Mar-2014

Commercial application of the second generation RHT catalysts for hydroprocessing the residue with low sulphur and high nitrogen contents

The RHT technology and the second generation RHT catalysts were applied in design of an 1.7 Mt/a VRDS unit at the SINOPEC Changling Branch Company.

Shao Zhicai, Zhao Xinqiang, Liu Tao, Dai Lishun and Nie Hong
Research Institute of Petroleum Processing, SINOPEC

Article Summary

The commercial application result demonstrated that the RHT catalysts showed good activity and stability in processing low-sulphur and high-nitrogen residue. The first long period run of unit for processing high Fe and high Ca content residue was achieved. The reasons for excessive pressure drop of R-101 were ascribed to Fe and Ca deposition as well as coke formation.

Introduction
The increasingly tightening requirements for the products specifications as well as the change in the demand for quality products have made residue hydroprocessing one of the most widely used technologies among various residue upgrading processes at many refineries1-3. In order to fulfill the specifications imposed by regulations and the increased demand for middle distillates, the main objectives of residue upgrading processes include hydrodemetallisation (HDM), hydrodesulphurisation (HDS), hydrodenitrogenation (HDN), and the Conradson carbon reduction (HDCCR). Therefore, it can be advantageous to operate the residue hydrotreater to achieve maximum conversion which will be sought after in order to produce better feeds for residue catalytic cracking (RFCC)4-5.

Over the past years the market trend towards lighter products has made it attractive to convert the residue fraction to lighter, more valuable products through the combination of residue hydrotreater and RFCC unit6-7. In view of the high commercial value and potential, the residue hydrotreating route was adopted in the inferior crude oil upgrading project of SINOPEC Changling Branch Co. In this project, an 1.7 Mt/a VRDS unit was constructed to provide feedstock for the downstream RFCC unit. The RHT technology licensed by the Research Institute of Petroleum Processing (RIPP) was applied in the project design and the corresponding second generation RHT catalysts were adopted in the 1st run8. In this paper, the performance of the RHT catalysts and the VRDS unit in the course of the operation will be discussed. An enhanced understanding apropos of the process is expected to assist the optimisation of the VRDS unit performance in the next run of the residue hydrotreater.

Characteristics of Feedstock
Before start-up, the feedstock samples of the VRDS unit were collected and analysed. Table 1 lists the main properties of the feedstock, from which we can figure out that the Yangtze River pipeline residue was greatly different from feedstock conceived in the design. It can be seen from the data depicted in Table 1 that the sulphur content of the feedstock was only 1.38%, whereas its nitrogen content was as high as 0.68%, denoting that this feedstock was a typical low sulphur and high nitrogen residue. However, we should bear in mind that in recent years almost all of the residue feedstocks delivered to the VRDS or RDS units in China are imported from theMiddle-East countries, which means that the conventional residue feedstock is characteristic of high sulphur (with S content>1.5%) and low nitrogen (with N content<0.3%).

Since the nitrogen is more refractory to conversion than sulphur in the hydrotreating process, in this case it is reasonable to assume that the big variations in the sulphur and nitrogen contents pose great challenges for the refiners.

Moreover, it is the first time for the application of the second generation RHT catalysts in the low sulphur and high nitrogen residue hydrotreating unit. As demonstrated in Table 1, the Fe and Ca contents were also higher than the designed values (envisaging a Fe content of 36 µg/g and a Ca content of 92.9 µg/g).
 
Different Processing Characteristics between Two Typical Residues
Just as illustrated above, the aim of residue hydrotreating unit is to provide suitable feedstock for the downstream RFCC unit which has strict specification requirements for the contents of metals, CCR and sulphur required by the unit.

As demonstrated in the literature2-3, the CCR content of the residue hydrotreating products is linearly associated with the content of hetero-atoms (in particular sulphur or nitrogen atoms). Figure 1 shows the simulated asphaltene structure units and the calculated bonding energies12.

Upon comparing with C—N bonds, the S—S bonds or C—S bonds have low bonding energy. The reaction rate of HDS is faster than that of HDN. So it is difficult to process residue with low sulphur and high nitrogen contents on the same graded catalysts. Besides, there are two main routes for desulphurisation reaction13-14, viz.: the direct desulphurisation (DDY) pathway by hydrogenolysis via C—S scission and the hydrodesulphurisation (HYD) pathway by hydrogenation involving an initial hydrogenation step to aromatic-ring followed by C—S scission. DDY is the main pathway in residue desulphurisation11 which is also the major advantage of the 2nd generation RHT catalysts. According to the mechanism of hydrodenitrogenation15-16, the hydrogenolysis of C—N bonds in aromatic nitrogen compounds occurs after heterocyclic ring   hydrogenation. Owing to the low sulphur but high nitrogen contents in the feedstock as depicted in Table 1, the nitrogen removal step will result in significant reduction of CCR content in comparison with the sulphur removal step. Hence, hydrogenation activity of the catalysts grading system should be strengthened to handle a residue with low sulphur and high nitrogen contents so as to acquire qualified hydrotreated products with required CCR content.

Except for the low sulphur and high nitrogen contents, the other noticeable property of the feedstock is the high Fe and Ca contents. Under hydroprocessing conditions, the oil soluble iron species in residue will be converted to FeS and easily deposited on the external surface of catalysts (or guard catalysts) particles or the interstices between catalyst (or guard catalysts) granules9. The organic Ca species, which behave in a similar way just as Fe species, will be converted to CaS which is prone to build up on the external surface of catalyst10. The crust-like layer created by the deposition of such solids on the catalyst bed can affect the operation by causing channelling to develop pressure drop in the reactor11. To tackle this problem, the decalcification facility was developed before the electric desalting unit in order to remove Ca species from the feedstock before entering the residue hydrotreater based on the discussion between RIPP and Changling Branch Co.

Modulation of Catalyst Grading
During hydroprocessing of residue using FBR (fixed-bed reactor), proper catalyst grading should be used to increase cycle length and prevent premature shutdown and it is common to use guard materials at the top of the first reactor to catch fouling precursors. To meet all  product requirements, the HDCCR and HDN activity of RHT catalysts should be strengthened depending upon the feedstock assay. For this purpose, catalyst grading scheme was adjusted by RIPP. Functions of every kind of the second generation RHT catalysts and the result of adjustment are listed in Table 2. The principles of the catalysts grading are presented below:
1) In order to strengthen the bed void fraction and metal storage capacity (FeS and CaS) in the first part of the reactor train, where the concentration of metals in the oil is high, volumes of guard catalysts and large particle HDM catalysts were increased correspondingly.

2) RDM-3B is not only a HDM catalyst, but also has better HDCCR activity than RDM-2B. On account of the relatively low Ni and V content in the feedstock, it is appropriate to increase the volume of RDM-3B and decrease the volume of RMD-2B consequently. This is in line with the suitable HDM activity and high HDCCR activity requirements.