A Natural Gas to Liquids Process Model for Optimal Operation

Industrial & Engineering Chemistry Research, 2020

Natural gas conversion to liquid transportation fuels through Fischer−Tropsch synthesis (FTS) is an alternative approach compared to the conventional crude oil refining. In a gas-to-liquid (GTL) plant, natural gas is first converted to synthesis gas (syngas) and then to liquid fuels in an FT reactor. The distribution of liquid fuels being produced in the FT reactor depends on several factors including the catalyst, reactor type, operating pressure and temperature, chain growth probability (α), and so forth. The change in α along the FT fixed-bed reactor is affected by the temperature and H 2 /CO ratio. In this paper, a typical multitubular FT reactor was simulated with a variable α along the tube length. To do so, the reactor was discretized to 1000 small segments and the α model was modified to fit the wax concentration in the reference FT reactor data with a wax production capacity of 6900 kg/h. The developed FT reactor model was then used for the plantwide simulation of a GTL flowsheet with a larger wax production capacity of ca. 16,719 kg/h. The distribution of chemical species based on variable and constant α was compared. With a similar feed syngas composition, the wax mass fraction is 45.34% in the variable α scenario and is 41.02, 55.7, and 73.57% when α is fixed at 0.9, 0.92, and 0.95, respectively. In addition, sectioning of the FT reactor into two stages was considered to investigate the potential of volume reduction to produce ca. 16,719 kg/h of wax. The results suggest that the staged reactor has nearly 12% lower volume compared to the base case.

Journal of Chemical Engineering & Process Technology, 2018

Natural gas is recognized as one of the cleanest and most abundant fossil fuels. In the past decades, the price ratio of crude oil to natural gas has continuously fluctuated. The gas-to-liquid industry has received continuous interest due to the abundant supply of conventional and unconventional natural gas (shale gas, etc.), as well as the environmental advantages of FT technology. The GTL process chemically converts natural gas to long chain hydrocarbons (naphtha, diesel, wax, etc.) through three main steps, i.e., natural gas reforming, Fischer-Tropsch synthesis (FTS), and products upgrading. In this work, a rigorous simulation model including gas sweetening, syngas production, FTS, and product fractionation is provided. Among different routes for natural gas reforming and different reactors for FTS in a GTL process, autothermal reforming (ATR) and slurry bubble column reactor (SBCR) were chosen respectively, due to their advantages over other solutions. Meanwhile, the economic and environmental analyses were also conducted for the sustainability assessment of provided GTL process using Aspen Icarus and WAR software.

Journal of Natural Gas Science and Engineering, 2013

Energy and the environment are two of the most important issues this century. Multifunctional reactor has been recognized as one of the most promising configurations for simultaneous production of gasoline and hydrogen; as well stabilizing the atmospheric green house gases level. This paper focuses on mathematical modeling and optimization of a novel multifunctional reactor (MR) in order to enhance the production of gasoline and reduces CO 2 and CH 4 emission. FischereTropsch synthesis is carried out in exothermic side and supplies the necessary heat for the benzene synthesis reaction. The proposed reactor configuration consists of two catalytic fixed beds separated by the solid wall and also two Pd/Ag membranes, one is used for ultrapure hydrogen production from the endothermic side and the subsequent is applied in order to selectively hydrogen addition to the exothermic side. A one-dimensional, steady-state heterogeneous model and the differential evolution (DE) method, as a strong and powerful optimization method, are applied to simulate and optimize the proposed reactor configuration. The results of optimized multifunctional rector (OMR) represent 40.91% enhancement in the yield of gasoline in comparison with conventional FischereTropsch reactor (CR). Moreover, 75% and 34.46% decrease in the yield of methane and carbon dioxide as undesired products, respectively. On the other hand, a favorable temperature profile along the reactor length of OMR is achieved in comparison with CR.

2010

In this study we used a commercial process simulator to model a gas to liquid (GTL) plant and evaluated different process alternatives from an economical point of view. These alternatives include different syngas unit configurations and the effect of removing CO 2 from the tail gas. The criteria used to compare different syngas configurations is payout time. Results show that the most economically attractive syngas technology for GTL applications is an ATR. A case study is conducted to investigate the effect of removing CO 2 from the tail gas. Results show that the carbon efficiency for an optimized ATR-based process with and without CO 2 removal unit is 77.3 % and 73.5 %, respectively.

American Journal of Chemical Engineering, 2015

The technical analysis carried out in this paper is aimed at dealing with element incorporation, structure generation and optimization of the gas-to-liquid (GTL) process. The GTL model developed did not include the desulphurization unit as well as the product upgrading unit. A detailed description of the sequential steps for analyzing the process is as follows: firstly, a base-case process is designed with parameters and operating conditions obtained from literature. Secondly, this flowsheet is simulated with computer-aided simulation package ASPEN Hysys V8.4 to evaluate the specific characteristics of the main equipment and streams entering and leaving units. Thirdly, the simulated base case was analyzed in terms of Thermal Efficiency (TE), Carbon Efficiency (CE) and product flow to upgrading. This process was carried out using the optimizer tool for steady-state modelling to account for multiple variables in the Hysys simulation with the aid of case studies to maximize a given objective function. This resulted in a CE of 82.41%, TE of 65.93% and a production of 19940 bbl/d of syncrude.

Energy Conversion and Management, 2018

In the present research, multi-objective optimization (MOO) was applied to a natural gas-to-liquids (GTL) process with a multi-stage Fischer-Tropsch (FT) reactor. A CO 2 stream was fed into the GTL plant's endothermic steam reformer (SMR) to adjust the H 2 to CO ratio of the SMR effluent. Three objective functions were considered in this paper; minimizing the amount of CO 2 released to the atmosphere, maximizing carbon efficiency, and maximizing wax production. The degrees of freedom for the GTL process with single-stage FT reactor were recycle ratio to process and recycle ratio to the FT reactor. In addition, for the GTL process with two-stage FT reactor, the synthesis gas split ratio between the stages was considered as another degree of freedom. Maximizing wax production rate results in lower carbon efficiency and higher CO 2 emission to the atmosphere. The amount of CO 2 intake to the SMR in all cases was 256 tons/hr. With the same amount of utilized CO 2 in the SMR, sectioning of the FT reactor to two similar stages notably increases the carbon efficiency and mass flow of wax compared to the single stage case.

Industrial & Engineering Chemistry Research, 2012

The aim of this work is to select the best individual or combined controlled variables (CVs) for a natural gas to hydrocarbon liquids (GTL) process based on the idea of self-optimizing control. The objective function is to maximize the variable income of the plant, and two modes of operation are studied. In mode I, where the natural gas flow rate is given, there are three unconstrained degrees of freedom (DOFs) and the corresponding individual self-optimizing CVs are selected as (i) CO 2 removal in fresh synthesis gas (syngas), (ii) CO mole fraction in fresh syngas, and (iii) CO mole fraction in recycle tail gas from the Fischer−Tropsch (FT) reactor. This set of CVs gives a worst-case loss of 1,393 USD/h. Adding one, two, and three measurements and controlling measurement combinations decrease the worst-case loss significantly, to 184, 161, and 53 USD/h, respectively. In mode II, the natural gas flow rate is a degree of freedom and it is optimal to increase it as much as possible to maximize profit. The variable income increases almost linearly until the oxygen flow rate becomes active. Practically, this is the maximum achievable income. Theoretically, it is possible to increase the natural gas flow rate to improve the objective function, but this results in large recycle flow rates to the FT reactor (similar to "snowballing") because its volume is the limitation.

Biomass and Bioenergy, 2011

Proceedings of the 2nd Annual Gas Processing Symposium, 2010

This study of gas to liquid (GTL) production uses an authothermal reformer (ATR) for synthesis gas production and a slurry bubble column reactor (SBCR) with Cobalt catalyst for the Fisher-Tropsch (FT) conversion to liquid fuels. The well-known Satterfield kinetics are used for the FT reactor; the remaining reactions are simulated assuming thermodynamic equilibrium. The process also includes high-pressure CO 2 removal. Important process parameters, which are subject to optimization, include feed rates of water and oxygen, inlet ATR temperature and recycle flows. In addition to determining the process and equipment design, optimization is used to find the controlled variables. The aim is to identify "self-optimizing variables" that indirectly give close-to-optimal operation with constant setpoints, in spite of disturbances.

The paper presents the conclusions obtained in the investigations of methanol synthesis, Fischer-Tropsch synthesis, and higher alcohols synthesis from syngas as a raw material in slurry reactors. The overview of the role of process engineering was made on the basis of the experience in optimizing process conditions, modeling reactors and working out new technologies. Experimental data, obtained with a laboratory-stirred autoclave and theoretical considerations were used to develop the kinetic models that can describe the product formation and the model of the simultaneous phase and chemical equilibrium for the methanol and Fischer-Tropsch syntheses in the slurry reactors. These models were employed in modeling of the bubble-column slurry reactor (BCSR). Based on these considerations, a computer simulation of the low-pressure methanol synthesis for the pilot-scale, BCSR, was devised. The results of the calculations and the conclusions could be employed in the process for designing an industrial plant. #