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Oct-2012

Platformer debutaniser revamp and optimisation

Recommendations following a study of high reformate RVP delivered a range of benefits including a significant increase in gasoline production

RAKAN BILAUS, SAJEESH PADMANABHAN and NEELAY BHATTACHARYA
Saudi Aramco
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Article Summary
Saudi Aramco carried out a study at Yanbu refinery to troubleshoot and resolve the high Reid vapour pressure (RVP) of reformate that was imposing blending constraints, resulting in reduced i-C5 and isomerate blending and RON giveaway. The study concluded with a debutaniser revamp that was implemented early in 2011. As a result, the reformate RVP saw a drop from 4.5 to 3.8 psia. However, during the summer season, several performance issues were raised. Optimisation and overcoming cooling limitations have resulted in improvements in performance and a further drop in RVP from 3.9 to 3.7 psia. The reduction in platformate RVP allows for an additional one thousand barrels of i-C5 to the gasoline pool, which has increased Yanbu Refinery’s revenue significantly.

Background

The high reformate RVP causes problems with gasoline blending, leading to high RVP in the summer months, resulting in reduced iC5 and isomerate blending. Consequently, gasoline production is reduced with a RON giveaway. In order to increase gasoline production and eliminate RON giveaway, the reformate RVP needed to be reduced from 4.5 psia to 3.8 psia. Accordingly, a study was initiated to look into the available options to upgrade Yanbu refinery’s platforming debutaniser column to achieve the targeted RVP at full column capacity.

Process background
Yanbu refinery is located on the west coast of Saudi Arabia with a capacity of 250 000 b/d. The refinery is responsible for processing Arabian Light Crude oil, which is received from the Crude Oil Terminal and pumped through the East & West pipeline to meet demand from the western region market for high-quality refined products.

The refinery’s platforming unit has the purpose of upgrading low-octane heavy naphtha from the naphtha hydrotreating unit to a high-octane material for motor fuel blending. The principal product is the high-octane reformate (sent to saturated gas concentration for vapour pressure adjustment), whereas a hydrogen-rich gas is produced as a by-product and sent to the naphtha hydrotreating unit and the light straight-run naphtha hydrotreating unit. The unit consists of the following sections:
•  Reaction section, composed mainly of three heaters and three reactors, in which the reforming reaction takes place
•  Compressor section, composed of three centrifugal compressors for the recycle gas service and two alternative compressors for the hydrogen make-up gas sent to the hydrotreating units
•  Product separation section, in which reformate is recovered from the hydrogen-rich gas by means of a chilling unit.

The saturated gas concentration unit processes the unsterilised platformate produced in the platforming unit and the treated LPG coming from the LPG Merox unit. Products are stabilised platformate, propane, butane and/or mixed LPG. Components lighter than propane are sent to the refinery’s fuel gas network.

The unit consists of the following sections (see Figure 1):
•  Debutaniser tower reboiled by means of the fired heater that separates the unstabilised platformate in LPG as overhead product and stabilised platformate as bottoms product
•  Deethaniser tower reboiled with medium-pressure steam that removes components lighter than C3 from LPG
•  Depropaniser tower reboiled with medium-pressure steam that splits LPG in propane as overhead product and butane as bottom product.

Unit operation
The debutaniser column was designed by Chiyoda in 1977 and the column internals were provided by Koch-Glitsch. When the platformer unit was converted to CCR in 2006, the column was deemed suitable for the new process flows. The only modifications implemented were new feed bottoms exchangers and new overhead trim coolers. The main function of the column is to stabilise the reformate and produce LPG as the overhead product. The main specifications were <1.0 mol% C4 in reformate and <1.0 mol% C5 in LPG. The reformate was estimated to have an RVP of 3.8 psia. However, the reformate RVP has been consistently at 4.5 psia at full unit capacity (see Figure 2). It is possible to reduce the reformate RVP by increasing the LPG draw to decrease the C4 components in the reformate. However, this will result in a higher C5 content in the LPG, causing it to be off-specification. Furthermore, the reflux temperature was having a considerable impact on column operations. The design temperature was 38°C but, over time, the reflux temperature has gradually shifted higher. During summer peaks, the temperature had been reported as high as 50°C.

Study basis and scope
PFDs, PI data system and Chiyoda mechanical drawings of the saturated gas concentration unit were used as a basis of the study. The study would provide the following outcome:
•  Provide detailed simulation of the unit with tray-by-tray rating calculations
•  Identify the hydraulics limitations contributing to low tray efficiency
•  Provide sensitivity analysis for reflux temperature
•  Provide changes to internals proposed by Koch-Glitsch to overcome limitations.

Outcome of study
Design check through simulation

The debutaniser column was evaluated for the new process conditions when the fixed-bed Platformer was converted to a CCR. As per their calculations, the column internals were adequate to produce a reformate RVP of 3.8 psia. In order to validate the process design, the debutaniser column was simulated with full capacity with the design conditions. The results clearly indicated that for the design process conditions a 30-tray debutaniser column is adequate for producing reformate with a RVP of 3.8 psia.

Hydraulics limitation assessment
Once the process calculations were verified, the next step was to perform a hydraulic check for the rated dimensions to check for any limitations. A detailed tray-by-tray analysis showed a high level (80-89%) of flooding in the rectification section. To verify the results, Koch- Glitsch was contacted to perform a detailed hydraulic check of the column. Their comments were as follows:
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