NEW CHEMICAL REDUCES COKE DUST EMISSIONS AT MISSISSIPPI REFINERY

Robert P. Bennett
Benetech Inc.
Aurora, Ill.

H.J. (Jim) Blevins, H.J. (Joe) Cuevas
Chevron U.S.A. Inc.
Pascagoula, Miss.

A new chemical dust-suppression system designed specifically for use with petroleum coke is reducing emissions and maintenance requirements at Chevron U.S.A. Inc.'s Pascagoula, Miss., refinery.

Dust levels have been reduced 90-95% and workers no longer are required to wear personal respirators inside the conveyor galleries. Routine cleanup requirements for the coke conveyors have been reduced, as have maintenance needs within the coke-handling facilities.

FUGITIVE DUST

As an emissions source, fugitive dust is not as well publicized or restricted as other environmental concerns, such as ground water runoff and air quality control. Fugitive dust is, however, closely watched by many environmental groups, regulatory agencies, and, in areas where problems exist, the general public.

Fugitive dust is considered one of the greatest environmental concerns for industrial facilities that handle or produce friable bulk solids. This type of emission presents several general problems:

• Dust becomes an environmental issue if carried off site as air-blown or water-blown particles.

• Dust on site is a concern for workers' health and affects general working conditions.

• Machinery requires additional maintenance to operate in a dust-filled environment.

• Cleanup of work areas requires additional time and energy.

CHEVRON SYSTEM

The refinery has a coker that was constructed in 1981. By 1983, the coker was producing about 4,000 tons/day of coke. This coke is transferred from the coke pit to the coke yard along about 10,500 ft of enclosed conveyor, and through several transfer stations, before "stack-out" (Fig. 1). ("Stackout" refers to the loading of coke into piles.)

The coke piles are contained in a 10-acre storage yard, and may remain in the yard for as long as 45 days. Because the facility is situated near one of the Gulf Coast's most prominent and oldest towns, fugitive dust control is a major consideration.

The initial system installed to control fugitive dust incorporated chemical dust suppression and elevated sprays for the storage yard. The chemicals, however, were designed to control fugitive dusting from other bulk products, such as coal, lignite, or limestone, but there were no products specifically designed for petroleum coke.

This system produced some favorable results, but Chevron wanted better control for this sensitive location.

NEW CHEMICAL

Benetech Inc. installed a coke dust-suppression system at Chevron's Pascagoula, Miss., refinery, in response to environmental concerns.

A new chemical, called "GDS-12," was developed specifically for use with petroleum coke. Natec Inc. started this research project in May 1991, after which its chemical division was acquired by Benetech in May 1993.

In 1991, in conjunction with Chevron's blending and shipping group, Benetech (as Natec) began to formulate a dust-suppressant product particularly for use with petroleum coke. The chemical needed to:

• Provide immediate and superior dust control

• Have a residual effect of at least 45 days

• Be economical, so that every ton of coke could be treated

• Be environmentally acceptable

• Keep moisture addition to a minimum.

Benetech's technical staff and chemists researched the properties of petroleum coke. They found that coke is extremely hydrophobic, contains metals such as vanadium, iron, copper and nickel, can have a very low pH, and is extremely friable.

It was determined that the difficulty of controlling coke dust is caused by two main factors: the particles' small size and ability to repel water.

The very fine particles are easily carried by the wind, and any type of crushing, dumping, conveying, stacking, or reclaiming of the coke causes the particles to become airborne. These extremely small particles tend to streamline around large water droplets, so a fine mist is required to wet the particles. In addition, the surface of coke particles is a mixture of organic and inorganic materials; the organic materials will repel water, making it hard to coat the surface.

Traditionally, water sprays have been used to control coke dust. (Water increases the particles' weight and acts as a binding agent between particles.) These systems require almost constant spraying to overcome the hydrophobic nature of petroleum coke.

Water-spray systems were improved with the addition of surfactants. Surfactant reduces surface tension and improves droplet atomization, which, in turn, improves water's wetting ability. These surfactant systems, however, still required spraying large quantities of water.

Benetech's research led to a new formula. The result was GDS-12, a combination of three components: an organic polymer family with humectant properties, a blend of surfactant additives, and organic polymer binders.

The surfactant blend demonstrated superior wetting capabilities. The addition of a binding agent and a humectant increased the chemicals' effectiveness. The humectant allowed the chemical to maintain an effective binder coating on the coke surface for an extended period.

CHEVRON STUDY

A field study was proposed to develop and implement improvements to Chevron's fugitive dust control methods and replace the current technology with GDS-12. This study was designed to allow Chevron to return to the original system if problems occurred, so that coke production would not be interrupted.

Chevron agreed to a 90-day trial period.

Benetech replaced the existing chemical technology, which consisted of a lignosulfonate-based spray and supplementary water sprays. This system applied the lignosulfonate at Transfer Tower No. 6 (Fig. 2). The coke yard received supplementary water spray and additional surfactant.

ORIGINAL SYSTEM

Some characteristics of Chevron's original dust-suppression system were:

• The use of surfactant and water sprays were required hourly-Petroleum coke is a porous material and retains a substantial amount of water. This not only decreases its BTU value, but increases its weight substantially. This weight increase boosted shipping costs.

• Pile height was limited to 35 ft because of loss of pile integrity and wind erosion - Pile integrity was extremely poor; the problem was compounded during rainy weather. This led to pile slippage and runoff onto the stacker and reclaimer tracks. The capacity of the storage area also was limited by the pile height.

• Large amounts of fugitive dust accumulated on equipment in the enclosed transfer and conveyor areas-Maintenance and cleanup were necessary in these areas to maintain conveyor operation. Workers inside the enclosed conveyor areas were required to wear respirators.

• Work areas surrounding the stack-out area needed substantial cleanup-This included cleaning the control room and nearby offices that became coated with coke dust. Reduced dust levels improved working conditions and reduced cleanup expenses.

• Intermittent spraying was needed to control windblown dust on the roads and access areas surrounding the coke pile-A spray trailer manually sprayed these areas.

DEMO TRIAL

In May 1991, Benetech began a 90-day demonstration trial at the Chevron refinery. The test system included GDS-12 application at Transfer Tower No. 6 and GDS-08 foam application at ship load-out. This was consistent with the previous system applications, using liquid foam at the same treatment location but substituting Benetech's formulated foam.

The installation of GDS-12 treatment facilities required the addition of a storage tank, a shed for the necessary pumping equipment, and spray nozzles. The pumping system is illustrated in Fig. 3, and the pumping shed is pictured in Fig. 4.

The major objectives of this demonstration period were:

• To provide superior dust suppression quickly

• To determine both the most effective location for application and the optimum feed rate for maintaining high performance with a residual effect of at least 45 days, at a low cost

• To optimize the spray patterns to obtain an economical means of complete coke coverage

• To reduce dust levels to below OSHA levels

• To keep moisture addition to a minimum.

The demonstration period was divided into a 60-day period for determining the optimum GDS-12 feed rate, followed by a 30-day period of operation at the optimum feed rate.

To enable direct comparison of the original system and GDS-12, the same application point was used for GDS-12. The supplemental water-spray system was left in place.

Before the test period began, Benetech inspected the application point to determine the most effective means of application. The spray was to be applied from two directions-above and beneath the coke flow-using one spray header. This setup would coat all sides of the coke.

The first step in testing was to stack-out the entire yard with coke treated with GDS-12. This primary stackout was performed while applying 0.1 gal/ton of GDS-12 over a 3-week period.

Over the next 5 weeks, the chemical application rate was reduced consecutively to determine the lowest effective rate. The supplementary GDS-08 foam application at ship load-out was optimized in the same manner.

MONITORING

Two methods were used to monitor dust levels: a RAM-1 monitor with strip recorder, and visual inspection. Moisture tests also were recorded on the coke piles.

The RAM-1 monitor detects dust-level concentrations in mg/cu m. The RAM-1 monitor was used to compare the different application rates, and to compare treated and nontreated coke. The results were not used to quantify dust levels.

After each successive reduction of the GDS-12 rate, the RAM-1 monitor was used to measured dust concentrations in three different locations: at ground level in Transfer Tower No. 6, at a ship load-out, and in the coke yard (used as a portable monitor).

Routine visual inspections are considered the most accurate and reliable means of monitoring outdoor fugitive dust. The coke piles in the storage yard were the focus of these inspections. Pile moisture also was recorded during the visual inspections.

RESULTS

The initial stack-out of the entire coke yard, using 0.1 gal/ton GDS-12, was successful. There was no visual evidence of dust production.

The application of GDS-12 formed a thin crust on the piled coke. The crust maintained pile integrity and minimized fugitive coke dust.

During the initial stackout of the coke yard, two additional tests were discussed. One involved stacking out the coke piles to 45 ft instead of the normal 35 ft. The second test involved the replacement of GDS-08 foam with GDS-12, so that only one chemical would be necessary.

Over the next 30 days, the application rate was consecutively reduced from 0.1 gal/ton to 0.075, to 0.065, and finally to 0.05 gal/ton. The results at these rates were excellent.

The use of water spray in the coke yard was reduced from hourly to once every 4 hr. There also was an 80% reduction in the total amount of chemical used, compared to the original system.

For the final 30 days of the testing period, the application rate was held constant at 0.065 gal/ton. This small increase over the minimum rate of 0.05 gal/ton was used as a buffer. Also, the two additional tests-stacking coke to 45 ft and replacing GDS-08-began.

A test pile was stacked out to 45 ft, the top 10 ft of which was treated at a greater application rate. As indicated previously, GDS-12 produced a thin crust on the outer layer of the pile. This prevented any additional wind erosion or slippage because of the increased pile height.

Further testing was done. It was determined that the same results could be accomplished at the normal application rate of 0.065 gal/ton.

At one point during testing, heavy rains were encountered, during which 5 in. fell within 3 hr. The outer crust formed by the GDS-12 maintained the pile integrity, even during this storm. In fact, pile integrity was maintained for 45 days.

The importance of these tests was that the results provided additional storage capacity for Chevron. For example, a coke pile that is 75 ft wide and 200 ft long increases storage capacity by about 115,500 cu ft of coke, or 2,450 tons. This corresponds to a 30% increase in the coke yard storage capacity.

Another benefit of greater pile integrity was the elimination of the machinery and manpower required to uncover the reclaimer/stacker tracks after pile slippage. Because the pile developed a thin crust that eliminated wind erosion, the spray trailer also was eliminated.

The roads and access areas no longer need to be sprayed, as the dust levels were reduced in these areas by 95%. The dust levels within the control room, offices, and equipment racks next to the stack-out area were reduced by more than 90%. This significantly reduced cleanup time and expenses.

Because of the outstanding performance of Benetech's GDS-12 during the 90-day trial period, Chevron has continued using it as its primary dust-control method.

SYSTEM ADJUSTMENT

Further improvements have been made by moving the application point further upstream, from Transfer Tower No. 6 to Tower No. 2 (Fig. 5). This change produced several immediate advantages:

• Dust levels in the enclosed conveyors were reduced significantly.

• Dust levels were reduced to below OSHA levels so that workers no longer are required to wear personal respirators inside the conveyor galleries.

• Routine cleanup requirements for conveyors were decreased.

  Additional benefits soon were realized as a result of the large reduction in cleanup and maintenance requirements. The improved working conditions were commented on by health and safety audit personnel, and, more importantly, Chevron's coke yard workers responded positively.

Copyright 1994 Oil & Gas Journal. All Rights Reserved.

Issue date: 12/26/94