HEX-REACTOR APPLICATIONS IN THE NETHERLANDS

Transcription

1 April 2004 ECN-RX HEX-REACTOR APPLICATIONS IN THE NETHERLANDS Study identifies significant energy savings with new type of reactor J.A. Hugill (ECN) J. Spaans (SynChem Plus) H. Keuken (Process Design Center) J.H. Vleeming (Process Design Center) Accepted for publication in NPT Procestechnologie, Volume 2, March/April 2004

2 Acknowledgement/Preface We gratefully acknowledge financial support from the Novem, project number /0030. Abstract The HEX reactor has not yet found significant application in industry, despite several advantages including energy savings. This study identified candidates for both short-term and long-term development. For a short-term demonstration, an oxidation process was identified with potential energy savings of about 0.6 PJ/y (for one plant) together with improved product yield, and a high repetition potential for similar processes. For longer-term applications, twentytwo large-scale processes were identified, with total energy savings of possibly 20 PJ/y (in NL). An evaluation of one process identified energy savings of up to 4 PJ/y (in NL), perhaps together with cost savings. 2 ECN-RX

3 CONTENTS LIST OF FIGURES 4 1. INTRODUCTION 5 2. THE SEARCH FOR A DEMONSTRATION PROJECT 6 3. EVALUATION OF A POLYMERISATION PROCESS 7 4. EVALUATION OF AN OXIDATION PROCESS 8 5. BROAD INVENTORY 9 6. EVALUATION OF AN ENDOTHERMIC PROCESS EVALUATION OF AN EXOTHERMIC PROCESS CONCLUSIONS 12 ECN-RX

4 LIST OF FIGURES Figure 1.1 Prototype HEX reactor (courtesy Heatric)... 5 Figure 3.1 A Prototype HEX reactor (courtesy Chart Heat Exchangers)... 7 Figure 5.1 A commercial compact heat exchanger (courtesy Heatric) ECN-RX

5 1. INTRODUCTION The HEX reactor (heat-exchanger reactor) combines the operations of chemical reactor and heat exchanger in one compact unit. Most recent developments use commercially-available compact heat exchangers (CHEs) as chemical reactors. But despite their advantages, there are few if any applications of HEX reactors in the chemical industry. Against this background, the project described here had two objectives: to identify applications for short-term demonstration projects, and to make a broad inventory of large-scale applications for longer-term development 1. Hex-reactors The HEX reactor offers advantages such as compactness, improved selectivity and yield, savings in costs and energy, less environmental impact, and better safety. In many cases the technology also enables a switch from batch to continuous operation. Energy savings may be direct (in the reactor itself) or indirect (e.g. improved yield leading to energy savings downstream). Non-catalytic HEX reactors have advantages for fast exothermic liquid-phase reactions. The technology has been researched for liquid-phase systems, but more remains to be done for gas-liquid systems. Catalytic HEX reactors have advantages for gas-phase reactions, but application to large-scale processes is a long-term objective. Although HEX reactors are based on proven technology (CHEs), there are many hurdles to be overcome before they are generally accepted in the chemical industry, including issues such as fouling/plugging, corrosion, catalyst design and replacement, fabrication of large-scale units for high process temperatures, and capital investment costs. Figure 1.1 Prototype HEX reactor (courtesy Heatric) 1 For reasons of confidentiality, the individual process applications are not explicitly identified in this article. ECN-RX

6 2. THE SEARCH FOR A DEMONSTRATION PROJECT The search was focussed mainly on processes with fast exothermic liquid-phase reactions, which currently use stirred-tank reactors. Research on this type of HEX-reactor application is relatively advanced, and many of these processes are small-scale. The total potential energy savings for this type of process in the Netherlands was estimated to be in the range 2-8 PJ/y. Processes using heterogeneous catalysis were excluded because such applications require longer development times. An initial list of about 100 companies/processes was screened to produce a short list of about 10 processes, using ranking criteria which included energy savings, repetition potential, expected interest of end-users, and practical constraints. For this list a more detailed ranking was done, based on contacts with the companies concerned, and considering additional factors such as costs, operability, control, safety, environment, and facilities at the end-user site. Finally two applications, a polymerisation process and an oxidation process, were selected for detailed evaluations. 6 ECN-RX

7 3. EVALUATION OF A POLYMERISATION PROCESS For a certain class of liquid-phase polymerisation process, a switch from (semi-)batch to continuous operation could lead to higher productivity and savings in cost and energy. HEX reactors also offer better safety, smaller plot size, less fouling problems, and reduction of waste streams. But fundamental understanding of the reaction mechanism is lacking, and the product quality is highly sensitive to the reaction conditions. To achieve the benefits of HEX reactors, it may be essential to leave the current operating window, which makes it a challenge to reproduce the product quality of the current processes. It was concluded that this process is not suitable for a short-term demonstration project, but rather for long-term development. The total potential energy saving was estimated to be less than about 0.25 PJ/y (in NL). Figure 3.1 A Prototype HEX reactor (courtesy Chart Heat Exchangers) ECN-RX

8 4. EVALUATION OF AN OXIDATION PROCESS This large-scale process produces several products by liquid-phase oxidation with air; the reaction is highly exothermic and currently takes place in tank reactors. Computer simulations showed that HEX reactors lead to a slightly improved selectivity, and to a beneficial change in the product distribution. However for a realistic reactor size, the reactors should work at higher pressure and use oxygen instead of air; these conditions also lead to a reduced recycle to the reactor. The use of multiple oxygen feeds also helps to reduce the reactor size. The potential energy saving was estimated to be 0.6 PJ/y for this one plant. Further work is required to optimise the reactor design and to establish the economic feasibility. However the application could be an interesting demonstration project, especially because of the high repetition potential for other liquid-phase oxidation processes. 8 ECN-RX

9 5. BROAD INVENTORY A list of more than 50 important chemical and refinery processes was compiled, based on production capacity but excluding processes for which HEX reactors are obviously not viable. This list was screened to produce a short list, using ranking criteria which included exo/endothermicity, energy savings, product quality and yield, long-term technical feasibility, safety, reactor size, and throughput. The short list has 22 processes, which were mainly characterised by a high heat of reaction, a large production capacity, and a high reaction temperature. For the short list, the total energy consumption is about 200 PJ/y (in NL), and the total (thermodynamic) room for improvement is also about 200 PJ/y. If just 10% of this theoretical maximum could be achieved with HEX reactors, the savings would be 20 PJ/y. For this list a more detailed ranking was done, partly based on contacts with the companies concerned, and two applications were selected for detailed evaluation. Both are large-scale processes with heterogeneously catalysed gas-phase reactions; one process is endothermic and the other is exothermic. Figure 5.1 A commercial compact heat exchanger (courtesy Heatric) ECN-RX

10 6. EVALUATION OF AN ENDOTHERMIC PROCESS HEX reactors are more compact than the conventional reactor, and offer better temperature control. They are well suited for thermally coupling the endothermic and exothermic reactions that occur in the process. Energy savings of about 2 PJ/y can be expected (in NL). However, although HEX reactors are technically more advanced, they do not bring step-change advantages over the heat-integrated designs already available. HEX reactors have little chance of acceptance until they can offer reductions in capital investment, which is the key factor for most producers. Other hurdles are catalyst design, reactor fouling, and (especially in view of the large scale and high temperatures) reactor fabrication. It is expected that HEX-reactor innovations in this process will come from smaller applications, where reactor size is more important, and where commercial breakthroughs can be expected. When HEX reactors are generally accepted for small-scale applications, then they will also be considered for large-scale operation. 10 ECN-RX

11 7. EVALUATION OF AN EXOTHERMIC PROCESS State-of-the-art technology is stepwise adiabatic operation with interstage cooling. Conversion is limited by chemical equilibrium. At the end of the first catalyst bed, the reaction rate decreases because the equilibrium temperature is approached. The effluent from the first catalyst bed is cooled and then reacted over a second bed. The limited conversion necessitates a large recycle loop. A HEX reactor allows operation with the optimum temperature profile (maximum reaction rate) over the whole reactor length, leading to a higher conversion per pass (i.e. a reduction in the size of the recycle loop, or a higher production rate). The energy saved by reducing the recycle was estimated as up to 4 PJ/y (in NL). Because the recycle loop is smaller, there may be an overall cost saving, even if the HEX reactor itself is more expensive than a conventional reactor. Considerable development work will be required to design a HEX reactor for the optimum temperature profile. ECN-RX

12 8. CONCLUSIONS The search for a demonstration process resulted in the identification of an oxidation process with potential energy savings of about 0.6 PJ/y for one plant, with a high repetition potential for other oxidation processes. For longer-term development, 22 large-scale applications were identified, with total energy savings of possibly 20 PJ/y (in NL). For one of these processes, energy savings up to 4 PJ/y might be achieved (in NL), perhaps together with cost savings. 12 ECN-RX