2.1.4. Steam Network

While most industrial facilities have a major steam source (e.g., a combined heat and power unit (CHP)), the utilization of waste heat from production units in means of steam production is also a common practice [43]. Moreover, the overall steam supply of these local producers can even exceed the major steam source production. Thus, the quality of the supplied steam varies significantly depending not only on the CHP unit's operation but on the whole production network [22,72]. It is therefore vital for process drive sizing to account for steam quality fluctuations.

When analyzing steam networks, as well as any other pipeline systems, momentum and heat transfer must always be considered [73]. Pressure loss caused by friction and/or various installed armatures and potential heat loss to ambient space can a ffect the steam quality significantly. When analyzing the heat transfer, steam–ambient temperature di fference plays the most significant role. The impact of heat loss on the medium quality is thus best observable during extreme ambient conditions, i.e., temperature peaks during winter. Even though most adverse e ffects of ambient temperature changes can be suppressed by e ffective insulation, Hanus et al. [26] showed that the effectivity of outdoor insulation decreases heavily after years of operation and it is in designer's best interest to investigate the true heat conductivity of the used insulation.

Effects of both pressure and heat losses vary greatly depending on pipeline geometry and mostly on the steam network topology and plant infrastructure, i.e., the resulting pipeline length. While the distance (pipe length) of the considered steam drive from the battery limit header can range in terms of tens of meters in some cases, there are units where the pipe length can reach hundreds of meters or even a kilometer. This is mostly the case of mid-twenty century refineries where individual units and their subunits were built far apart for safety reasons [26].

Lastly, it is necessary to assess the impact of steam drive alteration on the steam transport velocity. Naturally, high transport velocities may cause serious erosion of pipeline walls and damage installed armatures. However, low velocities in the pipelines may result in excessive heat loss and decrease in steam quality or even condensate production [26]. Hence, the velocity changes with the steam consumption of the drive, and pipeline suitability assessment is inevitable.

The process described in the previous text draws high-pressure steam from a steam network using a CHP unit as the main steam producer. As illustrated below (Figure 2), steam crossing the fluid catalytic cracking (FCC) unit battery limit is first supplied to the modeled system (propylene recovery unit) and then to other parts of the FCC unit. To account for all above-mentioned aspects, pipeline topology, geometry, and heat transfer properties were incorporated in the Aspen Plus ® process model.

**Figure 2.** Simplified plant steam network. Legend: Red line= HPS pipelines, orange line= middle-pressure steam (MPS) pipelines, dotted red line = turbine exhaust; blue line = condensate.

#### 2.1.5. Combined Heat and Power Unit (CHP) as Marginal Steam Source

Industrial combined heat and power plants (CHP) traditionally employ steam boilers and steam turbines as the cogeneration technology; some of them including gas turbines or combustion engines [40,74]. Their task is to cover the steam network imbalance on all pressure levels (i.e., to serve as a marginal steam source) while the cogenerated electric energy is utilized in the industrial facility or sold to an external grid. Seasonal steam demand variations influence their operation and the resulting backpressure power production decreases in summer below a pre-set acceptable level, and has to be compensated by other means (condensing power production). The reason for this specific system feature is explained in more detail in Section 3.2. Techno-economic assessment of a process steam drive sizing inevitably impacts the CHP operation and should be evaluated correctly. Last, but not least, the CHP operation is influenced not only by internal steam demand, but by external factors as well (energy managemen<sup>t</sup> strategy of the refinery, changeable energy prices and their uncertainty, etc.) [75].

The considered CHP comprised high-pressure steam boilers (9 MPa, 530 ◦C) and a set of backpressure and extraction condensing turbines exporting high-pressure (HPS, 3.5 MPa), medium-pressure (MPS, 1.1 MPa), and low-pressure (LPS, 0.5 MPa) steam to the industrial facility. Constant marginal heat production e fficiency of the CHP and constant marginal backpressure and condensing power production per one ton of steam were assumed. The e ffect of a change in the production unit steam balance on a certain pressure level was transposed to the corresponding change in backpressure electric energy production and fuel consumption in the CHP. A detailed description of the CHP unit operation and its further characteristics can be found elsewhere [26,47].
