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Hydrotreater revamps for ULSD fuel

Scope and capital investment for revamping an existing diesel hydrotreater to meet the 15wppm sulphur standard. The base design is typical of hydrotreaters commissioned in the early 1990s to meet on-road specifications of 500wppm sulphur

R E (Ed) Palmer, Mustang Engineers and Constructors Inc
Salvatore P Torrisi, Criterion Catalysts & Technologies
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Article Summary
In mid-2006, thanks to the US Environmental Protection Agency (EPA), 80% of refiners’ on-road diesel pool will have to meet a new ultra-low sulphur diesel (ULSD) specification of no more than 15wppm sulphur. To ensure this target is not exceeded at the ultimate point of distribution, and to allow for some manufacturing flexibility, revamped or new facilities will need to achieve a desulphurised product sulphur level significantly below the mandated target. Sulphur limits and regulations of similar severity are being implemented in other areas of the world such as Europe and certain parts of Asia.

For an existing diesel hydrotreater, a number of options will directionally improve sulphur removal, including:
- Higher activity catalyst
- Increased reactor temperature
- Installation of high-efficiency reactor feed distributors
- Increased catalyst volume
- Recycle gas scrubbing to remove hydrogen sulphide.

Revamp design basis
The feed basis for this study is identical to the original design and consists of a blend of two-thirds straight-run gas oil and one-third light-cycle oil (LCO). Table 1 shows the feed constituents and blended properties. Key design criteria for the original facility are shown in Table 2. This includes existing reactor operating pressure, catalyst volume and treat gas rate. The design product sulphur for the original design was 470wppm.

A simplified flow diagram of the base cases is shown in Figure 1. Other study basis criteria were:
- Product sulphur content of 8wppm
- Utility systems such as steam, cooling water, power, instrument/plant air, and the pressure relief system have sufficient capacity for the revamp
- The existing DCS has capacity to accommodate all new instrumentation
- Plot space is available near the hydrotreater unit for new equipment 
- The refinery is currently consuming all available reformer hydrogen.  Incremental hydrogen will be purchased from a pipeline
- Hydrotreating kerosene for cloud point control in the winter is not required
- The existing hydrotreators are not operating at charge rates significantly above the original design.

The original design treat gas rate of 1400 SCFB was assumed for this study.  Higher recycle rates, if hydraulically feasible, would extend the catalysts cycle or allow higher throughputs. For this study, Criterion Catalysts provided the reaction conditions in Table 3 for several options required for the production of ULSD. All reaction conditions are based on an assumed distribution of higher boiling sulphur compounds. For a definitive design, pilot testing of the feedstock is recommended. The base case reaction parameters are also shown for comparison.

In all revamp cases, the current catalyst was replaced with Criterion’s most active desulphurisation catalyst, DC-2118. Three options were considered. The first (Revamp A) consisted of purifying the reformer hydrogen with a pressure swing adsorption (PSA) unit. Combined with the purchased hydrogen, this resulted in the total make-up volume having a purity of 99.9 vol%. Revamp cases B and C consisted of adding incremental catalyst volume to the base case design. The incremental hydrogen volume was also supplied by pipeline purchases. Purification of the reformer make-up was not considered.

Using the reaction conditions specified, computer simulations were developed for each revamp alternative. Hydrogen make-up rates were calculated based on the sum of chemical hydrogen consumption, solution losses and purge gas, if any, from the simulations. Additional process information for each revamp is presented in Table 4.

The simulation results were used to rate existing equipment and determine what modifications were required as well as the size and other design information for new items. All shell and tube exchangers, air coolers, pumps and fired heaters were rigorously rated. Pressure drop across each catalyst bed was calculated using the Ergun(1) equation with the Larkin(2) two-phase correction parameter. An allowance was also included for fouling. The resulting information was used to develop hydraulic calculations across the reactor loop to establish an operating/design pressure and temperature profile, check the performance of the recycle compressor, and to identify other limitations within the system.

Table 5 summarises the detailed revamp scope for each case. Process flow diagrams of the hydrotreater revamps are shown in Figures 2 and 3 (equipment and lines shown in bold are new or modified).

Revamp requirements
Each revamp case shares the following common requirements, including state-of-the-art high-activity catalyst, increased hydrogen make-up compression capacity and the addition of reactor quench.

All cases take advantage of the latest advances in catalyst technology and are based on using Criterion’s DC-2118 CoMo catalyst. Catalyst beds were assumed to be dense loaded with 1/20” catalyst to provide maximum cycle length and to ensure even contacting between the catalyst and oil. Also to ensure maximum cycle length, the existing feed distributors were upgraded. Good feed distribution and proper catalyst installation is critical for the production of ULSD. Design and cost information for new feed distributors and inter bed quench internals was provided by Shell Global Solutions.

Producing ULSD requires a significant increase in chemical hydrogen consumption compared to what is needed to make diesel meeting the 500wppm specification. This is due to additional, marginal saturation of aromatics and the removal of sulphur from aromatic sulphur compounds. To provide the incremental make-up capacity, a new spared reciprocating booster compressor is added upstream of the existing first-stage make-up cylinders. Due to design pressure limitations, the existing first-stage make-up compressor intercooler and knock-out drum were re-piped to service the new booster compressor. A new intercooler and knock-out drum designed for a higher pressure were specified to replace this equipment. The existing make-up cylinders also required replacement because of the higher capacity and operating pressure. 
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