1.10 Propylene
Propylene has typically been produced as a byproduct either from the steam cracking of naphtha to produce ethylene or from the fluid catalytic cracking to produce gasoline. With the shale gas boom and the excess of NGLs such as ethane, the production of ethylene has switched the feedstock from naphtha to ethane. This action has eliminated the production of propylene as a byproduct, opening an opportunity for the development of on‐purpose propylene production processes. The alternatives to produce propylene from shale gas include two options via methanol and one using the propane obtained from the purification of shale gas [32,33]. The processes to produce propylene via methanol are the MTO route and the methanol to propylene (MTP) process [33]. The MTO process is described earlier in the ethylene section. MTP follows a similar path. First, natural gas is transformed into syngas gas using a reforming alternative, and then the syngas is transformed into methanol. As opposed to the MTO process, where crude methanol is sent to the MTO reactor, methanol has to be purified for its use as feedstock for the MTP process. Therefore, the crude methanol obtained from the methanol synthesis reactor is sent to a flash unit and purified using a distillation column. The purified methanol is then fed to a reactor, where it is converted to dimethyl ether and water. Then, the outlet stream of the reactor is sent to a fixed bed catalytic reactor to produce propylene. The effluent from the fixed bed reactor contains propylene, gasoline, and LPG, as well as water. It is sent to a flash unit to remove water and the remaining stream is purified using distillation columns.
Another alternative for the production of on‐purpose propylene is the propane dehydrogenation process, in which a depropanizer column is used to separate C4+ compounds that may be present in the fresh material. The purified propane enters a cold box to refrigerate the effluent from the propylene production reactor. Then, the propane stream is mixed with hydrogen and sent to a fired‐heater before being fed to a fluidized catalyst bed reactor. The reaction is highly endothermic. The outlet stream of the reactor contains propylene, propane, light gases, ethane and ethylene, and some heavier hydrocarbons. The reactor effluent is cooled, compressed, and sent to a cool box where hydrogen is separated from the hydrocarbons. The liquid stream from the cold box is sent to a selective hydrogenation process (SHP) to further improve the production of propylene. The effluent from the SHP is fed to a deethanizer column to remove light gases. Finally, the remaining stream is fed to a C3‐splitter column to produce the propylene. The propane obtained at the bottom of the splitter column is recycled to the depropanizer column [32].
These processes represent an excellent opportunity for the independent production of propylene instead of obtaining it as a byproduct of other processes.
1.11 Process Intensification Opportunities
The incentive for shale gas monetization can also be viewed as an opportunity to develop intensified processes for shale gas transformation technologies. Recent efforts to design intensified processes have been observed. Process intensification is understood here as a search for more competitive process alternatives via the development of more compact flowsheets (i.e. with fewer pieces of equipment or smaller sizes of the same number of equipment units) and/or with a reduction on the consumption of basic resources (raw materials, energy) through more efficient designs.
The first efforts to develop formal design methodologies for intensified processes were due to the work by Gani and his research group [34-36]. Such initial methodologies make use of the concepts of tasks and phenomena, giving rise to the concepts of phenomena building blocks (PBBs), which represent the tasks involved in a process unit such as reaction, heating, cooling, mass transfer, and so forth; the combination of PBBs provides simultaneous phenomena building blocks (SPBBs), which are used to model the operations involved in a process. For instance, in a distillation column, the following SPBBs can be observed. Each tray shows a mixture with two phases, with contact, transfer, and separation between the two phases (vapor and liquid), while the condenser adds cooling and the reboiler adds heating to the previous SPBBs, as shown in Figure 1.2 for a column with five trays.
Figure 1.2 Simultaneous phenomena building blocks in a conventional distillation column.
Using these concepts, Lutze et al. [36] developed a methodology for process design of intensified processes, consisting of three stages. In the first one, a basic process flowsheet is synthesized. In the second stage, SPBBs are developed and a superstructure that contains all the possible tasks of the system is formulated. The problem is then solved as a mixed‐integer nonlinear programming (MINLP) model to obtain the intensified structure that minimizes a given objective function such as the total annual cost of the system. Babi et al. [34] applied an extended formulation of that model that included sustainability metrics to a case study dealing with the production of dimethyl carbonate. In the work by Castillo‐Landero et al. [37], such a methodology was taken as a basis, but instead of formulating an MINLP model to search for the optimal intensified configuration, a sequential approach with gradual intensification of the process was conducted until a final structure with a minimum number of equipment units was obtained. One advantage of this procedure is that one can assess individual levels of process intensification so that a structure that favors a given metric of interest can be selected.
Interesting challenges arise when shale gas processes are considered for process intensification. Let us take, for instance, the basic flowsheet for the production of ethylene from shale gas, or natural gas, shown in Figure 1.3. As discussed earlier, this process shows a fairly simple structure but an adverse profitability, which poses a particular incentive to explore potential benefits that an effective process intensification task could provide. Nonetheless, noticeable challenges exist for its transformation into an intensified process that combines the process tasks, namely, reaction and several separation tasks. First of all, the reactor performs a catalytic, gas‐phase reaction task, which consists of a complex reaction mechanism. Secondly, the separation tasks consist of a combination of compression for water condensation and absorption for CO2 removal (typically carried out with an amine such as MEA). Combination of absorption and membrane could possibly be considered. The distillation train, finally, is highly energy intensive. Given the tasks identified for this process, and the aim to intensify it, the use of membrane units would be of special consideration. Membrane units could conceptually be designed first to carry out the individual tasks. Then, combinations of reaction and separation tasks based on such membrane units could be considered. The resulting structure would include innovative gas‐phase membrane‐reactive‐separation units. The design of effective membranes that, among other things, could separate the gas mixture that requires cryogenic distillation systems could provide a significant impact on the process economics. It should be mentioned that some efforts to combine reactive distillation with membrane separation have been reported (e.g. [38]). However, the reaction for which the intensification with membrane units has been considered has been typically implemented for liquid‐phase reactions. In such cases, membranes aid in improving the effectiveness of the process, for instance, by releasing one of the products of the reversible reaction to improve its yield. The problem posed by shale gas processes is the gas‐phase reaction that requires special membrane materials for its effective application. The aspects outlined here taking the OCM transformation technology as an example provide a clear incentive toward the design of more competitive alternatives based on more compact, innovative shale gas‐intensified technologies.
Figure 1.3 Basic flowsheet for ethylene production from shale/natural gas.
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