Technology

Introduction

1. Background
   
   Deep Catalytic Cracking (DCC) process is very similar to conventional FCC process, but the operating condition of DCC process is regulated to realize deep cracking reactions, so the DCC unit has more severity on operating condition compared to that of FCC process. DCC converts various heavy hydrocarbon feedstocks by employing special zeolite catalysts to obtain propylene as the main product. By circumventing limitations of conventional FCC processes, DCC technology can typically produce several times higher light olefin yields, such as ethylene & propylene, than that of conventional FCC, and generate high octane number naphtha containing more aromatics at the same time.
    
2. Process scheme 
   
   The process scheme of DCC, shown in Fig. 1, is similar to that of FCC consisting of reaction-regeneration, fractionation and absorption and stabilization sections. Feedstock dispersed with steam is fed to the system, then contacted with the hot regenerated catalyst: either in a riser plus fluidized bed (DCC-I Mode for maximum propylene operation) or in a riser (DCC-II mode for maximum isoolefin & gasoline operation) for the catalytical reaction to take place. Reactor effluent are sent to a fractionator for separation. 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.
   
   Figure 1 - DCC Process Scheme
   
    
3. Key operating parameters 
   
   Table 1 lists the main features of DCC in comparison with FCC.
   
   Currently, the commercial and typical DCC technology can be categorized as DCC-I (maximum propylene operation) and DCC-II (maximum isoolefin & gasoline). Besides feedstock property and catalyst, the main operating parameters of DCC-I and DCC-II are reaction temperature, residence time, catalyst to oil ratio, and steam dilution. The reaction temperatures of DCC-I and DCC-II are around 560℃and 530℃ respectively. DCC uses longer residence time to enable the cracked naphtha to undergo secondary reaction forming light olefins. The heat of reaction of DCC is two to three times that of FCC due to the high conversion and large quantity of gas produced, resulting in a higher catalyst circulation rate and higher catalyst to oil ratio to provide the heat of reaction. High yields of the desired olefins are favored by low oil partial pressure. It encourages higher gas olefinicity and decreases the coke formation. DCC needs more steam than FCC but still much less than in steam cracking.
   
   Table 1 - General comparison between DCC and FCC
   
    

Technology characteristics

1. Operation flexibility
   
   DCC-I Mode (for maximum propylene operation) and DCC-II Mode (for maximum isoolefin & gasoline) can be smoothly shifted in any DCC commercial unit.
    
2. Feedstock adaptibility
   
   DCC can process various types of heavy feeds. The following feeds have been applied in the commercial DCC units: VGO, hydrotreated VGO, hydrogen cracking bottoms (HCB), deasphalted oil (DAO), coker gas oil (CGO), atmospheric residue (AR), hydrotreated AR and Blending of VGO and VTB, etc. The more favorite products distribution can be obtained when the paraffinic base feedstock is adopted.
    
3. Main performances
   
   Production of propylene, butylenes and naphtha with high content of aromatics and high octane number is maximized by using Heavy Oil (VGO, deasphalted oil, coker gas oil, atmospheric residue, vacuum residue, others).
   
   DCC Type 1 (DCC-1) typically produces 6.1% ethylene, 20.5% propylene, 14.3% butylenes, 5.4% isobutylene and 50-80% naphtha (Table 2). ​​Daqing paraffinic feedstock showed the highest propylene yields of 24.83wt% for DCC-I, while the propylene yield was 14.57wt% for DCC-II operation with a naphtha yield of 35.72wt%.
   
   Table 2 - Comparison of DCC and FCC- Products’ Yield

   Product (wt%)	FCC	DDC-1	DCC-2
   Ethylene	0.9	6.1	-
   Propylene	6.8	20.5 (24.83*)	14.57*
   Butylene	11	14.3	-
   Isobutylene	3.3	5.4	-
   Naphtha	Base	50-80	35.72*
   LCO	Base	Lower	-

   *From Daqing paraffinic feedstock.
   
   DCC naphtha is rich in BTX especially toluene and xylenes. The total BTX in the narrow cut was 57.56vol%, 21.87vol% and 30.33vol% respectively. The concentration is high enough to be utilized as BTX plant feed.
    
4. Running period
   
   Running period of DCC unit is normally around 3-5years, the same as FCC unit.

DCC catalysts

A series of DCC catalysts for different DCC types and feedstocks have been formulated and commercially available. The new generation DCC catalyst has been put into application on several commercial DCCUs, and resulted in higher propylene selectivity, higher propylene yield, and less impurities contents than the former ones. The specifications of DCC catalyst is listed in Table 3.

Table 3 - DCC Catalyst specifications

Product yields from commercial applications with DMMC-1 and MMC-2 catalysts are presented in Table 4.

Table 4 - Commercial application of DMMC-1 and MMC-2 catalysts

DCC Commercial Practices

DCC is the heart of a commercial platform with several commercial versions (Fig. 2). 

Figure 2 - DCC Platform

 As of April 2021, twenty-two DCC units have been licensed to China and overseas markets such as Thailand, Saudi Arabia and India, etc. Another four DCC units are under design and construction. The capacity of the biggest one is 4.60 MMT/a.

References

1. Yujian Liu et al., Mar 2011, Advances in DCC Process and Catalyst for Propylene Production from Heavy Oils, China Petroleum Processing and Petrochemical Technology, 13.
2. SINOPEC Research Institue of Petroleum Processing Co., Ltd., 26^th Apr 2021, DCC(Deep Catalytic Cracking) Technology.
3. Jinquan Zhu, Shi Shou, 16^th Aug 2021, DCC Technology, Fluid Catalytic Cracking Technology Department, SINOPEC RIPP.