Regeneration of Coked Zeolite from PMMA Cracking Process by Ozonation

Fig. 1. Schematic diagram of the experimental set-up for the regeneration of coked
zeolites.

250 ^◦C, 350 ^◦C and 450 ^◦C to estimate the strength of acid sites [27].

2.3. Experimental set-up and procedure for regeneration of coked zeolites

   Ozonation of coked ZSM-5 samples (800 mg) was performed in a glass tube reactor (4 mm i.d., 18 cm length) at temperatures varying from 20 ^◦C to 150 ^◦C. It was inserted in a heated stainless steel cylinder whose temperature was adjusted by a PID controller. A laboratory ozone generator (HTU-500 ozone generator, Azcozon) was used to produce ozone from pure oxygen, in the range of concentration 16–50 g/m3.

   The schematic diagram of the experimental set-up is given in

Fig. 1.

   Spent zeolite particles of about 3 mm in diameter were loaded in the reactor after half part was cut and kept as reference. The sample was first heated under oxygen flow (up-flow mode, flow rate from 12.7 to 69.9 l/h) until desired temperature was reached. Then the gas flow was switched to O₃/O₂ mixture (16–50 g/m3 of ozone content). Time on stream (TOS) was varied from 0.5 h to 4 h. Inlet and outlet ozone concentrations were measured by the potassium iodide-trap method (IOA Standardized Procedure 001/96).

   After treatment, particles were crushed and mixed for carbon analysis by flash combustion technique (same for reference half pellets).

2.4. Experimental set-up and procedure for activity tests

   PMMA cracking tests were carried out in 100 ml batch reactor (Parr instrument) using 0.4 g of crushed zeolite extrudate and 40 g

Fig. 2. Evolution of BET surface area of zeolite extrudates as a function of carbon content.

of PMMA. Pure nitrogen was continuously flowed through the reactor for 5 min in order to eliminate oxygen. Then the gas inlet and outlet were closed and the reactor temperature was set to 300 ^◦C. The reaction was subsequently performed for 120 min at 300 ^◦C under autogenous pressure, resulting in gas, liquid and solid products. After cooling and degassing, the reactor content was weighted again to give the gas yield. Then solid and liquid products were separated. The solid residual was washed with 50 ml of MMA, then centrifuged and dried at 80 ^◦C for 24 h to remove all interstitial liquid before weighing. The liquid product was distilled in a rotary evaporator at 95 ^◦C under vacuum pressure (200 mbar) to obtain the light fraction.

3. Results and discussion

3.1. Physicochemical properties of zeolite extrudate samples

   Regeneration experiments were performed on two different lots of coked ZSM-5 extrudates:

-bottom particles from continuous PMMA cracking reactor, containing 2–4% of carbon (sample A),
– top particles, containing 9–11% of carbon (sample B).

3.1.1. Surface area and porosity

   Typical textural properties of coked and regenerated samples are given in Table 1 and compared to those of fresh zeolite.

   Coke deposition was found to occur in both micro- and mesopores, resulting in a sharp, almost linear, decrease of the surface area with respect to carbon content (cf. Fig. 2). The most severely coked sample exhibited nearly no microporosity.

   The specific surface area of the partially regenerated samples aligned with the previous data in Fig. 2, showing that the ozone treatment restored the textural properties of the zeolite, without any significant structural modification. The sample A recovered a