Pyrolysis residence time distribution

The major industrial source of ethylene and propylene is the pyrolysis (thermal cracking) of hydrocarbons.137-139 Since there is an increase in the number of moles during cracking, low partial pressure favors alkene formation. Pyrolysis, therefore, is carried out in the presence of steam (steam cracking), which also reduces coke formation. Cracking temperature and residence time are used to control product distribution. 

This chapter comprises part of an experimental program undertaken to provide precise kinetic and product distribution data for the steam pyrolysis of a number of pure olefins, diolefins, and naphthenes over a wide range of temperatures and residence times, including those encountered in industrial practice. Runs were carried out in the bench-scale flow reactor (1), which has been shown (2) to provide yield and conversions data that are in excellent agreement with pilot plant and commercial data. 

Because of the broad scope of direct biomass pyrolysis, the basic technologies and principal products are tabulated in Table 8.12 to facilitate easy comparison. The conversion conditions and major products shown in this table are typical, but subject to considerable variation. There are several commonalities among the different pyrolysis methods. Pyrolysis time and temperature are clearly the key operating parameters that have the most influence on product yields and distributions. Moderate but optimized temperatures are needed at short residence times to maximize liquid yields, whereas long residence times and... 

During the pyrolysis process, the final conversion mainly depends on three phenomena the heat transfer from the reactor to the feedstock, the feedstock movement in the reactor and the kinetics of pyrolysis reactions. The heat transfer rate determines the rate of temperature increase of the feedstock. The feedstock movement behaviour determines the residence time of the feedstock particles in the reactor. In turn the heating rate and the residence time control the quantity of energy transferred and thus the ten Jerature distribution throughout the feedstock in the reactor. Once the tenqserature distribution is known, the kinetic behaviour of the feedstock determines the final conversion at the reactor outlet. 

A vortex tube has certain advantages as a chemical reactor, especially if the reactions are endothermic, the reaction pathways are temperature dependent, and the products are temperature sensitive. With low temperature differences, the vortex reactor can transmit enormous heat fluxes to a process stream containing entrained solids. This reactor is ideally suited for the production of pyrolysis oils from biomass at low pressures and residence times to produce about 10 wt % char, 13% water, 7% gas, and 70% oxygenated primary oil vapors based on mass balances. This product distribution was verified by carbon, hydrogen, and oxygen elemental balances. The oil production appears to form by fragmenting all of the major constituents of the biomass. 

The products obtained from thermal cracking of plastics depend on the type of plastics, feeding arrangement, residence time, temperatures employed, reactor type, and condensation arrangement [42]. Reaction temperature and residence time have strong influence on the yield of pyrolysis products and the distribution of their components for plastic samples. Jude et al. conducted smdies on thermal cracking of LDPE in a batch reactor resulted in the production of a broad range of hydrocarbon compounds where the yield of aromatics and aliphatics (olefins and paraffins) deeply depended on the pyrolysis temperature and residence time. 

The presence of oxygen, hydrogen, water vapor, carbon oxides, and other compounds in the reaction atmosphere during pyrolysis may either support or inhibit numerous reactions with coal and with the products evolved. The distribution of weight and chemical composition of the products are also influenced by the prevailing conditions (i.e., temperature, heating rate, pressure, residence time, etc.) and, last but not least, the nature of the feedstock (Wang and Mark, 1992). 

The optimum yield of liquid product is obtained in the coal-to-liquids process at approximately 1075 F. A typical product distribution for coal-to-liquida processing using this coal is 56% char, 35% tar, 7% gas, and 2% water. The residence time for this process is also kept as short as possible which maximizes the yield and prevents further cracking of the liquid product. The effect of flash pyrolysis on the liquid yield is shown by the fact that the tar and light oil yield for this coal from... 

Pyrolysis Exhibit 7-4 illustrates a pyrolysis furnace whose produa tubes are placed in the center of the radiant section because of a relatively short residence time, high heat transfer rate, and need for even temperature distribution in the tubes. An integral waste heat recovery system that employs the use of a steam drum and a transfer line exchanger (TlX) is also shown. Steam decoking is required to clean the internal walls of the process tubes. 

The temperature profile is the most important aspect of operational control for pyrolysis processes. Material flow rates, both solid and gas phase, together with the reactor temperature control the key parameters of heating rate, highest process temperatures, residence time of solids and contact time between solid and gas phases. These factors affect the product distribution and the product properties. Solid residence time is another important factor in the bio-oil yields. A short residence time enhances biooil yields, while a longer residence time increases char production (Antal and Gronli, 2003). 

The yield of products from catalytic pyrolysis depends on catalyst type and catalyst to feed ratios besides pyrolysis temperature and residence time (Ojha and Vinu, 2015). Different types of catalysts have different properties such as surface acidity, specific surface area, pore size, and pore size distributions which also determine the yield and selectivity of various products. The range of different functionalities of the catalysts should be matched to the various pyrolysis feedstocks as each feedstock may have a preferred pyrolysis catalyst. Therefore research has been precise in developing particular catalysts for specific raw material (depending on pyrolysis reactor). The section below describes the properties and effect specific catalysts have on pyrolysis vapor upgrading. 

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