SAFETY AND ENVIRONMENTAL IMPACTS

SAFETY AND ENVIRONMENTAL FACTORS

Although light olefins are colorless gases with a mild odor that is non irritating to the eyes or respiratory system, these are hydrocarbons and therefore flammable. All vessels must be designed for handling the liquids and gases during operation at the temperature and pressure that exists, and safety and relief (depressurizing) valves must be provided to relive excessive pressure. Releasing the hydrocarbons in the air in large amounts must be avoided because of health and fire hazards. If hydrocarbons must be released in the air it is done under blanket of steam. To protect the plants and personnel in case of fire, a complete fire-fight system is provided with tanks grouped to minimize fire and provided with foam makers and deluge systems. An olefin plant produces liquid, gaseous, and solid wastes that must be disposed of in an environmentally safe manner. Liquid waste generated within the complex consists of waste water streams of relatively low organic content, and process waste of high organic contents. Wastewater from various units and operations are serrated according to the wastewater characteristics, such as type of contaminants, concentration, special treatment, or permanent requirements. A segregated sewer allows for the most efficient treatment.

Atmospheric emissions from the facility are either controlled or fugitive in nature. Controlled emissions are released from process venting, waste incineration, decoking operations, and heater firing. Fugitive emission may occur from product loading and storage and equipment and valve leaks.  Solid wastes are treated in solid waste disposal area to reduce their volume and/or toxicity prior to final disposal in a secure landfill. Combustible wastes can be incinerated in a slagging rotary kiln to reduce volume and toxicity.

Safety

The concern for operating safety asserts itself during the design of a plant. The design and specifications are critically reviewed while the plant is being erected and even after it is completed. In particular, this applies to the general layout in the area of controls and instrumentation. Plant safety in operations depends seriously on the quality and competence of the operating and support personnel.

All safety and operating reviewals are condensed eventually in the issues of operating manuals for the plant. These manuals lay particular stress on

·        The mode of operation at design and reduced loads;

·        Emergency procedures -applicable, for instance, in the case of a sudden outside power failure for the quick and safe release arid disposal of plant inventories of liquids and vapors;

·        The prevention of ignition through above-ground dissipation of flammable materials escaping from the damaged lines and apparatus by the use of steam, foam, and water sprays;

Find finally

·        In serious; incidents, localization of the cause of an incident and disaster prevention.

The subject of safety of olefins plants is rendered complicated by the extreme range of pressures and temperature encountered which extend from about 17 kPa to over 12 M Pa and from -160°C to over 1000°C.

ENVIRONMENTAL IMPACTS OF OLEFINS PLANT

The environmental pollution resulting from the operation of hot section of ethylene plant can be classified into three broad categories,

·        Air pollution

·        Water pollution

·        Noise pollution

AIR POLLUTION

The major source of air pollution in on ethylene plant in the stack gases coming out of the cracking furnace, Gaseous pollutants of environmental concern include carbon monoxide and oxides of nitrogen, I he high furnace operating temperature and a slight oxidizing atmosphere, preclude the formation of carbon monoxide in any significant quantity. Rndiant siclewoll burners are inherently low In the NO_x generation due to the uniformity of combustion temperature and absence of strong recirculation patterns In general, higher furnace temperatures, one! Larger burner capacity will tend to increase the NO_X level of a particular furnace.

Water Pollution

The liquid waste effluent from the hot section of ethylene plant include oily fractions from leaks, spills, tank draw off ; and other sources include waste caustic wash water, condensate waters from distillate separators and coke from equipment tubes, towers and other locations. The major source of water pollution is the spent caustic, solution from the caustic scrubbing column, whose disposal can greatly affect the economics of the plant.      

The main remedial measures for reducing oily wastes are,

·        Reduction of oil leakages by preventive maintenance of pipelines and equipments.

·        Preventing formation of emulsions or where they exist, isolation and separate treatment.

The spent caustic, containing reaction products (sodium carbonate and sodium sulfide) and some dissolved hydrocarbons is drained from the botto0m and sent to proper treatment for neutralization, hydrocarbon removal and stripping or oxidation of sulfide, before final dumping in the sewer.

Environmental

 Stringent environmental laws requires that nitrogen oxides (NO_x), and sulfur oxides emission from furnaces be drastically reduced. In many parts of the world, regulations requires NO_x be reduced to 70 vol ppm or lower. on a wet basis. Conventional burners usually produce 100 to 120 vol ppm of NO_x.

Since NO_x production depends on the flame temperature and quantity of excess air, achieving required limits may not be possible through burner design alone. Therefore many new designs, incorporate DENOX units that employ catalytic methods to reduce the NOx limit. Platinum containing monolithic catalysts are used. Each catalyst performs commonly for a specific temperature range, and most of them work properly on 400°C.