In petroleum refineries, the Hydrotreating process mainly removes Sulfur from Naphtha, Kerosene, and Diesel to produce environment-friendly petroleum products in an oil refinery. On the other hand, during the operation of hydrotreating plants, numerous pollutants are produced which can be minimized by applying the best operational and engineering controls. Moreover, owing to the stringent environmental regulations air emissions, effluent, and solid waste control are becoming challenging for refineries. These pollution problems should be taken as an opportunity to increase profits by reducing, re-using, and producing valuable products from pollution products.
1. Emissions To Air
Air emissions from hydrotreating plants may arise from fired heater flue gases, flaring, fugitive emissions, catalyst regeneration, maintenance activities, etc.
a. Fired Heater
is the major source of emissions of CO, CO2, NOXs, SOXs, Volatile Organic Compounds, particulate matter, etc.
- Calculate and improve specific energy consumption per ton of the feed processed. Energy optimization is the best tool that can reduce fuel consummation and hence emissions rate.
- Reduce fuel utilization by using hot feed and recovering heat from the hot streams. In addition, measure the efficiency of heat exchangers and rectify any problem.
- Monitor and control draft, flame impingement, flame pattern, and equal heat distribution across the furnace.
- Ensure complete combustion in the furnace. Higher excess air will waste the heat while the low rate of air will allow the unburned fuel into the atmosphere.
- Use pre-heated fuel and air that would increase the burning efficacy of the furnace by about 5~10 %.
CO2 (Carbon Dioxide) is the major emission from the oil refinery. It is a greenhouse gas and is produced as a result of the combustion processes of refinery fuels.
- Higher heater efficiency will reduce the CO2 emission rate. Optimize excess airflow and ensure complete combustion. Control excess air at the designed rate.
- The use of only Oxygen rather than air will reduce CO2 and other pollutants. In addition, would require lesser fuel and less production of CO2.
- CO2 from the flue gases can also be recovered using the Amine treatment method. CO2 can be liquefied and sold due to its various industrial uses.
CO (Carbon Monoxide) is produced as a result of partial combustion.
- It can be reduced through the complete combustion of the fuel.
- High CO content shows energy loss because of incomplete fuel burning.
- Improperly calibrated analyzers and poorly mixed fuel and air may cause incomplete combustion.
SO2 (Sulfur Oxide) produces acid rain and particulate matter.
- The only source of SOx is fuel. It can be reduced by using clean and treated refinery fuel. Almost all the sulfur in the fuel is converted to SOx.
- If high sulfur fuel is used and a higher rate of SOx in the flue gases is produced then the wet scrubbing technique or any SOx control method can be applied to the flue gases which reduces SOx and particulates.
- Fuel oil normally contains Sulfur up to 2% on the other hand refinery fuel gas treated in the Amine unit contains less than 100 ppm.
NOX (NO-Nitric Oxide, NO2-Nitrogen Oxide) causes smog and acid rain, affects the tropospheric ozone, and results in global warming.
- The NOXs are produced during burning from the oxidation of nitrogen present in the fuel called fuel NOx and the Nitrogen in the air called thermal NOx.
- High temperature, residence time, and high oxygen concentration result in higher NOX.
- Oil burning normally leads to higher levels of NOX releases, due to high nitrogen content in the fuel (0.03 – 1 %).
- Replace the standard burners with the latest ultra low-NOx burners that produce 10~15 ppm NOXs as compared to standard gas burners which produce >100 ppm.
Particulate Matter is a serious concern due to its adverse health effects.
- It is produced in the furnace in the form of coke, soot, metals, nitrates, and sulfates.
- Optimized operation of the heater and the use of clean fuel gas will reduce the particulates.
- By reducing NOX, SOx, and VOCs, particulate matter can be reduced.
- Wet scrubbing can capture more than 90 % of particulates along with the removal of NOx and SOx.
- In addition, particulate matter at DHT units is produced during catalyst replacement. A vacuum cleaning machine can be applied to collect the catalyst dust.
b. Flaring In Oil Refinery
In an oil refinery, flaring is required for burning off purge gases and other releases like an emergency, pressure safety valves, startups, and shutdowns.
- The highly suggested solution is to install a flare gas recovery system in the flare header of an oil refinery. This will not only save the environment but also save millions of dollars of gases from the refinery.
- An effective maintenance and calibration plan of pressure safety valves can minimize the malfunction of PSV.
- During start-up utilize as minimum as possible makeup gas required for the desired pressurization rate. In addition, after the feed cut in route the bleed gases to the fuel header as early as possible.
- Avoid sudden level changes in the vessels that are blanked with fuel gas, otherwise more gases will flow toward the flare.
- In case of emergency due to system depressurization bulk flow of toxic gasses and liquid hydrocarbons are released to flare. Emergency situations can be minimized by adopting the best troubleshooting skills and training the plant staff.
c. Fugitive Emissions or Leakage
Emissions of gases or vapors due to leakages from pump seals, flanges, valves, piping, sampling points, etc. These can be minimized by monitoring, identifying, auditing, and rectifying the leakages.
- Installation of a good leakage detection system.
- Routine inspections of process equipment, flanges, pumps, valves, piping, compressors, etc. A portable detection meter can be used for leakage identification.
- The best-scheduled inspection and maintenance can reduce the risk of leakages. A leak survey audit can also help in the timely, identification of the leakages.
d. In Situ Catalyst Regeneration
In catalyst regeneration, Carbon and Sulfur accumulated on the catalyst are burned off.
- During this process, Coke, SO2, CO, CO2, particulate matter, and VOC (volatile organic compounds) are produced. The gases should be treated before discharging into the atmosphere. It is preferred to perform ex-situ catalyst regeneration.
e. Recycle Gas and Stripper Off Gases
Both of these gases contain a high concentration of H2S and Ammonia and are treated with Amine.
f. Releases During Turnaround
Petroleum Refinery process units are completely shut down for maintenance and repair after regular intervals. Many emissions including, H2S, VOC, particulate matter, hydrocarbon fumes, etc. are released. These pollutants can be minimized by;
- Recovering as many as possible liquids and transferring them to slop tanks for recycling.
- Hydrocarbon liquids from process lines and equipment should be drained to close the drain and recover later on.
- Liquids produced during steam out, water flushing, and Nitrogen purging along with hydrocarbons should also be drained to close the drain for recovery.
- Gases can be recovered if the flare gas recovery unit is alive otherwise have to burn in the flare.
2. Effluent Waste
Effluent is generated from the normal unit operation, process upsets, maintenance activities, rainwater, spillage, leaks, etc. It is better to minimize the wastewater production or Effluent treatment plant that must be efficient to treat and reuse the wastewater and recycle the value streams. The hydrotreating unit generates a flow of wastewater of 30~50 Liters per ton of Diesel processed. The wastewater contains Sulfur compounds, NH3, hydrocarbons, suspended solids, etc. from spillage leaks.
- Sour water from High-Pressure Separator and Stripper Receiver. Sour water is treated separately in Sour Water Treatment Section to remove contaminants, after treatment reuse this water in the process.
- Avoid over-injection of wash water in the system. For the stripper receiver, adjust the stripping steam flow at a minimum keeping in view the flashpoint and Copper Strip result of diesel product within the range.
- Water to oily water drainage system from the feed, product coalescers boot, and salt driers draining. Treat the water efficiently at a waste treatment plant to recover hydrocarbons. In time replacement of coalescer’s cartridge elements and salt loading in the drier will avoid the hydrocarbon draining to OWD.
- Using a steam re-boiler instead of living stripping steam to the Product Fractionation Column (Stripper) will stop sour water and effluent water from draining the product coalescer and salt drier.
- Diesel waste from sampling can be reduced by relying on online analyzers and adjusting sampling frequency on genuine requirements.
- Lubricating oils used in gearboxes of pumps or compressors can be sold out for recycling or reuse.
- Aqueous Neutralization soda (Na2Co3, 2~5 %) used for the austenitic stainless steel process lines and vessels can be reused or apply recommended practices for safe disposal.
- Leakage from pumps, valves, flanges, etc. can be reduced through regular inspections and maintenance.
- Implement spill response plans for immediate cleanup and recovery if possible.
- Ensure maximum recovery of process liquids by draining them into the closed drain or slop tanks and avoid massive discharge of process liquids into the oily water drainage system.
- Hydrocarbons contaminated water from scheduled cleaning or handover activities for maintenance and hydrocarbon-containing effluents from process leaks should be treated in the wastewater treatment plant.
3. Solid Wastes
- Mechanical wastes like old equipment, valves, or pipes are produced during the replacement of the revamping of the unit. This waste can be minimized by reusing the equipment at other locations. In addition, during revamping avoid unnecessary replacements.
- Waste generated from maintenance activities like damaged parts or replacement of disposable parts like coalescing elements, filter cartridges, etc. Avoid the use of disposable parts, if possible use parts that can be reused.
- Semi-solid sludge is normally removed from the process vessels during maintenance work in an annual turnaround. There must be a process in the oil refinery to treat the sludge and recover the valuables or can be sold to the contractor for treatment or use. Landfilling must be avoided.
- Metals particles due to corrosion in the process may result in the production of tons of solids in a year. Corrosion waste is collected at the reactor catalyst and high-pressure separator. The corrosion rate can be minimized by selecting the best available material for construction or avoiding a corrosive environment.
- Spent catalysts generated 50~200 MT/year depending upon the plant capacity. Spent catalysts may contain molybdenum, nickel, cobalt, platinum, palladium, vanadium iron, copper and silica, or alumina. Reuse the catalyst as many times as possible after regeneration or rejuvenation.