Quenched coking

In industrial practice, however, the most widespread technique consists in passmg a mixture of hydrocarbons and steam through tubes placed in a furnace. The hydrocarbons, which are raised to high temperature, are pyrolysed and the resulting products are separated after a rapid quench. Coke deposits are periodically removed by controlled combustion. This is the technology of steam cracking, which is the main focus of this chapter. 

Toluene Solubles Determined for a Quenched Coking Reaction in a Continuous Coker... 

The automation and safety interlock design techniques for steam stripping, water quench, coke cooling, hydraulic decoking and oil/gas preheating operations of the coke drums not only reduce work intensity and ensure safe operation, but also create conditions to reduce the drum-cycle time to 16-18 hours. 

Trials with oxygen enrichment of the furnace blast air were first carried out in the United Kingdom as long ago as 1964. In general, it is considered too expensive to use routinely, but a small amount of enrichment is used in several smelters. At Cockle Creek in 1975 oil was injected into the tuyeres and replaced up to 12% of the coke. There were no adverse results and oil injection continued until the rising cost of oil rendered it uneconomic. Subsequently, in 1986 the Harima smelter became the first ISF to inject solids on a consistent basis. Up to 6% of the lump coke charge was replaced by injected dry quench coke dust. This practice continued until November 1995. 

An additional area of concern relates to the quality of the coke produced from hydrotreated vacuum resid feeds. For a variety of reasons, premature furnace fouling, inclusion of semi-coked liquids in the quenched coke drum, and carry-over of coke fines into the fractionator, the delayed coking of 100% hydrotreated feed has not been successfully achieved in commercial operations. 

The Ube plant consists of four complete trains of Texaco quench-type gasifiers. During normal operation, three gasifiers are on line and one is on standby. Each gasifier consumes 500 t/day of coal to generate syngas for 350 t/day of ammonia. Up to the middle of 1990 the Ube plant gasified 2.2 million t of coal and petroleum coke. 

The reaction gas is rapidly quenched with injected water at the point of optimum yield of acetylene, which happens to correspond with the point of maximum soot production. Coke will deposit on the walls of the burner and must be removed from time to time by a scraper. 

The flame-space walls are stainless steel and are water cooled. No mechanical coke scraper is required. A water quench cools the cracked gas stream rapidly at the poiat of maximum acetyleae and this is followed by a secondary water quench. The primary quench poiat can be adjusted for variation ia throughput, to accommodate the depeadeace of acetyleae yield oa resideace time ia the flame space. 

The carbon black (soot) produced in the partial combustion and electrical discharge processes is of rather small particle si2e and contains substantial amounts of higher (mostly aromatic) hydrocarbons which may render it hydrophobic, sticky, and difficult to remove by filtration. Electrostatic units, combined with water scmbbers, moving coke beds, and bag filters, are used for the removal of soot. The recovery is illustrated by the BASF separation and purification system (23). The bulk of the carbon in the reactor effluent is removed by a water scmbber (quencher). Residual carbon clean-up is by electrostatic filtering in the case of methane feedstock, and by coke particles if the feed is naphtha. Carbon in the quench water is concentrated by flotation, then burned. 

A notable example of controlled water reuse was utilization of secondary sewage effluent from the Back River Wastewater Treatment Plant in Baltimore by the Sparrows Point Works of Bethlehem Steel (6). The Sparrows Point plant was suppHed primarily by weUs located near the brackish waters of Baltimore harbor. Increased draft on the weUs had led to saltwater intmsion. Water with chloride concentration as high as 10 mg/L is unsuitable for many steelmaking operations. Rollers, for example, are pitted by such waters. However, treated effluent from the Back River Plant can be used for some operations, such as coke quenching, and >4 x 10 m /d (10 gal/d) are piped 13 km to Sparrows Point. This arrangement has proved economical to both parties for >40 yr. 

Another modification to the vertical coke oven process, dry quenching of the coke as first developed in the GIS (13), started becoming of interest in the 1960s and has continued to grow steadily. Using dry quenching, the coke is discharged into a hot car and is quickly transported and dumped into a... 

Coke quenching ParHculate matter Baffles and water sprays... 

The coke is taken to a quench tower, where it is cooled with a water spray or by circulating an inert gas (e.g., nitrogen), a process known as dry quenching. The coke is then screened and sent to a blast furnace or to storage. 

An electrostatic precipitator is used to remove more tar from coke oven gas. The tar is then sent to storage. Ammonia liquor is also separated from the tar decanter and sent to wastewater treatment after ammonia recovery. Coke oven gas is further cooled in a final cooler. Naphthalene is removed in a separator on the final cooler. Light oil is then removed from the coke oven gas and is fractionated to recover benzene, toluene, and xylene. Some facilities may include an onsite tar distillation unit. The Claus process is normally used to recover sulfur from coke oven gas. During the coke quenching, handling, and screening operation, coke breeze is produced. The breeze is either reused on site (e.g., in the sinter plant) or sold offsite as a by-product. 

Heat recovery tor steam generation, pre-heating combustion air, and high efficiency burners Adjustable speed drives, heat recovery coke oven gases, and dry coke quenching Efficient hot blast stove operation, waste heat recovery for hot blast stove, top gas power recovery turbines, direct coal injection... 

The bottom section of the main column provides a heat transfer zone. Shed decks, disk/doughnut trays, and grid packing are among some of the contacting devices used to promote vapor/liquid contact. The overhead reactor vapor is desuperheated and cooled by a pumparound stream. The cooled pumparound also serves as a scrubbing medium to wash down catalyst fines entrained in the vapors. Pool quench can be used to maintain the fractionator bottoms temperature below coking temperature, usually at about 700°F (370°C). 

Install high efficiency Feed Nozzles Lower Preheat Temperature Inject Naphtha Quench to Riser Increase Stripping and Dispersion Steam Switch to a Coke Selective Catalyst... 

One method of maximizing the LCO end point is to control the main fractionator bottoms temperature independent of the bottoms pumparound. Bottoms quench ( pool quench ) involves taking a slipstream from the slurry pumparound directly back to the bottom of the tower, thereby bypassing the wash section (see Figure 9-9). This controls the bottoms temperature independent of the pumparound system. Slurry is kept below coking temperature, usually about 690°F, while increasing the main column flash zone temperature. This will maximize the LCO endpoint and still protect the tower. 

In the blast furnace, the reaction of the nitrogen in the blast with coke leads to the formation of poisonous chemicals such as hydrogen cyanide and cyanogens, and each cubic meter of the blast furnace gas contains from 200 to 2000 mg of these compounds. The blast furnace gas is scrubbed with water in the dust collection system the cyanide compounds dissolve in the water, which is then discharged after the compounds have been destroyed. Another poisonous emission in blast furnace operations is hydrogen sulfide. The sulfur present in the coke is converted into calcium sulfide in the slag, the water-quenching of... 

Particulate emissions due to pushing of coke from the oven into the quench car... 

Particulate from the coke mass during coke quenching... 

The major consumption of water in coke plants is for cooling purposes in a variety of cooling and condensing operations. For the coke quenching operation alone, about 120 to 900 gal of water are required per ton of coke.1 The various sources of process wastewater include the following ... 

Largest sources coke handling, charging, pushing, quenching... 

Spray Cooling - Cooling of Ingot in Continuous Casting - Cooling of Nuclear Cores - Cooling of Turbine Blades - Coke Quenching... 

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