Deep Catalytic Cracking(DCC)

Transcription

1 Deep Catalytic Cracking(DCC) Introduction Propylene, only next to ethylene, is one of the most fundamental petrochemical raw materials. Recently, the world demand for propylene has been increasing rapidly and the annual growth percentage of propylene is even higher than that of ethylene. As a co-product of steam cracking with its yield approaching half of the ethylene yield, propylene from steam cracking cannot keep up with the required propylene/ethylene balance. R&D for increasing propylene production from heavy hydrocarbons has been carried out actively in the oil refining and petrochemical fields. Deep Catalytic Cracking technology (DCC), developed by SINOPEC, broke the limitation of conventional FCC process and can doubly increase the yield of propylene, and has been highlighted in the oil refining and petrochemical industries. Process Description Deep Catalytic Cracking(DCC)is a catalytic conversion technology which uses heavy hydrocarbon feedstocks, such as VGO, VTB or VGO blended with DAO to produce light olefins(ethylene, propylene and butylenes), LPG, gasoline, and middle distillates etc. The technology mainly targets maximizing production of propylene (DCC-Ⅰ) or maximizing production of iso-olefins (DCC-Ⅱ). The process scheme of DCC is similar to that of conventional FCC consisting of reaction-regeneration, fractionation and gas concentration sections. Feedstock dispersed with steam is fed to the system, then contacted with the hot regenerated catalyst either in a riser plus fluidized dense bed reactor (for DCC-Ⅰ) or in a riser reactor (for DCC-Ⅱ) and is catalytically cracked. Reactor effluent proceeds to the fractionation and gas concentration sections for stream separation and further recovery. The coke deposited catalyst is stripped with steam and transferred to a regenerator where air is introduced and the coke on the catalyst is removed by combustion. The hot regenerated catalyst is returned to the reactor at a controlled circulation rate to achieve the heat balance of the system. The schematic diagram of DCCU reaction-regeneration system is listed in Figure 1.

2 Figure 1 The schematic diagram of DCCU reaction-regeneration system Features of Technology Characteristics of Technology DCC has two reactor operating modes: DCC-I ( Riser plus fluidized dense bed reactor, maximum propylene operation mode) and DCC-II ( Riser reactor, maximum isoolefins operation mode ). DCC can process different heavy feeds (VGO, deasphalted oil, coker gas oil, atmospheric residue, vacuum residue, etc.), and paraffinic feedstocks are the best feeds for DCC. In DCC-I mode, over 20wt% propylene yield can be obtained from paraffinic feedstocks. The naphtha and middle distillates streams from DCC unit can be used as blending components of high octane number commercial gasoline and fuel oil, respectively. As employing specially designed and patented zeolite catalysts, the reaction temperature of DCC is higher than that of conventional FCC, but much lower than that of steam cracking. Flexibility of process operation. Easy to obtain the shift of DCC operation modes by regulating the operating conditions and catalyst formulations. Easy of separation and recovery of product streams through a similar absorption/fractionation system of conventional FCC. It is not necessary to use cryogenic separation system of steam cracking for the separation and recovery of DCC product stream. Impurity hydrocarbons are trace in DCC lighter olefin products, and hydrotreating is not needed. Table1lists general comparison between DCC and FCC, and a block flow diagram of DCC complex is shown in Figure 2. Table 1 General comparison between DCC and FCC

3 Process FCC DCC Feedstock A wide range of heavy oils A wide range of heavy oils preferably paraffinics Catalyst Various types of Y zeolite catalyst A modified pentasil structure zeolite containing catalyst Hardware Reactor Riser Riser and/or bed Regenerator Base Similar Main fractionator Base Higher vapor/liquid ratio Stabilizer/absorber Base Bigger Compressor Base Larger capacity Operating conditions Reaction temp. Base Higher by 30~50 o C Regeneration temp. Base Similar Cat/oil ratio Base 1.5~2 times Residence time Base More Oil partial pressure Base Lower Dilution steam Base More To diesel blending Gas Ethylene Ethylene Purificat. recovery To ethylene plant unit Light ends unit Propylene To storage C 3 & C 4 C 3 cut C 3 Purificat.& split splitter LPG to storage C 4 cut Gasoline Fractionator selective H 2 Other hydrogen. Gasoline feeds Vacuum H 2 gas oil VGO DCC reactor C 4 cut to MTBE plant HTU & regenerator Decanted oil to fuel Figure 2. Block flow diagram of DCC complex Performances Reaction temperature is one of the most important factors in the cracking reactions. In comparison with pyrolysis, DCC can significantly reduce the cracking temperature via the reduction of cracking activation energy by using the specially designed catalyst. The optimum reaction temperature of DCC is normally within the range of 520~580 o C varying with the crackability of the feedstocks and the required product slates, and DCC-I (maximum propylene mode) and DCC-II (maximum iso-olefin mode ) employ the upper range and the lower range of the above-mentioned temperatures, respectively.

4 The crackability of the feedstocks can have great effect on the operation parameters and the product slates. The higher UOP K value and hydrogen content of the feedstock result in the higher crackability,and hence the higher light olefin yields. Some representative feedstocks and their olefin yields under DCC-I and DCC-II are shown in Table 2 and Table3, respectively. Table 2 DCC-Ⅰlight olefin yields from representative feedstocks Number VGO VGO+Resid VGO+ DAO VGO Feedstock Paraffinic base Paraffinic base Interm. base Naphthenic base Density, g/cm UOP K Hydrogen, t% Olefin yield, wt% Ethylene Propylene Butylenes Feedstock Table3 DCC-Ⅱlight olefin yields from representative feedstocks Number VGO Paraffinic base VGO+Resid Paraffinic base VGO+ DAO Interm. base VGO Naphthenic base Density, g/cm UOP K Hydrogen, wt% Olefin yield, wt% Propylene Isobutylene Isoamylenes Safety and Environmental The DCCU emissions of waste water, waste gas, waste residue, powder and noise are similar to that of the conventional FCC, so the measures and equipments of relative environmental protection are similar for both DCC and FCC. Catalysts A series of DCC catalysts for different DCC types and feedstocks listed in Table 4 was formulated and commercially available. MMC, the new generation DCC catalyst, has been put into stream on several commercial DCCU, and resulted in both higher propylene selectivity and higher propylene yield than the former ones. Table 5 lists the properties of MMC catalyst.

5 Table 4 DCC catalysts Designation DCC Commercializati Type on Date Features CHP-1 I 1990, 11 High bulk density, high propylene selectivity CHP-2 I 1992, 9 Medium bulk density, high propylene selectivity CRP-1 I 1994, 6 High hydrothermal stability CRP-S I 1995, 5 Less active for fresh catalyst startup CIP-1 II 1994, 6 Higher activity and bottoms crackability CIP-2 II 1998, 9 Higher activity and bottoms crackability, metal tolerance CIP-3 I&II 1998, 10 Higher bottoms crackability, high propylene selectivity CIP-S II 1998, 9 Less active for fresh catalyst startup, metal tolerance MMC-1 II 2002, 11 Higher activity and bottoms crackability, higher propylene selectivity MMC-2 I 2002, 9 higher propylene selectivity Table 5 MMC catalysts Items MMC-1 MMC-2 Pore volume, ml/g Specific area, m 2 /g ABD,g/ml Cracking Index (520 o C Reaction,800 o C/4h aged),wt% AI,wt%/h PSD 0-40 μm, v% μm, v% APS,μm Economics In order to evaluate and quantify the petrochemical opportunities opened to refiners,a linear program model of a typical US Gulf Coast refinery was established using Haverly Systems GRTMPS. The base case is a typical transportation fuels operation. Case Ⅱ utilizes the base case configuration,but opens petrochemical options and lets the model decide the most profitable mix of products. Case Ⅲ allows the same petrochemical opportunities,but the FCC has been replaced with a DCC maximum propylene operation. The base model for this study includes typical process units consistent with the average of all Gulf Coast refineries. The capacities of the individual process units are based on a 100,000 BPCD crude unit. The evaluation results of the model are listed in Table 6. There are significant differences between the

6 Case Ⅱ and Case Ⅲ. The propylene and para-xylene productions increase by 182.7% and 15.0% respectively in the Case Ⅲ, however, the premium and regular gasoline productions reduce by 4.5% and 4.9% respectively. The economic analysis shows that Case Ⅲ has a US$94,129 per calendar day advantage when the gross incomes are compared based on the productions shown in Table 6. Table 6 Case comparison Case Base case Case Ⅱ Case Ⅲ Case Ⅲ / Case Ⅱ Products (BPCD) Propane 4,755 6,005 7, % Butane 2,785 3,801 3, % Premium unleaded 17,507 2,330 2, % Regular unleaded 36,159 45,014 42, % Jet fuel 27,611 14,710 13,643, -7.3% Diesel 8,190 17,031 12, % Fuel oil Ethylene (TPA) , % Propylene (TPA) 113, , , % Benzene (TPA) - 63,500 73, % Paraxylene (TPA) - 99, , % Butadiene (TPA) - 1,825 1,825 - Commercial Experience The first commercial DCC unit was put into production in 1990, and nine DCC units have been licensed with a total capacity of 3.58MMTA, and the biggest one among them is 800KMTA. A 4.50MMTA DCC unit for maximizing propylene production will be constructed and put into production in Technological Services Turn-key project or process design package can be submitted as well as technical consultant, training and on-site service for start-up.