Computational fluid dynamics investigation for austenitic AISI 904L stainless steel corrosion in a biodiesel stream piping

Abstract

A Brazilian biodiesel plant uses the transesterification of soybean oils with the methanol and sodium methoxide catalyst excess to convert these oils into alkyl esters and glycerin. Hydrochloric acid solution is injected in the piping to neutralize free NaOH and reduce saponification of oils. After the hydrochloric acid solution, several failures of pipelines of AISI 316L and 904L steels were registered. The objective of this work is to study the austenitic stainless steel corrosion observed in the downstream piping the hydrochloric acid injection, applying computational fluid dynamics and to evaluate pH and chloride profiles using different geometries for the acid solution injection in order to avoid or mitigate the corrosive attack. The obtained results showed a good agreement with in situ observations of the pipe degradation. All the studied geometries presented evidence of a weak mixture between the polar and non‐polar species, even under turbulent flow. The elbow geometry is more effective to reduce the local corrosive attack compared to two other simulated geometries.

Corrosion failures of austenitic and duplex stainless steels in a biodiesel plant

“…3) was not efficient for the complete mixing between biodiesel and acid water. The pH is not neutralized efficiently for the orthogonal and diagonal geometries used in acid dosing systems in biodiesel plants even at 1.50 m the downstream point of insertion of acid solution [16]. Thus, even after the mixer A-14 (Fig.…”

“…3), chloride concentration gradients were still observed in the regions close to the pipe wall. The chloride concentrations remain high on the walls of the orthogonal and diagonal geometries in a long straight direction on the same side of acid injection [16]. Some regions of the tube remain wetted with biodiesel containing low chloride concentration and others were into contact with acidic water with high concentration of chloride.…”

“…But this study has a novelty aspect of describing corrosion failures in a very specific medium consisting of a biodiesel and acid water where the proximity to the acid injection site plays a crucial role in the corrosive process and failure. Ferreira et al [16] studied the orthogonal, diagonal and elbow geometric arrangements of injection of acid solution used in a biodiesel plant and proved that the elbow geometry presents the lowest values of chloride wall concentration, gradient of chloride mass fraction, and shear stress on the walls, coherent behavior with the geometric arrange of an emissary tube that protects the walls of external pipe against direct contact of the acid injection. The three Brazilian biodiesel plants studied in this work adopted the diagonal geometric arrangement of acid solution injection, which generate corrosion failures.…”

“…Presently, the Brazilian biodiesel industries are implementing the elbow geometry with an emissary tube. Among the three geometric arrangements evaluated by Ferreira et al [16] by using computational fluid dynamics investigation, the diagonal geometry presents the highest chloride concentration on the pipe walls and chloride mass concentration gradient.…”

Avaliação do desempenho do misturador estático Kenics aplicado à neutralização em linha de uma corrente de biodiesel

“…No presente trabalho, simulações de fluidodinâmica computacional (CFD) são utilizadas para avaliar a influência do misturador Kenics no processo de dosagem de ácido clorídrico em uma corrente de biodiesel com catalisador alcalino em dois arranjos desenvolvidos para uma tubulação que liga as etapas de transesterificação e decantação de uma da unidade de produção de biodiesel localizada em Montes Claros, MG. O uso do misturador estático se justifica pelo fato de que o arranjo geométrico de injeção da solução ácida que não favorece a mistura radial entre as correntes e pode resultar em ataques corrosivos freqüentes devido à elevada concentração de íons cloreto nas paredes da tubulação (Ferreira et al, 2017).…”