Propylene is an important light olefin used in the manufacture of a wide variety of products. Apart from being a (petrochemical) raw material widely used in motor fuels for octane improvement, it is in the rubber and plastic industries where the largest of propylene share finds its application, as intermediate in production of polypropylene, acrylonitrile, propylene oxide, cumene/phenol, oxo alcohols, acrylic acid, isopropyl alcohol, and oligomers. This alkene is the second simplest unsaturated organic compound and its demand is ever increasing, mainly due to the increase of polypropylene production. The largest outlet, accounting for 66% of propylene demand globally, is polypropylene.
In the current study a 13X zeolite extrudates have been chosen as the adsorbent and isobutane as desorbent to separate a mixture containing 32% of propane and 68% of propylene. Adsorption equilibrium isotherms on the 13X zeolite extrudates has been determined gravimetrically at 50, 100 and 150 ºC, up to 5 bar for propane and propylene; and up to 3.5 bar for isobutane. The Dual Site Langmuir (DSL) model was fitted to the adsorption equilibrium data and isosteric heat of adsorption as a function of adsorbate loading was calculated by the van’t Hoff equation. Additionally, gas phase SMB cycles were designed and performed experimentally for production of polymer-grade propylene (> 99.95% eluent free bases) from 0.32/0.68 propane/propylene feed mixture, with a recovery of about 90% of the olefin in the extract.
For this purpose a senior LSRE team, making use of its own resources, designed and assembled a new pilot SMB unit able to separate gas mixtures. Due to the high complexity of the unit that was proposed to be assembled a detailed planning was required. Therefore, prior to the acquisition of the material and equipment, the unit was designed using the software “SolidWorks”. This software allowed the determination of the exact amount of material required including the Swagelok fittings and tubing necessary for the assembly.
One should notice the high number of different items, more than eighty. Some of the items were required in very high numbers, such as 1/8”-Port Connectors: 252, 1/8” Union Elbow: 224, and ISO_BSP Parallel Thread Male Connectors: 130. Besides the number of required parts, this planning stage also allowed to study the best spatial distribution of the parts. In particular, the distribution of the columns inside the oven, and the layout of the gas distribution lines (inlets – feed and eluent, outlets – extract and raffinate), in order to reduce the dead volumes between columns, and between each column and the recycling streams.
The pilot gas-phase SMB unit contains eight columns in stainless steel, each with an internal diameter of 2.2 cm, and a length of 10 cm, with a metal sieve (0.5 micron, Swagelok, SS-2F-05) placed at the bottom end of each column. The gas-phase SMB unit can work between 1 and 7 bar of pressure, temperatures in the range of 303 – 523 K, and feed flow rates within 0.1-6 SLPM. The feed is obtained from the mixture of up to three individual gas streams, through the accurate control of three mass flow controllers. While, the eluent is obtained from a single gas stream controlled by an accurate mass flow control, with a maximum flow of 2 SLPM. The individual feed gas streams are mixed in a union cross before entering one of the columns. The eight columns are stainless steel with 2.12 cm internal diameter and 10 cm length, with a metal sieve placed at the bottom end to hold the adsorbent bed. The internal temperature of each column is acquired by type K thermocouples placed in the middle of the columns. The thermocouples are placed approximately at the centre (radial direction) of the adsorbent bed. The columns are placed inside of an air-forced convection oven (Termolab) equipped with resistances of 10 kW of power, and 2 PID controllers (Eurotherm), that allow the control of the temperature within 303 and 523 K with ±2 K accuracy. The inlet (feed, eluent, and 2 recycle entries) and outlet (raffinate, extract, and 2 recycle exits) streams are separated in 8 individual streams, by 2 manifolds 1/8”, and each individual stream is connected to one column. Sixty-four solenoid valves are automatically actuated from the computer interface, to control the schedule of gas admission/exit of each column, accordingly with the SMB switch time. At the exit of each column a filter was placed to prevent damage to the check-valves, vacuum pumps, or other parts of the unit due to fines released as a consequence of adsorbent attrition. The direct connection between two consecutive columns is done via one lift check valve to prevent the back flow between the columns, and to ensure flow in only one direction. The system pressure is assessed by four pressure transducers with range of 0 – 7 bar. The pressure transducers are place at the top of every two columns, and the obtained data is acquired automatically by the computer interface. Two recycle loops (one before the feed stream and another before the eluent stream) are forced by means of two vacuum pumps with the purpose of re-pressurizing the exit of the columns before the feed or the eluent streams, to ensure perfect mixture. The flows of raffinate and extract are controlled by means of two Coriolis mass flow meters, up to a maximum of 2000 g/h. The internal composition profile of the SMB unit can be analysed through a sampling point that is located at the top of Column 7. Sampling at this point at different times of one cycle allows assessing the composition of different points in the four sections of the SMB. Additionally, the composition of the unit outlet streams, raffinate and extract, can also be analysed. This analysis at different times of one step allows assessing the composition of the raffinate and extract streams. These analyses are performed by gas chromatography using a capillary column, and a flame ionization detector (FID). The gas chromatograph is coupled with an automatic valve system and an 8-loops trapping selector with microelectric actuator. The 8-loops trapping selector with 50 μL loops allows to sample during the experiment at selected times. The GC is operated isothermally with an oven temperature of 373 K, a carrier gas flow rate of 5 ml min-1 of helium, and Split Ration of 60.
A LabView interface was developed and a relay controller from OMRON was programmed for switching the 64 valves automatically. The LabView interface allows the control of the feed, eluent, raffinate, and extract flow-rates by providing a certain set-point value to them. Additionally, the acquired signals of temperature (9 signals: 8 columns + oven), pressure (4 signals), and the actual flow-rates (5 signals) are presented by graphical and numerical displays in the interface, allowing the operator to have a clear knowledge of the operation conditions at any moment. The data acquisition is done by four Bus-Powered Multifunction Data Acquisition units. All the data, time, pressures, temperatures and flow rates, are automatically stored to a file.