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Application of Shell Reactor Internals in Huizhou Refinery
Shao Weidang, CNOOC Oil & Petrochemicals Co., Huizhou Refinery and Xiao Song Qing, China Sales Manager, Shell Global Solutions This paper briefly presents the industrial application of patented Shell reactor internals in a 4 MTPA high pressure Hydro cracker Unit(HCU) in CNOOC Huizhou Refinery. By using Shell’s reactor internals, the pressure drop across the reactor top bed remained stable, while the radial temperature for catalyst bed was uniform. These demonstrate that the reactor internals have perform better on the mixture and re-distribution of hot and cold streams.

In order to meet more stringent environmental requirements, it has become a trend to produce clean fuel products by the hydrogenation process in fuel refinery. Crude processing capacity of CNOOC Huizhou Phase I refinery project is 12 MTPA, which processes crude oil extracted by CNOOC from the Bohai Sea. The processing capacity of the high pvessure (HP) hydrocracker unit (HCU) is 4 MTPA, which is the biggest in terms of unit capacity, and the feedstock is mainly VGO2#, VGO3# and CGO. Shell is the process licensor and the catalysts are provided by Criterion Catalyst & Technologies. The reactor internals have played an important role in fully utilising the catalyst activity and extending the catalysts’ lifecycles. Domestic as well as international organisations have carried out extensive research on the Shell internals used in the HCU. This paper brings insights into is Shell's patented reactor internals in Huizhou Refinery's HP HCU.

Application of Shell Proprietary Reactor Internals
• Inlet Distributor

The mixture of feed oil and cycle hydrogen in the HCU first passes through the inlet distributor on the reactor top, which prevents generation of vortex while it enters the reactor inlet. The inlet distributor diffuses the reactor feed to the entire reactor section, eliminates the vertical impact across the top distribution plate from gas and liquid to ensure stable operation and promotes uniform mixing of gas and liquid phases through disturbance. The oil and gas mixture enters the distributor from the top and then splashes out after colliding with the round baffle in middle of the inlet diffuser, finally diffusing uniformly across the distribution plate on the reactor top.

Filter on Reactor Top Bed
The reaction oil-gas mixture of the domestic HCU first passes through the inlet distributor and then falls onto the catalyst bed on the top distribution plate. Scale basket, installed on top of some HCUs, is not used any more, while for Shell proprietary reactors the filter is added on the reactor top. The filter is loaded with refining catalyst DN3551 to further filter particles. Most oil from the inlet distributor is routed from the filter side through the refining catalyst.

It enters the top distribution plate via a circular channel at the filter center while most of the gas is routed through the large circular channel. Such design can ensure that the pressure drop is very low and can be negligible when the oil-gas mixture passes through the filter. Shell compared the following three cases,
a) Oil and gas from the reactor top enter the catalyst bed directly
b) A scale basket is installed on the reactor top
c) A Shell proprietary filter is installed at the reactor top.
Shell analysed the relationship between the catalyst bed pressure drop and catalyst operation time (results shown in Fig 1). Pressure drop with Shell proprietary filter is more stable and rises slowly, which extends the catalyst operation cycle to a great extent. With Shell proprietary filter on the top, the Huizhou Refinery HP HCU has been operating since its startup on April 24, 2009 and has processed 7.6 million tonnes of feedstock. The catalyst bed pressure drop is basically stable as depicted in Fig. 2.

Before July 2010 pressure drop of catalyst bed #1 was basically stabilised at 0.05MPa but after July 2010 the total pressure drop of catalyst beds #1, #2 and #3 raised slightly; however the trend was slow. Current total pressure drop in the three catalyst beds is stabilised around 0.22MPa under full-load production. This demonstrates that besides performing the function of top bed grading materials, the Shell proprietary reactor filters play a significant role in mitigating the pressure drop across the catalyst beds and also increasing the catalyst cycle life.

Distribution Plate
The distribution plate is required to ensure uniform distribution of reaction oil and gas across the entire catalyst bed. Bubble cap with many open slots and a central tube is used for domestic HCUs. Suction occurs in the annulus between the bubble cap and central tube when the gas passes through the slots.

High-speed gas flow breaks liquid into liquid drops which are carried to the central tube for spraying across the next catalyst bed. In general, large viscosity feed and small hydrogen-oil ratio always result in small oil-gas spraying angle, which would lead to uneven distribution of gas and liquid across the bed. This will cause poor utilisation of catalysts or even catalyst coking because of excessively large radial temperature difference.

Patented Shell “HD” distribution plate is used at the Huizhou Refinery HP HCU. Most oil and gases flow through the center of the HD tube while liquid oil flows into the tube through small holes on the “HD” tube side. Meanwhile, high speed gas that flows within the “HD” tube forms the suction force to break liquid within the holes into atomized drops. The oil-gas mixture flows downward quickly to collide with the baffle at the bottom of the “HD” tube and splash mist so that the distribution of reaction oil and gas across the catalyst bed is uniform, thus avoiding uneven fluid distribution.

Shell tested “HD” bubble cap and ordinary sieve plate to study the type with the best oil-gas distribution effect. Results depicted in Fig 3 indicate that the HD distribution plate is better than the others. The reactor at HP HCU at the refinery has large inner diameter of 4.4m. Large inner diameter results in poor distribution of oil and gas across the catalyst bed and occurrence of large radial temperature difference. Six flexible E+H thermocouples are installed at the inlet of each of the catalyst beds. Since the startup, with patented Shell “HD” distribution plate, the radial temperature difference of most catalyst beds is less than 2 degrees and an individual one is more than 2 degrees but still less than 3 degrees, Table 1 (on the next page) shows the temperature distribution at the inlets of the first and last beds of reactors in Row A, which demonstrates that “HD” distribution plate has better oil and gas distribution effect.

In addition, as the oil and gas in the reactor, are heavy and viscosity is high at the upper bed of the reactor a bigger HD tube is designed. Since hydrocracking reaction occurs on the catalyst bed, a smaller HD tube is designed for the bottom of the reactor which has low viscosity, light oil and gas.

Hydrogen Quench Box
The overall thermal effect of hydrocracking reaction is exothermic. Quench hydrogen should be injected into the catalyst bed to control the bed temperature rise (i.e. reaction depth). The structure of a quench hydrogen box plays a key role in the uniform mixing of hot oil and gas flowing from the catalyst bed and quench hydrogen. In general, the structure of the quench hydrogen box of a domestic hydrocracking reactor is as follows; quench hydrogen is injected into the reactor quench hydrogen compartment from the quench hydrogen tube for mixing with oil and gas from the catalyst bed, then flows through transverse plate opened with two or more large holes for pre-mixing, through a sieve plate with small holes for further mixing, and finally enters the re-distribution plate between the catalyst beds. Quench hydrogen box used by Huizhou Refinery is of Shell's proprietary technology and its specific structure is shown in Figure 4. The working process is that, quench hydrogen is injected from the quench hydrogen header, whose internal wall is installed with many nozzles at a certain angle. The installation direction of the nozzles is consistent with that of the cyclone guiding baffle in the middle mixing plate of the quench hydrogen box. As the nozzles are at a certain angle, quench hydrogen injected into the reactor forms rotary flow and mixes with oil and gas from the upper part of the reactor so as to ensure their complete mixing. The mixed oil and gas rotate into the middle mixing plate of the quench gas box and its guiding baffle, and then form rotary stream to flow downward. Fluid flows downward from “oxhorn” tubes around the mixing plate. Oxhorn nozzles do not point to the centre of the mixing plate, but like quench hydrogen nozzles at a certain angle. Therefore, liquid also rotates downward and mixes with oil and gas rotating downward. Together these fall on the serrate baffle, which is below the mixing plate. Two phases of oil and gas further collide and mix via the serrate baffle and then enter the redistribution plate below. The rotary flow of cold and hot fluid not only extends the retention time of the cold and hot fluid in the quench hydrogen box, but also increases the fluid turbulence. This will promote the mixing of cold and hot fluids and ensure uniform fluid temperature across the redistribution plate. Table 2 (on the next page) shows the radial temperature distribution at the inlet of a catalyst bed with different quench hydrogen flows at Huizhou Refinery HP HCU reactor #5 bed-inlet.

Note that for three different quench hydrogen flows, the radial temperature difference of the bed is within 3 degrees, which indicates that the quench hydrogen injected into the reactor can be fully mixed with hot fluid from the upper catalyst bed.

Like HD, “Oxhorn” tube is on the upper bed of the reactor. As the oil and gas in the reactor are heavy and viscosity is high, “oxhorn” nozzles are designed bigger in size for the upper bed. However, since hydrocracking reaction takes place on the catalyst bed and the oil and gas are light and viscosity is low at the bottom of the reactor, the nozzles that are designed are smaller in size.

In addition, the structure of Shell quench hydrogen box is compact and occupies smaller space. Shell quench hydrogen box is used for HP HCU with the height of 1.1 m, while domestic quench box used for IP HCU with the height of 1.7 m. It means that with the same reactor volume, the use of patented Shell internals can save more space to load catalysts.

Outlet Collector of Reactor
The outlet collector at the bottom of the reactoris intended to collect the resultant oil from the reaction. However, to prevent the catalysts and ceramics from flowing out of the reactor, the maximum gap of the outlet collector does not exceed the size of the catalyst or ceramic ball. The domestic outlet collector at the bottom of the hydrogenation reactor is similar to patented Shell outlet collector.

Huizhou Refinery's 4 MTPA HP HCU currently has the largest domestic single-processing loading capacity, loading of about 353 tonnes catalyst per reactor. Shell patented reactor internals are very essential in ensuring uniform distribution of reaction materials across the catalyst bed, full catalyst activity, safety and stability when raising the reactor catalyst bed temperature for uniform mixture of hot and cold fluids and avoiding large radial temperature difference of the bed.

Since the startup of Huizhou Refinery HP HCU, the radial temperature distribution across the catalyst bed is uniform without obvious rill flow or hot spot, and the catalyst activity is fully indicated.

These demonstrate the successful application of Shell proprietary reactor internals in Huizhou Refinery HP HCU.

Another significant characteristic of Shell proprietary reactor internals is that filter, distribution plate and quench hydrogen box are connected through wedges for convenient assembly and disassembly, while domestic bolts are generally used for the connection. When unloading internal passage plates of the reactor, bolt seizure always hinders the normal removal. In short, the unique design and excellent performance of Shell proprietary reactor internals play a very important role in the safe, smooth and long-cycle operation of Huizhou Refinery HP HCU.