Hydrogen
Carbon dioxide gas
Condensing system
To methanator
Re ux drum
CO absorber
re ux
Amine regenerator
CW
Shift e uent
Raw hydrogen 100–130˚F 200–300ps ig
Shift e uent
Rich solvent
Lean solvent
Figure 7 Typical amine treating system for CO 2 removal
water and may lead to corrosion in different locations than the previous operation. Amine treating system changes Extending beyond the hydrodeoxygenation unit, support process units may also experience a change in expected cor- rosion. One unit of concern is amine treating/amine regen- eration (see Figure 7 ). Higher levels of CO 2 in hydrotreating recycle gas change the operation of an amine absorber. Amine absorption will be different between hydrogen sulphide (H 2 S)-dominated recycle gas and CO 2 dominated. For primary and secondary amines, the heat of reaction for CO 2 is higher than for H 2 S. This will cause a higher tem- perature ‘bulge’ in the absorber and require more reboiler energy in the regenerator. The use of tertiary amines may require additives to speed up the removal of CO 2 to an acceptable level. A change in amine selection, concentration, or circulation would need to be investigated to ensure efficient acid gas removal. Additionally, trace contaminants coming in with the feed can cause foaming and amine carryover. Purge rates of sour water in the amine regenerator over- head also need to be considered, depending on how much ammonia is absorbed by the amine solution. The final point for consideration is the amount of heat stable salt anions (HSS - ) present in the recycle amine stream. FCC processing may lead to increased accumulation of small organic acid anions such as formate and acetate in the amine solution. The most significant issue seen with FCC processing of renewables has been the transport of surface-active con- taminants via LPG streams to LPG amine treaters. LPG treaters that operate normally on petroleum-based feed develop severe amine carryover problems running renew- able feedstocks in FCC units as these surface-active agents build above the amine/LPG interface in the LPG absorber. Studies are ongoing for both prevention and mitigation of the carryover problem. While these bespoke examples are the most likely areas for changes in damage mechanisms to occur, this should not be considered an exhaustive list of areas of concern.
Special attention should be given to other areas of a facil- ity when changing feedstocks from traditional petroleum to bio-oils. Considerations for pretreatment units, sour water stripping, sulphur recovery, and others would require addi- tional study. Emulsion behaviour in treaters/separators Due to the different chemistry of the renewable feeds, the formation of trace levels of surface-active compounds can occur during thermal processing operations or if some bio- logical degradation occurs in upstream tankage. The sur- face-active components can turn up in unexpected areas of the refinery due to reprocessing of slops containing material from renewable feeds. The surface-active compounds can impact phase separa- tion and foaming behaviour in many of the separation and wet treating processes in the refinery, including desalters, wet treaters, amine treaters, water washes, separators, and fractionators. Vessel modifications, internals replacements, and/or new chemical treatments may be required to main- tain acceptable separation performance of these systems. Blending For renewable feeds that have been co-processed through hydroprocessing and FCC unit units, the quality and blend- ing of finished renewable fuels mainly use the same correla - tions and blending procedures as for fossil fuels. Typically, the cetane index and number are higher for renewable diesel products due to the high paraffin con - tent of the renewable products. However, the high paraffin content may also result in cold flow property blending chal - lenges due to wax crystallisation. The cold flow property challenges can be managed by either cold flow additives or the use of dewaxing (cracking/isomerisation) catalyst in the hydrotreating reactors. Renewable jet is another product that can be obtained from co-processing in hydroprocessing by fractionation of the final product. For hydroprocessing units with significant jet production, co-processing typically requires more isom- erisation and cracking than for diesel production.
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PTQ Q3 2022
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