This paragraph summarizes the development of the simulation model based on MTO/MOGD technology. Studies of Avidan (1988) and Tabak & Yurchak (1990) were used as references.
The design was complemented using other literature sources and general engineering principles.
5.2.1 Description of the model
Simulation and flow diagrams of the MTO/MOGD model are shown in Figure 29 and 30, respectively.
Figure 29 Simulation diagram of the MTO/MOGD model.
Figure 30 Flow diagram of the MTO/MOGD model.
Methanol enters the process in atmospheric conditions. Its pressure is increased in pump P-101 and it is preheated in heat exchanger E-101. Heater E-102 stabilizes the feed temperature prior to isothermal stoichiometric reactor R-101 where methanol is first converted to dimethyl ether and then to light olefins. Heat of the reactor effluent is utilized in heating the reactor feed and some of the heat is transferred to waste water exiting the process in heat exchanger E-103. The remaining heat is removed in cooler E-104 before sending the olefin mixture to a separation stage. Light hydrocarbon compounds are separated from the mixture in flash column FLASH1 and compressed in compressor C-101. Liquid flow is treated in decanter D-101 where water is separated from hydrocarbons. Pressure of the liquid olefin stream is also increased in pump P-102. The hydrocarbon streams are connected in mixing valve V-101 and cooled in heat exchanger E-105 and cooler E-106. Light gases are separated from the mixture in distillation column DIST1 and durene in distillation column DIST2. Olefin mixture exists DIST2 as the top product and its pressure and temperature are increased in pump P-104 and heater E-114 before sending it to further treatment.
Isothermal stoichiometric reactor R-103 converts light olefins into gasoline and distillate range hydrocarbons under high pressure. Valve V-106 lowers the pressure prior to a separation phase.
Distillation column DIST3 separates gasoline and distillate fractions. LPG is further separated from the gasoline fraction in DIST5 and the bottom product of the column is directed to gasoline blending. The distillate blend is upgraded in a hydrotreatment process. Pressure and temperature of the hydrocarbon mixture is adjusted to reaction conditions with pump P-105 and heater E-115. Hydrogen flow from an external source is compressed and cooled in compressor C-103 and heater E-122 and fed to reactor R-104. Hydrocarbons undergo hydrogenation reactions where olefins are saturated to paraffins. After the reactor, pressure of the stream is lowered with pressure reduction valve V-107 and the temperature is reduced in coolers E-116 and E-117.
Excess hydrogen is removed from the fuel products in flash column FLASH3 in atmospheric pressure. Finally, kerosene and diesel cuts are separated in distillation column DIST4 after elevating the pressure in pump P-106 The separated fuels are brought to atmospheric conditions by the means of pressure reduction and cooling.
Isomerization process of durene has the same principle as in the MTG model. The durene-rich stream is first pumped to the operation pressure in pump P-103, passed through heat exchanger E-107 and heated to the reactor temperature in heater E-108. Stoichiometric hydroisomerization
reactor R-102 operates isothermally and converts durene into its isomers in the presence of hydrogen. After the reactor, the effluent is cooled in four steps in heat exchanger E-107 and coolers E-109, E-110 and E-111. The cooled product is led to flash column FLASH2 where hydrogen is separated from the hydrocarbons. The hydrogen is recycled back to the reactor feed via valves V-104 and V-105, recycle compressor C-102 and cooler E-112. A small amount of the gas flow is purged from the process from valve V-104 and a corresponding amount of fresh hydrogen is fed though valve V-105. The liquid product from FLASH2 with a reduced durene content is blended with the olefinic gasoline in mixing valve V-102 and it is brought to atmospheric conditions with pressure reduction valve V-103 and cooler E-112.
5.2.2 Process specification
To simplify the complex system of various hydrocarbon compounds, kerosene was estimated to consist only of C12 paraffins and olefins. Diesel, on the other hand, was assumed to include C16, C18 and C20 hydrocarbons. Gasoline range compounds were chosen based on the MTG model excluding isooctane since alkylation of C4 compounds was not considered in this model.
In the MTO/MOGD model, methanol conversion to DME and further conversion to light olefins occurs in reactor R-101, unlike in the MTG model. Due to the highly exothermic reaction system the reactor was assumed to be equipped with a cooling jacket. MOGD reactor R-102 converts C2-C6 olefins to kerosene and diesel range olefins via oligomerization under isothermal operation. The yields in both reactors were set to correspond to experimental yields reported by Avidan (1988) by adjusting fractional conversion of each reaction.
Modeling of hydrotreatment of the distillate fractions was modeled based on the work of Ahmad et al. (2011). In their study, diesel hydrotreating process was modeled and optimized. Hydrogen feed to the processes was estimated to be equimolar to the hydrocarbon flow, as suggested by Gong (2017). The hydrogenation reactions were assumed to produce both normal and isoparaffins over a γ-alumina catalyst (Gruia, 2006). An example of a hydrogenation reaction of distillate olefin is shown in Figure 31.
Figure 31 Hydrogenation reaction of 1-dodecene.
The modeling principle of hydroisomerization of durene was similar to the one presented in the case of the MTG model. Alkylation process was not considered in the model since the production rate of gasoline is significantly lower than in the MTG model.