Further understanding and evaluation of the delayed coking process

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Further Understanding and Evaluation of the Delayed Coking Process Abstract: The economic viability of delayed coking and catalytic cracking was compared; under current price conditions, catalytic cracking offers slightly better benefits than delayed coking. Through ethylene cracking tests and comparisons between hydrogenated delayed coking gasoline and conventional straight-run gasoline, it was found that hydrogenated delayed coking gasoline can be used as a feedstock for ethylene production. Changes in the circulation ratio have a significant impact on the product distribution, but little effect on economic efficiency. Keywords: delayed coking, process, understanding, economics. 1 Introduction Delayed coking is a fairly mature process for processing vacuum residue, and it has been used as a common method for further processing over the years. In recent years, as the properties of crude oil have deteriorated (i.e., sulfur content has increased), the trend in coking capacity has been rapid. Wax oil from delayed coking is generally used directly as a feedstock for catalytic cracking in China, but it is not favored for catalytic cracking due to its high nitrogen content. Some delayed coking units, in order to increase production capacity at the lowest cost, use a low recycle ratio or even a zero recycle ratio, which results in an increased amount of coking wax oil of lower quality; this constitutes one of the challenges faced by such coking units. Over the next 5 years, delayed coking units in our country will continue to develop, making it even more important to understand and assess the role of these units within oil refineries. 2 Comparison between delayed coking cracking and catalytic cracking: In the past, catalytic cracking used straight-run wax oil as its main feedstock, while coking relied on vacuum residue; the difference in feedstock prices made it difficult to compare the two methods ； However, as more slag oil is used in catalytic cracking, the comparability between the two becomes greater. In recent years, catalytic cracking technology for processing nearly 100% desulfurized residue (Daqing desulfurized residue) has been developed, enabling good comparability between the two. The following compares the two using Daqing residue oil as an example. The data listed in Tables 1 and 2 show the properties of the feedstocks and the product distributions for the delayed coking unit of Daqing Petrochemical Company, the residue catalytic cracking unit of Qiange Oil Refinery, and the catalytic cracking unit of Yanshan Petrochemical Company. In terms of the nature of their products, as technology advances and requirements for product quality increase, not only does diesel produced through catalytic cracking need further processing, but gasoline from this process also requires further treatment due to its high olefin content, similar to how gasoline and diesel from coking processes need further processing. 　　Table 3 shows a comparison of the economic benefits of the two, with prices shown excluding tax. For simplicity, coker wax oil is further processed via catalytic cracking to reduce the number of products. Regarding the selection of product prices, coker dry gas can be used as a raw material for hydrogen production, with a price of 1,000 yuan per ton; catalytic dry gas, used as a regular fuel, has a price of 800 yuan per ton ； The properties of catalytic gasoline are superior to those of coker gasoline (which becomes high-quality ethylene feed after hydrogenation); considering a price difference of 80 yuan per ton between the two. However, the properties of catalytic diesel are inferior to those of coker diesel, with a price difference of 50 yuan per ton between them. At the prices listed in Table 3, the sales revenue from catalytic cracking at Qiange is 40.68 yuan per ton of feedstock higher than that from coking; at Yanshan, it is 54.04 yuan per ton higher. However, considering that the unit processing cost for catalytic cracking is higher than that for coking (by about 20 yuan per ton), and given that the catalytic cracking feedstock at Yanshan Petrochemical contains 14.9% wax oil (with the price difference between wax oil and residue being over 300 yuan per ton), the economic efficiency of catalytic cracking using only residue is only slightly higher than that of delayed coking. In the previous years, when there was a large difference in prices between gasoline and diesel, as well as between gasoline and naphtha, and when there were no requirements regarding the olefin content in gasoline, catalytic cracking was very profitable. However, now that the price differences have narrowed, the economic benefits of these two methods have become more similar. However, not all slag reduction methods, such as the one used in Daqing, can be incorporated in large quantities into catalytic cracking feedstocks to replace delayed coking; therefore, delayed coking still plays an irreplaceable role in catalytic cracking. Table 1: Properties of the raw materials used in the three units
Daqing Petrochemical, Qian’guo, Yanshan (85.1% slag mixture)
Density (20°C)/g·cm³: 0.920, 0.921, 0.920, 0.919
Viscosity (100°C)/mm²·s: 103.1, 105.7, 114.4, 77.92
Freezing point/°C: 35, 25
Residue, %: 9.8, 8.5, 98.2, 7.19
Elemental content, %:
C: 86.54, 86.91, 86.85
H: 12.42, 12.55, 12.62
Metal content/μg·g⁻¹:
Fe: 12.41, 1.83, 3.9
Ni: 6.57, 8.85, 5.6
V: 0.1, <0.1
Ca: <0.1, <0.1
Na: 1.48, 3.03, 3.2

Table 2: Product distribution in coking and catalytic cracking
Product distribution, %
Daqing Petrochemical, Qian’guo, Yanshan (85.1% slag mixture)
Dry gas: 5.30%, 5.04%, 4.15%
Liquefied gas: 6.10%, 11.09%, 9.39%
Gasoline: 26.72%, 49.95%, 54.10%
Diesel: 45.04%, 11.80%, 28.74%
Oil slurry: 0.10%, 7.45%, 5.05%
Charring: 0.93%, 11.00%, 10.82%
Petcoke: 15.3%
Losses: 0.61%, 0.51%, 0.75%

Table 3: Economic benefit comparison
Product name | Catalytic product price/ yuan·t⁻¹ | Product revenue/ yuan·t⁻¹ | Coking product price/ yuan·t⁻¹ | Coking product revenue/ yuan·t⁻¹
Dry gas (Qian’guo, Yanshan): 800, 40.00, 33.20; 1,000, 53.00
Liquefied gas: 1,950, 214.50, 183.11; 1,950, 118.95
Gasoline: 1,900, 949.05, 780.90; 1,820, 486.30
Diesel: 1,700, 200.60, 488.58; 1,750, 788.20
Oil slurry: 1,200, 128.88, 60.60
Petcoke: 300, 45.90
Total: 1,533.03, 1,546.39, 1,492.35

3. Relationship between delayed coking and ethylene feedstock
With the development of the petrochemical industry, both the production of resins and polyesters require large amounts of naphtha as a raw material. However, a large portion of the crude oil in our country has a low naphtha content, which is far from sufficient to meet the raw material demands of the ethylene industry and catalytic reforming. To this end, it is necessary to obtain naphtha from vacuum wax oil and vacuum residue, and hydrocracking and delayed coking are the most suitable methods. The free-radical reaction mechanism of coking determines that coker gasoline and coker diesel contain more alkanes and fewer aromatics. 　　Table 4 shows the properties of the catalytic gasoline from the two refineries after hydrogenation; it also lists the properties of A’s Marsilene naphtha for comparison. Among them, Refinery A processes almost entirely imported crude oil; its coking feedstock (vacuum residue) is extremely complex, and the Yemeni Masyra crude oil listed in the table is one of the imported crudes processed by this refinery ； The coker naphtha from Refinery B is obtained by delayed coking of the residue from Luning pipeline crude oil. When comparing the coker naphtha from the two refineries, since Refinery B uses coking feedstock that is high in aromatics, resins, and asphalts, its coker naphtha contains slightly less alkanes and slightly more aromatics; however, the overall difference is not significant. 　　Table 5 shows the results of the steam cracking evaluation tests on these three types of naphtha. The yields of ethylene, propylene, and total trienes in the coker gasoline were all high, comparable to those of conventional naphtha. Extensive industrial experience shows that naphtha from delayed coking, after hydrogenation, is an excellent raw material for ethylene production. However, given the scale and operating methods of the coking units in domestic refineries at present, only a limited amount of coker gasoline can be produced, which can serve only as a supplement. To enable coking units to provide more ethylene feedstock, one approach is to expand the scale of delayed coking units, while another is to change the operating methods of delayed coking in order to appropriately raise the dry point of coker gasoline and increase its yield. Table 4 Properties of coker gasoline after hydrogenation
Parameter: Plant A, Plant B, Masila naphtha
Density (20°C)/g·cm-3: 0.702, 0.90705, 0.7278
Hydrogen content, %: 15.41, 15.22, 15.13
Composition analysis, %:
Alkanes: 70.54, 69.62, 55.94
Naphthenes: 25.43, 23.77, 41.14
Aromatics: 4.01, 6.61, 2.82
Olefins: 0.02, 0.1
Boiling range/°C:
Initial boiling point: 444, 158, 50%; 97, 108, 115
90% boiling point: 140, 146, 137
Dry point: 173, 172, 152

Table 5 Evaluation of steam cracking performance (industrial conditions)
Parameter: Plant A, Plant B, Masila naphtha
Furnace type: SW-28U, USRT-III, SW-28U
Furnace outlet temperature/°C: 850
Yield of products per pass, %:
Ethylene: 30.98, 28.80, 28.51
Propylene: 16.47, 16.86, 15.22
Total trienes: 52.90, 50.59, 49.77

Conclusions:
(1) The economic benefits of delayed coking are currently slightly lower than those of residue catalytic cracking. As the quality requirements for gasoline and diesel increase and their prices become more similar, the economic benefits of delayed coking will gradually approach those of catalytic cracking. 　　(2) The gasoline from delayed coking, after hydrogenation, is an excellent raw material for ethylene and should be made full use of. 　　(3) Not all slag reducers can be added in large quantities to catalytic cracking feedstocks; therefore, delayed coking still plays an irreplaceable role in catalytic cracking. As the trend of crude oil discoloration and deterioration becomes more apparent day by day, along with rising requirements for product quality, the role of delayed coking will become even more significant.