Jul-1997

Converting VGO HDS units to moderate pressure hydrocracking

An introduction to the technology of MAK Moderate Pressure Hydrocracking (MPHC), together with details of a successful revamp at the Chiba refinery of Kyokuto Petroleum Industries, Japan

David A Pappal, Mobil Technology Company
Michael G Hunter, Mobil Technology Company
Lucas R Groeneveld, Akzo Nobel Chemicals BV

Article Summary

Market demand for middle distillate transportation fuels is expected to maintain steady growth in the future. Together with this increasing demand, there will be continuous pressure to lower sulphur content and improve burning quality by increasing the average cetane index of the gas oil pool. Many refiners have found it profitable in recent years to modify existing vacuum gas oil (VGO) hydrodesulphurisation (HDS) units for mild hydrocracking (MHC) service.

This is accomplished through the use of specialised MHC catalysts together with changes in the operations strategy for the unit which involve running at near maximum reactor temperatures and accepting shorter catalyst cycles between regenerations. Mobil Technology Company has developed and commercialised a new technology called Moderate Pressure Hydrocracking (MPHC) which is now being offered for license to the refining industry through the Mobil-Akzo Nobel-Kellogg (MAK) hydrocracking alliance.

This process overcomes several of the key limitations experienced with MHC type operations and has been applied to the revamp of a 28000bpsd VGO HDS unit at Mobil’s joint venture Kyokuto Petroleum Industries (KPI) refinery in Chiba Prefecture, Japan.

Moderate pressure hydrocracking is a single-pass hydrocracking process for the partial conversion of vacuum gas oils to low-sulphur distillates and unconverted oil which is highly upgraded relative to the raw feed. Operating at lower pressure significantly reduces capital investment and results in lower operating costs and substantially less hydrogen consumption. Furthermore, the process requirements for MPHC are within the range of many existing VGO desulphurisation units. Typically, the operating conditions for MPHC range between 50 and 100 bar total pressure, 340–425°C reactor temperature and between 350 and 1000 normal cubic metres of recycle gas per cubic metre of feed.

The two companies have been actively engaged in moderate pressure hydro-cracking research for more than 10 years. Mobil’s first commercial MPHC installation, a VGO HDS retrofit, was successfully started up in 1983. The company operates five hydrocrackers, two of which are partial conversion MPHC designs that process vacuum gas oils into middle distillate products.

This research and operating experience has led to an advanced capability to apply hydrocracking conversion technology to heavy feedstocks under moderate pressure conditions. Akzo Nobel has commercialised a family of hydrotreating and hydrocracking catalysts which are combined to achieve the optimal balance between activity and selectivity for each specific refining application. Akzo Nobel zeolite-based hydrocracking catalysts have been selected for application in both of Mobil’s MPHC units as well as nine additional units around the world.

Process description
MAK Moderate Pressure Hydrocracking is a single stage, single pass process configuration consisting of a reactor section and fractionation section designed to meet specific project objectives. The advanced Spider-Vortex reactor internals design technology allows the application of multi-bed reactors while maintaining stable operations and maximising catalyst utilisation. In most cases, catalyst requirements are such that only a single reactor vessel is needed.

Both high temperature and low temperature high pressure separators are utilised in the reaction section to enhance operability and heat integration with the fractionation section. An amine contactor is utilised in the reactor section to remove hydrogen sulphide from the recycle gas enhancing hydrogen partial pressure and catalyst HDS performance. Due to the low light ends make from MPHC, a simple stripper followed by the fractionating column can be specified for product recovery. Low pressure separators upstream of the stripper tower can be specified if hydrogen recovery from the flash gas is a design objective. Lower design pressure and a minimum equipment count result in substantial capital cost savings.

The general equipment requirements for the MPHC unit are the same as for many existing VGO HDS units. The primary difference is the need for increased reactor volumes to provide sufficient catalyst to achieve the desired conversion and cycle life. The advanced reactor design capabilities developed by Mobil and available through the MAK hydro-cracking technology alliance allow for the efficient and cost effective addition of catalyst volume while minimising incremental pressure drop in the reactor section.

Reactor design
Reactor performance is vital for the safe, reliable and profitable operation of any hydrocracking process. Multiple catalyst bed reactor designs which do not have adequate quench and redistribution internals are subject to reactant mal-distribution which can lead to reactor temperature instability (temperature excursions and runaways), poor catalyst efficiency (loss of catalyst cycle life) and decreased yields.

These issues are as important in moderate pressure partial conversion hydrocracking as they are in high pressure high conversion hydrocracking. Mobil has maintained active research and development programmes in the area of hydroprocessing reactor design for more than 30 years. These programmes have resulted in the commercialisation of the Spider-Vortex quench and redistribution reactor internals together with advanced design techniques to ensure the optimum performance of hydrotreating and hydrocracking process facilities.

The commercially demonstrated improvements in temperature distribution and catalyst utilisation have translated into better yields, longer catalyst cycles and more efficient use of limited hydrogen resources throughout the refining system. A complete description of the Spider-Vortex development programme can be found elsewhere.