A. Kister and M. EL-HALWAGI, Texas A&M University, College Station, Texas (U.S.); and C. DUEWALL, Bryan Research & Engineering, Bryan, Texas (U.S.)
Due to the vital role of distillation in hydrocarbon processing and the significant energy consumption associated with distillation systems, there is a critical need to continue to optimize the performance of existing infrastructure and generate insights for new unit designs. Furthermore, it is necessary to enhance the various objectives (e.g., economic, energy, environmental) of these systems. This article proposes a combined approach using exergy loss analysis and product optimization as a rapid method for evaluating and improving existing distillation processes to reduce their costs and environmental impact. Distillation exergy analysis identifies thermodynamic irreversibility and evaluates retrofit possibilities after which the retrofit is economically and environmentally optimized. This two-step process is applied to the deethanizer column of a natural gas liquids (NGLs) fractionator saving 8,000 tons per year (tpy) of carbon dioxide (CO2) equivalent emissions and increasing profits by $2 MM.
Distillation stands as the dominant separation technology, bearing responsibility for 90%–95% of all liquid separations spanning across petroleum refineries, chemical processing plants, natural gas purification plants and many more.1 Although highly successful at achieving these desired separations, distillation columns are also known to exhibit very low thermodynamic efficiencies, resulting in the consumption of enormous quantities of energy.2 Estimates suggest that distillation columns account for approximately 40% of the energy used in refineries and 30% of the energy consumed in chemical plants.3 The substantial energy demands of these distillation columns contribute significantly to large CO2 footprints, negatively impacting the sustainability of these processes. Several process troubleshooting, integration and optimization techniques have been proposed to diagnose and improve the performance of distillation systems.4
With global CO2 and greenhouse gas (GHG) emissions consistently reaching record levels each year, drastic efforts are being implemented worldwide to rapidly reduce industries’ carbon footprint and transition to a more sustainable future. While extensive research has focused on optimizing new column designs and seeking alternatives to distillation, significantly less effort has been put into improving existing distillation columns. In the U.S. alone, more than 40,000 distillation columns are in operation, many of which were designed and built well before sustainability became a primary consideration.5 Some of these columns may have been designed with some degree of energy efficiency in mind; however, historically this was exclusively related to operational expenditures (OPEX) and was only significant during periods of high energy prices. Even for those columns initially designed to be optimal from a sustainability perspective, decades of operation involve changes in feed composition, product specifications or overall use, rendering what was once considered optimal as insufficient.
To address these sub-optimal designs, the authors propose a two-part method to rapidly analyze existing distillation columns for potential improvements (FIG. 1). This approach involves performing an exergy loss analysis to identify regions of excessive thermodynamic irreversibilities within the column, followed by economically and environmentally optimizing the selection of product specifications. Retrofit optimization typically requires discrete variables for decision-making and specialized optimization methods. Exergy analysis simplifies the complex optimization problem into a simple nonlinear programming problem.
Considering the magnitude of CO2 emissions from existing distillation columns, systematically reducing their energy consumption using this comprehensive method has the potential to yield significant environmental and economic benefits. This article will discuss a deethanizer distillation column in an NGL fractionation train and showcase how combined exergy analysis and retrofit optimization may be applied to yield large savings.
NGL FACILITY OVERVIEW
Natural/shale gas production has experienced tremendous growth over the past two decades, gaining global significance due to its crucial role in meeting rising global energy demands and in providing a substantial resource for monetization to value-added chemicals.6 With the recent discovery of abundant reserves of unconventional shale gas resources, the International Energy Agency (IEA) has estimated that, at current production rates, global natural gas reserves would be sufficient for more than 230 yr.7 Given the plentiful supply of natural gas and its status as a cleaner, more versatile energy alternative compared to other petroleum derivatives, natural gas usage will remain highly prevalent in the coming years.8
Natural gas primarily consists of methane (CH4); however, upon extraction from various wells, it is typically accompanied by impurities in the forms of CO2, hydrogen sulfide (H2S), nitrogen, water and heavier hydrocarbons. While most of the impurities are undesirable, it is profitable to recover the heavier hydrocarbons (everything heavier than CH4) and utilize them for petrochemical sales or gasoline blending, making them incredibly valuable byproducts.8 These heavier byproducts are collectively referred to as NGLs and typically include ethane, propane, butane, isobutane and natural gas.9 NGLs are usually processed from natural gas at the wellsite through an NGL fractionation train, which consists of a series of distillation columns designed to separate and purify the individual hydrocarbon components. These systems must be robust enough to handle the potential variability in feed composition.10
Due to the rapid nature in which many of these fractionation facilities were built over the past few decades, and the emphasis on increasing capacity to handle what seemed like ever-growing throughputs, little focus has been placed on optimization. With modern sustainability goals in mind, this presents an excellent opportunity to analyze these existing plants for possible improvements.
For this reason, the proposed method of exergy analysis combined with product optimization will be applied to a deethanizer column on an NGL fractionation train. The strong impact that the deethanizer products have on the downstream columns renders the deethanizer as a particularly suitable candidate for the optimization work.
A process flow diagram (PFD) of the NGL fractionation train that will be discussed throughout this case study is shown in FIG. 2. The feed to the train consists of 50,000 barrels per day (bpd) of NGL, the equivalent of 8,145 lb-mol/hr. The full NGL feed is introduced to the first column of the train, the deethanizer. The deethanizer separates the ethane and residual CH4 overhead as a product stream. The bottom stream from the deethanizer, containing C3s and heavier components, is then fed to the depropanizer. This column is designed to separate the C3s from the C4s and heavier components. The overhead C3s from this column are sold as a product.
The bottom stream from the depropanizer (C4s and heavier components) is then fed to the debutanizer. This column removes C4s overhead, leaving a bottom product of C5s and heavier components. The bottom stream is sold as a product, while the overhead C4s product stream [isobutane (iC4) and normal butane (nC4)] is sent to a C4-splitter. The C4 splitter separates the iC4 and nC4 isomers, with iC4 being extracted overhead and nC4 coming out of the column bottoms.
This article exclusively analyzes the available exergetic and optimization improvements for the deethanizer. Extensive process simulationsa were conducted for the deethanizer with the SRK thermodynamic model. Many results were generated using tools developed with an Excel add-ina, alongside the built-in optimization tool. To ensure coherent and comparable results, certain product specifications were kept constant across all simulations. This consistency ensures that product quality remains unchanged between any two simulations of the same column, enabling a direct comparison among the different cases being investigated. The product specifications are discussed separately for the exergy loss analysis and optimization sections.
As the first column of the NGL fractionation train, the feed composition to the deethanizer frequently varies due to the quality of upstream wells or different production requirements sometimes related to LNG production.6 As a result, three distinct Y-grade feed types were individually tested: high C2 feed, mid C2 feed and low C2 feed, as outlined in TABLE 1. This was done to ensure that the recommended modified column would be successful in handling a diverse slate of feed compositions.
The feed to the column enters on Stage 6 at 80°F (27°C) and 343 psig. The column consists of 33 equilibrium stages along with a partial condenser and partial reboiler. The initial case for each of the three feeds was simulated to serve as a benchmark upon which improvements could be made (FIG. 3).
Due to ethane’s low boiling point [–128°F (–88.9°C)], the deethanizer operates at a high pressure to avoid using excessively cold condenser utilities. In the case study, the overhead column pressure is set to 320 psig, resulting in a condenser operating at 31°F (–0.6°C) and a reboiler operating at 190°F (88°C). Consequently, the condenser relies on refrigerated propane at 21°F (–6.1°C), and the reboiler relies on low-pressure steam (LPS) at 250°F (121°C). From a sustainability standpoint, producing refrigerated propane demands significant energy consumption in the refrigeration compressors, making it costly in terms of emissions. Similarly, the generation of steam—albeit LPS, which is comparatively more sustainable than medium-pressure steam (MPS) or high-pressure steam (HPS)—also negatively impacts the process sustainability, as the heat needed to generate the steam is also sourced from burning fossil fuels (TABLE 2). Therefore, to enhance the sustainability of the deethanizer, any modifications implemented should aim to reduce the reliance on refrigerated propane by the condenser and/or the LPS in the reboiler, without increasing consumption of the other. This case study is based on a real plant.
Exergy analysis methodology. Exergy loss analysis is a comprehensive method of analyzing distillation columns. Exergy, expressed in units of energy, is a thermodynamically adjusted quantification of energy representing a combination of the first and second laws of thermodynamics. Consequently, it accounts not only for the quantity of energy but also for its quality, as determined by entropy. Considering these entropic effects is critical for correctly describing the energy requirements of a thermal process.
The exergy of any stream may be calculated based on its properties according to Eq. 1.11 Simulating the column in a commercial process simulator provides the internal column liquid, vapor flowrates and composition needed to calculate exergy. The results included in this work use the author’s company’s softwarea to simulate the column as well as provide the associated enthalpic and entropic properties of the vapor and liquid on each stage. These stage-by-stage exergy balances can determine the amount of exergy lost in each stage.12 These may be plotted to reveal precisely where exergetic losses occur throughout the column, as shown in FIG. 4 (Eq. 1):
Ex = H − T0S (1)
where: H is enthalpy T0 is absolute reference temperature S is entropy Ex is exergy.
Exergetic losses represent thermodynamic irreversibilities that negatively impact the efficiency of the column. However, some of these irreversibilities are required to ensure the presence of sufficient driving force for separation. Accordingly, one may split exergetic losses into two categories: losses necessary for performing the separation and excessive losses. The goal is to target the excessive losses.
Reducing the excessive exergetic losses is typically achieved by evenly distributing exergy losses throughout the column.12 This may be accomplished by altering the internal flowrates of the column through the application of various revamp strategies. These revamp strategies include mechanical changes such as altering the thermal condition of the feed, changing the feed stage or adding side reboilers/condensers, as well as operational changes such as altering the operating pressure or the column reflux. While simulating these modifications, it is critical that key product specifications are maintained to guarantee a consistent product quality. For the exergy portion of the case study, product compositions were held constant at purities corresponding to 95 vol% ethane in the overhead and a ratio of ethane to propane in the bottoms at 0.05.13,14
Once the appropriate revamp strategies have been selected, the new resulting exergy loss plot for the modified column may be analyzed with the hope that overall exergetic losses have been reduced. Generally, reducing the overall exergy losses reduces a column’s energy consumption, which can positively impact sustainability. However, this is not always the case, making it crucial to evaluate changes based on their true environmental impact rather than focusing solely on exergetic savings.
EXERGY ANALYSIS CASE STUDY RESULTS
This section reports the results and observations of applying the described exergy analysis methodology to the example deethanizer column. An exergy loss plot is shown in FIG. 5 for the high C2 feed case. While the analysis primarily focuses on the high C2 case, very similar results were observed for the other two feed types. The plot reveals a poor distribution of exergy losses throughout the column with large exergy loss peaks near the feed stage and bottom reboiler, and almost no exergy losses throughout the middle stages of the column. The total sum of column exergy losses in the original column is 5,630 British thermal units (Btu)/hr.
Since it is desirable to lower the overall exergy losses, attempts will be made to apply revamp techniques to more evenly distribute the exergy losses throughout the column by shifting some of the losses away from the large peaks in the plot.
The exceptionally high exergy loss peak at the feed stage (Stage 6) indicates a mixing inefficiency, possibly due to improper feed location or conditions. To remedy this, revamp strategies that target the feed to the column were considered. The first of these involved lowering the feed stage to the column, taking advantage of the very low exergy losses throughout the middle stages of the column (FIG. 6). Lowering the feed stage indeed shifts many of the exergy losses from the top of the column towards the previously underutilized center section. This more even distribution notably reduces the exergy loss peak at the feed stage, suggesting successful targeting of the mixing inefficiency.
Based on these plots, a minimum in exergy losses can be identified between feed Stages 15–20. The effects of lowering the feed stage from the original Stage 6 to a modified Stage 15 on the condenser and reboiler duties are displayed for all three feed types in TABLE 3. It is evident from the table that this modification effectively reduces the exergy and energy duties required by the column across all feed types. Therefore, subsequent analyses in this study assume a modified feed stage of 15.
We may now address the second major exergy loss peak leading up to the reboiler. These exergy losses may be shifted away from the bottom of the column by adjusting the thermal condition of the feed. This was investigated by testing various degrees of feed preheat, as displayed in FIG. 6, resulting in a reduction of the bottom reboiler duty. As the vapor fraction of the feed increases, the high exergy losses near the bottom reboiler start to shift up the column, contributing to a more even distribution. A minimum exergy loss is identified at a feed vapor fraction of 28% (FIG. 7).
Although this preheat may be optimal from an exergy perspective, it is not from a sustainability perspective. TABLE 4 illustrates how different extents of preheating impact the duties of the reboiler and condenser, and subsequently influence the overall cost and emissions associated with utility generation. The discrepancy arises from the significant environmental and economic costs of refrigeration production—a factor not fully accounted for by exergy analysis, which only considers the magnitude of temperature differences from ambient conditions [70°F (21°C)]. In this case, exergy overvalues the hot LPS at 160°F (71°C), which is 90°F away from ambient, compared to the refrigerated propane at 30°F (–1°C), which is only 40°F away from ambient, despite the opposite trend in economic and environmental impact. This can lead to an optimal solution according to exergy that does not align with real-world profit and sustainability objectives.15
Takeaway. Overall, exergy analysis has been demonstrated as an excellent tool for comprehensively identifying thermodynamic irreversibilities within a column that can be used to implement positive revamp strategies. By lowering the feed stage from 6 to 15, significant reductions in reboiler and condenser utilities may be attained for the deethanizer. This led to a reduction of GHG emissions of up to 8% (or 8,000 tpy of CO2e) and an annual profit increase of up to 8% (or $1 MM). The preheat example also demonstrates the critical step of ensuring that these improvements translate into genuine sustainability and profit benefits rather than merely optimizing exergy metrics. Part 2 of this article will build upon the exergy analysis step by optimizing the product splits for economic and environmental criteria. HP
NOTE
Bryan Research & Engineering’s ProMax® Optimization Tool
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