I. Introduction

1. Background

DCC, a novel process using various heavy feedstocks for propylene production, is derived from the petrochemical  application extension of Fluid Catalytic Cracking (FCC), and was  firstly chosen & published in 'Refining Processes Handbook' by Hydrocarbon Processing's  editing team in 1990's. Up to now, DCC has been commercially proved as an advanced  technology with the highest propylene production yield from heavy feedstocks, which is in  a leading position in the world. With breaking the limitation of conventional FCC process,  DCC technology can typically produce light olefin (ethylene & propylene etc.) several times  more than conventional FCC, and generate higher octane number naphtha with higher  aromatics content at the same time.

2. Process scheme

The process scheme of DCC, as shown below, is similar to that of FCC consisting of  reaction-regeneration, fractionation, and absorption and stabilization sections. Feedstock  dispersed with steam is fed into the system, then contacts with 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 iso-olefins & gasoline operation) and catalytic reaction  takes place. Reactor effluent is charged 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 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.

3. Key operating parameters

Main features of DCC in comparison with FCC are listed in Table 1.  Currently, the commercial and typical DCC technology can be categorized into DCC-I  (maximum propylene operation) and DCC-II (maximum iso-olefins & gasoline). Besides  feedstock properties and catalyst, main operating parameters of DCC-I and DCC-II include  reaction temperature, residence time, catalyst to oil ratio, and steam dilution ratio. The  reaction temperatures of DCC-I and DCC-II are around 560°C and 530°C respectively.

DCC with longer residence time enables the cracked naphtha further secondary reaction  for light olefins formation. The reaction heat of DCC is two to three times as high as that of  FCC due to higher conversion and larger quantity of gas produced, resulting in higher  catalyst circulation rate and higher catalyst to oil ratio to generate needed reaction heat.  High yields of desired olefins are favored by low hydrocarbon partial pressure. It produces  more olefinic gas and less coke. DCC needs more steam than FCC but much less than  steam cracking.

Table.1 General comparison between DCC and FCC

II. Technology characteristics

1. Operation flexibility

DCC-I Mode (for maximum propylene) and DCC-II Mode (for maximum iso-olefins &  gasoline) can be smoothly switched in any DCC commercial unit.

2. Feedstock adaptability

DCC can process various heavy feeds. Following feeds have been applied in commercial  units: VGO, hydrotreated VGO, deasphalted oil (DAO), coker gas oil (CGO), atmospheric  residue (AR) and blends of VGO and VTB, etc. More favorite products distribution can be  achieved when processing paraffinic feedstock.

3. Main performance

Daqing paraffinic feedstock showed the highest propylene yield of 24.83wt% for DCC-I  while the propylene yield was 14.57wt% for DCC-II operation with a naphtha yield of  35.72wt%.

4. Running cycle

Running cycle of DCC unit is normally 3-5 years, similar to FCC unit.

III. DCC catalysts

A series of DCC catalysts for different DCC modes and feedstocks have been formulated  and are commercially available. The new generation DCC catalyst has been applied on  several commercial DCC units and results in higher propylene selectivity, higher propylene  yield, and less impurities in products than former ones. The specifications of DCC catalyst  are listed in Table 2.

Table.2 DCC Catalyst specifications

IV. Commercial Experience

Eighteen DCC units have been licensed in China and overseas market including Thailand,  Saudi Arabia and India etc. Seven more are under design and construction. The largest  single unit has a capacity of 4.60 MTA.