Jul-2013

Predicting catalyst lifetime

A simulation model compares accurately with actual operating data to provide reliable predictions of catalyst lifetime in naphtha reforming

S REZA SEIF MOHADDECY and SEPEHR SADIGHI
RIPI

Article Summary

The catalytic reforming of heavy naphtha (heavy straight-run gasoline, or HSRG) is a favoured process in petroleum refineries for producing high-octane gasoline. From the viewpoint of process operation, there are three kinds of widely used catalytic reforming units: semi-regenerative, continuous catalyst regeneration (CCR) and cyclic regenerative processes. Semi-regenerative catalytic naphtha reforming is the oldest type and it is generally carried out in three or four fixed-bed reactors in series with intermediate preheaters operated adiabatically at temperatures between 450°C and 520°C, total pressure between 10 and 35 bar, and molar hydrogen-to-hydrocarbon ratios between 3 and 8. Among all commercial technologies for the catalytic reforming process, those using fixed-bed reactors in series are the most common configurations due to the relative simplicity of scaling up an operation. However, the main disadvantages of fixed-bed reactors are coke formation, active phase sintering and poisoning during the catalyst’s life cycle. Consequently, the main problem with fixed-bed reactors is the loss of catalyst activity over time, which reduces the length of continuous operation.

The catalyst’s life cycle is further complicated by numerous technical, environmental and organisational issues. In principle, different companies can be involved in each of the life cycle steps. The life cycle of a catalyst starts with the initial production of fresh catalyst oxide, which is pre-sulphided prior to being used in the refinery process. During its use when the catalyst deactivates and it does not meet performance targets within the limits of the reactor’s operating conditions, the reactor is shut down. Therefore, the catalyst’s life cycle typically involves a long chain of operations, normally performed by different specialised companies.^1,2

Catalyst life is dependent essentially on process conditions and also, to a large extent, on the distribution of species such as sulphur and nitrogen present in the oil, both of which can change during a typical commercial run. Moreover, depending on the process used, the catalyst’s life cycle may vary from a few seconds, as in fluid catalytic cracking (FCC), to several years, as in ammonia synthesis; therefore, a reliable prediction of catalyst life has been extremely difficult.

Before the catalyst is loaded in the reactor, many laboratory experiments are carried out to identify its operability. An accelerated method for a one-day laboratory test is proposed here for the prognosis of activities and an estimation of the lifetime of the catalyst. The method is based on knowledge of the deactivation behaviour of catalysts under different reaction conditions, including extreme conditions at high liquid hourly space velocities (LHSV). Hence, the method enables prediction of lifetime and performance using data obtained from a one-day test in laboratory conditions. The curve of LHSV versus lifetime is an individual property of a given catalyst, which has to be established experimentally. Any point from this curve can be used for an estimation of lifetime.^3,4 Since deactivation of the catalyst takes place over a long time scale, it is difficult to mimic this in a pilot plant or at lab scale. Consequently, any study on an industrial scale to predict the life cycle of a catalyst plays an important role regarding economic, operational and environmental issues.^5,6

Works on the life cycle of an industrial-scale catalytic naphtha reforming unit are scarce. In our present work, the semi-regenerative catalytic reforming process of a commercial-scale oil refinery was simulated using the Petro-Sim simulator (KBC Profimatics, 2009). After validating the simulation, the lifetime of the next cycle was predicted by the previous simulated cycle lives, and the results were compared with actual test runs of the plant at the end of the cycle.

Process description
A commercial fixed-bed catalytic naphtha reforming unit called Platformer, licensed by Chevron and with a nominal capacity of 16 500 b/d, was chosen as a case study. The feed to the plant, prior to entering the catalytic reformer, should undergo hydrodesulphurisation (HDS) in the hydrotreatment unit. Then, the produced naphtha, called Platcharge, is introduced to the catalytic reforming process. The most commonly used types of catalytic reforming units have three or four reactors, wherein each has a fixed catalytic bed. For such a unit, the activity of the catalyst reduces during operation due to deposited coke and loss of chloride. Hopefully, the activity of the catalyst can be periodically regenerated or restored using in situ high- temperature oxidation of the coke followed by a chlorination process. Therefore, the catalyst of a semi-regenerative catalytic reformer is regenerated during routine shutdowns of the process every 18-24 months. Normally, the catalyst can be regenerated three or four times. Then, it must be returned to the manufacturer for reclamation of platinum and/or rhenium.

In Figure 1, Platcharge is preheated by the first furnace (H-1). It then enters the first reactor (R-1), where naphthenes are dehydrogenated to aromatics. Next, the product stream from the first reactor passes through the second reactor (R-2), and the outlet stream of that enters the third reactor (R-3). Similarly, the product stream from the third reactor enters the fourth reactor (R-4). The overall reforming reactions are endothermic, so a preheater (H-1, H-2, H-3 and H-4) is provided ahead of each reforming reactor.

Next, the product stream from the fourth reactor enters a separator, V-1, in which hydrogen produced during the reforming process (the gas stream) is recycled, then mixed with Platcharge. Finally, the liquid product leaving the separator is introduced to the gasoline stabiliser, in which LPG and light gases are separated from gasoline. The vapour pressure of the gasoline can be set according to market requirements.
The distribution of catalyst in the reactors is shown in Table 1. The normal operating conditions of the unit are shown in Table 2.

Process simulation and validation
Catalytic reforming is often modelled and simulated based on the number of reactive species and the type of kinetic model used.^7,8 The presence of many components, either as reactants or as intermediate products in the reactive mixture, with resulting new reactions, creates challenges for modelling the process. To reduce these complications, reactants in the mixture are classified in certain and limited groups called pseudo- components. Additionally, Arhenius and Langmuir-Hinshelwood kinetics are widely used for kinetic-based catalyst modelling and simulation of the catalytic naphtha reforming process.^9-13 Petro-Sim software is a simulator capable of simulating commercial-scale catalytic reforming units,¹⁵ and so enables us to simulate reactors with different catalyst weights and sizes.16,17 In this research, Petro-Sim has been used to simulate and analyse a catalytic reforming unit.