State-of-the-art process flow scheme

https://doi.org/10.1016/j.seppur.2021.118959

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The Process Flow Diagram (PFD) considered as benchmark state-of-the-art configuration for NH₃-based capture processes and used in this work as basic scheme for the implementation of advanced configurations is shown in Fig. 1. The description of the process and the purposes of each process section and unit operation have been detailed elsewhere [14]. Such process flow scheme is the result of implementing the patents related to the CAP [10], [48], [49], [50], [51], including process sections that allow for the control of NH₃ emissions and for the recuperation of NH₃, as well as simple process modifications broadly applied in solvent-based capture processes that allow to decrease the overall energy consumption with minor impact on the process complexity and operability, e.g. the RSS. More specifically and most relevant to this work, the benchmark PFD considers:

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Fig. 1. Benchmark state-of-the-art flow scheme of the CAP used as reference configuration of NH₃-based capture processes. Boxes with dashed lines are introduced for the identification of the different sections of the process, i.e. A to G. Labels provide the name of the process section or, if within a unit operation, the equipment or equipment section identification. The operating variables whose value govern the performance of the process are indicated in the flow scheme and have been colour-coded according to the process section to which they belong, i.e.

for CO₂ absorber operating variables,

for CO₂ desorber operating variables and

 for FG-WW column operating variables. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)

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The absorber pumparound (within the CO₂ absorption section (F) in Fig. 1), a slip stream of the cold CO₂-rich stream that is cooled, chilled and sent to the top of the CO₂ absorber in order to minimize the NH₃ slip to the gas.

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The FG post-conditioning section (A), which is able to further decrease the NH₃ concentration in the CO₂-depleted FG to values that meet the regulatory limits at the stack. It consists of a Flue Gas Water-Wash (FG-WW) column that is an NH₃ absorber, followed by an acid-wash scrubber using an aqueous solution of H₂SO₄.

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The CO₂ Water-Wash (CO₂-WW) section (B), which is used to minimize gaseous NH₃ losses from the CO₂ desorber, consists of a Direct Contact Cooler (DCC) that uses water to remove NH₃ from the CO₂ gas stream produced in the CO₂ desorber.

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The solvent recovery section (C), whose goal is to minimize the NH₃ make-up requirements, is composed of two different distillation columns, namely: (i) the NH₃ desorber, whose purpose is the recuperation of NH₃ (and CO₂) from the NH₃-rich solution exiting the NH₃ absorber; and, (ii) the appendix stripper, which recovers NH₃ (and CO₂) from the solvent stream purged from the CO₂ absorber/CO₂ desorber loop and from the liquid stream purged from the CO₂-WW section. Both the NH₃ desorber and the appendix stripper operate at atmospheric pressure and generate an almost pure water stream at the bottom of the column, requiring ca. 100 °C at the reboiler.

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The RSS (within the CO₂ desorber section (E)), a split of the cold CO₂-rich stream that bypasses the rich/lean heat exchanger and that is directly sent to the top of the CO₂ desorber instead. In addition to control the temperature in the CO₂ desorber thus the NH₃ slip to the CO₂ stream, its optimization, together with the pressure in the CO₂ desorber, allows minimizing the energy demand while avoiding solid formation downstream in the process [14].

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The solvent is regenerated and CO₂ is stripped-off in the CO₂ desorber (E), which operates at pressures between 7.5 and 30.0 bar and that reaches temperatures in the reboiler between 115 and 150 °C.

The operating variables that govern the overall performance of the benchmark state-of-the-art CAP are indicated in Fig. 1 [14]. They are enumerated below, and arranged according to the capture process section to which they belong:

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CO₂ absorber (F) decision variables, i.e. the apparent NH₃ concentration in the CO₂-lean stream, cˆNH3 [molNH3kgH2O−1], the CO₂ loading of the CO₂-lean stream considering apparent species, llean [molCO2molNH3−1], the liquid-to-gas flowrate ratio for the CO₂-lean liquid and inlet gas streams entering the CO₂ absorber, Llean∕Gin [kg kg⁻¹], the temperature of the pumparound stream, Tpa [°C], and the pumparound split fraction, fs [-].

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CO₂ desorber (E) decision variables, i.e. the pressure in the CO₂ desorber, PCO2des [bar], and the cold-rich bypass split fraction, fcr [-].

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FG-WW column (A) decision variables, i.e. the apparent NH₃ concentration in the NH₃-lean stream, cˆNH3FG−WW [molNH3kgH2O−1], the liquid-to-gas flowrate ratio for the NH₃-lean liquid and the CO₂-depleted FG entering the NH₃ absorber, (L∕G)FG−WW [kg kg⁻¹], the temperature of the NH₃-lean stream, TleanFG−WW [°C], the temperature of the pumparound stream, TpaFG−WW [°C], and the pumparound split fraction, fsFG−WW [-].

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