**1. Introduction**

Butyric anhydride is an important intermediate product, which is mainly used in the production of cellulose acetate butyrate, butyl butyryl lactate, and spices. The existing production of butyric anhydride is a reversible anhydride exchange reaction of butyric acid and acetic anhydride. The boiling point profile of the reaction systems shows that reactive distillation (RD) can be used to produce butyric anhydride. RD is an intensified process which couples the chemical reaction with distillation separation in one piece of equipment, thus equipment costs are reduced. RD can also improve the conversion and yield of the reversible reaction by removing the product from the reaction system in time; in addition, the concentration of reactants increases and the reaction rate also increases as the products are moved out. In the past decades, the batch RD process has been used to produce butyric anhydride [1], with the purity of butyric anhydride being high. The main disadvantages of the batch process include difficulty of operation, long production period, high labor costs due to the difficulty of automation, and high energy consumption due to the need to steam out the transition components [1].

In recent years, continuous manufacturing processes have been introduced to produce fine chemicals, such as butyric anhydride, due to the numerous advantages over batch-wise manufacturing; these include lower capital and operating costs, improved controllability and product consistency, and lower environmental footprint [2–4].

Gao [1] introduced continuous RD technology for producing butyric anhydride based on the analysis of the physical properties of the reaction system; results revealed that the continuous RD process is feasible for the production of butyric anhydride.

The operation of the continuous RD process can be classified into two types, namely, neat and excess. For neat operation, reactants are fed into the RD column at the same ratio as the stoichiometric of reaction, while the composition of the column overhead and bottom products are not determined in this situation, since strict flow control cannot be guaranteed in practical production; for example, Xu et al. [5] compared the neat and excess operations for synthesizing N-propyl propionate, the dynamic simulation results show that the product purity requirements cannot be ensured during neat operation; Daniel et al. [6] showed the same result with a process combining a continuous stirred tank reactor and an RD column for production of isoamyl acetate on a pilot scale.

Therefore, the excess operation is often adopted in practice; this requires one feed for the reactants according to the reaction stoichiometric, which helps avoid the disadvantage of the neat operation, while an additional distillation column is needed to separate the excess reactant from the product. Chung et al. [7] studied the influence of the excess ratio on the reactant conversion and energy consumption, and the results show that there is an inherent trade-o ff between the number of reactive trays and energy consumption; Chua et al. [8] compared di fferent choices of excess reactant for producing isopropanol with the RD process.

As a result of the reliability of the excess operation, Li et al. [9] proposed a two-column RD process to produce butyric anhydride with an excess feed of butyric acid, thus the reliability of the process was validated by simulation and experiments.

In this paper, a novel continuous process with a single RD column (SRDC) is proposed to produce butyric anhydride using the excess operation, where the second distillation column in the traditional RD process for separating excess reactant is removed based on the analysis of the boiling point profile of the reaction system. In order to ensure the stable composition of the RD column, an excess reactant is usually needed in the traditional RD process, and for this reason a second distillation column is needed to separate the product from the excess reactant. SRDC completes the separation of the excess reactant and the product in the stripping section or the rectifying section of the RD column. Compared with the traditional RD process, product purification is removed and the total annual cost (TAC) of butyric anhydride production is greatly reduced.

The rest of this paper is organized as follows. Section 2 summarizes the reaction kinetics and thermodynamic data needed for the simulation of the RD process. The traditional two-column RD process for the production of butyric anhydride is simulated in Section 3. Section 4 analyzes the boiling point profile of the reaction system and presents a novel SRDC process with internal material circulation for butyric anhydride production, where two di fferent applications are simulated and compared. Finally the paper is concluded in Section 5.
