Process Intensification and Integration for Sustainable Design. Группа авторов

H₂O Endothermic CH₄ + H₂O → CO + 3H₂ Catalytic Partial oxidation O₂ Exothermic

Catalytic/non catalytic Dry reforming CO₂ Endothermic CH₄ + CO₂ → 2CO + 2H₂ Catalytic

      Although these processes may be used separately, combinations of two or more of the main reforming options have been proposed to enhance the overall performance of the reforming task. One such process is the autothermal reforming (ATR) in which the exothermic nature of the POX reforming is combined with the endothermic SR [21].

      In all of these reforming alternatives, energy and water usage and generation are key points to consider when selecting the appropriate technology. Studies regarding heat and mass integration potential for the SR, POX, and ATR options can be consulted in the work of Martínez et al. [21] and Gabriel et al. [22].

1.7 Methanol

Typically, methanol is used as an intermediate to produce other chemicals such as acetic acid, formaldehyde, and MTBE, among others [23]. The production process for methanol consists of three stages, reforming, synthesis, and purification. In the first stage, the main goal is to transform methane into syngas. For this purpose a reforming process is selected. One important factor to consider when selecting the reforming process is that the ratio of H₂ to CO to feed the methanol synthesis reactor has to be equal to 2.

      For the synthesis of methanol, compression of the syngas obtained from the reforming stage is needed. Then, the compressed syngas is fed to a catalytic reactor in which the following reactions take place:

      The synthesis reactor operates at 83 bar and 260 °C. The outlet of the reactor is cooled and sent to a flash unit to separate the unreacted syngas and recirculate it. Additionally, a fraction of the recycled syngas is purged, with a potential use as fuel. The crude methanol obtained from the flash unit is purified using one or two distillation columns [23].

This process has been analyzed to assess its environmental impact and its safety characteristics [23,24]. The main drawbacks of the process are the high pressure required for the operation of the synthesis reactor and the wasted fraction of non‐recycled syngas. Ortiz‐Espinoza et al. [24] studied the effect of different operating pressures for the methanol synthesis reactor on the safety, environmental, and economic characteristics of the methanol production process using POX reforming. The high operating pressure is related to the profitability of the process, but safety properties may be hindered by such operating conditions. Greenhouse emissions are an additional item of relevance for consideration. Figure 1.1 shows the results of the analysis conducted by Ortiz‐Espinoza et al. [24], in which values of three metrics used for profitability, inherent safety, and sustainability are reported for different reactor pressures and recycling fractions for the unreacted syngas. Such metrics were the return on investment (ROI) for economic performance, process route index (PRI) for inherent safety, and total emissions of CO₂ equivalents for process sustainability. One can observe the gradual trend of the three metrics that reflect their conflicting behavior. In summary, the economic potential of the process is better at high pressures and high recycling fractions, but if safety is of primary concern, a lower pressure would favor the process characteristics.

Figure 1.1 Safety, sustainability, and economic indicator for different pressures and recycling fractions in the methanol production process.

      It should also be noticed that the methanol synthesis reaction is exothermic; therefore, heat integration options may be considered to further enhance the environmental and economic performance of the process.

1.8 Ethylene

Ethylene is a major building block used in the chemical industry to produce a wide variety of important chemicals. The increasing availability of shale gas has boosted the ethylene industry as several ethylene production plants have been planned to be built in the United States [5]. The alternatives to produce ethylene include processes that use NGLs as feedstock, such as ethane cracking or propane dehydrogenation [25], and processes that transform methane to ethylene [26,27]. Among the processes that convert methane to ethylene, two important options are the oxidative coupling of methane (OCM) and the methanol to olefins (MTO) technology. OCM is a direct process in which methane and oxygen are fed to a catalytic reactor, with the products of the reaction being separated in a purification stage that consists of the removal of water and CO₂ and a cryogenic distillation train [26]. Although this process is known for the low yield achieved in the reactor, which render a process option with low profitability, the development of new catalyst structures has made possible the construction of demonstration facilities for this technology that offer better economic perspectives [28].

The other alternative for ethylene production is MTO, which is a more complex process as it involves several stages. First, the reforming of natural gas and the production of methanol take place. After the methanol synthesis, the crude methanol is sent to a catalytic reactor where low‐weight olefins are produced. In the reactor a variety of components are produced, such as ethylene and propylene, butylene, C₅s, hydrogen, low‐weight hydrocarbons, water, and CO₂. The effluent of the reactor is then sent to separation and purification units, which start with CO₂ removal and dehydration units. Then, the remaining stream is sent to a distillation train consisting of demethanizer, deethanizer, and depropanizer columns, as well as C₂ and C₃ splitters. A column to separate C₄s and C₅s is also needed. The overall process is very energy intensive, as it involves a reforming stage and a large distillation train.

      Even when the MTO technology has been reported to be more profitable than the OCM option [26], the latter technology is less complex and avoids the need to transform the natural gas to intermediate products such as syngas. That provides an incentive to develop improvements to this technology in order to enhance its overall performance and profitability. Proposed ideas to achieve such improvements include the use of membranes in the CO₂ separation system and modifications to the ethylene fractionation column to reduce heating and condenser duties [29,30].

1.9 Benzene

      Benzene is an important starting molecule in the petrochemical industry. The production of benzene from shale gas was considered in Pérez‐Uresti et al. [31], and a process based on the direct methane aromatization (DMA) route was designed. In this process, methane is fed to a DMA reactor operating at 800 °C and atmospheric pressure. The main products of the reaction are benzene and hydrogen. The effluent from the DMA reactor is sent to a membrane unit to separate the hydrogen. Then, the remaining stream is cooled and compressed to be separated in a flash tank. The gas stream obtained from the flash separator is methane‐rich and is recycled to the DMA reactor. The liquid stream is fed to a distillation column where benzene is obtained as a top product. Although the DMA process competes with the traditional production routes based on catalytic reforming