Fixed-bed pyrolysis

Thermochemical Liquefaction. Most of the research done since 1970 on the direct thermochemical Hquefaction of biomass has been concentrated on the use of various pyrolytic techniques for the production of Hquid fuels and fuel components (96,112,125,166,167). Some of the techniques investigated are entrained-flow pyrolysis, vacuum pyrolysis, rapid and flash pyrolysis, ultrafast pyrolysis in vortex reactors, fluid-bed pyrolysis, low temperature pyrolysis at long reaction times, and updraft fixed-bed pyrolysis. Other research has been done to develop low cost, upgrading methods to convert the complex mixtures formed on pyrolysis of biomass to high quaHty transportation fuels, and to study Hquefaction at high pressures via solvolysis, steam—water treatment, catalytic hydrotreatment, and noncatalytic and catalytic treatment in aqueous systems. 

Table 11.3 shows the yield of gas, oil/wax and char from the fixed bed pyrolysis of the main polyalkene plastics, polyethylene and polypropylene, found in municipal solid waste [7-15], Table 11.4 shows the gas, oil/wax and char from the pyrolysis of other plastics [7-9, 14-17], All of the plastics produced an oil/wax and gas and in some cases a char. The product yield related directly to the type of plastic, the reactor type... 

Figure 11.5 shows that the functional group compositional analysis of the pyrolysis oil/waxes derived from the fixed-bed pyrolysis of PVC, PS and PET is very different from the polyalkene plastic pyrolysis oil/waxes. The spectra of the PVC pyrolysis oil/wax shows that the characteristic peaks of alkanes and alkenes are present as described for the polyalkene plastics. Since the PVC plastic polymer is based on a similar backbone structure to the polyalkene plastics, a similar degradation product oil/wax composition may be expected. However, the spectra for PVC in Figure 11.5 show that there are additional peaks in the region of 675-900 cm and 1575-1625 cm The presence of these peaks indicates the presence of mono-aromatic, polycyclic aromatic and substituted aromatic groups. Benzene has been identified as a major constituent in oils derived from the pyrolysis of PVC whilst other aromatic compounds identified included alkylated benzenes and naphthalene and other polycyclic aromatic hydrocarbons [19, 32, 39]... 

The pyrolysis oil/wax derived from the fixed-bed pyrolysis of PS produced a strong presence of both aromatic and aliphatic functional groups. The presence of CH3 and CH2 are indicated by the peaks between 3000 and 2800 cm and the peak in the region of... 

It is essential to centralize the feed and keep it away from the reactor walls. Sticking of particles to the inner snrface of the walls tnms rapid pyrolysis into fixed-bed pyrolysis. The growth of the stnck mass leads to blocking of the reactor. Carbonization on walls is an impediment to drawing a mass balance. 

Beis, S. H., Onay, O., Kockar, O. M. Fixed-bed pyrolysis of safflower seed influence of pyrolysis parameters on product yields and compositions. Renew Energ 2002, 26, 21-32. 

Ate , F., Piitiin, A.E., Piitiin, E., 2005. Fixed bed pyrolysis of Euphorbia rigida with different catalysts. Energy Conversion and Management 46 (3), 421—432. 

The application of a selective pyrolysis process to the recovery of chemicals from waste PU foam is described. The reaction conditions are controlled so that target products can be collected directly from the waste stream in high yields. Molecular beam mass spectrometry is used in small-scale experiments to analyse the reaction products in real time, enabling the effects of process parameters such as temperature, catalysts and co-reagents to be quickly screened. Fixed bed and fluidised bed reactors are used to provide products for conventional chemical analysis to determine material balances and to test the concept under larger scale conditions. Results are presented for the recycling of PU foams from vehicle seats and refrigerators. 12 refs. 

Somewhat related is a process proposed and demonstrated on labscale by the University of Siegen (Germany). The process is called the (Herhof)-Integrierte Pyrolyse und Verbren-nung (IPV) process and is decribed in detail by Hamel et al.60 In this process, biomass is converted with high-temperature steam to pyrolysis gas in a fixed-bed reactor. The generated carbon from this reactor is led to a stationary FB combustor from which the hot ash is returned to the first-mentioned reactor. The ash works catalytically to reduce the tar content of the gas produced. The gas is further cleaned and conditioned using a scrubber and electrostatic filter from which the catch is returned to the FB combustor. 

Operation conditions. A specially designed oxygen mixer and the interlock system assure safe operation outside tbe flammability envelope. Simple design and high quality material specifications result in two to three year intervals between major turnarounds and on stream times as high as 98+% over one year. The heat of reaction is recovered as high-pressure steam (18+ barg), which can be used by other parts of tbe VCM process (e.g., pyrolysis). Wastewater does not contain suspended catalyst fines therefore, there is no need to settle them and then dispose as a hazardous waste. Copper levels in fixed-bed effluents are below 0.3 ppm thus, no treatment is required. 

Vertical or shaft reactor Fixed bed reactor Gravity flow Mainly for catalytic reactions, converting vapours from a first reactor Rubber pyrolysis Eoster Wheeler... 

E. A. Williams, P. T. Williams, The pyrolysis of individual plastics and a plastic mixture in a fixed bed reactor. Journal of Chemical Technology and Biotechnology, 70 (1), 9-20, 1997. 

According to this concept, Masuda et al. [75] studied the catalytic cracking of the oil coming from a previous thermal pyrolysis step of polyethylene at 450°C in the bench-scale fixed-bed reactor shown in Figure 3.11. The catalysts employed were different zeolite types REY (rare earth exchanged zeolite Y), Ni-REY (nickel and rare earth... 

A full-scale pyrolysis-catalytic process in which the catalytic cracking zone is directly connected to the pyrolysis zone was developed in Japan (Fuji Process) [19]. In this process, after separation of PVC and impurities by wet techniques, waste plastics are thermally pretreated at 300°C for dechlorination and then introduced into the pyrolysis reactor and thermally cracked at 400°C. Subsequently, degradation products are fed directly to the fixed-bed reactor using a ZSM-5 catalyst. 

The carbonization of polypropylene is similar to that of polyethylene. During slow pyrolysis of polypropylene and for a temperature increase from 400 to 700°C, the yield in the liquid phase remains higher than 80% with a very small increase in the yield of gas phase (less than 20%). On the other hand, in flash pyrolysis of polypropylene, an increase of temperature from 550 to 700°C leads to a decrease of the yield in the liquid phase down to 40% with an increase of the gas phase up to 60%. As presented for PE (see Section 2.1), Sawagushi et al. [21] present the results of steam gasification of PP in a fixed bed reactor (Figure 10.8). 

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