Modelling of Naphtha Cracking for Olefins Production João Miguel Monteiro Marcos [email protected] Instituto Superior Técnico, Lisboa, Portugal December 2016 Abstract The production of ethylene and propylene from naphtha via thermal cracking is a cornerstone of the chemical industry. This process is carried out in furnaces operating at high temperature, and optimal operation of these furnaces is necessary to maintain profitability. In the present work, a mathematical model of a naphtha cracking furnace was developed in gPROMS®, in which the main focus was the development of a SRT-VI coil model. This furnace model was used to validate kinetics present in literature against typical data for naphtha cracking. A tuning of the implemented kinetics was carried out, by adding new components, new sets of reactions and tuning the kinetic parameters in the reaction scheme, leading to good predictions, having yield deviations of less than 5% for the main products (light olefins - ethylene and propylene) and around 15% for aromatics. Keywords: modelling, steam cracking, naphtha, ethane, propane, kinetics, furnace. 1. Introduction Hydrocarbon steam cracking is one of the production of polyethylene, ethylene oxide, vinyl most important processes in the petrochemical acetate, and ethylbenzene and ethylene industry, producing highly valuable olefins such as dichloride [3]. ethylene, propylene from lower value feedstocks, Propylene is considered a co-product of this which usually have fossil fuel origin, ranging from process, reporting 109 million tonnes in 2014. It is gaseous feedstocks, like ethane and propane to used for the production of polypropylene, liquid, heavier feedstocks, such as naphtha, gas propylene oxide, cumene and isopropanol [4]. oil and gas condensates. Naphtha is the most For the optimisation of an olefins plant, the widely used, due to availability, low cost and developing of predictive furnace models, capable potential for producing high yields of olefins [1]. of describing cracking and coking phenomena, Ethylene is the major product of a stream becomes essential, and will be the main focus of cracking unit. With a world production of around the current work. 1.48 million tonnes/year in 2014 [2], and an annual growth at an average rate of 4%, it is the largest volume building block, and it is mainly used for the 1 2. Background 600 − 860 ℃, which are maintained through heat The method for the production of olefins from transfer with a firebox, where fuel burners reach naphtha is through thermal hydrocarbon cracking temperatures between 1000 − 1200 ℃. The heat reactions, which occur in the presence of steam in transfer mechanism is mainly radiation. Due to the the radiant coil of the furnace, at high endothermicity of the cracking reactions, high temperatures ranging from 700℃ at the inlet of the heat fluxes are required (75 − 85 푘푊/푚2of coil) coil to 900℃ at the outlet. [1]. For the cracking of naphtha, and depending The cracking reactions occur via free-radical on the operating conditions (which heavily mechanisms, and for the cracking of naphtha, influence the severity of cracking), the product resulting in yields of ethylene between 25 − 35% stream is made up of (in 푤푡. %) 25 − 35 % of and propylene between 14 − 18% [1]. ethylene, 14 − 18 % of propylene, 4 − 6 % of butadiene as well as ~14 % of methane and 5 − 2.1 Steam Cracking process 10 % of aromatics, namely BTX. A simplified flowsheet of the steam cracking Typical cracking coil dimensions are usually process that be found in Figure 3. The cracking 40 − 90 푚 in length, diameters from3 − 20 푐푚. [1] furnace serves as the reactor and “heart” of the process, thus being the most important unit of the plant. It is comprised of two main sections: the furnace section and the radiant section, as seen in Figure 1. The hydrocarbon feed enters the furnace in the convection section, where it is pre-heated by heat exchange against flue gases. It is then mixed with dilution steam, up to ratios of Steam:Oil (푘푔푠푡푒푎푚/푘푔ℎ푦푑푟표푐푎푟푏표푛) of 0.25 − 0.40 for gaseous feed and 0.40 − 0.55 in the case of naphtha. This reduces hydrocarbon partial pressure, leading to reduced rates of coke formation (thus avoiding decreased heat transfer coefficients and increased pressure drops), being able to increase the run-time of the furnace. The Figure 1- Diagram a steam cracking furnace in a typical olefin plant. resulting mixture is further heated to temperatures of 500 − 680 ℃, which favour the cracking reactions. [1] [3] This mixture of feed and steam, in the gaseous state, enters the radiant section of the furnace, where radiant coils act as tubular reactors, submitting the hydrocarbons to cracking reactions for periods of 0.1 − 0.5 푠. This section of Figure 2- Industrial cracking coil. the furnace operates at temperatures between 2 Figure 3- Simplified flowsheet of the steam cracking process. After exiting the furnace, the resulting stream, To compress the cracked gas, a series of 4 to in the gaseous phase, and with a high content on 6 compression stages with inter-stage coolers is light olefins is subjected to a series of treatments used, allowing the cracked gas to reach pressures to remove condensates, water and other up to 35 bars, while maintaining temperatures undesired components before the fractionation below 100 ℃. The condensates, as well as water step. and other heavier components are removed The cracked gas then leaves the radiant coil during this cooling process, alongside with at 800 − 860 ℃ (COT), and is cooled during a 퐻2푆 and 퐶푂2, which are removed by contacting period of 0.02 − 0.1 푠 to 550 − 650 ℃ to prevent with an alkaline solution (acid gas removal). [1] [3] further cracking of valuable reaction products as The resulting gas needs to be dried in order well the formation of coke. This cooling process to remove water (up to < 1 푝푝푚), in order to occurs in the transfer-line exchangers (TLE), by proceed for the fractionation equipment. indirect quenching. Finally, the cracked and now purified gas is An oil quench follows, which is used to reduce chilled and separated into its product streams the temperature down to around 230 ℃. (ethylene, propylene, crude 퐶4 and pyrolysis Next, a primary fractionator (gasoline gasoline and gas oil), by means of a series of fractionator) is used in order to separate the distillation columns. pyrolysis fuel oil (heavier hydrocarbons) from the In order to further increase light olefins yield, main stream. hydrogenation reactions occur, in which To be further processed, the hydrocarbon acetylene, methylacetylene and propadiene are product stream is then cooled to near ambient converted to ethylene and propylene in catalytic temperature by means of water quench tower, in hydrogenation beds. [1] [3] which it contacts with a large descending water stream. 3 2.2 Steam cracking reactions In order to be able to have predictive models, It is widely accepted that the largest part of capable of describing the cracking reactions and gas phase hydrocarbon pyrolysis occurs through prediction product distribution, the need arises to a free radical mechanism, characterized by a vast develop kinetic models, which describe the number of species and reactions. cracking phenomena through a series of The kinetic mechanism is summarised by the reactions. In the current work, mechanistic models following reaction classes [5] [6]. will be used, based on schemes of free radicals. Two kinetic schemes (available in literature) for 1) Initiation and termination reactions the cracking of naphtha will be used: the scheme These reactions involve either the C-C bond described by Joo [7], comprising a total of 231 scission, forming two smaller radicals (Eq. 1a), or reactions between 79 chemical species up to 퐶9; a new bond (C-C or C-H) as two radicals come and the scheme described by Towfighi [8], together and produce a single molecule (1b). containing 150 reactions and involving 54 components up to 퐶 . ∗ ∗ 8 푅1 + 푅2 → 푅1 + 푅2 (1a) ∗ ∗ 푅1 + 푅2 → 푅1 + 푅2 (1b) 3. Implementation The current work was developed in gPROMS 2) Propagation reactions ProcessBuilder®, which was used for model After the initiation step, radical species development, flowsheeting and simulation. undergo a series of propagation reactions in An external physical property package was which keeping the reaction chain going. These used (Multiflash) as well as a stoichiometric matrix reactions can be of different types: compression scheme (LSKM). A. Hydrogen abstraction 3.1 Model Equations Smaller reactive radicals abstract a This section describes the model hydrogen atom from another molecule, creating equations in mathematical furnace model both a new molecule and new radicals. used in this work. The furnace is composed of ∗ ∗ 푅1 + 푅2 → 푅1 + 푅2 (2) several sub-models, which together describe all phenomena occurring in the furnace. B. Radical addition For the tube model, which is treated as a Radicals react with olefins, thus forming PFR, the mass balance is as follows (Eq. 5): less saturated compounds and a new radical. 푑 [푁 퐴] = 푀푊 퐴푟 (5) ∗ 푑푧 푖 푖 푖 푅1 + 푅2 = 푅3 → 푅1 − 푅2 − 푅3 (3) Where 푁 represents the mass flux for C. Radical isomerisation 푖 Responsible for the transfer of the active component , 퐴 is the cross-section area of the radical position within the molecule. tube, 푀푊 is the molecular weight of component and 푟푖 is the overall reaction rate (rate of ∗ ∗ 푅1 − 푅2 − 푅3 − 푅4 ↔ 푅1 − 푅2 − 푅3 − 푅4 (4) formation/disappearance) of component . 4 The reaction rate of a given reaction j, 푟푗 can be related with are the forward and backwards reaction constants for a given reaction j, 푘푓,푗 and 푘푏,푗 , respectively, and the component molar concentration of the reactants and products by (Eq. 6): 푟푗 = 푘푓,푗 ∏ 퐶푟푒푎푐 − 푘푏,푗 ∏ 퐶푝푟표푑 (6) The energy balance is as shown in (Eq.6): 푑 Figure 4 - Schematic of the modelled SRT-VI coil.