## 1 Introduction  
Biofuels have become a recent focus for scientific research due to the fact that they are a sustainable alternative to fossil fuels and that their combustion is of minor environment impact. Specifically, biodiesel has been found to be a suitable alternative to conventional diesel. Biodiesel has similar properties to diesel, can be used in any conventional motor, is obtained from renewable sources (vegetable oil and animal fat), is biodegradable, can reduce most exhaust emissions, and has excellent lubricity and a higher flash point than diesel [1]. Biodiesel, which is mainly obtained from vegetable oils via the transesterification process, is formed by a mixture of fatty acid methyl esters (FAME). It is produced in industrial plants comprising several sections that carry out the pre-treatment of raw materials, the transesterification reaction, and the post-treatment of the products. For example, the complete biodiesel production line in a plant equipped with technology of the American Society of Agricultural Engineering comprises the following: two pre-treatment sections processing the crude vegetable oil (a degumming section and a refining section); a central section carrying out the transesterification process; and three post-treatment sections (an ester washing section, a glycerol separation section, and a methanol recovery section) [2]. Mainly comprising chemical reactors, the transesterification section plays the most important role in the biodiesel production process, with reactor operating conditions having a crucial impact on both the process variables and the quality specifications [3]. Modern plants generally use continuous reactors instead of batch systems, with the continuous stirred-tank reactor one of the most commonly used [2; 3; 4; 5]. A constant supply of raw materials is required for continuous industrial biodiesel production processes to satisfy global demand. According to the \"_Organization for Economic Co-operation and Development_\" and the \"_Food and Agriculture Organization of the United Nations_\", global biodiesel production is projected to increase from the 36 bln L recorded in 2017 to 39 bln L by 2027 [6]. However, in some situations, the vegetable oil used in the biodiesel production process is not available. The availability of feedstock for producing biodiesel depends on the regional climate, geographical locations, local soil conditions, and agricultural practices of any country [7]. The oil yield from the crop itself is one of the key factors in deciding the suitability of a feedstock for biodiesel production [8; 9]. For example, palm oil is mainly the raw material of the biodiesel production in Asia [8; 10]. However, the oil palm yield is affected by a huge amount of pests (caterpillars, leaf miners, grasshoppers, moths, beetles, weevils, rats) and diseases (basal stem rot, upper stem rot, fusarium wilt, red ring disease, sudden wither, bunch rot), which, even if they do not kill the palm tree, considerably reduce palm yield [11]. When a nominal raw material is not available or is not suitable for the process, a feed oil change is necessary to maintain the production. It is a matter of significant impact given that such a change during the continuous operation of the transesterification reactor causes a disturbance of the process, which cannot be interrupted, even in the presence of disturbances. An appropriated mathematical model can be used to predict the effect of disturbances in biodiesel reactors, including these caused by oil change, and help to reduce the negative effects by the design of a control system [3; 4; 12; 13; 14; 15]. Nonetheless, the continuous reactor model previously reported does not consider the dynamics of feed oil changes. In this sense, [PERSON] et al. presenting a rigorous mechanistic model of the biodiesel transesterification reactor [3]. The model was developed by applying mass and energy balances to a continuous stirred-tank reactor, but only considers the transesterification of palm oil. [PERSON] et al.