Goran B. G. Nyman, Arvid Tokerud
ABB Flakt
Oslo
ABB Flakt's seawater flue gas desulfurization unit in Statoil's refinery at Mongstad, Norway, has completed its first year of successful operation.
The refinery is one of the most advanced in Europe, with a high degree of cracking and bottoms processing capability (OGJ, Mar. 12, 1990, pp. 33-36).
Because seawater already contains 900 mg/l. sulfur as a natural constituent, seawater can be used for the absorption of sulfur dioxide (SO2) from flue gases and returned to the sea, without detrimental effects on the environment.
The process, called the Flakt-Hydro flue gas desulfurization process, is a joint development of ABB Flakt and Norsk Hydro A.S., Norway's largest industrial company.
REFINERY SULFUR EMISSIONS
SO2 emissions from the refining industry are modest compared to the power industry, for instance, a refinery may be a significant source of SO2 at the local level.
Refineries remove sulfur from the light product streams.
Fortunately, most of the sulfur is highly concentrated and can be recovered. But as markets demand less and less sulfur-containing fuel, the "bottom of the barrel" becomes increasingly more difficult to get rid of.
Burning the heavier oil residues is a common solution, but it results in SO2 emissions. The concentration of the SO2 in the offgases is too low to encourage recovery of the sulfur.
But flue gas desulfurization, commonly used in the power generating industry, may be a good solution.
When considering flue gas desulfurization, most people envisage huge stacks of limestone and gypsum. There are, however, other ways of capturing sulfur.
Scrubbing flue gases with seawater is a proven industrial desulfurization process. There are more than 5,500,000 actual cfm (acfm) of flue gases currently being cleaned Of SO2 by seawater scrubbing.
MONGSTAD SEAWATER SCRUBBER
The first seawater scrubber introduced at a refinery was installed in 1988 at the Mongstad refinery. The process heat exchangers are cooled by seawater. A part of the seawater is utilized for absorption Of SO2 in a once-through operation.
Fig. 1 shows a simplified flow diagram of the Mongstad seawater scrubber.
The scrubber is designed to treat 300,000 acfm of flue gas from a fluid catalytic cracker (FCC) and a flow of 25,000 acfm from a sulfur recovery unit (SRU). The flue gas from the FCC unit is conducted through CO boilers into an electrostatic precipitator, where catalyst fines from the FCC unit are removed.
The FCC flue gas is combined with the dust-free flue gas from the SRU, downstream of the precipitator. The gases enter two flue gas quenchers, arranged in parallel, at the gas inlets to the scrubber.
To protect the corrosion-resistant lining of the scrubber from heat deterioration, the quenchers cool the flue gas by injecting seawater through spray nozzles in a once-through operation.
The scrubber removes SO2 and SO3 by absorption, in a packed bed with countercurrent flow. The spent seawater flows by gravity to a mixing device, where it is combined with the rest of the spent cooling water. The mixture is then returned to the sea.
The seawater quality and the depth and water replenishment conditions at the exit point make a water treatment plant redundant at the Mongstad refinery.
The scrubbed flue gas passes one layer of mist eliminators and leaves the scrubber at the top. The treated gas is ducted to a mixing chamber in the bottom of the stack. Here the cold scrubbed gas is mixed with hot flue gas produced in an auxiliary burner.
The burner is fired with almost sulfur-free refinery fuel gas. This increase in the scrubbed flue gas temperature achieves plume dispersion and buoyance in the atmosphere. Fig. 2 shows the Mongstad flue gas scrubber.
The flue gas scrubber is designed to produce an hydraulic resistance less than the inherent overpressure after the CO boilers and sulfur incinerator. Therefore, a main booster fan is not required to convey the flue gas through the scrubber.
PROCESS DESCRIPTION
Seawater is suitable for absorption Of SO2 for two main reasons:
- Seawater is alkaline by nature because it contains bicarbonates, which act as a buffer upon the addition of acid.
- The absorbed SO2 is transformed into sulfate ions, a natural constituent of seawater.
The alkalinity (buffering capacity) of seawater is easily understood by comparing the effects of the addition of acid on seawater with the effects on fresh water. Fig. 3 shows why acid rain is an inland problem; fresh water has almost no buffering capacity.
The reactions taking place in the scrubber are illustrated by simplified Equations 1 and 2.
In Equation 1, SO2 is absorbed in water and reacts with oxygen to form sulfate ions (SO4 2-) and hydrogen ions (H+). An increase in the concentration of hydrogen ions results in lower pH.
In Equation 2, bicarbonate ions (HCO3-) from the seawater react with the hydrogen ions, thereby reducing the acidic effect of absorbed SO2-.
The quality and water replenishment conditions of the receiving waters will dictate the possible need for a seawater treatment plant. The effluent from the scrubber can be aerated to force the oxidation Of SO2 and to saturate the water with oxygen.
Because of the absorbed SO2, the pH value of the effluent will be lower than the pH value of the receiving waters. Because of the low pH, the CO2 has been stripped and the carbonate content reduced.
Low carbonate content means low buffering capacity, so the pH value of the effluent will quickly increase when blended with the receiving waters, which have high buffering capacity.
Aeration of the effluent will facilitate stripping Of CO2 and reduce the mixing zone affected by a reduced pH value.
FIRST YEAR OF OPERATION
The unit installed at the Mongstad refinery has been in operation since September 1989. The performance of the plant has been measured and the results are good, as can be seen from the following data:
- Removal efficiency, SO2 = 98.8%
- Removal efficiency, SO3 = 82.8%
An emergency stack is installed upstream of the electrostatic precipitator to enable continuous operation of the FCC unit during operational disturbances of the electrostatic precipitator or the scrubber unit.
Between January 1990 and January 1991, the unit was bypassed for only 98 hr. The unit was in operation 8,076 hr. The availability for this period is 8,076 - (98 + 8,076), or 98.8%.
It should be noted that the reasons for the shutdown have been corrected.
ENVIRONMENTAL CONSIDERATIONS
Utilization of seawater scrubbing introduces a discharge to the ocean. However, as supported by literature studies and bioassay testing of effluent, there are no significant effects from the discharge.
SULFUR
At first glance, the seawater scrubber appears to take the pollutant from the air only to transfer it to the sea. However, because sulfate is a natural ingredient in seawater, the absorption process may be considered a shortcut for depositing offgas sulfur into the sea, without allowing it to detour through the atmosphere and vegetation.
Without influence from human activity, there are over 3.5 million tons of sulfur in the atmosphere. This sulfur (Most Of it SO2) is in equilibrium with HSO3 and SO3 2- in the sea, as shown in Equation 3.
Sulfite (SO3 2-) is readily oxidized to sulfate (SO4 2-) and the oceans contain more than 1 trillion (1015) tons of sulfur, as sulfate. If it is imagined that all the sulfur in the sea were spread out as a layer, the total ocean area of the world would be covered by a 5 ft-thick layer of sulfur.
If all the sulfur in all the known oil and coal reserves were added to this layer, the thickness would only increase by the thickness of a sheet of paper.
The increase in sulfate concentration in the effluent seawater is marginal and well within natural variations.
WATER PH
As an environmental problem, low pH-value water will be limited to a zone close to the discharge point. This will occur only when the plant is running.
An effluent with a low pH value is quickly neutralized by seawater. Contrary to discharges of, for example, trace metals, it is impossible to demonstrate a "rest" of low pH in the recipient water when the discharge stops.
The U.S. Environmental Protection Agency (EPA) has promulgated water quality standards for coastal waters exposed to industrial discharges. The effect of a low-pH discharge on the recipient water will depend on the size of the area affected during plant operation.
EPA has therefore defined an initial mixing zone (IMZ). At the boundary of the IMZ, the pH value of the discharge shall not vary more than -0.2 units from the natural pH value. Initial mixing is defined to be complete when the momentum-induced velocity of the discharge ceases to produce significant mixing of the effluent.
The EPA water quality standards can be met by aerating the effluent prior to discharge or by increasing the velocity of the discharge.
At the Mongstad refinery, the effluent is simply mixed with the rest of the spent cooling water before being discharged through a submerged diffusor.
This diffusion process is sufficient to meet EPA standards for pH at the IMZ boundary.
DUST AND TRACE METALS
The dust leaving the precipitator upstream of the seawater scrubber contains trace metals-mostly nickel and vanadium. However, some of the dust will be caught in the scrubber.
Control of the release of the dust and trace metals to the ocean is a matter of proper sizing and selection of the dust collector upstream of the scrubber.
BENTHIC SURVEY
The University of Bergen, Norway, has done a benthic survey before and after the deployment of the seawater scrubber at Mongstad. Investigations were carried out in March 1989, and in March and October 1990.
The immediate area around the scrubber effluent discharge point was monitored by three stations. Samples of the bottom fauna and sediment were taken from each station and analyzed.
The major results were:
- No evidence of harmful impact on the marine bottom fauna at the seawater scrubber outlet was found. The bottom fauna was rich, with a moderate-to-high diversity and evenness in both sampling periods.
- The mean sulfate content had increased after startup of the unit. However, the content in the sediment was within natural variations.
- The concentrations of all metals were within natural variations.
PROCESS ADVANTAGES
When seawater is available, removal Of SO2 from refinery flue gases can easily be achieved by scrubbing the gases with seawater.
- The process is simple.
SO2 is washed out of the flue gas in a once-through operation, i.e., there are no clogging problems.
There are no critical levels or other process parameters to control.
The simplicity of design means highly reliable plants.
- No chemicals are required.
The process uses only seawater and air.
Operating costs are hence low.
- No land disposal is needed.
- The absorbed SO2 is converted to sulfate, a natural constituent of seawater.
- It is a safe choice.
The seawater effluent quality is tested and can be controlled by designing the plant to suit the site-specific conditions.
ABB Flakt has finished the basic engineering for a Flakt-Hydro seawater flue gas desulfurization unit for the Lagoven refinery at Amuay, Venezuela.
The unit will treat flue gases from two boilers burning flexicoke, and is preliminarily scheduled to be on-line during the second quarter of 1993.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
