Steam cracker products

One important application of selective hydrogenation of alkynes is their removal from the industrial steam cracker products. These can contain several percents of alkynes as byproducts. They are particularly unwelcome in that they poison the catalysts used for the downstream polymerization of the olefins. Selective hydrogenation of these steam cracker cuts has two advantages. It removes acetylenes and converts them to desired alkenes, thereby increasing the overall yields (see Section 11.6.1). 

Finally, it is important to address the topic that is not only scary, but keeps many industrial chemists awake at night cost (see Figure 2.4). The highly optimized production processes for polyolefins, combined with the low costs of the raw materials, lead to bulk prices as low as 1 per kilogram, and sometimes even lower. This, in turn, leads to the realization that almost any other chemistry is more expensive than that produced from steam cracker products there is no room for anything else - the low-hanging fruits are gone ... 

Propylene is also a product of the steam-cracking process (up to 15 wt% of the steam cracker product mix. Section 6.6). In addition, it is obtained by catalytic dehydrogenation of propane (Section 5.3.1) and as a by-product of the FCC process (Section 6.7.2). The technical relevance of propylene as a feedstock for the synthesis of industrial chemical processes has ever increased since the 1960s. The increasing value of propylene can be realized from the fact that propylene was seen in former times as an undesirable by-product of the steam-cracking process and as such it was... 

Apart from the described radical reaction pathways, there are several important side and consecutive reactions that also proceed in the cracking furnace. The higher the product concentration in the stream (i.e., at high feedstock conversion), the higher is the probability of these side and consecutive reactions. Important side and consecutive reactions include isomerization, cyclization, aromatization, alkylation, and also condensation reactions. The aromatic compounds found in the steam cracker product stream are formed, for example, by cycloaddition reactions of alkenes and dienes followed by dehydrogenation reactions. Moreover, monoaromatic compounds transform into aromatic condensates and polyaromatics (see also Scheme 6.6.2) by the same reactions. Typically, more than 100 different products are found in the product mixture of a commercial steam cracker. 

Scheme 6.6.2 Set of different reactions taking place in the cracking furnace that result in more than 100 products in the steam cracker product stream. Adapted from Moulijn, 2001.

Steam cracker units operating on naphtha produce a wide range of products. Their complete utilization is crucial to realize competitive operation of the cracker. As a consequence, steam cracker units are typically operated in networks "Verbund ) of other plants that consume all steam cracker products on the same production site. 

High-purity 1,3-butadiene is recovered from steam cracker product streams. By using dimethyl formamide as a solvent, butadiene can be extracted from the C4 cuts. 

They are classified apart in this text because their use differs from that of petroleum solvents they are used as raw materials for petrochemicals, particularly as feeds to steam crackers. Naphthas are thus industrial intermediates and not consumer products. Consequently, naphthas are not subject to governmental specifications, but only to commercial specifications that are re-negotiated for each contract. Nevertheless, naphthas are in a relatively homogeneous class and represent a large enough tonnage so that the best known properties to be highlighted here. 

Synthetic Fuels. Hydrocarbon Hquids made from nonpetroleum sources can be used in steam crackers to produce olefins. Fischer-Tropsch Hquids, oil-shale Hquids, and coal-Hquefaction products are examples (61) (see Fuels, synthetic). Work using Fischer-Tropsch catalysts indicates that olefins can be made directly from synthesis gas—carbon monoxide and hydrogen (62,63). Shape-selective molecular sieves (qv) also are being evaluated (64). 

Production estimates for propylene can only be approximated. Refinery propylene may be diverted captively to fuel or gasoline uses whenever recovery is uneconomic. Steam-cracker propylene production varies with feedstock and operating conditions. Moreover, because propylene is a by-product, production rates depend on gasoline and ethylene demand. 

Worldwide propylene production and capacity utilization for 1992 are given in Table 6 (74). The world capacity to produce propylene reached 41.5 X 10 t in 1992 the demand for propylene amounted to 32.3 x 10 t. About 80% of propylene produced worldwide was derived from steam crackers the balance came from refinery operations and propylene dehydrogenation. The manufacture of polypropylene, a thermoplastic resin, accounted for about 45% of the total demand. Demand for other uses included manufacture of acrylonitrile (qv), oxochemicals, propylene oxide (qv), cumene (qv), isopropyl alcohol (see Propyl alcohols), and polygas chemicals. Each of these markets accounted for about 5—15% of the propylene demand in 1992 (Table 7). 

Since the bulk of butadiene is recovered from steam crackers, its economics is very sensitive to the selection of feedstocks, operating conditions, and demand patterns. Butadiene supply and, ultimately, its price are strongly influenced by the demand for ethylene, the primary product from steam cracking. Currently there is a worldwide surplus of butadiene. Announcements of a number of new ethylene plants will likely result in additional butadiene production, more than enough to meet worldwide demand for polymers and other chemicals. When butadiene is in excess supply, ethylene manufacturers can recycle the butadiene as a feedstock for ethylene manufacture. 

Table 6 compares the total production of butylenes in the United States, Western Europe, andjapan. Included in this table are relative amounts of productions from different processes. In the United States, about 92% of the butylene production comes from refinery sources, whereas only about 45% in Western Europe andjapan are from this source. This difference arises because the latter cracks mostiy petroleum distillates in the steam crackers that produce larger amounts of butylenes than the light feedstocks cracked in the United States. 

Significant products from a typical steam cracker are ethylene, propylene, butadiene, and pyrolysis gasoline. Typical wt % yields for butylenes from a steam cracker for different feedstocks are ethane, 0.3 propane, 1.2 50% ethane/50% propane mixture, 0.8 butane, 2.8 hill-range naphtha, 7.3 light gas oil, 4.3. A typical steam cracking plant cracks a mixture of feedstocks that results in butylenes yields of about 1% to 4%. These yields can be increased by almost 50% if cracking severity is lowered to maximize propylene production instead of ethylene. 

Many heterogeneous catalysts have been commercialized to dimerize ethylene to selectively yield 1-butene or 2-butene (66—70). Since ethylene is generally priced higher than butylenes, economics favor the production of butylenes from steam crackers, not from ethylene. An exceUent review on... 

Recycles are meticulously accounted for because they load equipment and draw utilities. An olefin plant sustaining relatively low conversion per pass often builds up large amounts of unreacted feed that is recycled to the steam crackers. With utilities charged to ultimate products, these recycles would seem to the model to be free. The model would likely opt for very low conversion, which usually gives high ultimate yield and saves feedstock. Assigning the utility costs to users causes the compressor to pay for the extra recycle and the model raises conversion to the true optimum value. 

Cracking n-hutane is also similar to ethane and propane, hut the yield of ethylene is even lower. It has been noted that cracking either propane or butanes at nearly similar severity produced approximately equal liquid yields. Mixtures of propane and butane LPG are becoming important steam cracker feedstocks for C2-C4 olefin production. It has been forecasted that world LPG markets will grow from 114.7 million metric tons/day in 1988 to 136.9 MMtpd in the year 2000, and the largest portion of growth will be in the chemicals field. 

The chemical value chain shown in fig. 26 results into a product tree over multiple steps starting from the oil refinery and a steam-cracker, chemical products are processed over multiple steps with increasing variety and complexity by adding further substances or additives. The chemical product tree is often reflected in the production structure of chemical produc-... 

Traditional olefin plants have more than one alias. One is even fraudulent. They are variously called ethylene plants after their primary product steam crackers because the feed is usiuilly mixed with steam before it is cracked or whatever aacker, where whatever is the name of the feed (ethane cracker, gas oil cracker, etc.). Olefin plants are sometimes referred to as ethylene crackers, biit only those who don t know any better, use that misnomer. Ethylene is not cracked but rather is the product of cracking. 

In 2000 two major petrochemical companies installed process NMR systems on the feed streams to steam crackers in their production complexes where they provided feed forward stream characterization to the Spyro reactor models used to optimize the production processes. The analysis was comprised of PLS prediction of n-paraffins, /xo-paraffins, naphthenes, and aromatics calibrated to GC analysis (PINA) with speciation of C4-C10 for each of the hydrocarbon groups. Figure 10.22 shows typical NMR spectral variability for naphtha streams. Table 10.2 shows the PLS calibration performance obtained with cross validation for... 

Steam crackers provide the traditional cost-effective approach for olefins production from lighter feed stocks such ethane, propane, naphtha, and AGO. However, these options typically provide higher E/P ratio. To meet the increasing demands of ethylene and particularly propylene, refiners and petrochemical producers are planning integrated facilities. The objectives are ... 

A major current refinery-petrochemical project is under construction for Saudi Aramco-Sumitomo Chemical at their Rabigh, KSA site. In conjunction with our JGC partner we have linked the upstream refinery expansion to a combined mega-DCC unit and mega-ethane steam cracker. Corresponding production rates are 1500 kta ethylene/950 kta propylene. The corresponding integrated layout is shown below in Figure 8.2. 

Larger scale DCC units (and associated FCC units) offer greater product recovery options. These are particularly attractive when linked to either an existing or new steam cracker. In such schemes several of the individual unit operations can be combined or substituted by a more efficient system. 

C4 Hydrorefining. The main components of typical C4 raw cuts of steam crackers are butanes (4-6%), butenes (40-65%), and 1,3-butadiene (30-50%). Additionally, they contain vinylacetylene and 1-butyne (up to 5%) and also some methylacetylene and propadiene. Selective hydrogenations are applied to transform vinylacetylene to 1,3-butadiene in the C4 raw cut or the acetylenic cut (which is a fraction recovered by solvent extraction containing 20-40% vinylacetylene), and to hydrogenate residual 1,3-butadiene in butene cuts. Hydrogenating vinylacetylene in these cracked products increases 1,3-butadiene recovery ratio and improves purity necessary for polymerization.308... 

A unique information with respect to the use Pd on HZSM-5 in a selective hydrogenolysis has been disclosed.501 The transformation of methylcyclohexane to n-alkanes with two or more carbon atoms is a useful transformation since these products are desirable components in synthetic steam-cracker feedstock. It was shown that these compounds are not obtained on catalysts with high (0.2 or 1%) Pd loading or without Pd. But on Pd on H-ZSM-5, with Pd content in the range of 10-100 ppm, the desired products are formed with high ( 78%) selectivity. 

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