Control VOC`s in Refinery Wastewater
Oil and water do not mix, like many of life’s one-liners this statement is basically true but not the whole story. Many hydrocarbon liquids, particularly aromatics, have significant solubilities in water:
Benzene 1800 PPMV
Ethyl Benzene 150
Petroleum refineries do not like salts in their feedstock since these corrode and foul process equipment. The first refining step is Desalting where a hot water wash extracts the salts. If feedstock contains aromatics then some will be in the desalter effluent and this is a major source of refinery wastewater containing VOCs.
Usually the desalter is the major source of contaminated process wastewater and typically also has the highest BTEX content. At several refineries the desalter effluent flow has been as high as 50% of the total wastewater flow and over 70% of total BTEX discharge.
The environmental community is concerned about releases of VOCs and HAPs (Hazardous Air Pollutants) to rivers and streams, to groundwater sources, as well as to the atmosphere. Since aromatics, such as benzene, are considered potential carcinogens, they have received considerable regulatory attention, and are classed as HAPs as well as VOCs. The National Emission Standards for Hazardous Air Pollutants (NESHAPs) require that discharges containing more than ten metric tons per year of a HAP, such as benzene, are subject to regulation - that’s an average of only 2.5 lbs/hr: above this threshold stringent levels of control are required. If other HAPs are also present then these also have to be controlled. For benzene discharges regulators require control device efficiencies exceeding 99%.
Other processing units are also sources of aromatics in the process wastewaters. Chemicals units producing aromatics being prime examples. Aromatics are totally soluble in other hydrocarbons and only partially soluble in water. Typical benzene in water levels are 20 PPMW to 200 PPMW, and dependent on feedstock other aromatics may be present in similar amounts.
The main effluent treatment facility often includes an activated sludge unit where bio-degradation converts the final traces of aromatics and other HAPs in the wastewater to carbon dioxide and water.
NESHAPs do not permit open process drains since HAPs could evaporate into the atmosphere prior to reaching the wastewater treatment facility. So it is necessary to provide separate closed drainage systems for HAPs contaminated wastewater. As enclosure of drainage systems is extremely expensive the HAP treatment unit is often located adjacent to the HAPs source.
Just as refineries vary in size so do HAP contaminated process wastewater flows: from 100 GPM at a small refinery to over 3,000 GPM at a large complex. A 500 GPM flow containing 50 PPMW benzene is an annual benzene discharge of 54 tons and therefore subject to regulation.
There are several techniques available to prevent or control HAPs and VOCs in wastewater discharges: these are described and evaluated below:
- Desalter Emulsion Breaker: The desalter water wash produces an emulsion that holds more benzene and other aromatics than water and if the emulsion is discharged with the washwater it increases the aromatics discharge. Desalters use heat, electrical fields and demulsifiers to minimize the emulsion. Dependent upon feedstock chemistry it can be advantageous to increase demulsifier usage or change demulsifiers to reduce the amount of emulsion discharge. One recent trial reported that changing demulsifier reduced benzene discharge by 50%.
- Activated Carbon: Direct treatment of the wastewater with activated carbon reduces aromatics content to below acceptable limits. In addition, the carbon also captures oil, grease and other organics. Working capacity of carbon in the liquid phase is about 5% of carbon weight - the spent carbon is returned to the carbon factory for high temperature kiln regeneration and reuse. Although effective, the operating costs are high. One study found that to treat 500 GPM of wastewater entailed $250,000 capital cost and annual operating costs exceeding $1,200,000 (freight to/from kiln, kiln fuel, carbon make-up, etc.).
Steam Stripping: Bringing wastewater to the boil by live steam injection effectively strips volatiles such that discharge contains less than 0.5 PPMW aromatics. If overheads condensate comprises equal amounts of aromatics and water they will phase separate: with over 95% of the hydrocarbons in upper phase which recycles to the refinery feedstock. Aqueous phase, with solubility levels of organics, recycles to stripper for cleanup.
- Steam stripping has several concerns: fouling of equipment with oil/grease: fouling of packing with salts, particularly those that precipitate at stripper operating temperatures: energy consumption, even with 75% heat recovery a 500 GPM unit requires 25,000 lbs/hr of steam: capital cost is substantial since stripping column diameter exceeds 10 feet.
- Air Stripping: Stripping wastewater with air is very effective and readily reduces total BTEX to less than the required 0.5 PPMW. Air stripping is best at around 100°F. As temperature drops packing height increases - at 60°F required packing height doubles to attain same discharge. Typical stripping air discharges contain 500 to 3000 PPMV aromatics and environmental regulations require aromatics capture before air is discharged. The VOC laden stripping air is passed through vapor phase carbon which retains the organics allowing cleansed air discharge. Note that, in the vapor phase, carbon holds several times the quantity of VOC held by liquid phase carbon. There are two carbon options: off-site or on-site regeneration. Off-site regeneration entails shipping the VOC laden carbon to a kiln for high temperature regeneration. On-site regeneration entails live steam desorption of the carbon - usually a bed requires steamout once a shift.
Areas of concern for air stripping are: safety, since in refinery upset conditions large quantities of hydrocarbons may get into the wastewater resulting in explosive conditions in air stripper and vapor phase treatment unit: fouling of air stripper packing with oil/grease: fouling of packing with compounds that precipitate, particularly those that react with oxygen: fouling of carbon by hydrogen sulfide, note that in an oxygen free situation carbon has very limited capacity for hydrogen sulfide, however, presence of oxygen enables chemisorption onto the carbon as elemental sulfur that fouls the adsorption pores thereby decreasing capacity for aromatics and other VOCs.
An improvement has been developed that utilizes the advantages of air stripping and addresses the concerns listed above. The improvement was conceived and patented by Texaco who worked with the carbon adsorption systems engineers of AMCEC to develop full scale units which AMCEC provides on an exclusive worldwide basis.
Nitrogen is used as stripping gas thus inerting the process. Since oxygen is not present the safety issue is answered. Lack of oxygen (typically now well below 1%) inhibits many concerns about salt and other foulants precipitating on to packing, particularly biological slime formation. Also lack of oxygen reduces chemisorption of hydrogen sulfide onto the carbon thereby extending its working life. Obviously nitrogen is expensive (a 500 GPM wastewater flow requires a stripper gas flow rate of about 2,000 SCFM) therefore, the cleansed gas from the carbon beds is recycled to the stripper.
On-site regeneration was selected. Live steam is used for regeneration in such a manner that carbon vessels (adsorbers) are not isolated from the nitrogen stripping loop with the subsequent need to purge with nitrogen after each steamout to ensure that oxygen is not present. On-site regeneration also avoids frequent transportation of spent carbon to the kiln for reactivation.
Since system is a Recovery process it is considered a process unit and therefore does not require the stringent permitting associated with hazardous waste units. Thus a win-win process has been developed.