Environmental Aspects Petroleum Refinery: Environmental Aspects

Environmental Aspects Environmental Aspects of the Following Processes will be Highlighted Desalting Distillation Catalytic cracking – fixed bed & FCC Hydrocracking Hydrotreating Alkylation Catalytic reforming Solvent extraction Dewaxing Deasphalting

Environmental Aspects Main sources of air emission from petroleum refinery Process vent means a point of emission from a unit operation (distillation, condensers, reactors…etc) having a gaseous stream that is discharged to the atmosphere either directly or indirectly. Fugitive emissions are emissions of gases or vapors from pressurized equipment due to leaks and various other unintended or irregular releases of gases, and is not routed to a pollution control device. Flue gases bi-products of the combustion of fossil fuels used by heaters and furnaces. Incomplete combustion of these fuels can produce deadly CO.

Desalting Desalting: remove corrosive salts along with some of the metals and suspended solids Generates Oily sludge High temperature saltwater waste stream, send to wastewater plant

Distillation Distillation involves the heating, vaporization, fractionation, condensation and cooling of feedstocks Light end (methane and ethane): contains sour gas which is sent to a sour gas treatment system. The sulfur can be recovered in this process as elemental sulfur Significant air emissions due to combustion of fuels in the furnaces used to heat the crude, from process vents, etc. Oily wastewater can be generated during condensation

Catalytic cracking – fixed bed & FCC The catalytic cracking process produces coke as a byproduct, which collects onto the surfaces of the catalyst. This form of fouling reduces the effectiveness of the catalyst. The catalyst therefore needs to be regenerated continuously or on a frequent and periodic basis, which is accomplished by burning the coke off of the catalyst surfaces at high temperatures. Oil remaining on the catalyst is removed by applying steam stripping Catalytic cracking units are a main source of wastewater and air emissions. The air emission sources include the process heater flue gas, fugitive emissions, and regeneration of the catalyst. High concentrations of carbon monoxide (CO) and carbon dioxide (CO2) generated in the regenerator.

Catalytic cracking – fixed bed & FCC Significant amounts of fine catalyst dust (alumina and nickel) are generated in FCC as a result of attrition from the constant movement and collisions between catalyst particles. Wastewater containing metal impurities from the feed oil is also produced from the steam injection step used to purge and regenerate the catalyst.

Hydrocracking Catalysts are usually regenerated off-site after about two to four years of operation, meaning that few emissions are generated from the regeneration process. The primary air emission sources are then process heater, vents, and fugitive.

Hydrotreating Designed to remove specific impurities such as sulfur, nitrogen, oxygen, halides and metals Off-gas streams are generally rich in H2S and light fuel gas. They are sent to the sour gas treatment and sulfur recovery units. The air emissions sources from hydrotreating are the process heater flue gas, vents, and fugitive emissions

Alkylation Acid concentration of about 88% is used , some of the acid must be removed and replaced with a stronger acid. Wastewater is generated from steam strippers, depropanizers and debutanizers, and can be contaminated with oil and other impurities. In the sulfuric acid process, the acid removed must be regenerated in a sulfuric acid plant. The sulfuric acid plant is usually not part of the alkylation unit and most often is located off-site

Catalytic reforming The major reactions in this process are • Dehydrogenation of naphthenes to aromatics • Dehydrocyclization of paraffins to aromatics • Isomerization • Hydrocracking Air emission sources include process heater gas and fugitive emissions, with relatively small emissions coming from catalyst regeneration Wastewater containing sulfides, ammonia, and mercaptans are derived from the stripping tower used to remove light ends from the reactor effluent stream.

Solvent extraction Solvent extraction employs solvents to dissolve and remove aromatics from lube oil feed stocks to improve viscosity, enhance oxidation resistance, improve color, and reduce gum formation. Most common: furfural and phenol Solvent recovery from the oil stream is done through distillation and steam stripping in a fractionator. The stream extracted from the solvent generally contains high concentrations of hydrogen sulfide, aromatics, naphthenes, and other hydrocarbon constituents. These are usually fed to the hydrocracking unit. The water stream from the fractionator contains oils and solvents.

Dewaxing Lubricating oil base stocks are dewaxed to ensure that the product has the desired viscosity at lower ambient temperatures Solvents used for this purpose are methyl ethyl ketone (MEK) with methyl isobutyl ketone (MIBK) and MEK with toluene. The solvent is recovered from the oil and wax by the sequentially executed processes of heating, two-stage flashing, and steam stripping. The solvent recovery stage produces solvent contaminated in water as a waste. This stream is sent to the wastewater treatment plant. The wax can be reused. Air emission sources are fugitive and contain solvents.

Deasphalting Propane is used to remove asphaltenes due to its unique solvent properties. Propane is recovered from the two streams using a flash system followed by steam stripping. The recovered asphalt can be blended with other asphalts or heavy fuels or it can be used as feed to the coker. The propane recovery stage produces propane contaminated water, which is sent to the wastewater treatment plant. Air emissions are from fugitive sources and process vents.

Wastewater treatment Refinery wastewater treatment plants are integrated systems that use both primary and secondary treatment. Primary treatment: separation of oil, water, and solids in two stages. First stage: API oil-water separator. API: allow sufficient time for free oil to float to the surface and be skimmed off, while solids settle to the bottom of the unit and are removed by mechanical scraping and depositing of the sludge into a collection hopper. USES DIFFERENCE IN SPECIFIC GRAVITY

Oil water separator OIL WATER SEPARATOR

Primary treatment Second stage: relies on physical or chemical methods to separate emulsified oils from the wastewater: Physical methods are typically a series of settling ponds designed to have long retention times to allow settling, or they use dissolved air flotation Chemicals such as ferric hydroxide or aluminum hydroxide are used to coagulate impurities into a froth or sludge, which can be skimmed off the surface.

Dissolved air flotation unit Dissolved air flotation (DAF) is a water treatment process that clarifies wastewaters (or other waters) by the removal of suspended matter such as oil or solids. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank or basin. The released air forms tiny bubbles which adhere to the suspended matter causing the suspended matter to float to the surface of the water where it may then be removed by a skimming device. Dissolved air flotation is very widely used in treating the industrial wastewater effluents from oil refineries, petrochemical and chemical plants, natural gas processing plants and similar industrial facilities. wastewater

Settling pond for sedimentation A settling basin, settling pond or decant pond is a place where very fine particles from water are removed by means of gravity. The dirty water enters the basin at one end and the cleaner water is taken out at the other end by decanting. The water must be in the basin long enough for the desired particle size to be removed. Smaller particles require longer periods for removal and thus larger basins. Sometimes a flocculant may be added to help smaller particles stick together and form larger particles Also known as settling pond or decant pond. Sometimes uses flocculent