3.1.1. Exergy Profit of Waste Acetic Acid Streams.

To define the exergy footprint/profit of the waste acetic acid stream, the potential downstream process and the boundary need to be defined. In this paper, the methanol carbonylation process is chosen as the process that produces the pure acetic acid and the waste stream containing the acetic acid. The potential downstream process—namely vinyl acetate production—utilises the waste acetic acid as a secondary input. Figure 6 shows the identified processing options.

**Figure 6.** Exergy profit evaluation of waste acetic acid stream.

The purge stream containing acetic acid is the waste stream under evaluation. The methanol carbonylation process mainly synthesises acetic acid as the main product. The waste stream (purge stream) contains a significant concentration of acetic acid (see Table 1). The mass and energy balance data

are retrieved from [79]. The waste stream is not likely to be marketable but can be used to retrieve a secondary raw material for the vinyl acetate (VAM) production process. Figure 7 shows the Aspen HYSYS [82] simulation of the VAM process. The parameters for equipment sizing can be retrieved from [83]. Tables 1 and 2 show the mass balance data of the two processes. The exergy to be added to the process includes the exergy inputs of the energy streams and the workstreams. In this work, the reference state of the materials is evaluated based on the Szargut method [76]. The detailed calculation steps of the chemical exergy are shown in [6]. For simplicity, an open-source online tool [77] is used to estimate the chemical exergy of materials in this paper.


**Table 1.** Data of acetic acid production, derived from [79].

The basic data for exergy calculations of various streams have been found in [6]. The feed used in the VAM production process is a pure acetic acid. As an input stream type, no EXAsset or EXLiability values are assigned to it.

The exergy liabilities and assets have to be determined first to compute the exergy profit of the waste acetic acid stream. The exergy profit is, therefore, the difference between the assets and the liabilities. The exergy asset and liability can be calculated based on Equations (2), (3) and (8):

$$\mathbb{E}\mathbb{X}\_{\text{asset},\ AA\ Waste} = \sum\_{j} \mathbb{E}\mathbb{X}\_{\text{output},j} \tag{10}$$

$$\mathbb{E}\mathbb{X}\_{\text{liabilities},\text{AA Waste}} = \sum\_{i} \mathbb{E}\mathbb{X}\_{\text{input},i} + \mathbb{E}\mathbb{X}\_{\text{add}} \tag{11}$$

$$\text{EX}\_{\text{profit, AA Waste}} = \text{EX}\_{\text{asset, AA Waste}} - \text{EX}\_{\text{liability, AA Waste}} \tag{12}$$

where EXadd represents the exergy to be added to the downstream process, *i* represents the set of input material *i* in the downstream process, and *j* represents the set of output material *j* in the downstream process. Note that the exergy mentioned here is the total exergy (chemical + physical).

**Figure 7.** Aspen HYSYS simulation flowsheet of vinyl acetate production, adapted from [83]. AA: acetic acid.



## *Energies* **2020**, *13*, 2132

Table 3 shows the calculation results, assuming a value of 100 kg/h of the purge stream. The detailed calculation steps are given in the Supplementary Materials (Part 1). It can be observed from the table that the exergy profit of the stream is −0.0497 MW. The original exergy content of the purge stream is −0.000198 MW. This shows that the exergy profit is lower than the exergy content of the stream without undergoing the downstream VAM process. The negative sign of the exergy indicates that extra work needs to be inputted to bring the materials to the reference conditions since it is not a spontaneous process. According to the results, it can be deduced that higher exergy needs to be invested in reusing the purge stream for the VAM production process, translating to an exergy footprint of 0.0497 MW.

**Table 3.** Calculation results of the waste acetic acid stream, on the basis of a value of 100 kg/h of the purge stream.

