Oct-1996

Direct-coupled cyclone and feed injection

Retrofitting key technology elements in vintage FCC units can bring many of the economic benefits associated with the use of advanced technologies

R J Glendinning and H L McQuiston, ABB Lummus Global

Article Summary

Since its introduction over five decades ago, the design of the fluid catalytic cracking unit (FCCU) has been subject to a process of continuous evolution. There have been many innovative process developments during the past five or six years. During this time, significant effort has been expended in the development of advanced riser termination devices, high-efficiency feed injection technology, short contact time cracking and improved catalyst regeneration techniques.

When built-in to a modern unit design, the improvements achieved in each of the above fields contribute towards the refiner’s capability to achieve the desired yield slates, product qualities and operating flexibility.

In most instances, existing unit designs may also be easily upgraded to incorporate the key elements of these advanced technology features. Many of the economic benefits which accompany the utilisation of improved technology are thus readily available to the refiner without having to resort to the construction of a grassroots design. This was the case in three recent revamps incorporating Lummus FCC technology at the following locations:
- Netherlands Refining Company, Europoort, Netherlands
- Valero Refining Company, Corpus Christi, Texas
- Star Enterprise, Port Arthur, Texas.

This article provides an overview of the technology features incorporated and describes the economic benefits derived from each of the upgraded FCC units.

It is important to note that the FCC technology discussed was originally developed by Texaco Development Corporation and acquired by Lummus on 1 January 1996.

Achieving state-of-the-art grassroots quality yields in a vintage FCC unit may often be accomplished by simply replacing an outdated reaction system with a new one incorporating the required advanced technology features. The most recent Lummus reaction system technology may be considered to consist of: advanced Micro-Jet feed injection nozzles, an optimised short contact time riser and a negative pressure direct-coupled cyclone system.

This simple reaction system replacement assumes that the unit can adequately regenerate and circulate catalyst and that the mechanical integrity of all components in the reactor and regenerator systems is suitable for the new design conditions. This may or may not be the case, depending on the condition of the unit being revamped.

The two key technology elements essential to achieving grassroots quality yields in vintage FCC units are high efficiency Micro-Jet feed injection nozzles and the negative pressure direct-coupled cyclones. An in-depth review of both of these features has been presented elsewhere [Glendinning, McQuiston and Chan, Advancements in Process and Control Technologies Improve FCC Profitability, Japan Petroleum Institute, Tokyo, October 1994] and is not repeated here. However, to understand the advantages of this technology, it is necessary to consider the underlying design philosophy for each of these two key elements of the reaction system.

Direct-coupled cyclones
The main function of the riser termination device is to achieve a rapid and efficient separation of the spent catalyst from the riser effluent vapour and to minimise vapour dilute phase residence times, thereby inhibiting the undesirable thermal cracking reactions which would otherwise lead to increased dry gas production.

Compared to a rough-cut cyclone, other proprietary rough-cut devices or close-coupled systems, a direct-coupled cyclone (DCC) system will produce the minimum vapour residence time between the exit of the riser and the reactor outlet. Depending on the actual operating conditions prevailing in the FCC unit, the total vapour residence time over the two stage cyclone set of a DCC system is typically in the region of 1–1.5 seconds.

There are currently two alternative approaches to the design of the DCC system. As can be seen from Figure 1, a direct-coupled cyclone system may be classified as being either a negative pressure system or a positive pressure system.

In the negative pressure design, the primary cyclone operates at a reduced pressure relative to the dilute phase of the reactor vessel, while in the positive pressure system the primary cyclone operates at a pressure greater than the dilute phase.

The pressure balance characteristics of any riser termination device (RTD) are important since they may have a significant effect on the rate of vapour entrainment from the RTD into the dilute phase of the reactor vessel with the circulating catalyst. This, in turn, will have an impact on the performance of the FCC unit. Irrespective of the RTD design, some entrainment of hydrocarbon vapour is inevitable.

However, positive pressure devices such as rough-cut cyclones may have dipleg blowdown rates in excess of 10 per cent of the riser effluent vapour, even with the dipleg submerged in the stripper bed. Blowdown rates of this magnitude are not considered to be acceptable, as this will lead to increased dry gas make and to higher regenerator temperatures due to increased coke laydown on the catalyst and hydrocarbon carry-over from the stripper.

The negative pressure system is known to minimise hydrocarbon blowdown to the reactor vessel via the primary cyclone dipleg. Theoretical entrainment rates have been estimated to be less than 2 per cent of the total riser effluent vapour rate. This has been confirmed by commercial radiotracer studies on the Lummus DCC system, where it has been shown that the quantity of hydrocarbon vapours present in the dipleg of the primary cyclone in a negative pressure system is minimal. In comparison to a positive pressure system, the negative pressure cyclone system will: