Dynamic Optimization of Lurgi Type Methanol Reactor Using Hybrid

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Dynamic Optimization of Lurgi Type Methanol Reactor Using Hybrid GA-GPS Algorithm: Optimal Shell Temperature Trajectory and Carbon Dioxide Utilization abdulaziz alarifi, Zhefu Liu, Fatih Erenay, Ali Elkamel, and Eric Croiset Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02918 • Publication Date (Web): 17 Dec 2015 Downloaded from http://pubs.acs.org on January 3, 2016

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Industrial & Engineering Chemistry Research

Dynamic Optimization of Lurgi Type Methanol Reactor Using Hybrid GAGPS Algorithm: Optimal Shell Temperature Trajectory and Carbon Dioxide Utilization Abdulaziz Alariﬁ*, Zhefu Liu, Fatih Safa Erenay, Ali Elkamel, and Eric Croiset University of Waterloo Waterloo, Ontario, Canada N2L 3G1 *Corresponding author: [email protected]

1. ABSTRACT At present, methanol is mostly produced from syngas, derived from natural gas trough steam methane reforming (SMR). In a typical methanol production plant, unreacted syngas is recycled for mixing with natural gas and both used as fuel in the reformer furnace resulting in carbon dioxide (CO2) emissions from the flue gases emitted into the atmosphere. However, CO2 can be captured and utilized as feedstock within the methanol synthesis process to generate liquid methanol. To do so, dynamic optimization approaches to derive the ideal operating conditions for a Lurgi-type reactor in the presence of catalyst deactivation are proposed to determine the optimal use of recycle ratio of CO2 and shell coolant temperature without violating any process constraints. In this context, this study proposes a new approach based on a hybrid algorithm combining genetic algorithm (GA) and generalized pattern search (GPS) methodologies to provide a sufficiently good solution to this dynamic derivative-free optimization problem. The hybrid GA-GPS algorithm has the advantage of sequentially combining GA and GPS logics; while GA, as the most popular evolutionary algorithm, effectively explore the landscape of the fitness function and identify promising basins of the search space, GPS efficiently search existing basins in order to find an approximately optimal solution. The simulation results showed that implementing the shell temperature trajectory derived by the proposed approach associated with 5% recycle ratio of CO2 improves the production of methanol by approximately 2.5 % compared to the existing operating conditions.

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2. INTRODUCTION Carbon dioxide is the primary greenhouse gas emitted to the atmosphere through fossil fuels (coal, natural gas, oil) consumption for energy supply and transportation. In addition, many industrial processes produce CO2 emission either as a result of fossil fuel combustion to create heat and steam and/or as a byproduct of chemical reactions.1 For instance, steam methane reforming (SMR) is widely used to produce syngas from natural gas, where reforming reaction takes place inside catalyst filled tubes and heated externally by firing fuel in a burner located in the furnace chamber.2 Flue gases discharged through an exhaust at the top of furnace to a heat recovery system where feed fuel and combustion air are preheated, and steam is generated. In the process of burning natural gas using air as the primary oxidant, the flue gas usually consists of mostly nitrogen and carbon dioxide with nearly 10−25 volume percent of the flue gas. It also contains small percentage of some pollutants, such as soot, carbon monoxide, nitrogen oxide.3 Consequently, research studies on carbon dioxide utilization focus on developing novel approaches for mitigating the contributions of CO2 emissions to global warming and ocean acidification by developing beneficial uses of CO2 in such areas where geologic storage of CO2 may not be an optimal solution.4 The advantage of CO2 fixation over CO2 disposal is that the production of chemicals with high economic value is possible.5 The reaction of carbon dioxide and methane in absence of steam (1), known as dry reforming of methane, to form syngas is the most appealing approach to convert two inexpensive materials into useful chemical building blocks.6

CH + CO → 2CO + 2H ∆H = 247.3 kJ mol

(1)

A challenging problem in commercialization of dry reforming process is that the reaction condition is more favorable for other side reactions such as methane decomposition (2),

boudouard equilibrium (3).7

CH → C + 2H ∆H = 74.8 kJ mol 2CO → C + CO ∆H = −172.5 kJ mol

(2) (3)

Coke deposition occurs by the exothermic boudouard reaction and the endothermic cracking of methane which leads to severe catalyst deactivation and the need for premature catalyst changeout.8,9 CALCOR process uses a special catalyst for reacting dry CO2 with natural gas, liquid

petroleum gas (LPG), and syngas to produce high purity carbon monoxide that contains less than 0.1% methane and H2/CO ratio as low as 0.4.10,11 combination of steam methane reforming and


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dry methane reforming (SMR+DRM) of natural gas is a competitive process to the conventional SMR process. SMR+DRM produces the best syngas ratio for production of liquid fuels with 22.1 H2/CO molar ratio; whereas, SMR of natural gas produces syngas with a H2/CO molar ratio of slightly above 3.12 Regardless of its advantages, SMR+DRM process has not been matured yet and still suffer from coking, severely shortening the lifetime of the applied catalysts. As the DRM technology has not been widely implemented for reducing CO2 emission, alternative approaches to address this issue are still under ongoing investigation.13 Chemical recycling of CO2 to renewable fuel, such as methanol, offers a future transportation fuel. G. Olah and his research group developed an improved Cu/ZnO based catalyst for methanol synthesis from CO2 and H2 at similar operating conditions and deactivation rate to those of commercial catalysts for syngas-based methanol synthesis.14,15 The first CO2-to-methanol recycling plant, (George Olah Renewable Methanol Plant) is operated in Iceland by Carbon Recycling International (CRI).16 This plant uses available local geothermal energy (hot water and steam) to supply the electricity needed to produce pure H2 by water electrolysis process with a daily capacity of 10 ton/day. The present technologies of water electrolysis have an efficiency of 50%-80% and require 50-79 KWh/kg of H2; hence they cost 3 to 10 times more than steam methane reforming even with a traditional method of generating electricity.17 The direct conversion of natural gas is more feasible and cost-effective at an industrial level than burning the fuel to generate electricity needed to electrolyze water and then producing hydrogen.18 For an SMR-based methanol plant, H2/CO molar ratio is obtained close to 3 which are not optimal for methanol synthesis process. Therefore recycling CO2 back to the reformer feed gas can be an option to increase methanol production by adjusting the stoichiometry of the make-up gas, which increases the reformer heat duty while reducing the amount of waste water discharged from the distillation system. A case study presented by Satish et al. showed that the methanol plant capacity can be expanded approximately 20% by recovering sufficient CO2 from steam reformer stack and recycle it as feedstock to the SMR.19 However, recycling CO2 to the reformer feed-gas increases the demand of supplying more heat. An alternative option is recycling CO2 into the syngas compressor suction feed rather than reformer feed. In a previous research, we have concluded that around 3% improvement in the plant production capacity can be achieved by injecting additional inter-stage CO2 into ICI’s multi-bed adiabatic reactor.20 The objective of this study is using dynamic optimization to derive



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the optimal recycle ratio of CO2 and shell temperature trajectory of an industrial methanol lurgitype reactor during four years catalyst lifetime. We kept the recycle ratio of CO2 within the design limit of the plant (maximum 10%) and the peak bed-temperature bellows 543K and then analyzed the effects of these decision variables on the reactor performance and the catalyst activity.

3. MODEL DEVELOPMENT Consider a mid-size methanol plant, e.g., up to 2500 ton/day, employs a steam methane reformer with an external source of hot gas to heat tubes in which catalytic reforming reactions take place. The feedstock to the reformer is a mixture of steam and natural gas with a typical steam-tocarbon ratio of 3:1 to avoid carbon deposition on the catalyst surface.21 Steam and methane are converted to syngas at an elevated temperature and pressure. The syngas is then mixed with recycled unreacted gas, pressurized in a compressor until the operational pressure is achieved, and then led to a water-cooled tubular reactor. The tubes packed with catalyst are externally cooled by boiler-water and a nearly isothermal temperature profile is formed in the catalyst bed. This type of reactors aims at recovering the heat of methanol synthesis reactions in the form of high pressure steam to the maximum possible extent. Figure 1 shows the schematic diagram of the methanol synthesis process. Methanol synthesis from syngas over Cu/ZnO/Al2O3 catalyst composed of three main equilibrium reactions:

CO + 2H ⇌ CH OH ∆H = −91 kJ mol CO + 3H ⇌ CH OH + H O ∆H = −50 kJ mol CO + H ⇌ CO + H O ∆H = +41 kJ mol


(4)

(5) (6)

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Steam Natural Gas

Steam

Interchanger

Steam Drum BFW

CO2 Recovery

CO2

Reformer

Blow Down Flue Gas Heat Recovery Syngas

77 bar Methanol Reactor

Syngas Air

Water

Product Separator

MeOH

Cooler Cooling Water

Water

Figure 1: Scheme of the methanol synthesis loop.

3.1 Reactor model The methanol synthesis reactor studied here is of Lurgi-type used in Shiraz Petrochemical Company.22 A one-dimensional heterogeneous reactor model is used to simulate the performance of industrial Lurgi-type methanol reactor. It employed a two phase model in which heat transfer through the solid- and fluid phase are considered separately.23 The continuity and energy equations for gas and solid phases are expressed as follows:24 Continuity equations:

'() '() = −+, + ./,) 12 (()3 − () ) ∀ - ∈ (0, 6) , 7 = 1,2, … , 9: '* ''()3 %3 = ;) (()3 , < 3 ) − ./,) 12 (()3 − () ) ∀ - ∈ (0, 6) , 7 = 1,2, … , 9: '* Energy equations: %&

'< '< BC (< = − +, => ?@,> + ℎ> 12 (< 3 − ?@,>

(7) (8)

(9) (10)

The reaction rate and heat of reactions terms are: NOPQ

;) (()3 , < 3 ) = =& 1I J K)L ML (()3 , < 3 ) LR


(11)


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NOPQ

;H (()3 , < 3 ) = =& 1I J ML (()3 , < 3 ) S−∆TU,L (< 3 )V LR

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(12)

Boundary and initial conditions:

() = (),)W ; < = [ () = (),\ ; < =

Dynamic Optimization of Lurgi Type Methanol Reactor Using Hybrid

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