Revamps 2023 Issue

Sources of energy inefficiency in a hydrotreater

Segment

Concern

Heat exchangers

• Fouling issues resulting in poor heat transfer • Hydraulic issues resulting in decreased heat transfer and/or increased pumping requirements • Layout or number of heat exchangers in preheat train failing to fully recover available heat

Furnace

• Fouling issues resulting in poor heat transfer • Poorly optimised air-to-fuel ratio reduces energy efficiency

Reactor

• Catalyst deactivation reduces product yield and quality, resulting in wasted process energy • Pressure drop build-up due to plugging of the catalyst bed increases recycle-gas-compressor power consumption • Liquid maldistribution reduces product yield and quality, resulting in wasted process energy

Main fractionator

• Fouling in heater and/or heat exchangers results in poor heat transfer • Coke deposition at transfer line and inlet • Poor heat integration in and around fractionator • Inefficient condensers waste energy • Recoverable heat left in product streams • Failure to optimise heat recovery from air and water coolers

Process control

• Poor control of process variables leads to wasted energy

Table 2

investment scheme. This scheme would commence by integrating up to 10% renewable feedstock into an existing hydrotreating unit through co-processing, requiring mini- mal or no capital expenditure. As biofuel mandates become more stringent in the future, a dedicated hydrotreated veg- etable oil unit exclusively designed for processing 100% renewable feeds could be pursued. To achieve a 100% renewable feed, a substantial volume of hydrogen is needed, typically ranging from 300 to 400 Nm 3 /m 3 . Consequently, a larger volume of make-up hydro- gen and quench gas flow is necessary, even when co-pro - cessing relatively small amounts of renewable feedstock. Due to these needs, it becomes crucial to carefully moni- tor the hydrogen balance within the refinery to prevent a decrease in unit capacity. In the future, transition from grey hydrogen to blue and green hydrogen is an option to reduce hydrotreating’s carbon footprint. Improved equipment energy efficiency The main sources of inefficiency in each section of the hydrotreater, including the heat exchangers, furnace, reac- tor, and main fractionator, are listed in Table 2 . When traditional shell-and-tube technology is utilised for feed/effluent heat exchange in the hydrotreating pro - cess, up to eight series units are often needed for maxi- mum energy recovery. Fewer units are required for both spiral and plate heat exchangers. In addition, with a closer temperature approach provided by these designs, the fired heater energy requirement and the effluent air-cooling capacity requirement are lower. Reactor pressure drop A build-up in reactor pressure drop increases power con- sumption by the recycle gas compressor and challenges the mechanical integrity of the catalyst support trays. Optimising the pressure drop for a hydrotreater helps pro- mote uniform flow through the bed and maintains a uni - form radial temperature profile.

Increased pressure drop is symptomatic of a decrease in catalyst void volume and can be caused by fouling of the catalyst bed. Fouling is the result of a build-up of deposits – either contaminants in the feed or reaction products on the catalyst. Commercially, refiners can grade materials to deal with pressure drops arising from the carry-over of solid parti- cles. Used in combination, these top bed materials increase void fractions, extend the zone of insoluble deposits, and trap very fine particles, iron scale, and metal complexes. Reactor internals for gas-liquid distribution Reactor internals play a major role in boosting hydrotreater performance, as internals help optimise gas-liquid distribu- tion in fixed-bed catalytic reactors. Optimal performance efficiency is critical for a hydroprocessing reactor operat - ing in the trickle flow regime to meet increasingly stringent clean fuels specifications. With the aid of computational fluid dynamics and mock-up units, a design for distributor trays can be formulated that optimises flow distribution over a broad range of operating conditions and catalyst loadings. Reduced reactor operating temperature and pressure drop with well-designed reactor internals also result in a lower CO₂ footprint associated with the operation of a specific hydroprocessing unit. Furthermore, liquid mald - istribution can lead to localised hot spots, coking, insuffi - cient quenching, and shortened cycle lengths. Adversely, the problem reduces product yield and quality, resulting in wasted process energy. Temperature runaway During the operation of hydrotreating units, particularly when processing chemically unstable feeds such as VGO or delayed coking gasoils, a critical factor to consider is the potential occurrence of temperature runaway in the cata- lytic bed. Temperature runaway, referring to a sudden and uncontrolled rise in reactor temperature caused by exo- thermic reactions, requires significant energy for cooling.

Revamps 2023