Pyrolysis distillation columns

Description Raw pyrolysis gasoline is prefractionated into a heartcut C8 stream. The resulting styrene concentrate is fed to an extractive-distillation column and mixed with a selective solvent, which extracts styrene to the tower bottoms. The rich solvent mixture is routed to a solvent-recovery column, which recycles lean solvent to the extractive-distillation column and recovers the styrene overhead. A final purification step produces a 99.9% styrene product containing less than 50 ppm phenyl acetylene. 

The extractive-distillation column overhead can be further processed to recover a high-quality mixed xylene stream. A typical world-scale cracker could produce approximately 25,000 tpy styrene and 75,000 tpy mixed xylenes from pyrolysis gasoline. 

In the Smuda process the pyrolysis reactor temperature is 350°C and the operating pressure is 4-5 psi. The pyrolysis gases from the pyrolysis vessel are sent directly to a distillation column. The distillation column has a typical temperature profile as follows top 140°C, Sulzer 250Y middle 322°C, Sulzer 350Y and bottom 331°C. 

The Smuda process uses a reflux return where longer paraffin chains that condense shortly after exiting the main chamber are allowed to flow back to the main chamber (the reflux effect ). Also the heavies from the bottom of the distillation column flow back to the pyrolysis chamber for re-cracking (Figure 15.10b). 

In a separate room (in case of explosion) the distillation and quenching colnmns are installed. In the two distillation columns, fonr distillation boiling cuts can be obtained from the pyrolysis oil. In the first column with a diameter of 150 mm and a length of 10.5 m, the fraction with a boiling point of more than 180°C is split off from the distillation residne. In the second column with a diameter of 80 mm and a height of 8 m cnts of 80°C, 110°C, and 140°C (xylene) are obtained. The xylene is nsed as qnenching oil also in the coolers. 

The pyrolysis oil stored in the oil drum is fed into the distillation column with reboiler and separated into three fractions light oil from the top, medinm oil from the middle and heavy oil from the bottom of the column. These three types of oils are stored in separate oil tanks and sent to the furnaces and cogeneration system in the plant as fuel through piping, or loaded onto a tank lorry for shipment. 

Simplification of the present liquefaction processes by removing several pieces of equipment (for example, the distillation column) will be one important solution. The present complex facilities are necessary to treat chlorine-containing plastics and PET materials from plastic containers and packaging. They provide only small amounts of oil on pyrolysis, but cause various difficulties. The amount of the former materials tends to decrease due to the replacement of PVC and PVDC materials with polyolefins, but the latter is... 

Almost half the amount of pyrolysis gas produced was burnt in the exhaust gas flare the other part was sufficient to support the energy demands of the process, being burnt in the radiation fire tubes. Waste heat from the reactor heating (cooler) was sufficient to heat the distillation column. 

The pilot plant has been running since late 1978. It was proved in a test run with a feed of more than 150 car tyres, that the process is self-sufficient in its energy needs, producing at a temperature of 720°C, even excess pyrolysis gas to supply other heat necessary. Waste heat from the reactor heating is sufficient to heat the distillation column. 

The production of VDF and PVDF is capital intensive as it requires R-142b as a monomer precursor, corrosion-resistant pyrolysis reactors, and low temperature distillation columns for the vinylidene fluoride monomer, high-pressure polymerization reactors, and finishing process. So far, mass production of PVDF has been limited to the United States, Europe, and Japan. Major PVDF producers are shown in Table 4. 

Simulate the vinyl chloride process (Problem 5.4) using Aspen Plus. Take the feed at room temperature and 20 psia. Operate the direct chlorination reactor at 65°C and 560 kPa. A distillation column removes the trichloroethane and the rest of the stream is sent to the furnace. Heat the stream to 1500 F so pyrolysis takes place. Cool the effluent from the furnace, and recycle the vapor (mostly HCl). Send the hquid (vinyl chloride and ethylenedichloride) to a distillation column for separation. 

A preferable feed temperature would be 35°C or higher, which could be achieved by completing the cooling and partial condensation of the pyrolysis reactor effluent with cooling water, but the introduction of vapor into the column would increase the refrigeration load of the condenser at —26.2°C. Upon making this specification, key differences (temperature, pressure, and phase) appear between the effluent from the pyrolysis operation and the feed to the distillation column. These are eliminated in the next synthesis step by inserting tempeia-... 

The purification of MMA has potentially several alternatives. The current process adopts multiple distillation columns to cut the lighter and heavier components in the stream after pyrolysis, while MMA and methyl isobutyrate, for example, are not easily separated because of their similar boiling points. On the other hand, the fresh production of MMA from crude oil also comprises a purification process, where MMA itself is used as a solvent in the extraction of MMA. Therefore an extraction process may be appHcable for MMA purification and solvents may be also available if the PMMA pyrolysis process is located near the fresh MMA production processes such as the Mitsubishi Rayon Co. Ltd. Process integration including not only heat and power but also materials such as... 

The production of VDF and PVDF requires significant investment in corrosion-resistant pyrolysis reactors and low temperature distillation columns... 

The EDC produced from the direct chlorination, oxychlorination and the recovered from the cracking step is required to be treated to reach more than 99.5% purity before entering the pyrolysis tmit. The by-products are removed in a sequence of two distillation columns. The first column removes the light wastes while the heavy wastes, mainly C2H3CI3 (1,1,1-trichloroethane), are removed in the second column. 

The stream leaving the pyrolysis unit contains the co-product HCl, imcracked EDC and VCM. This stream is treated in a sequence of two distillation columns. In the first column, HCl is distilled off at the top and sent to the oxy-chlorination unit. The bottom product is fed to the second column to purify the VCM product fi om the unconverted EDC. The unconverted EDC leaves the bottom of the column and is recycled back to the EDC purification section. 

For the designed plant with no heat integration, there are 38 control degrees of freedom in this process. These degrees of freedom represents the available manipulated variables in the process and can be characterised as follow four feed valves, direct reaction and oxy-reaction coolers valves, direct reaction and oxy-reaction product valves, oxy quench cooler valve, three decanter product valves, pyrolysis preheater and heater valves, pyrolysis product valve, pyrolysis quench cooler valve, HCl heater valve, eight valves for the heating and cooling systems of the four distillation columns, thirteen valves for the base, top and reflux streams of the four distillation columns. 

The principal role of the two distillation columns that follow the pyrolysis section is to recover all of the produced vinyl chloride, recycle the by-product HCl and recover the uncracked EDC. HCl and EDC are recovered in the first and second columns, respectively. Therefore, the presence of HCl and EDC in the vinyl chloride product stream is required to be reduced as much as possible to prevent yield loss as well as reducing the presence of vinyl chloride in the recycled streams of HCl and EDC. Therefore, three control objectives are required to be considered (1) vinyl chloride composition in HCl recovery stream, (2) vinyl chloride composition in EDC recovery stream and (3) impurities, HCl and EDC, compositions in vinyl chloride product stream. The recommended manipulated variables for these control objectives are the reflux flow or the distillate flow to control the top stream composition and the reboiler duty or the bottom flow for controlling the bottom stream composition. 

We consider in the following a world-scale steam cracker plant with a production capacity of 125th ethene (Figure 6.6.8). The plant runs on light-run naphtha that is heated in the convection zone of the crack oven to 600 °C. The naphtha is mixed with water vapor (4.5 MPa, 257.5°C) to realize a steam-to-naphtha ratio of 0.45. This mixture is introduced to the main crack oven, which is an 80m tubular reactor at 850°C. The residence time of the feedstock in this hot section of the crack oven is 0.5 s. Following the crack oven, the product mixture is quenched to 200°C. In a first distillation column light components (C1-C5) are separated from the heavier pyrolysis products (Cs+). 

Cyclopentadiene was obtained by pyrolysis of dicyclopentadiene (pract., 90%) which was purchased from Fluka. Pyrolysis was carried out following the previously described general procedure.2, Dicyclopentadiene (80 mL) was placed in a round-bottomed flask equipped with a magnetic stirbar and a Vigreux column fitted with a distillation head through which cold water was circulated. The contents of the flask were slowly heated with stirring at 160°C in an oil bath, and ca. 60 mL of cyclopentadiene (bp 38-42°C) was collected in a receiver cooled in an ice-salt bath. The cyclopentadiene was used... 

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