Li Yanjun; Zhang Shuhong; Ren Lei; Shen Haiping; Wu Lei; Wang Xieqing

(1. SINOPEC Research Institute of Petroleum Processing, Beijing 100083;2. SINOPEC Engineering Incorporation, Beijing 100101)

Abstract: In order to utilize petroleum resources efficiently and greenly, and solve the problems of high coke yield, highsulfur coke utilization, and environmental protection concerns in China’s refineries, a resid contact cracking and coke gasification integrated technology is being developed by the Research Institute of Petroleum Processing (RIPP). Based on the three technical characteristics including thin films cracking, partial oxidation, and rapid cracking, this technology not only can reduce the production rate of coke and dry gas formed during the process, but also can increase the liquid yield.Moreover, the in-situ low-temperature gasification technology is used to solve the clean utilization of high-sulfur petroleum coke, which can play the role of “Utility Island” and is a green and low-carbon technology for low-quality heavy oil upgrading.

Key words: contact cracking; coke gasification; green low carbon; resid

1 Introduction

Starting from August 2011, the China Petrochemical Corporation (Sinopec Corp.) has formally introduced the green low-carbon concept into its development strategy.Green low-carbon production is an important aspect of building the core competitiveness of enterprises.As for the refining process, ensuring the cleanliness of the production process and the large-scale production equipment and integration of the process are important ways to achieve the green and low-carbon scheme[1].

The world’s oil resources are becoming increasingly heavy and inferior[2], which has brought about many problems to refining enterprises. For inferior residual oil with a Conradson carbon residue (CCR) value of more than 20%and the nickel and vanadium content exceeding 150 μg/g[3],the coking process is one of the most important conversion processes in petroleum refinery. The delayed coking process is used to process inferior residue, which has been playing a huge role in China’s refining industry because of its relatively simple process and low investment. However,the coke yield formed in delayed coking process is about 1.5 times the Conradson carbon residue (CCR) of raw material[4]due to its own limitations including large liquid phase, long residence time, and low temperature reaction after heating the residual oil to the coke drum. In China,the petroleum coke production was 28.201 million tons in 2018[5], 45.92% of which was high-sulfur petroleum coke, while in Sinopec this proportion reached more than 70%. With an increasing sulfur content of crude oil,especially the increased proportion of Middle East crude oil that is processed in the oil refineries, the coking unit of the whole refining industry is facing the problem of oil products with high sulfur content. According to the “Air Pollution Prevention Law of the People’s Republic of China” implemented in 2016, it is prohibited to import,sell and burn petroleum coke that does not meet the quality standards. At present, the maximum sulfur content of petroleum coke (green coke) stipulated by the product standard in petrochemical industry should be no more than 3%[6], which means that high-sulfur petroleum coke with sulfur content more than 3% cannot be produced. In addition, due to the semi-intermittent operation of delayed coking, it is necessary to deliver coke regularly, which will inevitably create the problem for disposal of “waste gas,wastewater, and solid wastes”, which could have greatly polluted the environment. The further utilization of coke needs to be transported to different places, which can also increase the transportation cost.

At present, the world’s industrialized coking process also includes Flexicoking and fluid coking for processing[7-8]residues by fluidization processing method, in addition to delayed coking. One of the difficult problems in fluidized processing of inferior residual oil is fluidization, because the residual oil is a component with high boiling point.This means that there is a gas-liquid-solid three-phase environment, and most of the heavy oil feed injected into a fluidized bed of the cracking reactor can initially form the liquid-solid agglomerates. Poor initial liquid distribution can cause agglomeration and fluidization problems. In order to solve such problems, fluidized coking or Flexicoking has adopted a variety of measures,such as using jet crusher in the reactor, using separation equipment such as elutriator to unload large coke particles from the system[9], setting multi-layer feed at different heights of the reactor, adopting specially designed nozzle[10], and strictly controlling the operating parameters such as temperature, space velocity, and tar ratio, which should match up to the raw material properties.

In order to utilize petroleum resources efficiently and greenly, it is necessary to grapple with the problems of high coke yield and the intricate utilization of high sulfur coke formed during processing of inferior crude in China’s refineries to meet environmental protection regulations.A new technology called Resid Contact Cracking and Coke Gasification Integrated Technology (RCGT) is being developed by the SINOPEC Research Institute of Petroleum Processing (RIPP). The RCGT technology is the upgrading of delayed coking technology, which can achieve higher liquid yield than conventional delayed coking. At the same time, the generated coke is partially oxidized, and the gas containing CO can enter the CO boiler to generate steam, which can solve the environmental pollution problem caused by the steam generation by coal from the boiler in the refinery. This technology can be applied after proper modification of the idle FCC unit in the refinery.

2 RCGT Introduction

2.1 Technological process

The Resid Contact Cracking and Coke Gasification Integrated Technology (RCGT) is an efficient and green conversion technology based on fluidization processing of inferior residual oil, which uses the main unit facilities like those of catalytic cracking unit. This technology uses inferior heavy oil as feedstocks and applies a special contact materials. After contact cracking reaction takes place between inferior heavy oil and contact materials,the gas, liquid products, and coke are generated. Coke is deposited onto the contact materials and becomes a spent contact materials. Coke on the spent contact materials is then converted into a CO-rich gas through partial oxidation. The gas is used as the CO boiler fuel to generate steam for refinery use and power generation.The principle flow chart of RCGT is shown in Figure 1. According to the process flow diagram, the inferior feedstock enters the fluidized cracking reactor through the nozzle and upon being exposed to the hot contact materials it would be converted into cracking products and coke. The coke deposited on the contact materials enters the regenerator and generates the CO-rich gas under partial oxidation conditions.

Figure 1 RCGT Process diagram

2.2 The technical characteristics of RCGT

2.2.1 Thin films cracking technology to reduce coke yield

When the inferior residual oil is processed by delayed coking unit, the raw material is heated to about 500 °C in the heating furnace prior to entering the coke drum for thermal cracking reaction. The reaction temperature in the coke drum ranges from 415 °C to 470 °C, and the gas phase residence time ranges from 200 seconds to 300 seconds. The main occurrence of raw material in the coke drum is the high capacity gas-liquid phase coking. As for RCGT, when the raw material is heated to a certain temperature, it is injected into the cracking reactor through the nozzle, so that it is evenly dispersed on the contact agent to form a very thin liquid film, which makes the vaporized cracking products transformed from the bubble shape through the general phase to the thin layer diffusion state, thus reducing the liquid residence time, reducing the secondary reaction, and decreasing the production of coke and dry gas. The schematic diagram of the two processes is shown in Figure 2.

Figure 2 Comparison diagram of mass transfer between RCGT thin-layer cracking and delayed coking

One of the difficult problems in fluidized processing of inferior residual oil is poor fluidization, because the residual oil is a mixture of components with high boiling point. After entering the cracking reactor, a large part of liquid phase exists in the cracking reactor to form a gas-liquid-solid three-phase. Gas-solid fluidization has no problem, but liquid-solid fluidization will lead to agglomeration and fouling problems. The RCGT technology uses special contact materials and specially designed reactor to achieve thin-layer cracking to avoid agglomeration caused by liquid-solid fluidization.

2.2.2 Rapid cracking technology to reduce dry gas yield

Through making an in-depth study of the contact cracking process of residual oil, it is recognized that the reaction of residual oil is mainly divided into two stages, with the schematic diagram of RCGT thin films cracking reaction process shown in Figure 3. The exposure of residual oil macromolecules to the thermal contact materials mainly occurs during thermal cracking reaction to generate medium-sized molecules and coke, which is the main stage of coke formation. The generated intermediate molecules can diffuse into the pores of the contact materials, and the intermediate molecules in the pores can react with the acid sites in the pores to generate small molecules and a small amount of coke, which constitutes the main stage of dry gas generation. Based on this understanding, the contact cracking reactor and process suited for treating inferior residual oil were developed,and the control technology of fast cracking low dry gas was formed.

2.2.3 CO production by partial oxidation at low temperature

Through conducting an in-depth study of coke partial oxidation reaction kinetics on the raw material, a coke partial oxidation process prediction model was established, and a reactor suitable for partial oxidation was developed. High CO concentration can be obtained by in-situ low temperature (below 800 °C) gasification of coke deposited on the contact materials using RCGT technology. The obtained gas can be used as fuel for steam generation or power generation in the CO boiler.The main reactions of coke partial oxidation on the raw material are as follows:

2.3 RCGT technical effect

Due to the thin film cracking of RCGT technology in the contact cracking process of inferior residual oil, the yield of dry gas and coke with low added value obtained in the contact cracking reaction products of inferior residual oil is greatly reduced as compared to that of delayed coking,and the liquid yield is improved. The comparison of pilot plant results and industrial data are listed in Table 1. It can be seen from the data presented in Table 1 that in comparison with the results of delayed coking process,the coke yield of RCGT technology decreases by 2.45 percentage points, the dry gas yield decreases by 1.67 percentage points, and the liquid yield increases by 4.5 percentage points, while the diesel-gasoline ratio also decreases. In a RCGT unit rated at 1.0 Mt/a, the coke can generate about 260 t/h of steam with a pressure of

Figure 3 The schematic diagram of RCGT thin films cracking reaction process

3.5 MPa.The comparison of economic benefits between RCGT technology and delayed coking is shown in Table 2. It can be seen from the data listed in Table 2 that the profit per ton of oil processed by the RCGT technology is by about 169 RMB more than the current operating mode of refinery delayed coking unit. The technical and economic analysis does not consider the investment caused by environmental factors, and if these factors are considered,the profit per ton of oil provided by the RCGT technology is higher than that of delayed coking process.

Table 1 Comparison of cracking product distribution between RCGT and delayed coking

Table 2 Comparison of economic benefits between RCGT technology and delayed coking

3 Conclusions

The Resid Contact Cracking and Coke Gasification Integrated Technology (RCGT) is an upgraded technology of delayed coking technology. This technology not only reduces the production rate of coke and dry gas during processing, increases the liquid yield, but also the associated in-situ low-temperature clean gasification technology can solve the problem of clean utilization of high-sulfur petroleum coke, realize the green production of refinery, and play the role of “Utility Island”. RCGT is a green and low-carbon processing technology for upgrading the heavy oil utilization process.

Acknowledgements:This work was financially supported by the SINOPEC Research Program (No.115015 and 117017-1).