Steam cracking liquid feeds

As feedstocks progress from ethane to heavier fractions with lower H/C ratios, the yield of ethylene decreases, and the feed per pound ethylene product ratio increases markedly. Table 3-15 shows yields from steam cracking of different feedstocks, and how the liquid by-products and BTX aromatics increase dramatically with heavier feeds. 

Liquid feedstocks for olefin production are light naphtha, full range naphtha, reformer raffinate, atmospheric gas oil, vacuum gas oil, residues, and crude oils. The ratio of olefins produced from steam cracking of these feeds depends mainly on the feed type and, to a lesser extent, on the operation variables. For example, steam cracking light naphtha produces about twice the amount of ethylene obtained from steam cracking vacuum gas oil under nearly similar conditions. Liquid feeds are usually... 

The hot regenerated catalyst supplies the bulk of the heat required to vaporize the liquid feed (and any recycle) to provide the overall endothermic heat of cracking, and to raise the temperature of dispersion steam and inert gases to the reactor temperature. 

A typical steam cracker consists of several identical pyrolysis furnaces in which the feed is cracked in the presence of steam as a diluent.The cracked gases are quenched and then sent to the demethanizer to remove hydrogen and methane. The effluent is then treated to remove acetylene, and ethylene is separated in the ethylene fractionator. The bottom fraction is separated in the de-ethanizer into ethane and C3, which is sent for further treatment to recover propylene and other olefins. Typical operating conditions of ethane steam cracker are 750-800°C, 1-1.2 atm, and steam/ethane ratio of 0.5. Liquid feeds are usually cracked at lower residence time and higher steam dilution ratios compared to gaseous feeds. Typical conditions for naphtha cracking are 800° C, 1 atm, steam/hydrocarbon ratio of 0.6-0.8, and a residence time of 0.35 sec. Liquid feedstocks produce a wide spectrum of coproducts including BTX aromatics that can be used in the production of variety of chemical derivatives. 

Application GT-Styrene is an extractive distillation process that directly recovers styrene from the raw pyrolysis gasoline derived from the steam cracking of naphtha, gasoils and natural gas liquids (NGLs). The produced styrene is high purity and suitable for polymerization at a very attractive cost compared to conventional styrene production routes. The process is economically attractive for pygas feeds containing more than 15,000 tpy of styrene. 

If large amounts of isobutyl alcohol are available as by-product of the Oxirane process for propylene oxide manufacture, it can also be obtained very easily form the isobutylene contained in Cl streams of steam cracking units. After the recovery of butadiene, the isobutylene of these streams is selectively hydrated to tertiary butyl alcohol. This process carried out in the liquid phase in the presence of a solid catalyst is certainly realized in fixed bed reactors. Very few details have been disclosed on this reaction which constitutes the first step of a new process to obtain methylmethacrylate from a spent butylene isobutylene feed (5 ). 

In steam cracking, a gaseous or liquid hydrocarbon feed like naphtha, LPG (low pressure gas), or ethane is diluted with steam and then briefly heated in a furnace, obviously without the presence of oxygen. 

The VPS overhead consists of steam, inerts, condensable and non-condensable hydrocarbons. The condensables result from low boiling material present in the reduced crude feed and from entrainment of liquid from the VPS top tray. The noncondensables result from cracking at the high temperatures employed in the VPS. Inerts result from leakage of air into the evacuated system. Steam and condensable hydrocarbons are condensed using an overhead water-cooled condenser. The distillate drum serves to separate inerts and non-condensables from condensate, as well as liquid hydrocarbons from water. Vacuum is maintained in the VPS using steam jet ejectors. 

Steam stripping is not adequate for the bottoms purity required. More positive stripping is obtained by charging the tower bottom liquid to a heating unit known as a reboiler. In a typical reboiler, 50% of the feed is vaporized and returned to the tower below the bottom plate. A fractionating tower equipped with a steam heated reboiler is shown in Figure 4. The reboiler may also be heated by a hot oil stream, such as a pumparound reflux stream from the primary fractionator of a cracking unit, or by a fired furnace. 

Feed to the unit may be all liquid, all vapor, or a mixture of liquid and vapor. Use of a reboiler causes the heavier liquid product to leave the system at a higher temperature than if steam stripping alone were used. For this reason, the use of reboilers is restricted to those systems in which the temperature is well below the level at which cracking takes place. In practice, materials boiling above about 400°F are seldom distilled in a reboiler type unit. 

Figure 7.7b shows the essential features of a refinery catalytic cracker. Large molar mass hydrocarbon molecules are made to crack into smaller hydrocarbon molecules in the presence of a solid catalyst. The liquid hydrocarbon feed is atomized as it enters the catalytic cracking reactor and is mixed with the catalyst particles being carried by a flow of steam or light hydrocarbon gas. The mixture is carried up the riser and the reaction is essentially complete at the top of the riser. However, the reaction is accompanied by the deposition of carbon (coke) on the surface of the catalyst. The catalyst is separated from the gaseous products at the top of the reactor. The gaseous products leave the reactor... 

Referring to the hardware in Figure 5—4, there are much larger facilities required for heavier liquids cracking than for ethane or propane. As you saw in Table 5—1, the yield of ethylene from the heavier feeds is much lower than from ethane. That means that to produce the same amount of ethylene on a daily basis, the gas-oil furnaces have to handle nearly five times as much feed as ethane furnaces. As the design engineer scales up these volumes, he or she has to worry about the size of the cubes necessary to heat up that much feed, the residence times best for each kind of feed, and the best pressure/temper-ature/steam mixture conditions. 

Refinery ethylene is usually made by the catalytic cracking of ethane, propane, or a mixed hydrocarbon stream, such as recovered natural gas liquids, naphthas, or gas oil [11]. Cracking conditions are quite severe 750-900°C and 0.1-0.6 second residence time for a low partial pressure hydrocarbon stream. A number of metal oxide catalysts have recently been evaluated for this purpose [12]. The usual diluent is steam, used at a weight ratio of steam to hydrocarbon of 0.2 1 for ethane feed, to progressively higher ratios with the higher molecular weight hydrocarbons of up to 2.0 1 for gas oil. 

One particular advantage of the CDTECH process is the ability of the alkylation reactor to accept a dilute ethylene feed. Because the alkylator operates in a mixed vapor-liquid phase, it is capable of utilizing dilute ethylene feeds, for example, offgas from a fluid catalytic cracking plant or dilute ethylene from a steam... 

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