cost v = 1000 + 22(p — 14.7)1-2 (p is the absolute pressure, psia), and the recondensation cost r = 144(80 — t)/x (t is the temperature, °F). The pressure is related to the temperature by... 

The flask is charged with about 3 1. of liquid ammonia (Note 1), the stirrer is started, and a rapid stream of acetylene gas (about 5 bubbles per second) is passed in for about 5 minutes to saturate the ammonia. The acetylene from a tank is sufficiently purified by passage through a sulfuric acid wash bottle a safety trap also should be inserted in the line. Sodium (92 g., 4 gram atoms) is cut in strips (about by 3 by 3 in.) so that they can be inserted through the side neck of the flask. One of these pieces of sodium is attached to the fish-hook and is gradually lowered into the liquid ammonia while a rapid stream of acetylene is passed in. fl he sodium should be added at such a rate that the entire solution does not turn blue. If it does, the sodium should l)i raised above the level of the ammonia until the color is discharged (Note 2). I lie rest of the sodium is added in a similar... 

Some of the steps that can be taken to help minimize the impact of SCC in ammonia storage are Complete stress relief, operation without air contact and the addition of small amounts of water (0.2%) as an inhibitor. Low-temperature carbon steels have considerably more resistance to SCC than normal carbon steels. This makes them the preferred material of construction for large atmospheric liquid ammonia storage tanks that operate at -33°C88. 

It is important to note the special design aspects of this example. Metal detection plates are installed on all sides to prevent any momentum-dominated two-phase release from going through the water spray. A two-phase release will impact the dike walls or deflection plates, causing the liquid ammonia to coalesce and rain out inside the dike, so that only vapor (without momentum) will come in contact with the water spray through the opening between the deflection plates and the tank. 

The liquid ammonia is stored in the tank at a temperature of 20°C under its vapor pressure of 851 kPa. Under these conditions the discharge calculations show that all of the ammonia in the vessel is released to the... 

When the 37.5-mm diameter pipe fails, the liquid ammonia is released from the tank and forms an unconfined pool on the ground. Figure 7.15 shows the growth of the pool for both the D/5 and F/2 meteorological conditions. As was shown in the carbon disulfide example, the pool formed at the D/5 conditions is smaller than the one for the F/2 conditions. The reasons in this case are the same as those discussed in the carbon disulfide example (see Section 7.5). Vaporization rate as a function of time is given in Figure 7.16. 

As discussed in Section 3.1.2.1, a liquid that is uncontained is one over which there is no control and which will result in potentially severe consequences. If a dike is placed around the tank containing the refrigerated liquid ammonia and the ammonia spill is confined within it, a much reduced hazard zone can be obtained because we have limited the surface area available for vaporization and additional postrelease mitigation measures can be applied. As pointed out in Chapter 3, combinations of postrelease mitigation measures will provide the best overall response to an accidental release. 

In this scenario, a dike 8.4 m by 8.4 m having a height of 1.5 m is placed around the refrigerated ammonia storage tank. The accidental failure of the 37.5-mm line occurs inside the diked area. All of the liquid ammonia released from the tank is contained inside the dike. The rate of flow of the liquid ammonia into the dike is the same as for the refrigerated example (Section 7.7.2). 

In another tank collapse incident, a tank containing a solution of ammonia and water was being emptied and cleaned. After employees emptied the tank of liquid ammonia, they added water to rinse the vessel. While the water was being added the sides of the tank were sucked in. The ammonia vapor remaining in the tank dissolved in the water so rapidly that air could not enter through the vent to prevent the collapse. [8]... 

A high-pressure bomb of about 1.1-1. capacity is charged with 82 g. (0.50 mole) of sebaconitrile 1 and about 6 g. of Raney nickel catalyst2 (Note 1) suspended in 25 ml. of 95% ethanol, using an additional 25 ml. of ethanol to rinse in the catalyst. The bomb is closed (Note 2), and about 68 g. (4 moles) of liquid ammonia is introduced from a fared, 5-lb. commercial cylinder (Note 3). Hydrogen is then admitted at tank pressure (1500 lb.), and the temperature is raised to 125°. The reaction starts at about 90°... 

Ammonia is delivered in small containers, tank trucks, tank cars, barges, and via pipeline. The most common small containers are cylindrical steel bottles and pressurized flasks that contain about 20 to 200 kg and polyethylene canisters and metals casks.74 Trucks have ammonia capacities up to 100 m3 whereas jumbo rail cars hold up to 150 m3. Liquid ammonia shipments by barge constitute a larger volume than by road or rail. 

Ancillary equipment, designed for at least 2.5 MPa (25 bar) meters and flow controls for pressurized ammonia feed and effluent streams centrifugal pumps for discharging into liquid ammonia supply piping and for liquid ammonia loading equipment for safe pressure relief for ammonia vapor and inerts (see Fig. 118 and [1268]). Design of pressure storage tanks and the related safety aspects are discussed in [1270]. 

Further safety provisions include surrounding the tanks by dikes or placing them in concrete basins to contain the liquid ammonia in the event of a total failure. Discussions of such secondary containment are found in [1285], [1291]-[1293]. 

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