**5. Conclusions**

The presented method for process steam drive sizing comprises both process and steam network including the marginal steam source operation assessment. A proper investigation of both process-side and steam-side design and operation parameters is necessary and should precede the drive sizing itself. Attention should be paid to key process parameters including the required power input and its hourly and seasonal variability as well as the variability of the driven equipment frequency. Driving steam quality and discharge steam pressure variations should be evaluated carefully, and their change due to frictional pressure and heat losses from steam pipelines should also be assessed. Driving and discharge steam pressure levels should be chosen according to the analysis of individual steam pressure levels' operation range and their anticipated change resulting from steam drive implementation. Their e ffect on the marginal steam source operation should be evaluated considering all previous findings.

The proposed method comprises process data evaluation and calculations in a linked MATLAB ® and Aspen Plus ® environment. Thereby, the potential of MATLAB in process data analysis and results evaluation and visualization is coupled with the ability of rigorous process simulation by Aspen Plus ®.

The industrial case study comprised the replacement of the existing condensing steam drive by a new backpressure one, driving the 1.25 MW compressor of the heat pump-assisted C3 fraction splitting process. Application of the proposed method and comparison with the results obtained using methods proposed by other authors revealed that:


3. The simplest sizing method combined with the ten-times-longer steam pipeline led to a C3 fraction splitting capacity decrease of around 20%, which was unacceptable.

Examination of the changes in the HPS and MPS network operation revealed that several capacity bottlenecks could be removed by the steam drive replacement, but new ones could arise, which requires active steam network management. The impact on the CHP operation included fuel savings of up to 3.72 kt/year, and a CO2 emissions reduction of 11.13 kt/year at the expense of an additional 17.29 GWh/year of power purchased from outer grid compensating for the power production decrease at the CHP. Warmer months' contribution to this cost was negligible as the CHP compensated for the lowered backpressure power production by the expensive condensing one. Despite this fact, the proposed steam drive replacement exhibited a simple payback period shorter than two years.

Further method improvement will be aimed in future work with the focus on implementing multi-objective optimization. The effects of steam drive implementation or replacement of the main steam network operation should be examined and assessed more closely.

**Author Contributions:** Conceptualization, P.F. and M.V.; data curation, P.F. and T.C.; funding acquisition, Z.L.; investigation, P.F. and T.C.; methodology, M.V. and P.F.; resources, T.C.; software, P.F; supervision, M.V. and Z.L.; validation, T.C.; visualization, P.F.; writing—original draft, P.F. and M.V.; writing—review and editing, Z.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the Slovak Research and Development Agency, Grant Nos. APVV-19-0170 and APVV-18-0134, and by the Slovak Scientific Agency, Grant No. VEGA 1/0659/18.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
