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Feb-2011

Diesel hydrotreater revamp

Major improvements to throughput and product specification required a revamp of catalyst and internals in a diesel hydrotreater

Torkil Hansen
Haldor Topsøe
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Article Summary
Bharat Petroleum (BPCL) was faced with a serious challenge when it needed to revamp a diesel hydrotreating unit (DHDS) in Mahul, India. This unit, originally installed in 1999, operated with 25–30% cracked feed at low pressure (41 barg). The new conditions required a 43% increase in load and a 97% reduction in sulphur.

This article describes how Haldor Topsøe enabled BPCL to make the most of what equipment they had by implementing specially tailored revamp layouts. This required extensive knowledge of system behaviour, catalyst and reaction patterns.

Goals and challenges
BPCL’s goals were:
• An increase in throughput from 4200 t/d to 6000 t/d
• To reduce the sulphur in their diesel product from 350 wppm to 10 wppm
• A minimum catalyst cycle length of three years
• For installation to be completed in just 40 days during a turnaround.
  The challenges were:
• Low-pressure operation, 41 barg
• Large variety in feed composition
• High radial temperature spread in existing reactor beds
• Congested plot space
• Vibrations in charge heater
• Power restrictions for recycle gas compressor and charge pump
• Various hydraulic limitations
• Low capital expenditure, requiring maximum reuse of existing equipment.

BPCL had made a precise identification of the unit’s hydraulics bottlenecks in advance. An eight-week study phase was arranged for quantifying all bottlenecks and analysing the possible layouts. It was clear from the start that a simple revamp with just more reactor capacity would not solve the associated problems. An innovative approach was required to handle all of the challenges, and a special design was developed in close cooperation with BPCL. The design minimised capital and operating expenditure, and resulted in the most favourable net present value (NPV). BPCL used the NPV evaluation method to calculate which design they preferred over time.

Feedstock
The primary feed (Feed 1) is shown in Table 1. Feed 2 was an alternative feed that would be run occasionally. Product specifications needed to be met for both feeds. The main product specifications are given in Table 2. The variation in sulphur content is driven by a required maximum 45 wppm specification and a preferred 10 wppm specification for some of the time.

Primary revamp recommendations
The study phase resulted in the following primary recommendations:
• New high-activity catalyst from Topsøe
• New Topsøe reactor internals
• One extra reactor to reach the required catalyst volume
• Place the new reactor in parallel service to the existing reactor
• Heat the second reactor train using reactor effluent from the first reactor
• Install vertical helix exchangers in the second reactor train to save plot space
• No replacement of existing equipment.

Thus, the concept was to reuse the existing unit and build a 
parallel reactor train into a limited plot space, and at the same time reduce the mass load on the fired charge heater (see Figure 1).

Implemented recommendations
All of the listed recommendations were accepted by BPCL during the design phase. Plot restriction is a major problem in many revamps, and equipment selection is an important parameter when it comes to minimising the required plot space. The original plot space did not allow for any major additions other than the new reactor. By choosing vertical heat exchangers, it was possible to build the unit upwards, thereby solving the plot problem.

Various types of heat exchanger were considered in order to handle the limited plot space. In the end, BPCL decided to remove a soda ash vessel to make space for a platform where shell and tube heat exchangers could be placed.

Catalyst selection
Catalyst selection was critical to the success of this revamp. Topsøe’s Brim series of catalysts has an extensive record of application in diesel and kerosene hydrotreaters.

The unit required a catalyst with maximum volume activity to bring down the reactor’s cost. The operating conditions, as well as the feed specifications, favour a CoMo-type catalyst over a NiMo one, as the primary constraint is to reach the sulphur specification of 10 ppm at around 40 bar operating pressure. Since the feed contained up to 30% cracked material, the use of a CoMo catalyst brought the further benefit of significantly reduced hydrogen consumption compared to using a NiMo catalyst. This meant the make-up gas compressors did not need to be revamped.

TK-576 Brim catalyst was installed in the unit. Since then, a new generation of Brim catalyst has been commercialised. By improving dispersion, the number of active sites for a given metal loading is increased. This results in higher activity for the catalyst or, alternatively, provides the possibility of maintaining the same activity with a lower metals loading on the catalyst.
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