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Influência da injeção de soda cáustica na carga à unidade de Visbreaker
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A implementação da legislação IMO2020 reduziu o teor de enxofre permitido no fuelóleo para 0,5 %(m/m), o que obrigou a Refinaria de Sines a modificar a dieta de crudes. Após esta alteração, registou-se uma mudança no comportamento da unidade de Visbreaker (VB), cuja alimentação é o resíduo de vácuo (RVII) proveniente da Destilação de vácuo 2. Os ciclos de operação do VB passaram de uma média de 8 meses para cerca de 3,5 meses, devido à necessidade de descoqueficação das serpentinas do forno da unidade. Esta dissertação teve como objetivo analisar a influência da injeção de soda cáustica (NaOH), utilizada no início do processo de refinação, na instabilidade operacional do VB e no aumento da formação de coque.
A metodologia consistiu na realização de ensaios laboratoriais de Flash Assay, comparando amostras de resíduo de vácuo, obtidas neste equipamento, antes e depois da adição de NaOH, bem como na análise de dados operacionais desde 2017 até 2024. Verificou-se que os crudes processados após 2020 são mais pesados, com uma redução de 15% nas frações leves e um aumento de 10% no resíduo de vácuo, que se reflete numa carga mais pesada. A análise dos dessalinizadores revelou que a variabilidade na concentração de sais aumentou no período pós IMO, resultando numa injeção média de NaOH de 14,5 ppm, ultrapassando assim o limite desejável de 8 ppm, com valores a atingir os 35 ppm. Como consequência, o teor de sódio no RVII aumentou, ultrapassando o limite recomendado de 30 ppm.
A análise da temperatura das serpentinas do forno do VB (VB-H1) demonstrou um aumento após 2020, com a temperatura final de ciclo a subir de 500ºC para valores superiores a 635ºC, indicando a instabilidade operacional e uma maior severidade no processo, aumentando a coqueficação. A pressão no soaker foi ajustada de 9 bar para 5 bar, de forma a mitigar a coqueficação nas serpentinas, com o impacto negativo de haver uma menor conversão do RVII em produtos leves.
Os resultados sugerem que a coqueficação acelerada das serpentinas do VB não é causada exclusivamente pela injeção de NaOH, mas sim por um conjunto de variáveis que incluem a alteração do mix de crudes, uma maior variabilidade na eficiência do processo de dessalinização e o aumento da temperatura nas serpentinas. Para mitigar este problema, é sugerido uma otimização na compatibilidade dos crudes processados, melhorias na eficiência dos dessalinizadores e a avaliação de estratégias para reduzir a deposição de coque, como a utilização de revestimentos nas serpentinas e o ajuste do caudal de alimentação à unidade.
The implementation of IMO2020 legislation reduced the maximum sulphur content allowed in fuel oil to 0.5 %(m/m), which forced the Sines Refinery to change its processed crude diet. Following this change, there was a significant change in the behaviour of the Visbreaker unit (VB), whose feed is vacuum residue (RVII) from Vacuum Distillation 2. The VB's operating cycles went from an average of 8 months to around 3.5 months, due to the need to remove the coils from the unit's furnace. The aim of this dissertation was to analyse the possible influence of the injection of caustic soda (NaOH), used at the start of the refining process, on the operational instability of the VB and the increase in coke formation. The methodology consisted of carrying out Flash Assay laboratory tests, comparing vacuum residue samples obtained in this equipment before and after the addition of NaOH, as well as analysing operational data from 2017 to 2024. It was found that the crude processed after 2020 is heavier, with a 15 per cent reduction in the light fractions and a 10 per cent increase in the vacuum residue, which is reflected in a heavier load on the VB. Analysis of the desalters revealed that the variability in salt concentration increased in the post-IMO period, resulting in an average NaOH injection of 14.5 ppm, thus exceeding the desirable limit of 8 ppm, with maximum values reaching 35 ppm. As a consequence, the sodium content in the RVII increased, exceeding the recommended limit of 30 ppm. Analysing the temperature of the coils in the VB furnace (VB-H1) showed an exponential increase after 2020, with the final cycle temperature rising from 500ºC to over 635ºC, indicating operational instability and greater severity in the process, increasing coking. The pressure in the soaker was adjusted from 9 bar to 5 bar in order to mitigate coking in the coils, with the negative impact of lower conversion of RVII into lighter products with greater economic value. The results suggest that the accelerated coking of the VB coils is not caused exclusively by the injection of NaOH, but by a number of variables including a change in the crude mix, greater variability in the efficiency of the desalination process and an increase in the temperature of the coils. To mitigate this problem, we suggest optimising the compatibility of the crudes processed, improving the efficiency of the desalters and evaluating strategies to reduce coke deposition, such as using coatings on the coils and adjusting the flow rate fed to the unit.
The implementation of IMO2020 legislation reduced the maximum sulphur content allowed in fuel oil to 0.5 %(m/m), which forced the Sines Refinery to change its processed crude diet. Following this change, there was a significant change in the behaviour of the Visbreaker unit (VB), whose feed is vacuum residue (RVII) from Vacuum Distillation 2. The VB's operating cycles went from an average of 8 months to around 3.5 months, due to the need to remove the coils from the unit's furnace. The aim of this dissertation was to analyse the possible influence of the injection of caustic soda (NaOH), used at the start of the refining process, on the operational instability of the VB and the increase in coke formation. The methodology consisted of carrying out Flash Assay laboratory tests, comparing vacuum residue samples obtained in this equipment before and after the addition of NaOH, as well as analysing operational data from 2017 to 2024. It was found that the crude processed after 2020 is heavier, with a 15 per cent reduction in the light fractions and a 10 per cent increase in the vacuum residue, which is reflected in a heavier load on the VB. Analysis of the desalters revealed that the variability in salt concentration increased in the post-IMO period, resulting in an average NaOH injection of 14.5 ppm, thus exceeding the desirable limit of 8 ppm, with maximum values reaching 35 ppm. As a consequence, the sodium content in the RVII increased, exceeding the recommended limit of 30 ppm. Analysing the temperature of the coils in the VB furnace (VB-H1) showed an exponential increase after 2020, with the final cycle temperature rising from 500ºC to over 635ºC, indicating operational instability and greater severity in the process, increasing coking. The pressure in the soaker was adjusted from 9 bar to 5 bar in order to mitigate coking in the coils, with the negative impact of lower conversion of RVII into lighter products with greater economic value. The results suggest that the accelerated coking of the VB coils is not caused exclusively by the injection of NaOH, but by a number of variables including a change in the crude mix, greater variability in the efficiency of the desalination process and an increase in the temperature of the coils. To mitigate this problem, we suggest optimising the compatibility of the crudes processed, improving the efficiency of the desalters and evaluating strategies to reduce coke deposition, such as using coatings on the coils and adjusting the flow rate fed to the unit.
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Palavras-chave
Visbreaker soda cáustica coqueficação das serpentinas fuelóleo IMO2020