**1. Introduction**

Acrylic acid is a widely used chemical intermediate in the polymer and textile industry. There are numerous technologies available for the production of both ester and glacial acrylic acid, however the most popular route is via the partial oxidation of propylene. Propylene is contacted with steam and air in a two-step reactor train, which uses different catalysts. In the first reactor, propylene is oxidized to acrolein followed by acrolein oxidation to acrylic acid in the second reactor [1]. The main partial oxidation reactions occurring in each reactor are indicated by Equations (1) and (2). Several side reactions can also occur, with the main byproducts being acetic acid and carbon dioxide [2].


*Energies* **2020**, *13*, 1971

$$\text{C}\_3\text{H}\_4\text{O} + \frac{7}{2}\text{O}\_2 \rightarrow 3\text{CO}\_2 + 2\text{H}\_2\text{O} \qquad\qquad\text{Carbon Dioxide}\tag{6}$$

Presently, acrylic acid production occurs in multitubular reactors consisting of up to 30,000 tubes with small diameters to ensure that thermal radial gradients are reduced whilst increasing the available heat exchange area for optimal and rapid heat removal by a circulating heat transfer fluid on the shell side [3]. Propylene and acrolein partial oxidation reactions are highly exothermic and therefore susceptible to thermal runaway, catalyst degradation and the promotion of unwanted side reactions. As such, effective heat removal is essential to maintain the required production rates and avoid irregular hotspots within the reactor. Figure 1 illustrates the typical reactor train and heat transfer fluid heat exchange network.

**Figure 1.** Typical reactor train and thermal fluid heat exchange network for acrylic acid production (BFW refers to boiler feed water).

The design, optimization and analysis of such a unit is a complicated task, since there are a large number of optimization variables to be considered. Proper representation of the reactor train in a process simulator is the first step towards heat integration, development of energy management strategies and overall understanding that could lead to intensification of the process [4]. Process simulation studies carried out on this process and reported in the literature have considered either adiabatic or fluidized bed reactor types with no in depth analysis of the more popular multitubular reactor configuration [2].

In this work, the rigorous design of a two-stage acrylic acid reactor train was undertaken and critical parameters identified for accurate representation of the unit in a process simulation platform. The study offers insight into the sensitivity of major performance criteria to the various operating parameters, many of which are unique to the multitubular reactor configuration.
