(657d) ZIF-8 As an Efficient Adsorbent for Ethane/Ethylene Separation By Gas Phase Simulated Moving Bed

Growth of global population, along with rising living standards lead to increasing demands for synthetic solid and liquid chemicals such as plastics and lubes with designed properties. Ethylene is the key building block in chemical industry as it is capable to catalytically co-oligomerize to form controlled polymeric hydrocarbon chains, and thus can be used in polymer production with designed physical properties, and also as a starting material/intermediate for the synthesis of other high demand chemicals. Annual production of ethylene achieves 150 Mt and expected to grow with rates of 4 to 6% per year making this olefin one of the most important chemical product in terms of volume, number of derivatives and sales value.

There are mainly two categories of feedstocks for ethylene production. One includes those derived from crude oil, and another includes those derived from natural gas. The raw materials from crude oil are mainly paraffins (alkanes), aromatics and naphtha, being the latter a mixture rich in paraffinic, naphthenic and aromatic compounds, with a boiling point between 20 and 200 ºC, besides having between 4 and 12 carbons in the carbonic chain. The composition of natural gas is basically methane with other higher alkanes, such as ethane, propane and butane. The composition of the final products is intrinsically related with the feedstock selected to produce ethylene, with implications on the costs of the olefin purification. The feedstock used depends on availability, price, and what other products are intended to obtain.

Thermal cracking is the main industrial method for producing ethylene at reaction temperatures around 850-900 °C. Light hydrocarbon feeds such as ethane, LPG or light naphtha is dehydrogenated to give complex mixture containing product alkenes such as ethylene, propylene and butadiene mixed with hydrogen, unreacted alkanes such as ethane and propane and other by-products. Heavier hydrocarbon feeds (full series and heavy naphtha as well as other refinery products) give some of these, but also give products rich in aromatic hydrocarbons and hydrocarbons suitable for inclusion in gasoline or fuel oil. The cracked product mixture are processed with successive distillation steps to initially pre-separate the ethane and ethylene mixture from other lighter and heavier molecules and finally separate ethylene from ethane with different olefin proportions, which can range from 50 to 80%.

Ethylene purity above 99.9% is required for polymerization processes due to presence of catalysts. The significant fraction of ethylene production cost is associated with separation steps primarily due to energy intensive cryogenic liquefaction of gaseous molecules, as well as similar relative volatility of the components. Separation processes consume more energy than thermal cracking of feeds and, in this way, determining the end price of many essential commodities in our daily lives.

The rising demand of ethylene combined with the large economic and energy production costs, make it crucial to search for more efficient and profitable separation processes. In this field, the separation of high value products based on adsorption at near ambient temperature appears as a strong substitute, to the currently practiced cryogenic distillation process in use, in particular cyclic adsorption processes, such as simulated moving bed (SMB) and pressure swing adsorption (PSA). Generally, it is possible to find an adsorbent for which the adsorption separation factor is much greater than the relative volatility, so in such cases an adsorption separation process may be economically more feasible.

More recently, a breakthrough was attained with the emerging of new MOFs with a paraffin selective peculiar characteristic, and some results in this type of materials are already available in the literature. ZIF-7, ZIF-8, ZIF-9 and ZIF-11 were the first examples of a microporous adsorbent exhibiting selective adsorption of paraffins over olefins offering the advantageous separation process configuration. These MOFs also display an interesting performance due to its structural flexibility.

Based on this new concept of paraffin selective adsorbents, we tested ZIF-8 granulates purchased from Materials Center of the Technische Universität Dresden, Germany, to separate ethylene from ethane, at high purities from a mixture having a composition very close to that obtained at the outlet of the industrial ethylene/ethane splitters.

The ZIF-8 material was characterized preliminarily by SEM/EDS, X-ray powder pattern diffraction, mercury intrusion porosimetry, and sorption of nitrogen at 77 K and carbon dioxide 273 K. Particle average diameter is about 1.1 to 1.4 mm and the crystal radius of 460 nm, observed by SEM analyses. The mercury intrusion porosimetry and the helium picnometry provided the apparent (339 kg/m³), solid (935 kg/m³) and skeleton (1328 kg/m³) density values. The textural properties, such as Langmuir surface area (1653 m²/g), micropore area (1635 m²/g), external surface area (17.9 m²/g) and micropore volume (0.58 cm³/g) were determined from the nitrogen and carbon dioxide adsorption equilibrium isotherms.

The adsorption equilibrium isotherms of the ethane, ethylene, propane and carbon dioxide at 303, 333 and 373 K on the ZIF-8 granulates were determined. The isotherms were obtained by a gravimetric method using a magnetic suspension balance (Rubotherm). The sample is placed in a basket within the microbalance measurement cell and regenerated, at 423 K and under vacuum, to remove any chemical species that have previously been adsorbed. Each point of the isotherm was obtained by increasing successively the system pressure. The presence of hysteresis was evaluated by measuring the adsorption and desorption isotherm branches. The Langmuir model was selected in order to describe the observed equilibrium data.

Two SMB cycles for ethane/ethylene separation with a four zone configuration, in open loop, were experimentally performed in the bench scale SMB unit with eight columns. Each column in stainless steel were individually packed with approximately 4.7g of ZIF-8. Activation of the adsorbent was performed by feeding 0.500 SLPM of nitrogen for 16 h at a temperature of 423 K. After the activation, the temperature of the oven was set to the desired value of 323 K. In each experiment, the desorbent gas cylinder (propane or carbon dioxide) was set to 600 kPa, to impose a higher pressure at the beginning of the Zone I, and the back pressure controller was configured to operate at 500 kPa and placed at the end of the Zone IV to keep the gas flow in only one direction.

The equilibrium data was assessed in order to allow the desorbent selection. It was concluded that propane would behave as a strong desorbent, since it presents an adsorption capacity higher than both components of the mixture, on the full ranges of the studied pressure and temperature. Its saturation capacity presents the value of 4.56 mol/kg, an affinity constant of 4.7×10^-8 kPa^-1, and a heat of adsorption of 35.1 KJ/mol. For carbon dioxide (weak desorbent), a lower adsorption capacity was observed when compared with the C₂ components, possessing a saturation capacity of 9.72 mol/kg, an affinity constant of 8.0×10^-8 kPa^-1 and an adsorption heat of 23.3 kJ/mol. The desorbent strength influences drastically the SMB performance therefore the importance of obtaining a suitable desorbent to ensure a separation as efficient as possible.

These experiments had as main focus the study of the influence of the desorbent strength on the ethane/ethylene separation using ZIF-8 sample, under similar operating conditions. In both cycles a 2-3-2-1 columns distribution and a switching time of 50 s were adopted. Propane was used as a desorbent for separation of 0.40/0.60 ethane/ethylene (run 1) feed mixture and a total feed flow rate of 0.127 SLPM, while carbon dioxide was used to separate 0.41/0.59 ethane/ethylene (run 2) feed mixture and a total feed flow rate of 0.081 SLPM. The desorbent flow rate in the run 1 was 0.780 SLPM, while in the run 2 was imposed a desorbent flow rate of 1.176 SLPM. The pressure range was very similar for the two experiments, since for run 1 the range was 495 to 520 kPa and for run 2 it was 500 to 540 kPa.

In the experiment, with carbon dioxide as desorbent, ethylene was obtained with 99.6% purity and 94.2% recovery, while in the experiment using propane as desorbent only 77.1% of ethylene was recovered with a purity of 82.7%. With the cycle using carbon dioxide desorbent it was possible to obtain better performances, but, above all, these experimental data allowed the validation of the mathematical model.

A process modelling software was used to simulate and to validate the gas-phase SMB model against the experimental data, and to compute the separation regions that lay within the purity and recovery restrictions, a purity of raffinate (ethylene) higher than 99.9% and recovery of ethylene of at least 98%, corresponding to an extract purity higher than 97.2%. The obtained regions are drastically different. While for the case in which propane is used as desorbent, the obtained separation region is non-existing; for the case considering carbon dioxide the separation, obeying to the desired specifications, is possible within a significant separation region.