5.1. INTRODUCTION
A catalytic reforming process converts a feed stream containing paraffins, Olefins and naphthene to aromatics. The product stream of the reformer is generally referred to as reformate. Reformate produced by this process has a very high octane rating. Significant quantities of hydrogen are also produced as a by-product. Catalytic reforming is normally facilitated by a bi-functional catalyst that is capable of rearranging and breaking long-chain hydrocarbons as well as removing hydrogen from naphthenes to produce aromatics. The idea of a Catalytic Reforming Unit is to have RON (Research Octane Number) as high as possible at the same time keeping the Olefins, Benzene & Aromatics under the specified limits. The different types of reformers are classified as a fixed-bed type, semi-regenerative type, cyclic type and the continuous regenerative type. This classification is based on the ability of the unit to operate without bringing down the catalyst for Regeneration. During the regeneration process, the refinery will suffer production loss. In the Continuous Catalytic Reforming unit, the reactors are cleverly stacked, so that the catalyst can flow under gravity. From the bottom of the reactor stack, the 'spent' catalyst is 'lifted' by nitrogen to the top of the regenerator stack. In the regenerator, the above mentioned different steps, coke burning, oxychlorination and drying are done in different sections, segregated via a complex system of valves, purge-flows and screens. From the bottom of the regenerator stack, catalyst is lifted by hydrogen to the top of the reactor stack, in a special area called the reduction zone. In the reduction zone, the catalyst passes a heat exchanger in which it is heated up against hot feed. Under hot conditions it is brought in contact with hydrogen, which performs a reduction of the catalyst surface, thereby restoring its activity. In such a continuous regeneration process, a constant catalyst activity can be maintained without unit shut down for a typical run length of 3 - 6 years. The
purpose of the CCR unit is to produce a high octane no. reformate. The octane no. of the gasoline coming from the AVU is around 66, whereas the required value of the octane no. is 87, 88 and 93. The whole CRU can be divided into three subunits as:
Naphtha Splitting Unit (NSU) Naphtha Hydro-treater Unit (NHU) Catalytic Reforming Unit
5.1.1.
NAPHTHA SPLITTING UNIT
This unit has been designed to split SR naphtha (144 MT/hr for BH and 95 MT/hr for AM) to C5-80 oC and 80-115 oC cut. Due to the restriction on Benzene content in the final product (motor spirit), the IBP of the heavier cut is raised to approximately 105 oC. NSU can be operated with naphtha directly from AVU (hot feed) and from OM&S (Cold feed), it can also be operated using both the feed simultaneously. For removal of benzene, the gasoline from storage tanks and CDU is sent to a column, containing 40 valve trays, which is called naphtha splitter. The bottom product of naphtha splitter is sent to the NHU.
5.1.2.
NAPHTHA HYDROTREATER UNIT
The purpose of Naphtha hydrotreater is to eliminate the impurities (such as sulphur, nitrogen, halogens, oxygen, water, olefins, di-olefins, arsenic and metals) from the feed that would otherwise affect the performance and lifetime of reformer catalyst. This is achieved by the use of selected catalyst (nickel, molybdenum) and optimum operating conditions except for water, which is eliminated in stripper.
In this unit, the naphtha coming from the NSU is mixed with H2 which comes from the reforming unit. This mixture is heated to 340 OC in the furnace and then passed to the hydrotreater reactor at a pressure of 22 kg/cm2.
In the reactor, there are two beds of catalyst. In one bed, the unsaturated hydrocarbons are converted to saturated hydrocarbons and in the second bed impurities like N, S, and O are converted to NH3, H2S and H2O respectively. The effluent of the reactor is sent to stripper section to eliminate the light end, mainly the H2S and moisture from the reformate feed. The light gases from the top of stripper are sent to amine wash unit. There is a reboiler attached to the bottom of the stripper, which maintains the heat requirement. The bottom product of the stripper is either sent to storage or the reforming unit.
5.1.3.
REFORMING UNIT
Feed for the Reforming unit (94 m3/hr at 14 kg/cm2 and 110 oC) is received directly from hydrotreater stripper after heat exchanger. The filters must be provided for the protection of the welded plate exchanger. Feed is filtered to remove any foreign particles. At the D/S of the feed filter, chloriding agent and water injection are done. CCl4 solution of 1% in reformate is dosed by pump. Dosing @ 1 ppm wt CCl4 in feed is done when continuous regeneration unit is down. Water injection (not on regular basis) is done to maintain Cl-OH equilibrium on the catalyst when regenerator is out of service.
Feed mixed with recycle H2 stream gets preheated in PACKINOX exchanger from 91C to 451C by the effluent from 3rd Reactor which gets cooled down from 497 C to 98C.
Due to the endothermic nature of the reforming reactions, the overall reforming is achieved in stages with inter stage heater provided to raise the temperature. There are three Reactors (15R-1, R-2 & R-3) each provided with reaction heater.
5.2. REACTORS
In the reactors, the feed contacts the reforming catalyst which is divided approximately in the ratio 15: 30: 55. In the CCR process, the catalyst circulates continuously in reactors, in the space between the external grid and the central pipe from the top to the bottom, from one reactor bottom to the top of the next one, from the last reactor to the regeneration unit for regeneration. From the regeneration unit, the regenerated catalyst returns to the first reactor.
Each reactor is a vertical cylindrical vessel with spherical heads. It is equipped with one inlet & one outlet nozzle for feed & effluent respectively. Catalyst enters the reactor through 12 nos. of 3" pipes, flows through the space between external grid and the central pipe from top to bottom and exits through 12 nos. of 2"pipes, slow moving bed of bimetallic catalyst and exits through the outlet nozzle at the bottom. The radial flow of feed is achieved by directing the flow through external grid to catalyst bed & exit is made to central outlet collector pipe. Gas tight baffle is provided on the outlet pipe to avoid short- circuiting of the feed to outlet pipe at the entrance.
Reactor effluent after passing through PACKINOX exchanger is cooled in air cooler to 65 C and then by trim cooler to 45C before entering the separator. The separated gas is compressed in the recycle gas compressor and a part is recycled to the reactors. The remaining gas is routed to a re-contacting section to improve hydrogen purity and recover liquid yield.