Process for n-butyl acrylate production using reactive distillation: Design, control and economic evaluation
Graphical abstract
Introduction
n-Butyl acrylate (n-BA) is an important bulk chemical used as precursor in the production of acrylic polymers, finding its main use in a variety of end products. In 2013, 43.7% of global demand came from coatings, 17% polyethylene, 17% adhesives, 10% textiles, and 12.3% from other end products (TranTech, 2014, accessed 02.10.2016).
n-BA is produced in the equilibrium reaction between acrylic acid (AA) and n-butanol (n-BuOH) catalysed by homogeneous or heterogeneous catalysts, with water obtained as co-product. At commercial scale, n-BA is produced in a multi-stage process using two reactors (Niesbach et al., 2013b). The most common strong acidic homogeneous catalyst used is sulfuric acid (Ohara et al., 2003). However, some key disadvantages of the homogeneous process are corrosion, catalyst removal from product, and catalyst disposal problems after neutralization. Usually, these lead to increased energy requirements, high maintenance costs, and a continuously increasing difficulty to comply with environmental regulations. In addition, both AA and n-BA polymerize very easily (Ohara et al., 2003), and this happens also during the esterification reaction (Niesbach et al., 2013a). Catalysed by heat, light and peroxides (Ohara et al., 2003), the risk of polymerization is high mainly at increased temperatures (Niesbach et al., 2013a). In view of new designs, exploiting these elements has a potential high impact on investment, footprint, and utility consumption. In addition, operation at mild temperature conditions and relatively low holdup reduce the risk of n-BA polymerization.
Recent studies focus on solid catalysis using various ion exchangers (Constantino et al., 2014, Ostaniewicz-Cydzik et al., 2014, Sert and Atalay, 2012, Sert et al., 2013). These studies can facilitate the design of processes that overcome the important drawbacks of the homogeneous catalysed process. Applying process intensification technologies as reactive distillation and simulated moving bed reactor, the capital investment and footprint are reduced, as well as the utilities requirement.
The literature is scarce in design, control and economic evaluation studies of reactive distillation-based processes for n-BA production using solid catalysts. To our best knowledge, there are only two studies dedicated to process development: one presenting the steady-state design and economic evaluation (Niesbach et al., 2013b), and another focused on design and control (Zeng et al., 2006).
In this work, the conceptual design, control and economic evaluation of a reactive distillation-based process is developed. Previous studies (Niesbach et al., 2013b, Zeng et al., 2006) report a process structure comprising a reactive distillation column and an overhead decanter. These processes achieve about 95.5% alcohol conversion, and deliver a waste water stream of 93.1–95.9%mass purity. Two improvements that can be made to this process are making better use of the alcohol and obtaining a purer waste water stream. The process presented in this study achieves these by introducing a flash unit that partially vaporizes the decanter aqueous stream, and a recycle that sends back to the column the alcohol-rich stream. The analysis starts with the description of chemistry, reaction kinetics, and basic thermodynamics. The topology analysis of the vapour–liquid equilibrium diagram is described in more detail as it plays an important role in process design. After sizing the column and the main equipment around it, the dynamic behaviour of the plant is analysed when operating conditions change. The system is tested for change in production capacity and change of purity of fresh reactants, for two proposed alternative control structures. The first alternative handles the acrylate product purity by inferential control using a temperature controller, while the second applies direct control using a concentration controller. Both alternatives use temperature to control the water purity. The study ends with an economic evaluation, general conclusions on the proposed design, and some observations on the key performance indicators and process parameters of other solid-based catalytic processes previously reported in literature: reactive distillation (Niesbach et al., 2013b, Zeng et al., 2006), conventional reactor-separation-recycle (Moraru et al., 2016), and two simulated moving bed reactors recently published (Constantino et al., 2016, Constantino et al., 2015).
The design proposes a plant with a capacity of 20 600 t/a of n-BA, assuming 8000 operational hours per year. The product specifications are ≥99.5%mass n-BA, ≤0.05%mass water, and ≤0.01%mass AA, while the rest is n-BuOH, which accounts as well for other impurities. Aspen Plus v8.4 and Aspen Plus Dynamics v8.4 are used as efficient Computer-aided Process Engineering (CAPE) tools to aid the design and analysis of the process.
Section snippets
Reaction kinetics
The equilibrium esterification reaction between n-BuOH and AA with formation of water and n-BA is described by Eq. (1). Due to operation at relatively low temperatures and low residence times, it is considered that this is the only reaction that takes place in the system.
This is a liquid phase reaction that can take place in the presence of a strongly acidic heterogeneous catalyst. Based on laboratory experiments using Amberlyst 131 and
Selected methods for calculating physical properties
Two thermodynamic methods, UNIQ-HOC and UNIQ-2 (default naming in Aspen), were simultaneously used to calculate the physical properties (e.g. thermal, transport, phase) required in the Aspen simulations. This approach is selected since one set of binary interaction parameters is not capable to accurately describe both the vapour–liquid and liquid–liquid equilibria. A complete list with the names of all property models used by these two methods is presented in the supplementary material.
Pure component properties
This
Process and equipment design
A pragmatic approach is adopted for design of the reactive distillation-based process. The thermodynamic analysis and information on reaction conditions allow revealing the area where reactive distillation is feasible. This is used to select the appropriate pressure-temperature conditions of operation. The amount of catalyst (2 000 kg) is chosen based on a previous study (Moraru et al., 2016), where it is shown that for a conventional reactor-separation-recycle process a space-time-yield of 1.25
Plantwide control
The control structure proposed for this process has the main objectives to keep the mass inventory in the plant and maintain the product and waste water purities at the required set-points, despite changes in operating conditions. The effect of two types of changes is studies, namely change in plant throughput and contamination of fresh reactant streams.
Calculation basis
The evaluation proposes the calculation of the economic potential (EP), Eq. (10), and some key economic performance indicators, namely, the specific cost of reactants, Eq. (12), utilities, Eq. (13), amortization, Eq. (14), and production, Eq. (15) — all in $/t of product. The components of EP are cost of product (n-BA) and reactants (AA and n-BuOH), cost of utilities (steam, cooling water, and waste water treatment) and amortization cost of the total capital investment, all in $/a. The
Published data on solid-based catalytic processes
Table 10 presents some key performance indicators and process parameters of several solid-based catalytic processes recently published in literature: two SMBR (conventional and enhanced) by Constantino et al. (2015) and Constantino et al. (2016), respectively; two RD with decanter by Zeng et al. (2006) and Niesbach et al. (2013b); and one conventional two-recycle RSR by Moraru et al. (2016). To these, the RD with decanter and flash presented in this study is added.
A fair comparison between
Conclusions
Production at industrial scale of n-butyl acrylate using the reactive distillation technology based on solid catalysis is feasible. The annual capacity of 20 600 t acrylate can be achieved in a reactive distillation column using Katapak™-SP12 with about 2 000 kg of catalyst. A rectification section of 1.6 m and a stripping section of 1.2 m, both of MellapakPlus™, are sufficient to achieve low concentrations of acrylic acid at the top and high purity n-butyl acrylate at the bottom. The process
Acknowledgements
C.S. Bildea gratefully acknowledges the financial support of the European Commission through the European Regional Development Fund and of the Romanian state budget, under the grant agreement POC P-37-449 (acronym ASPiRE).
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