Ethane, cracking

This chapter describes the basic process of ethane steam cracking operations to produce ethylene and the integration with downstream operations. The approach to economic analysis for various types of ethane cracking operations is described and the economic analysis for ethane cracking in a standardised approach is developed. The production of olefins from other feed stocks and the economics of production are developed in later chapters  

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). 

Immediately after cracking, the cracked gases are reduced in temperature as quickly as possible to stop the cracking processes and prevent the cracked gases forming coke. This quenching is often referred to as the transfer-line-heat-exchange (TEE). Excess steam is condensed and the water recycled (not shown). Heat from the TEE is recovered as process steam. 

The gases are then compressed further and passed through a drier to the separation train. The cracked gases now contain only hydrocarbons and hydrogen. 

Ethane cracking produces a range of by-products as well as ethylene. However, relative to other feed stocks, the amount of byproducts is small and in many small-scale ethane cracking operations these are used as fuel in the pyrolysis furnace. 

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The most important commercial use of ethane and propane is in the production of ethylene (qv) by way of high temperature (ca 1000 K) thermal cracking. In the United States, ca 60% of the ethylene is produced by thermal cracking of ethane or ethane/propane mixtures. Large ethylene plants have been built in Saudi Arabia, Iran, and England based on ethane recovery from natural gas in these locations. Ethane cracking units have been installed in AustraHa, Qatar, Romania, and Erance, among others. 

When a mixture is cracked, one or more components in the feed may also be formed as products. Eor example, in the cocracking of ethane and propane, ethane is formed as a product of propane cracking and propane is formed as a product of ethane cracking. Therefore, the "out" term in the above equation contains the contribution or formation from other feed components and hence does not represent tme conversion. Eor simple mixtures, the product formation can be accounted for, and approximate tme conversions can be calculated (29). Eor Hquid feeds like naphtha, it is impractical if not impossible to calculate the tme conversion. Based on measured feed components, one can calculate a weighted average conversion (A) (30) ... 

Fig. 3. Weight of coke formed (AQ and coking rate (r) in ethane cracking as a function of time (51).

Cracking temperatures are somewhat less than those observed with thermal pyrolysis. Most of these catalysts affect the initiation of pyrolysis reactions and increase the overall reaction rate of feed decomposition (85). AppHcabiUty of this process to ethane cracking is questionable since equiUbrium of ethane to ethylene and hydrogen is not altered by a catalyst, and hence selectivity to olefins at lower catalyst temperatures may be inferior to that of conventional thermal cracking. SuitabiUty of this process for heavy feeds like condensates and gas oils has yet to be demonstrated. 

The Cj plus bottoms from the demethanizer then go to the deethanizer. A propylene-propane bottoms product containing 90-92% propylene is obtained which may either be sold, used directly as propylene- 90, or further purified. The ethylene-ethane overhead from the deethanizer is separated in the splitter tower yielding a 99.8% overhead ethylene product at -25°F. The ethane bottoms at -l-18°F may either be sent to fuel gas or used as feed to an ethane cracking furnace. Overall ethylene recovery in these facilities is about 98%. The product is of very high purity with less than 50 parts per million of non-hydrocarbon contaminants and a methane plus ethane level below 250 ppm. 

The following are typical operating conditions for an ethane cracking unit and the products obtained ... 

The production of ethylene by gas crackers, mostly from C2, C3, and some C4 feeds, amounts to about 40% of the world ethylene capacity. This results in a small coproduction of benzene compared to benzene co-produced in naphtha and gas oil crackers, which account for 60% of the world s ethylene production capacity. A typical overall benzene yield from ethane cracking is on the order of only 0.6% of the ethane feed, and the yield of benzene from propane cracking is on the order of 3% of the propane feed. In contrast, the... 

Do these results also suggest that five-coordinate carbonium ions are not essential to explain alkane cracking The evidence is mixed. Kazansky and van Santen (132) reported low-level calculations and found a metastable carbonium ion (CH3-H-CH1) formed from ethane and a zeolite Brpnsted site, but this species was so high in energy that it did not appear to be thermally accessible. More extensive work by van Santen (133) shows, however, that the transition states leading from this species do not relate to ethane cracking Blaszkowski, Nascimento, and van Santen (134) found other transition states for ethane cracking (Fig. 26) that are similar to carbenium ions albeit with stabilization from the lattice. 

As one progresses from ethane through heavier, lower hydrogen content feedstocks the yield of ethylene decreases and causes feed requirements to increase markedly. For example, cracking heavy gas oil would require slightly over three times the amount of feed (per lb of ethylene) that is needed for ethane cracking. The extra feed required for heavier stocks is, of course, distributed among the various by-products. 

Ethylene also may be produced from other paraffinic or naphthenic hydrocarbons. The reactions are highly endothermic (34.400 keal/kg mole of ethane cracked at approximately 900°C) and proceed in the direction indicated at temperature exceeding approximately 620°C without a catalyst. 

Energy consumption Energy consumptions are 3,300 kcal/kg of ethylene produced for ethane cracking and 5,000 kcal/kg of ethylene for naphtha... 

Commercial plants Technip has been awarded four ethylene plants ranging from 500 kty up to 1,400 kty using either ethane or liquid feedstocks. While over 300 cracking furnaces have been built, and 15 units operate worldwide, numerous expansions over the nominal capacity based on progressive separation techniques are under way, with up to an 80% increase in capacity. For ethane cracking, front-end hydrogenation scheme is also available. 

The feedstock used in the Middle East is illustrated in Figure 1.6. What distinguishes cracking operations in the Middle East from those of other regions is the dominance of ethane cracking over other feedstocks. 

Both 1-butene and 2-butene can be used as a monomer for specialist polymers. Of interest to integrated cracking and polymer production operations is 1-butene for co-polymerisation with ethylene to produce LLDPE (linear low-density polyethylene/. For ethane cracking operations where the C4 stream maybe insufficient, 1-butene can be made by from ethylene by dimerisation. ... 

Ethane cracking is conducted across the world. The scales of operation range from the smallest, less than 50,000 t/y, when small amounts of ethylene is required, for example for a stand-alone styrene plant, to the largest ethylene production operations of over 1 million tonnes of ethylene. The block flow layout for a small stand-alone ethane cracking operation is illustrated in Figure 7.1. 

Figure 7.1 Typical flow-sheet for ethane cracking...

The gases are now passed to a splitter column which separates the ethane and ethylene. Ethane cracking has a relatively low pass conversion and there are relatively large amounts of ethane present in the ethylene stream. After separation, the ethane is recycled to the feed section where it is cracked to extinction. The ethylene is passed to downstream units for production of other chemicals and resins. 

In larger-scale ethane cracking operations, or those integrated into large chemical complexes, the useful by-products can be separated and used. In this instance the pyrolysis furnace is fired by fuel oil. Note that different process licensors have differing approaches to the layout of the unit operations. A typical situation is illustrated in Figure 7.2. 

Table 7.1 presents typical data from a large ethane cracking operation designed to produce 500,000 tonnes per year of ethylene with a flow-sheet similar to that given in Figure 7.2. 

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