Steam cracking Residence time

Olefins are produced primarily by thermal cracking of a hydrocarbon feedstock which takes place at low residence time in the presence of steam in the tubes of a furnace. In the United States, natural gas Hquids derived from natural gas processing, primarily ethane [74-84-0] and propane [74-98-6] have been the dominant feedstock for olefins plants, accounting for about 50 to 70% of ethylene production. Most of the remainder has been based on cracking naphtha or gas oil hydrocarbon streams which are derived from cmde oil. Naphtha is a hydrocarbon fraction boiling between 40 and 170°C, whereas the gas oil fraction bods between about 310 and 490°C. These feedstocks, which have been used primarily by producers with refinery affiliations, account for most of the remainder of olefins production. In addition a substantial amount of propylene and a small amount of ethylene ate recovered from waste gases produced in petroleum refineries. 

Advanced Cracking Reactor. The selectivity to olefins is increased by reducing the residence time. This requires high temperature or reduction of the hydrocarbon partial pressure. An advanced cracking reactor (ACR) was developed jointly by Union Carbide with Kureha Chemical Industry and Chiyoda Chemical Constmction Co. (72). A schematic of this reactor is shown in Figure 6. The key to this process is high temperature, short residence time, and low hydrocarbon partial pressure. Superheated steam is used as the heat carrier to provide the heat of reaction. The burning of fuel... 

The thermal cracking of a light ffaction of mixed plastics waste was carried out in a fluidised bed reactor and the fractions obtained were analysed by elemental analysis, gas chromatography and ashing. The main components of the waste were PE and PP with a small amount of PS and the bed was fluidised by pyrolysis gas, nitrogen or preheated steam. Experiments conducted at different temperatures and residence times were compared by calculating the crack severity for each experiment. The results obtained revealed that the amounts of ethene and propene obtained by pyrolysis with steam were comparable with those obtained using a commercial steam cracker. 

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. 

Steam-cracking reactors typically consist of several steel tubes, perhaps 100 m long and 4 in. in diameter in a tube furnace with reactants and steam fed through the several tubes in parallel. The ceramic fined furnace is heated by burning natural gas at the walls to heat the tubes to 900°C by radiation. The reactor is fed by ethane and steam in a ratio of 1 1 to 1 3 at just above atmospheric pressure. The residence time in a typical reactor is approximately 1 sec, and each tube produces approximately 100 tons/day of ethylene. We will return to olefins and steam cracking in Chapter 4. 

Modem steam cracking reactors use 4-in. steel tubes 100 ft long in a tube furnace heated to -850°C. Pressures are approximately 2 atm (sufficiently above 1 atm to force reactants through the reactor), and residence times are typically 1 sec. Water (steam) interacts very httle with the hydrocarbons by homogeneoirs reactions, but more water than alkane is typically added to reduce coke formation. 

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. 

The preheated feedstock enters the bottom of the fractionator, where it is mixed with the recycle oil. The mixture is pumped up to the charge heater and fed to the soaking drum (ca. atmospheric pressure, steam injection at the top and bottom), where sufficient residence time is provided to complete the thermal cracking. In the soaking drum, the feedstock and some product flows downward passing through a number of perforated plates while steam with cracked gas and distillate vapors flow through the perforated plates countercurrently. 

Cracking temperature and vapor residence time were the most important parameters controlling the cracking reactions. Within the range of conditions tested, other variables such as type and area of cracking surface, the vapor concentration of the feedstock and presence of steam made little difference to the yields of BTX and ethylene. Steam is used as a diluent and... 

Description Fresh feedstock and recycle streams are preheated and cracked in the presence of dilution steam in highly selective PyroCrack furnaces (1). PyroCrack furnaces are optimized with respect to residence time, temperature and pressure profiles for the actual feedstock and the required feedstock flexibility, thus achieving the highest olefin yields. Furnace effluent is cooled in transfer line exchangers (2), generating HP steam, and by direct quenching with oil for liquid feedstocks. 

Furnaces with very short residence time (Short Residence Time technolog> developed b> Lummus) adapt ideally to the cracking of gas oils on account of their tube diameter, which is larger than that of standard equipment, the low partial pressure of steam, and decreased coking. 

After a residence time of about 0.020 s, the reaction products leave the reaction zone at about 900 C and undergo rapid quench by the injection of a heavy oil in a quench cooler devdoped by the Ozaki Chigh-pressure steam. The quench oil is recovered and recycled, while the cracking effluents are fractionated to separate the naphtha and tars. Tlie gaseous products are then treated m a special compression and treatment system required to remove large amounts of acetylene. CO and H2S obtained in the procsess. The rest of the recovery section is conventional. 

A thermal cracking unit for waxes consists of a furnace, a primary separation column, a stabilization column and a distillation section. The feedstock is vaporized, mixed with steam to 40 per cent weight, and enters a tubular furnace in which the residence time is a few seconds (2 to 10 s) at 500 to 600°C. Once-tbrougb cbnversion is relatively low (15 to 30 per cent) to avoid side reactions. Operation is at atmospheric pressure or ghtly above. Direct quench, or quench with a heat transfer fluid, generates steam. Primary fiactionation allows the recycling of the unconverted part of the feedstock. 

Kureha crude oil steam cracking technology was devdoped jointly with Union Carbide for the manufacture of ethylene (see Section 2.13.4). By operating at very temperature and with very short xomact dmes (0LOO3 to 0.010 s), approximately equal amounts of acetylene and ethylene can be produced from a number of crude oils. This is illustrated by Table 52 for Indonesian and Arabian crudes, cracked in the presence of steam at 2000 in a steam to feed weight rado of about 5, and with residence time of O.OOS s. In these conditions, the temperature at the reactor exit before quench reaches 1150-C. /... 

The propylene (steam-cracked C3 cut) is dimerized in the presence of nripropylalo-mioum as catalyst at 130 to 200 C and 20.10 Pa absolute. Residence time is about 15 min. Molar selectivity is as high as 90 to 95 per cent for once-through convc. sion... 

Ethane enters the pyrolysis section, which comprises a series of cracking furnaces. The ethane is heated as quickly as possible to the cracking temperature and maintained at this temperature for the minimum residence time. In order to lower the hydrocarbon partial pressure and mitigate coke forming in the pyrolysis tubes, steam is added to the ethane prior to entering the pyrolysis section (not shown). 

The reference test was conducted in a stainless steel reactor assembly which was sized to duplicate the Kureha reactor geometry. The experimental operating conditions compared favorably with the actual plant conditions. In particular, the steam temperature, S F ratio, residence time, oil feed rate, and heat input were matched very closely. However, the reactor exit temperature was somewhat lower than that of the operating plant. The experimental gas yields for ethylene, ethane, propylene, and propadiene agreed very well with the plant. There were slightly lower experimental values for hydrogen, methane, acetylene, and total gas, which indicated a less severe crack. 

Since investment costs had to be kept low, the convection section, the fire box, and the burners were left unchanged. To fulfill these requirements, the new LSCC design was calculated for the same naphtha, P/E, hc-throughput, steam dilution, and crossover temperature. The following improvements in cracking parameters were calculated for the LSCC design pressure drop = 31% less, residence time = 28% less, and MCP = 43% less. 

In quite a different application, a novel approach for producing olefins via a hydrocarbon-steam cracking process, without the use of a catalyst, was demonstrated to benefit from the use of a honeycomb monolithic catalytic reactor [28]. A typical problem associated with cracking processes of this type is maintaining the appropriate combination of heat transfer and residence time, which, if not balanced, will lead to either poor conversion... 

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