CO 2 EMISSION IN INDUSTRIAL HYDROGEN PLANTS

Transcription

1 GPE-EPIC 2 nd International Congress on Green Process Engineering 2 nd European Process Intensification Conference June Venice (Italy) CO 2 EMISSION IN INDUSTRIAL HYDROGEN PLANTS HADI EBRAHIMI, AKBAR ZAMANIYAN, ALIREZA B. SARAND S. JODA Gas Research Division, Research Institute of Petroleum Industry (RIPI), Tehran, Iran. P.O. Box : ebrahimih@ripi.ir Abstract.Natural gas and hydrocarbons obtained from several refinery units are good source of hydrogen. A well known process of hydrogen production in the refinery is simulated here with versatile analysis of unfavorable CO 2 produced beside hydrogen. A new method for removing of the CO 2 is presented after the complete simulation. The results show that a unit such as syngas could be added to the process for managing of CO 2. Key-words. CO 2 emission, Process Simulation, Hydrogen 1. Introduction Hydrogen is a key component for several chemical processes such as hydrogenation, hydrocracking, and hydrotreating, or even for fuel. There are many ways for production of hydrogen such as reforming of hydrocarbons (e.g. natural gas), and thermal gasification of biomass and waste 1. In the world, more hydrogen is produced from natural gas. Reformers are common tools for converting natural gas or hydrocarbon feeds to hydrogen in the catalytic fixed bed reactors. Table 1 presents two feed characteristics of a reforming reactor in the hydrogen unit of a refinery. These feeds, with a large amount of methane, are light in comparison to the liquid hydrocarbons. Besides hydrogen as a main component, a large amount of unfavorable CO 2 may be produced. In most cases CO 2 is purged to the atmosphere because of its useless and harmful nature. Environmental concerns over rising CO 2 concentrations in the Earth s atmosphere are driving efforts to reduce anthropogenic emissions of CO 2 2. Damm and Fedorov 2 considered and suggested several concepts for CO 2 capture such as identification, development and demonstration of advanced energy conversion processes that are amenable to distributed CO2 capture. It was shown that when small scale energy conversion systems are designed to capture CO2 emissions, they could be nearly as efficient as their polluting counterparts. In the wake of the December 1997 Kyoto Protocol, which if implemented would oblige the industrialized countries to reduce greenhouse gases (consisting of CO 2 ) by , a number of proposals have been offered to reduce CO 2 emissions. 1

2 Table 1.Two feeds of the reformer as a good source of hydrogen. Component %moles (Refinery H 2 unit) %moles (Natural Gas) Hydrogen CO 0 0 CO Nitrogen Methane Ethane Propane i-butane n-butane i-pentane n-pentane n-hexane Table 2. Characterization of feed and product in the H 2 production unit. Material Feed: kg/hr Feed+water Fuel 3490 Air sum Product: CO 2 exited from stack 9739 CO 2 from acid gas of Amine Package sum of outlet CO H 2 produced 2551 Carbon dioxide is used by the food industry (for production of carbonated soft drinks, and soda water), the oil industry, and the chemical industry 3. It is also used in many consumer products that require pressurized gas because it is inexpensive and nonflammable, and because it undergoes a phase transition from gas to liquid at room temperature at an attainable pressure of approximately 60 bar, allowing far more carbon dioxide to fit in a given container than otherwise would. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. Aluminum capsules are also sold as supplies of compressed gas for airguns, paintball markers, for inflating bicycle tires, and for making seltzer. Rapid vaporization of liquid carbon dioxide is used for blasting in coal mines. High concentrations of carbon dioxide can also be used to kill pests, such as the Common Clothes Moth. There are several methods for recovery of CO 2 obtained from the refineries or petrochemical industries and preventing from entering to the environment. Carbon dioxide is used in enhanced oil recovery where it is injected into or adjacent to producing oil wells, usually under supercritical conditions. But the refinery should be close to the oil well that it is not occurred in most cases. Moreover, in mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points. In the chemical industry, carbon dioxide is used for the production of urea, carbonates and bicarbonates, sodium salicylate, and the new useful processes. In this paper, a well known process of hydrogen production in the refinery is simulated and presented here with versatile analysis of unfavorable CO 2 produced in the process. A new method for removing of the CO 2 is presented after the complete simulation. 2

3 2. Industrial Hydrogen Units An industrial well known hydrogen unit in the refinery is simulated here. Box A in Figure 1 presents the process that has several main parts: Reformer for reforming of the hydrocarbon feed with steam to H 2, CO, and CO 2 High and low shift converter for converting of CO to CO 2 Purification step consisting of Amine or PSA unit Methanator, for converting of the remained CO to methane to prevent the catalyst damage of the following units. The feed is sent to a reformer for producing of syngas, i.e. H 2 and CO in a rector. All hydrocarbons should be converted to H 2, CO, and CO 2 by the three main reactions presented in equations (1) to (3). Hydrogen concentration is increased prior to separation, through the use of water gas shift (WGS) reactors where CO is partially consumed down to a low percentage as it reacts with water to produce more H 2 and CO 4 2 according to the reaction (2) PSA besides Amine package achieves separation of CO, CO 2, CH 4 and H 2 O from H 2, by adsorption of these components on a solid adsorbent at a relatively high pressure. The adsorbed species are then desorbed from the solid, by lowering the pressure and purging with high purity product hydrogen. The resulting PSA waste gas contains significant amounts of hydrogen and methane and is thus burned as a source of heat for the reformer. Continuous flow of hydrogen product is maintained by using multiple adsorption beds, whose adsorption/desorption cycles are properly synchronized Simulation of Existing Industrial H 2 Unit Each unit (reformer with its furnace, High and Low Shift Converter, Amine, and Methanator) was simulated separately and linked with each other. A heterogeneous one dimensional model was used for the reactor tubes 5, 6. All equations of material, momentum, and energy balance of the tubes are prepared in the literature 5, 6. Iterative Newton raphson was used for solving simultaneous nonlinear equations 7. (I) SR: CH 4 + H 2 O CO + 3H 2 (1) (II) WGS: CO + H 2 O CO 2 + H 2 (2) (III) MR: CH 4 + 2H 2 O CO 2 + 4H 2 (3) Shift converters are fixed bed reactors. In this paper a heterogeneous one-dimensional model 8 was used. Different resistances associated with the catalyst pellets are considered in this model. Equation (2) is occurred mainly in the shift converter reactors Both Amine and PSA packages could be used for purifying H 2. However, according to the industrial plant, Amine is used for the simulation. Here, PSA or Amine package is considered as a splitter to simulate hydrogen plant, simply. After separation unit, the purified stream is entered into a Methanator reactor to convert the remained CO to methane. The reverse 3

4 reactions of equations (1) to (3) are taken place in the methanator. For high and low shift converters and methanator reactors, the Gear method was employed as an iterative method Results and Discussion a. Simulation of industrial hydrogen unit. An industrial hydrogen plant is simulated here. The plant uses the off gas of CRU Platformer unit, HP Amine unit, and propane as the feed of reformer and produces H 2 for Platformers and other uses. The feed characteristics of the reformer is presented in Table 1, column 2. The process has several main units shown in Figure 1, Box A. Figure 1. New Hydrogen Production Unit. First, the feed is reformed by steam in reactors of the reformer. On the furnace side of the reformer, fuel and air are combusted to generate the heat required for the endothermic reactions. Flue gas of the furnace passing through the recovery section is exported to atmosphere that has a high amount of CO 2. Product of the reformer usually consists of H 2, CO, CO 2, and N 2. CO is converted to CO 2 in one or more shift converter, typically two ones: high and low, discussed before. CO 2 produced in the system should be removed from the H 2 product that is a nowadays challenge for existing hydrogen plants in the refineries all over the world. It is mainly happened in an Amine or PSA unit. The remained CO is converted to CH 4 in the Methanator reactor. The amount of H 2 and CO 2 unfavorable produced in an industrial refinery H 2 unit is presented in Table 2. According to the Table CO 2 is obtained in the system almost ten times as much as the H 2 that is considerable. This CO 2 is leaked out in the atmosphere that would be a challenge for the green world. Results of the industrial plant consisting of outlet compositions of four units are presented in Table 3. Comparison of experimental and calculated (based on the modeling and simulation) are reported in this Table. Errors are almost acceptable. 4

5 Table 3. Hydrogen mass flow rate and error of industrial plant and simulated results Industrial Plant Simulated Error kg/hr kg/hr % Reformer Outlet High Shift Converter Outlet Low Shift Converter Outlet Amine Outlet Methanator Outlet b. Removing CO 2. According to the large amount of purged CO 2, a solution should be made. In this work, the method of using CO 2 for a syngas unit was employed. The unit, a primary reformer, is presented in Figure 1, Box B. By adding natural gas, steam, and Fuel with Air, the whole CO 2 of the first unit is removed. The product has H 2 /CO of near 1 that it can be used for different applications such as Fisher Tropsch synthesis in a GTL process and oxo synthesis for 2-Ethyl Hexanol production. The CO 2 produced in the flue gas of the second reformer could be recycled to the main CO 2 stream (Figure 1). The amount of feed to the reformer (Natural gas) was adjusted so that H 2 /CO ratio would be 1. The values of the added process (a new reformer) are presented in Table 4. Two reformers could be suggested as a new reformer for producing syngas and removing CO 2, presented in Table 4. If the primary reformer is used (same as in the H 2 plant, presented in Figure A. Box A), we can produce syngas with H 2 /CO of 1 and steam to carbon ratio of 8, however, there is also a large quantities of CO 2 is exited by the product. Although the CO 2 of flue gas is recycled to the new unit and the CO 2 of the product could be used in another process instead of purging to the atmosphere, the process is not suitable. `But, the authothermal reformer as a secondary reformer can remove all CO 2 and producing a syngas of H 2 /CO of 1 and steam to carbon ratio of In fact an authothermal reformer is a proper tool for using the injected CO 2 for producing of H 2 /CO near 1. Table 4. Characteristics of the second reformer Reformer Type Primary Autothermal Inlet: CO 2 Injected, kg/hr CO 2 of Flue Gas, kg/hr Feed Water, kg/hr Fuel For new Reformer Steam/Carbon Feed O 2 /CH Outlet: Syngas, kg/hr H 2 /CO CO 2 in product

6 6. Conclusion A large amount of CO 2 as a green house gas is harmful for atmosphere. An industrial H 2 unit was simulated for considering of both H 2 product and unfavorable CO 2 emission. The simulation was based on a comprehensive modeling of each part. According to the problem of CO 2 produced in the industrial H 2 plant, a process was presented. All CO 2 was removed by the adding of an autothermal primary reformer. References 1. Weil S., Hamel S., Krumm W., 2006, Hydrogen energy from coupledwaste gasification and cement production-a thermochemical concept study, International Journal of Hydrogen Energy 31, Damm, D. L. Fedorov A.G., 2008,Conceptual study of distributed CO2 capture and the sustainable carbon economy Energy Conversion and Management, 49 (6), Pierantozzi R., 2001, Carbon Dioxide : Kirk-Othmer Encyclopedia of Chemical Technology. John Wiley & Sons, Inc. 4. Takenaka S., Shimizu T., Otsuka K., 2004, Complete removal of carbon monoxide in hydrogen-rich gasstream through methanation over supported metal catalysts, Int. J. Hydrogen Energy, 29, Xu and G.F. Froment, 1989, Methane steam reforming: II. Diffusional Limitations and reactor simulation, AIChE Journal, 35, Zamaniyan, A., Ebrahim H. Soltan Mohammadzadeh J.S., 2008, A Unified Model for Top Fired Methane Steam Reformers using Three Dimensional Zonal Analysis", Chemical Engineering and Processing: Process Intensification, 47 (5), Mullges G.E., Uhlig F., 1996, Numerical Algorithms with FORTRAN, Springer,Berlin, Germany, pp Elnashaie S.S.E.H.and Alhabdan F.M.,1989, Mathematical modeling and computer simulation of industrial water-gas shift converters, Ma&Z. Cornput. Modelling 12 (8), Gear, C The simultaneous numerical solution of differential-algebraic equations. IEEE Trans. Circuit Theory CT-18,