Conventional industrial furnaces (steel industry, metallurgy, ceramic, glass, forging and the like) use large quantities of natural gas. In order to save energy, most furnaces include an energy recuperation system on the exhaust gases to preheat the combustion air to temperatures up to about 1000° C. (in the case of regenerative burners). Enriching combustion air with oxygen also leads to not only a reduction of the volume of combustion products, but also to energy savings. Unfortunately, this is generally achieved with an increase in NOx emissions due to a higher flame temperature, and sometimes hot spots will occur in the furnaces, which can be harmful to the load being heated. Conventional regenerative burners have the same pros and cons as the oxygen-enriching air technique.
Heating hydrocarbons for processing, such as in endothermic reactions, is accomplished in a number of ways, including, but not limited to, direct heating and indirect heating, such as through the use of superheated steam. A continuing challenge is to heat a hydrocarbon, such as a continuous hydrocarbon process feed stream, uniformly so that undesirable “hot spots” of temperatures higher than specified do not occur in the heating unit. Such hot spots cause difficulties, such as coking or degradation or unwanted polymerization of a reactant or product.
These challenges are particularly present in the heating to make monomer streams, such as styrene streams, where hot spots undesirably cause coking. It would be particularly desirable if monomer streams could be directly heated to avoid the complexities of using indirect heating such as through superheated steam and the like. Furthermore, there are limits to the maximum temperature of superheated steam. If hydrocarbon or monomer heating is no longer dependent upon the amount of steam needed to heat or reheat the process streams to and/or from reactors, more energy saving devices may be installed to lower the energy required to process the hydrocarbons or monomers.
In more detail, conventionally, the energy needed for the reaction to convert ethylbenzene to styrene is supplied by superheated steam (at about 720° C.) that is injected into a vertically mounted fixed bed catalytic reactor with vaporized ethylbenzene. The catalyst is typically iron oxide-based and contains a potassium compound (KOH or K2CO3) which act as reaction promoters. Typically, 1-2 kg steam is required for each kilogram of ethylbenzene to ensure sufficiently high temperatures throughout the reactor. The superheated steam supplies the necessary reaction temperature of about 550-620° C. throughout the reactor. Ethylbenzene conversion is typically 60-65%. Styrene selectivity is greater than 90%. The system is generally operated under vacuum.
After the reaction, the products are cooled rapidly (perhaps even quenched) to prevent polymerization. The product stream (containing styrene, toluene, benzene, and unreacted ethylbenzene) is fractionally condensed after the hydrogen is flashed from the stream. The hydrogen from the reaction is typically used as fuel to heat the steam (boiler fuel).
It is a continuing goal of the industry to heat hydrocarbon streams, especially monomer streams, uniformly and within relatively strict temperature limits to achieve the necessary temperatures, but also to avoid localized hot spots and consequential degradation of the hydrocarbon, such as to coking products.