Performance Update: Low Pressure Wet Air Oxidation Unit at Grangemouth, Scotland Roger Matthews

BP Chemicals Ltd., P.O. Box 21, Bo’ness Road, Grangemouth, Scotland FK3 9XH

In response to a requirement to provide a low-capital, low operating cost solution for the treatment of steam cracker spent caustic, BP Chemicals Ltd. (BPCL) and Stone and Webster Engineering Ltd. tSWElJ have jointlly developed a Low Pressure Wet Air Oxidation Process.

The first of these units has been in operation at the BPCL Grangemouth, Scotland facility for the last three years. In that time, it has demonstrated totally effective suode removal, sutpassing design targets, and produc- ing a neutralized effluent well within the allowable dis- charge consents for the site.

m e Grangemouth unit is designed to treat the spent caustic @fluent from two crackers with a combined ca- pacit?/ of 600 KTA (current(y being expanded to 700 KTA).from both liquid and gas, feedstock.$. A sign@ant feature of the installation has been the extreme& high level qf reliability and availability, ensuring troub1e;free operation and full compliance with effluent consenl levels mandated by the regulatory authorities. At the same time, the unit has demonstrated very low opemt- ing and maintenance costs.

me design basis and initial commercial experience was presented at the AIChE Spring National Meeting in April 1994 [l]. mbis paper concentrates on the opera- tional history of the Grangemouth unit, highlighting the process developments and improvements that have been made in the last three years.

BACKGROUND

As part of the specification for the BPCL KG Ethylene Plant, the design contractor (Stone and Webster Engineer- ing 1,td.) were required to evaluate all available technolo- gies for the treatment of spent caustic waste streams to meet the allowable discharge consents for the Grangemouth fa- cility. The petrochemical complex occupies a coastal loca- tion adjacent to the BP Exploration, Crude Stabilization and Gas Separation Plant (1,000,000 barrels/day), and BP Oil Refinery (200,000 barrels per day). Aqueous waste streams from all units on the petrochemical site are coHected and discharged via a common outflow into a tidal estuary. The area is ecologically important because of the high popula- tions of wading birds, and is designated as a Site of Special Scientific Interest (SSSI).

The company’s general philosophy on effluents is that of waste minimization at the source. All new site projects are therefore required to provide their own recovery and/or treatment facilities to ensure that the quality of the overall

site effluent is not compromised. In the case of the new spent caustic treatment unit, since this would treat effluent from the older steam cracker as well, a significant reduc- tion in certain pollutants (principally sulfide) was required.

Having examined the available technologies, HPCL and SWEL agreed to collaborate on the development o f a low pressure process to avoid the higher capital cost typical of medium pressure and ,high pressure wet air oxidation processes. The development programs, undertaken in 1991 and 1992, consisted of detailed research laboratory oxida- tion trials together with computer modelling to enable scale-up of the laboratory findings t o full-size commercial plant. Design and construction of the Grangemouth unit paralleled the development program, enabling the unit to be brought on-stream in February 1993.

Complete oxidation o f sulfide t o sulfate in an alkaline environment proceeds through a number o f intermediate steps which can be represented by the following equa- tions:

(1) 2 Na,S + 3 o2 = 2 Na,SO,

(3) Na,S20, + 2 0, + 2 NaOH = 2 Na,SO, + H,O (4) 2 Na,SO, + 0, = 2 Na,SO,

( 2 ) 2 Nd,S + 2 0 2 + H2O = Na2S20, + 2 NdOIf

Oxidation of thiosulfate to sulfate (reaction 3) is the rate determining step for overall conversion to sulfate. This is also the step which has maximum effect on chemical oxy- gen demand (COD) since thiosulfate ions require 5 oxygen atoms to be fully oxidized. The relative percentages of COD removed at each intermediate step of the reaction process are illustrated in Figure 1.

. . . ._.I... ..__I ............. _ _ _ .................. .- ..................... _ .........................................

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................................ .,

. . . . . .

. ” ..

-. .... ..

ScSde.ti! ThmsuWe Sul(ite.Barlfae Sulfate

srvcicr

FIGURE 1 Relative chemical oxygen demand of different chemical anions.

Environmental Progress (Vol. 16, No. 1) Spring 1997 9

TABLE 1. Design Targets

1. 100% sulfide removal 2 . 3. 4. 5 .

> 50% conversion to sulfate COD of final effluent minimized H,S, mercaptan and organics removal from offgas pH reduction of final effluent to 6.5-7.0

Design targets for the unit were therefore set to achieve full sulfide conversion and acceptable COD reduction per- formance. These targets are summarized in Table 1.

Since commissioning in February 1993, the unit has op- erated continuously, always attaining full sulfide removal and achieving up to 80% conversion to Sulfate. In so do- ing, the unit has allowed subsequent neutralization of the oxidized product from pH 13 to pH 7, without release of hydrogen sulfide or sulfur precipitation, and has brought about up to an 80% reduction in the chemical oxygen de- mand (COD) for this stream.

PROCESS DESCRIPTION

The Grangemouth Unit comprises three sections: (1) A Gasoline Washing Stage. ( 2 ) The LPWAO Reaction System, and subsequent neu-

(3) An Off-gas Deodorizing Unit. tralization.

Gasoline Wash

The presence of even trace concentrations of organic compounds has been demonstrated in the laboratory to re- duce oxidation rates as shown in Figure 2.

An effective means of removing light polymer or con- densed gasoline from the aqueous spent caustic stream is therefore essential to maintain unit performance. A schematic of the gasoline wash unit is shown in Figure 3. Debutanised, hydrogenated gasoline, which is readily available on site and has a high affinity towards the or- ganic polymers present, is used to extract the light diene polymer (sometimes referred to as yellow polymer). In the Grangemouth plant, the “contaminated” wash gasoline is then processed with the DAC (debutanised aromatic con- densate) stream to recover separate gasoline and fuel oil fractions.

Interface level control of these wash systems is fre- quently problematic-caustic itself acting as a potential emulsifier to create a diffused “rag” zone between the

100 - 90 ! 8o . +“Model“Caushc

0 30 60 90 120 150 180 210 240 Time (mins)

FIGURE 2 Effect of organics on sulfate yield.

SPENT CAU8lIC FROM TOWER

I

FIGURE 3 Gasoline wash system.

phases. The LPWAO reaction unit feedtank has therefore been designed to provide adequate residence time t o al- low the emulsion to break for complete phase separation.

LPWAO Reaction System

The Reaction System consists of three reactors arranged in series, with the option of either co-current o r counter- current operation (Figure 4). Each reactor comprises two reaction zones, designed to operate as a continuously stirred tank reactor (CSTR). Two reactors, which together allow a residence time of approximately 6 hours, are re- quired to give the minimum design performance. In prac- tice, all three reactors are streamed whenever possible.

Air is introduced through spargers at the base and mid- point o f the reactors, typically at 4-6 times [2 ] the stoi- chiometric requirement for complete oxidation. Operating at low pressure allows the use of “plant” air from the cen- tral factory facility, thereby avoiding the need for separate air compression. (Since all steam crackers require air at around 6 bar (gauge) pressure for furnace decoking, cata- lyst reactivation and general use, it will almost certainly be available). The correspondingly low temperatures also avoids the necessity for expensive construction materials. In the Grangemouth plant. the three reactors are con- structed from stress-relieved carbon steel. rather than the more typical nickel-based alloys required in medium and high pressure units.

The low pressure/low temperature process is also inher- ently safer than those using elevated pressures and tem- peratures, and appreciably safer than processes using oxy- gen or ozone.

Of course, lower temperatures give lower reaction rates, so residence time must be increased to compensate. Effec-

,- ---c

i x FACTOR” AIR

FIGURE 4 LPWAO reaction system (co-current illustrated).

10 Spring 1997 Environmental Progress (Vol. 16, No. 1)

tivc mixing of air and spent caustic is also essential to en- sure efficient mass transfer across the liquid/air boundary.

Neutralization of the reactor product is achieved using splfuric acid in a quick response balancing circuit. Al- though the presence of carbonate helps to buffer the solu- tion, there is the risk of end-point overshoot. The equip- ment is therefore designed in stainless steel, with auto- matic acid shut-off from dual on-line pH meters. Neutral- ization by direct injection of carbon dioxide gas is being considered.

Once neutralized, the treated spent caustic stream joins the oily water streams from the ethylene plant, passing through corrugated plate interceptors and a dissolved air flotation unit. This latter unit also helps to increase (albeit marginally) the conversion to sulfate. The exiting stream has a typical total oil loading of around 10-15 mg/L which requires no further treatment to meet the site's consent level.

offgas Deodorizing unit The spent gases from the LPWAO reaction section con-

tain trace quantities of mercaptans and organic species. This offgas could be routed into the firebox of a furnace or boiler, but because of its physical location, the Grange- mouth unit includes a catalytic thermal oxidizer to deodor- ize the offgas.

Operating Experience

Since commissioning in early 1993, the unit has operated continuously, achieving at least minimum design perform- ance at all times. However, several operational problems have been encountered, although all have been resolved or addressed.

The most significant o f these related to the design of the reactor air spargers. With a design objective of achieving the largest possible film contact area [3, 41, the air sparg- ers were specified to produce bubbles of only a few mi- crons diameter. In the early months of operation, fouling caused by insoluble inorganic carbonates and polymerized organic material severely reduced air injection capacity, with consequent reduction in oxidation efficiency.

Laboratory work indicated that the oxidation efficiency would only be marginally affected by increasing bubble size, and that a much simpler design of sparger, using 100-mesh screen, would still be effective. At the same time, the small quantity of required preheat steam was routed via the air supplies to ensure that the nozzles remained hot enough to prevent solidification of organic polymer. This modification was undertaken "on-the-run", one reactor at a time, as a simple conversion of the original spargers. It has proven to be completely effective, with no recurrence of this problem since implementing the change in mid-1933.

It has been our experience that the LPWAO reactors op- erate best in co-current mode (i.e. upward flow of liquid). However, from initial observations of temperature profiles, it was clear that significant backmixing was occurring across the perforated baffle plates separating each reactor zone. Clearly, this indicated that actual residence times would be less than design, and this served to support the laboratory data that predicted a somewhat higher conver- sion to sulfate.

A simple redesign to replace the perforated baffle plate with a chimney tray has now been incorporated in one of

the reactors. Initial results have indicated an improvement in conversion to sulfate, with operating experience now much closer to theoretical performance. The same modifi- cation will be incorporated in the remaining two reactors at their next scheduled inspection outage.

The importance of effective removal of free-organic ma- terial from the spent caustic feed was ably demonstrated on one occasion during the early months of operation fol- lowing failure of the interface level control. The oxidation product proved to be a very stable, light polymer emul- sion, causing significant fouling. Fortunately, this was con- fined to the first reactor, and the unit remained on-stream on two reactors whilst the fouled reactor was cleaned.

Improvements to the gasoline wash system (to achieve better mixing and interface control) have prevented a re- currence. However, some organic material does separate from the aqueous phase in the LPWAO feed tank, and this material has to be periodically decanted and recovered. A further modification is planned to install an additional in- terceptor vessel upstream of the feed tank. By increasing residence time, more complete phase separation can be anticipated, and the attendant risk of inadvertently feeding organics to the reactors minimized.

A significant proportion of the dissolved organics are air-stripped in the reactors, along with mercapvans and other volatile sulphur species. However, we have not found appreciable levels of mercaptans even in the feed. These are destroyed by catalytic oxidation in the off-gas deodor- izer unit.

Maintenance

Since the unit u5es factory air at 6 bar (gauge) pressure, and steam from the plant's MP main, there are few items o f equipment requiring maintenance attention Those modifi- cations undertaken since commissioning have been inex- pensive and easy to implement, requiring shutdown of only one reactor at a time.

At 120"c, some corrosion of stress-relieved carbon steel might be expected in this quite aggressive environment (Figure 5). The reactors were therefore designed with a generous 6 mm corrosion allowance.

All three reactor vessels have now undergone a thor- ough internal inspection-the first two in 1994 and the third in November 1995. Only minor corrosion of the vessels has been noted, generally limited to pitting to a maximum depth of 1.0 mm. Some galvanic action is present on

0 10 20 30 40 50

Concernahon NaOH wl?4

.Area "A" DArea "B" OArea "C"

FIGURE 5 Caustic soda service chart.

Environmental Progress (Vol. 16, NO. 1) Spring 1997 11

TABLE 2. Typical Reactor Performance Sulfide Sulfate

Stream mg/L mg/L PH Feed 5990 480 12.8 ex “A“ reactor 1450 2500 12.8 ex “B” reactor 150 6300 12.7 ex “C” reactor 0 12950 12.6 Neutralized effluent 0 16000 7.2

sparger distributors, but this can be easily rectified by a change in material.

Performance Against Design

Since commissioning in early 1993, the LPWAO unit has consistently achieved complete oxidation of sulfide en- abling subsequent neutralization without H * S release or sulfur deposition.

The extent of complete oxidation to sulfate has generally been dictated by the number of reactors on-line. With only two reactors in operation (residence time around 6 hours), approximately 60% of sulfide is oxidized to sulfate. With all three reactors available (the normal operating mode), conversions of up to 80% are common. This has generally been achieved against a significantly higher than expected sulfide loading (around 6000 mg/L compared with the de- sign 4000 mg/L (as S ” ) . Typical results are given in Table 2.

Operating costs for the unit are extremely low. The gasoline wash system provides a very effective way

of removing hydrocarbons from the spent caustic stream. Conventionally, hydrocarbons removed by separation in the effluent treatment unit are recovered as slop, and are returned to the cracker for processing with substantial quantities of water. Gasoline washing recovers > 97% of the hydrocarbons from the spent caustic stream, which are readily (and cheaply) processed to recover the gasoline and fuel oil fractions. Operating costs of the LPWAO unit itself are estimated at just S300/day, or around 17 cents per tonne of ethylene produced. This figure includes the cost of com- pression power, steam for pre-heat and sulfuric acid for neutralization. Direct operating and maintenance costs (labor and materials costs) are also insignificant.

At Grangemouth, the aqueous discharge flow from the petrochemical complex is around 3 million gallons per day (dry weather), coming principally from cooling water

blowdown. The oxidized and neutralized effluent from the LPWAO unit therefore contributes approximately 200 mg/L sulfate to the combined discharge, less than one-tenth of the concentration in the seawater into which it flows.

It is appreciated that other operators may be subject to more stringent environmental controls, particularly where their plants are located inland. However, even where 100% conversion to sulfate is required, the LPWAO process pro- vides an extremely effective means of achieving the bulk of the task, with in-pipe treatment providing final polish- ing.

CONCLUSION

The LPWAO unit developed jointly by Stone and Web- ster Engineering Ltd. and BP Chemicals Ltd. for the treat- ment of spent caustic satisfactorily achieves its design in- tent of eliminating sulfide and allowing subsequent neu- tralization without release of obnoxious H ,S or precipita- tion of sulfur. In so doing, the unit enables HPCL to achieve the environmental consent levels for aqueous dkcharge from their complex at Grangemouth, Scotland.

The significance of this unit is demonstrated by the re- sults of the annual Environmental Survey undertaken by the University of Stirling [ 51. For the first time in twenty years, this independent survey has revealed that the previously sterile area around the aqueous effluent outfall is now populated by the same marine life as elsewhere in the es- tuary-providing clear evidence of the sustained improve- ment in effluent quality.

LITERATURE CITED

1.

2.

3.

4.

5.

Small, K., “Performance of a Low Pressure Wet Air Oxidation Unit Treating Steam Cracker Spent Caustic,” presented at the ALnerican Institute of Chemical Engi- neers’ Spring National Meeting, Atlanta, Georgia, Pa- per No. 25b (April 1994). Chen, K. Y., and Morris, J. C., 5th International Wa- ter Pollution Res. Conf. (July-Aug. 1970, 111.) Suslick, K., “Ultrasound: Its Chemical, Physical and Biological Effects,” VCM: New York (1988). Abegg, O., Erdol und Kohle. Erdgas. Petrochemie, pp.

McLusky, D. S., “Survey of the Grangemouth to Bo’ness Intertidal Area of the Forth Estuary,” unpub- lished report by University of Stirling Department of Biological and Molecular Sciences (1995).

621-626 (1961).

12 Spring 1997 Environmental Progress (Vol. 16, No. 1)