Beating BTEX
Methods of refinery wastewater BTEX control and improvements made.

By Mike Worrall

Reprinted from Hydrocarbon Engineering, November 2007

Benzene, Toluene, Ethyl Benzene and Xylenes (BTEX) are the most water soluble Hydrocarbon Carbons processed by a typical refinery. Consequently, they are to be found in refinery wastewater.

Table 1. Water solubilities
Hydrocarbon Level, ppm
Benzene 1800
Toluene 470
Ethyl Benzene 150
Xylenes 150

Society is concerned about releases of VOCs (Volatile Organic Compounds) and HAPs (Hazardous Air Pollutants) to rivers and streams, to groundwater sources, as well as to the atmosphere. Since BTEX, particularly benzene, are considered potential carcinogens, they have received considerable regulatory attention, and in the USA are classed as HAPs as well as VOCs. The US 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 small quantities of several HAPs are present, and their total exceeds 10 tons, this necessitates the need for control. For benzene discharges, regulators require high control device efficiencies.

Aromatics are totally soluble in other hydrocarbons, and only partially soluble in water. Therefore, partition coefficients have an impact. In a typical refinery wastewater, the benzene in water levels are 20 to 200 ppm, and dependent on crude feedstock other aromatics may be present in similar amounts.

API separators, dissolved air flotation units, etc. will have removed oil/grease, solids, etc. before water soluble HCs can be removed.

NESHAPs, however, do not permit extensive open process drainage systems, 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, a treatment unit is often located adjacent to major HAP sources.

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 ppm benzene and other aromatics is an annual BTEX discharge of 54 metric tons and therefore subject to regulation.

Several techniques are available to control HAPs and VOCs in wastewater discharges. These are described and evaluated below.

Activated Carbon (liquid phase)

Percolating the wastewater through beds of activated carbon, which capture the BTEX, 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 $400,000 capital cost and annual operating costs exceeding $1,5 million (carbon removal, 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 the discharge water contains less than 0.5 ppm aromatics. If overhead 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 steam 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 7000 lbs/hr of steam).
  • Capital cost is substantial since stripping column is of large diameter.


Gas Stripping

Stripping wastewater with gases, such as air, is very effective and readily reduces total BTEX in the water to less than the required 0.5 ppm. Gas stripping is best at around 100oF. A lower temperature increases required packing height. At 60oF required packing height doubles to attain same discharge. Typical stripping gas discharges contain 500 to 2500 ppm aromatics and, environmental regulations require control of aromatic vapors before gas is discharged.

When air is the stripping gas, the stripper and treatment unit operate in an open loop with cleansed air discharging to atmosphere.

The BTEX rich gas is controlled either by oxidization (destruction to CO2 and water) or adsorption onto vapor phase activated carbon.

Oxidation

Oxidation is an effective control method, however, requires heating BTEX rich air to over 1500oF which, even with 75% heat recovery, entails considerable fuel costs. And, with today's interest in "green", it should be noted that the carbon footprint of oxidization is many times that of in-situ regenerated vapor phase carbon adsorption.

Adsorption

The BTEX laden stripping gas is passed through vapor phase carbon which retains the organics allowing cleansed gas discharge. Note that, in the vapor phase, carbon holds several times the quantity of BTEX and other VOCs held by carbon in the liquid phase. Adsorption is temperature sensitive process, and carbon rapidly loses working capacity above 120oF. Moisture content of the VOC Laden Gas (VOCLG) is important as will be seen from the water adsorption isotherm below (Figure 1):

Figure 1. Water Adsorption Isotherm

With RH levels below 35% carbon has a slight affinity for water vapor. Above 65% RH the capacity to hold water increases tenfold and water vapor is attracted to the same adsorption sites in the carbon pores as organic compounds. Because of the vast difference in heat of adsorption between water and organics (1350 compared to 200 BTU/lb), it is not possible to displace water with an organic unless substantial external energy is provided. Thus with high RH conditions carbon may become "water logged" and unable to capture organics.

For BTEX vapors, this issue is resolved by cooling the VOCLG to about 90oF which condenses much of the water vapor. After demisting, VOCLG is heated to 115oF, which lowers RH to 40%. Thus VOCLG enters carbon at a suitable temperature and RH for effective adsorption.


Carbon options

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. Please refer to Figure 2.

Figure 2. Open loop air stripper /carbon bed

To provide continuous 24/7 operation, system will be comprised of 2 or more adsorbers. One adsorbing while other is off-line for regeneration.

In-situ regeneration utilizes live steam to heat the carbon, this releases BTEX vapors, which the steam carries to a condenser. The process condensate gravity separates into organic and aqueous phases. Organics return to the refinery. Aqueous phase recycles to inlet of air stripper.

Areas of concern for gas stripping in a refinery situation are:

  • Safety, particularly if air is used, 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 from the wastewater: 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 by Texaco who worked with the carbon adsorption systems engineers of AMCEC who developed full scale units we call BRUs (Benzene Recovery Units). Please refer to Figure 3.

Figure 3. Closed loop nitrogen stripper/carbon beds

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 onto 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.

In-situ regeneration uses live steam 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. Also, In-situ regeneration avoids frequent unloading and transportation of spent carbon to the kiln for reactivation.


BRU case study

  • Wastewater Flow: 500 gpm
  • Temperature: 90 to 110oF
  • BTEX: 100 ppm
  • Required BTEX removal: to < 0.5 ppm

HAP contaminated wastewater from an API separator is pumped into stripping tower and descends through 20 feet of high efficiency packing - 2.5 in. diameter polyethylene open type spheres. The stripped water discharges with < 0.5 ppm BTEX so meeting the water discharge regulations.

Nitrogen circulates at 2000 scfm so that BTEX concentration at exit from stripper is about 950 ppmv. Nitrogen temperature equalizes to that of incoming wastewater and the nitrogen exits stripper at 100% relative humidity (RH). As discussed previously, for effective vapor phase carbon adsorption the VOCLG must be at less than 50% RH and as cool as practical. To satisfy this requirement when handling BTEX vapors, the VOCLG is cooled to less than 90oF, demisted and then heated to 110oF.

Two carbon beds are provided - each bed sized to adsorb for an eight hour shift at full incoming BTEX load. The carbon captures more than 98% of the BTEX so the nitrogen stripping gas recycled to the stripper contains less than 20 ppmv. Stripper is sized to operate with 50 ppmv BTEX in the nitrogen stripping gas, so the 20 ppmv does not upset stripping performance. While one carbon adsorber captures BTEX the other is being counter-flow live steam regenerated (desorbed) or is in standby mode.

Desorption steam displaces the nitrogen in the adsorber being regenerated pushing it through the condenser into the main nitrogen stripping gas loop - raising loop pressure from 1 psig to 5 psig which does not impact stripper nor adsorber performance and avoids loss of nitrogen. Steam slowly heats the carbon bed to about 230oF releasing much of the adsorbed BTEX. The steam/BTEX vapors flow to a heat exchanger for condensation and cooling. Provided steam flow is within the condenser design capability and coolant flow there is no passage of steam or BTEX vapor past condenser and into main nitrogen stripping gas loop.

Condensed steam and BTEX are gravity separated into organic and aqueous liquid phases. Organic layer is pumped to refinery feed. The small quantity of aqueous phase, containing about 1000 ppmw organics, is pumped into wastewater feed entering the stripper.

At completion of desorption, the steam flow ceases and as steam in desorbing vessel cools, nitrogen is drawn back through the condenser from the main nitrogen stripping gas circulation loop. This reduces loop pressure back to 1 psig. Desorbed vessel is then cooled by a slip stream flow of nitrogen from main loop, after 30 minutes bed is sufficiently cool for return to adsorption service.

Steam consumption is around 1000 lbs/hr. Electric power, including that for the air cooled dehumidification and desorption condensing units, but excluding wastewater pumps, is about 50 Kw. Nitrogen consumption is about 5 scfm which is mainly used by the blower shaft seals.

Today, the capital cost for equipment and controls for this Case Study, built to refinery standards, but excluding foundations, installation, pumping systems, etc., is about $2,000,000. (Figure 4.)

Figure 4. Refinery BRU unit

There are about a dozen BRUs operating at various US refineries. Operating experience has been good with little system downtime. Carbon beds (each 5,000 lbs) are replaced every twelve months, as pores are fouled by higher boiling compounds.

Recently, we have evaluated a number of other applications for VOC/HAP control for the petroleum industry, two of which are discussed briefly below.


Tanker ballast water

Several facilities load crude oil into ocean tankers in environmentally sensitive locations. After crude is unloaded at the refinery, sufficient water is loaded (as ballast) to ensure vessel stability in the return sea journey to the crude source. This ballast water often picks up 10 to 20 ppm of BTEX from the crude compartment walls of the vessel during the ocean voyage.

Upon reaching the crude loading terminal, the contaminated ballast water is offloaded and, after treatment, will be discharged to the ocean.

The ballast water collects in an equalization tank from which a steady flow is pumped through an API separator for oil/grease/solids removal. For a large terminal, the equalized water flow is in the 1000 to 2000 gpm range: at 10 ppmw BTEX, that's 125 to 250 lbs/day (20 to 50 tons per year).

BTEX contaminated water is gas stripped, using either air or nitrogen, to extract the aromatic vapors. The BTEX rich gas passes through beds of activated carbon, with insitu live steam desorption, in similar manner as described previously.

Product tanker venting

There are a number of facilities where ocean tankers are loaded at a refinery with large quantities of BTEX liquids and or gasoline.

The gas/air in the storage compartments (head space) has BTEX/gasoline vapors at their vapor pressure; i.e. as much as 25% of the head space volume will be hydrocarbon vapors.

As the tanker is loaded, the head space gas/air is displaced and must be controlled before discharge to atmosphere.

Vapor phase carbon adsorption with insitu steam regeneration is used as the control technique, thus avoiding oxidation or flares.


Conclusion

From the above examples, it will be appreciated that vapor phase carbon adsorption is a reliable and highly effective means to control HAP/VOC emissions in the petroleum industries.