Methane cracker

Figure 5.51 Schematic of the process of a methane cracker/fuel cell system, as proposed by Poirier et al. [80].

A typical ethane cracker has several identical pyrolysis furnaces in which fresh ethane feed and recycled ethane are cracked with steam as a diluent. Figure 3-12 is a block diagram for ethylene from ethane. The outlet temperature is usually in the 800°C range. The furnace effluent is quenched in a heat exchanger and further cooled by direct contact in a water quench tower where steam is condensed and recycled to the pyrolysis furnace. After the cracked gas is treated to remove acid gases, hydrogen and methane are separated from the pyrolysis products in the demethanizer. The effluent is then treated to remove acetylene, and ethylene is separated from ethane and heavier in the ethylene fractionator. The bottom fraction is separated in the deethanizer into ethane and fraction. Ethane is then recycled to the pyrolysis furnace. 

Methane and fuel oil produced by the cracker are recycled as fuel. All the ethane and ... 

Energy cost. The fixed heat loss of 20 X 106 Btu/h can be expressed in terms of methane cost (5.380/lb) using a heating value of 21,520 Btu/lb for methane. The fixed heat loss represents a constant cost that is independent of the variables, hence in optimization we can ignore this factor, but in evaluating the final costs this term must be taken into account. The value for 7 depends on the amount of fuel oil and methane produced in the cracker ( 7 provides for any deficit in products recycled as fuel). 

The separation section of a gas oil cracker looks like a small refinery, as you can see in Figure 5 or in Figure 5—5. In addition to the fractionators and treaters used in the purification section of the simpler ethane cracker, there are facilities to separate the heavier coproducts. In the front end of the separator facilities in Figure 5-4, the cold box option for handling the liquefaction of the gases is shown. Temperatures as low as -220°F are achieved in this super-refrigerator. At those low temperatures, Freon wont do the job. Liquid air, methane, ethylene, or ammonia are often used as the refrigerant in much the same way Freon has been used in an air conditioner. 

Other catalytic reactions carried out in fluidized-bed reactors are the oxidation of naphthalene to phthalic anhydride [2, 6, 80] the ammoxidation of isobutane to mcthacrylonitrilc [2] the synthesis of maleic anhydride from the naphtha cracker C4 fraction (Mitsubishi process [81, 82]) or from n-butane (ALMA process [83], [84]) the reaction of acetylene with acetic acid to vinyl acetate [2] the oxychlorination of ethylene to 1,2-di-chloroethane [2, 6, 85, 86] the chlorination of methane [2], the reaction of phenol with methanol to cresol and 2,6-xylenol [2, 87] the reaction of methanol to gasoline... 

In those days, pitch chemistry had not advanced sufficiently to understand fundamentally the foregoing phenomena. The preparation of mesophase pitch with low softening point (the so-called "soft mesophase pitch") was based on direct experiment. Nevertheless through extensive and serious efforts, it became possible to prepare soft mesophase pitches from naphtha tars, decant oils from fluidized catalytic crackers (FCC), atmospheric-reduced crude oils, and other pitch-like materials. A typical example of these preparation procedures is the following. A purified FCC or naphtha pitch is heated at 400°C for one hour under methane to convert the pitch to a mesophase content of 23.6% (27,28). The mesophase separated by sedimentation has a softening point of 226°C it is spun at 320°C, and the fiber is stabilized in air and finally carbonized by rapid heating at 100 to 1600°C/min (29). 

The fiwdstocks used for pyrolysis vary widely and range from light saturated hydrocarbons such as ethane, propane, and even ethane/propane blends, to heavier petroleum cuts such as petrochemical naphtha and light and heavy gas oils. In this respect, the situation is clearly in favor of fight hydrocarbons in the United States, a country that is rich in natural gases containing methane as well as ethane and propane, and vHiich still mainly uses the latter two to manufacture ethylene, hi Europe and Japan, by contrast, petroleum cuts traditionally supply the steam cracker feedstocl (Table Zl). 

The first separation unit is a tower that separates methane and hydrogen from the C2+ gases (de-methaniser). These are used as fuel in smaller cracking operations, but can be further separated in the larger scale crackers to produce a fuel gas and hydrogen. 

Table 7.2 presents the data for a plant which extracts propylene and pyrolysis gasoline, but recycles the rest of the products. Of the byproducts the mixed C4 stream is recycled to the feed-side of the cracker furnaces, with the hydrogen and methane recycled to the fuel-side. The same quantum of operating allowances for feed and fuel are included in the statistics. 

After separation of the mixed olefins the product work up is similar to that in a steam cracker using LPG feedstock. Small amounts of carbon dioxide are removed and the hydrocarbon gases are dried before passing to a de-ethaniser column. The C2- fraction is passed to an acetylene removal unit before methane is removed from the C2 stream. This comprises 98-i-% ethylene, the remainder being ethane. The C3+ stream is split between the C3 fraction (98% propylene) and C4+. The work up of the C4 stream to produce linear butenes (not shown in the figure) is likely to be less problematic than the corresponding C4 stream from steam crackers, which is highly complex and cannot be separated by fractionation alone. The process produces little product above C5. 

A less effective, but more economically viable method, would be to recycle all low-value hydrocarbon by-products to the cracker furnace. This particularly focuses on methane which within the confines of an operation is typically valued relative to the fuel oil price. However, this equally applies to ethane and propane which are generally recycled to the feedstock side of the cracking furnace. Depending on the relative value, it may be optimal for minimising carbon emissions in some operations to use ethane as a fuel rather than a feedstock. 

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. 

H-Oil unit are processed for sulfur recovery and then sent for separation through the gas recovery facilities associated with the steam cracker. Remaining unconverted residue from the H-Oil operation is used as a fuel oil component for plant fuel. Ethylene is manufactured by steam cracking of ethane, propane, naphtha, and distillate, and products from these operations are separated in conventional gas recovery facilities. Hydrogen for H-Oil is partially supplied by by-product recovery from steam cracker and H-Oil off-gases supplemented by steam reforming of methane. The heavy oils produced in steam cracking of naphtha and distillate are blended with the H-Oil residue to yield plant fuel. 

For the thermal cracking, a tandem furnace can be operated where a methane-air flame is used to heatup firebrick to w 1400 °C. The air is then turned off and the methane decomposes on the hot brick until the temperature drops below 800 °C. The H2 enriched effluent gas stream is taken to heatup the second furnace. A continuous process is thought of as being possible also, but has not yet been practiced on a conunercial scale [55]. A small-scale catalytic propane cracker has been realized at the Duisburg University, Germany (see section 9.4.). 

Methanol is endothermally cracked over a catalyst to produce synthesis gas plus traces of methanol, ether, and methane. A two-stage membrane separation system extracts the hydrogen from a CO-rich fuel that fires the cracker (see Fig. 5-3). 

Extremely high rates of temperature change (10 K/s) show that it is possible to intermpt the methane cracking chain before solid carbon is produced from acetylene. Thus the problem of coke formation can be eliminated. In any case possible formation of coke in liquids is not so detrimental problem as in the conventional hydrocarbon crackers - in view of low wettability in many liquids coke could be relatively easier separated. 

The table on p. 358 shows typical overall yield patterns (%w/w), with ethane/propane recycle, for a number of cracker feedstocks n-butane gives high ethylene yields, isobutane more propylene and methane. In all cases, the cracker product stream is cooled rapidly to below 400°C to minimize further reactions. After further cooling and separation of condensed hydrocarbons and water, the gases (H2,Ci—CJ are compressed, scrubbed with aqueous alkali to remove CO2 and other acidic contaminants, and dried over solid beds. Thereafter, the C2 and C3 alkanes and alkenes are separated by distillations at pressures up to 3 5 atm., with refrigerated condensers for the early columns in the train. Selective hydrogenation to remove acetylenes and dienes (most frequently over a... 

Rather recently, in Western Europe, methane from natural gas or from refineries has been preferred to feed steam reformers. Natural gas liquid has not been available to steam crackers as in the United States. 

The cracker yields 75% ethylene gas leaving 25% gas effluent. While the methane and ethane hydrocarbon gas streams from the waste gases were fully recovered, a large part of the remaining gases - propylene, butane, propane and pentanes including higher aromatics and paraffin - had to be disposed of either via the flare or the site incinerator. 

Carbon can be formed via the whisker mechanism as for methane (reaction (9)), but it may also be formed by thermal cracking of the hydrocarbons (reaction (10)) taking place above 600-650 C. In fact, a steam reformer without a catalyst would operate like a pyrolysis furnace (steam cracker) for ethylene production. 

Ethylene. The largest potential chemical market for n-butanc is in steam cracking to ethylene and coproducts. n-Butane is a supplemental feedstock for olefin plants and has accounted for 1 to 4 percent of total ethylene production for most years since 1970. It can be used at up to 10 to 15 percent of the total feed in ethane/propane crackers with no major modifications. n-Butane also can be used as a supplemental feed at as high as 20 to 30 percent in hea y naphtha crackers. The consumption of C s has fluctuated considerably from year to year since 1970, depending on the relative price of butane and other feedstocks. The yield of ethylene is only 36 to 40 percent, with the other products including methane, propylene, ethane, butadiene, acetylene, and butylenes. About 1 to 2 billion lb of butane are consumed annually to produce ethylene. 

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