Cracking reheat

On the negative side, the MCFC suffers from sealing and cathode corrosion problems induced by its high-temperature molten electrolyte. Thermal cycling is also limited because once the electrolyte solidifies it is prone to develop cracks during reheat-... 

The BP Chemicals polymer cracking process is based at Grangemouth in Scotland and uses mixed plastics as the raw material. The reactor uses a fluidised bed which operates at 500 °C in the absence of air, and under these conditions the plastics crack thermally to yield hydrocarbons. These vaporize and are carried away from the bed with the fluidising gas. Solid impurities such as metals from PVC stabilisers accumulate in the bed or are carried away in the hot gas to be captured by a cyclone further along in the plant. PVC decomposes to HCl and this is neutralized on a solid lime absorbent to yield CaCl2 which is disposed of in landfill. The purified gas is cooled to condense most of the hydrocarbon which can be employed as commercially useful distillate feedstock. The light hydrocarbons which are less easy to condense are compressed, reheated and recycled as fluidising gas. 

By cooling the gas is condensed and then available as hydrocarbon feedstock for other processes (some 85% of the MPW input). The light hydrocarbon gas (15% of the MPW input) that remains after cooling is compressed, reheated and returned to the reactor as fluidising gas. It can also be used as a fuel for the cracking process, though other recovery options are being studied as well. 

During the strike, the sulfur plant was shut down for minor repairs. I had to supervise its start-up. Mainly, I had to reheat the adiabatic-combustion chamber to 1800°F, before restoring the flow of H2S. This was done by burning a controlled amount of methane or natural gas, with a carefully regulated flow of air. The idea was to slowly heat up the combustion chamber with hot flue gas by 100 to 200°F per hour. This slow reheat was needed to avoid cracking the refractory bricks, because of uneven heating. To carry away a portion of the heat of combustion of the natural gas, we used pipeline nitrogen. 

Cristobalite is transparent. We do not normally consider devitrified glass as a transparent material. However, once fused silica has cooled below 250°C, B-cristobalite is transformed into a-cristo-balite. This substance is the white opaque material we usually associate with devitrified silica. When fused silica is reheated into the devitrification range, the a-cristobalite turns back into B-cristobalite. However, because a-cristobalite has many fissures and cracks, the opacity remains when it is reheated back into B-cristobalite. 

Compressed gas at 450 psig is dried and then chilled. A multistream heat exchanger chills the tail gas to -265°F. Liquid condensates are separated at various temperatures, such as -30°F, -65°F, -100°F and -140°F, and are reheated against incoming cracked gas. The partially vaporized streams are sent to a deethanizer stripper operating at about 320 psig. The bottoms C3+ stream is sent to propylene and heavys recovery. 

The best joins are those that are made without delay. Should any interruption occur in feeding the molten rod into the space between the ends, the whole join must be reheated to avoid cooling cracks. 

Contact coking. Contact coking is a continuous process in which residual stocks are cracked by contact with a stream of hot coke granules circulated through a reactor and a reheater in a manner analogous to the circulation of catalyst in a TCC unit (215,287,304). The feed is preheated to about 800°F. and introduced into the top of the reactor. Volatile fractions are vaporized, while unvaporized oil is retained as a liquid film on... 

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