Offgas Cooling and Heat Recovery

The first step in treating metallurgical offgas is cooling in preparation for electrostatic dust precipitation. Electrostatic precipitators operate at about 600 K. Above this temperature, their steel structure weakens. Below this temperature, sulfuric acid forms from small amounts of SO3 and H20(g) in the furnace offgas - causing corrosion of the precipitator. 

Gas cooling is mostly done in waste heat boilers. Fig. 4.2. These boilers cool the gas and recover its heat in useful form - steam (Abeck, 2003 Peippo et al, 1999). 

Most offgas dust falls out in the waste heat boiler. It is collected and recycled to smelting. It falls out due to low gas velocities in the large boiler chambers. 

An alternative method of cooling metallurgical offgas is to pass it through sprays of water. Spray cooling avoids investment in waste heat recovery equipment but wastes the heat of the gas. It also generates acidic waste liquid that must be neutralized and treated for solids removal/recycle. 

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Reactor effluent is cooled in a series of exchangers (3) to recover waste heat and to condense (4) the hydrocarbons and steam. Uncondensed offgas—primarily hydrogen—is compressed (5) and then directed to an absorber system (6) for recovery of trace aromatics. Following aromatics recovery, the hydrogen-rich offgas is consumed as fuel by process heaters. Condensed hydrocarbons and crude styrene are sent to the distillation section, while process condensate is stripped (7) to remove dissolved aromatics and gases. The clean process condensate is returned as boiler feedwater to offsite steam boilers. 

The offgas is cooled in a waste heat boiler and de-dusted in two electrostatic precipitators before being fed to the existing acid plant. Total sulphur recovery is in excess of 99%. 

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