Plastics Process Analysis, Instrumentation, and Control. Группа авторовЧитать онлайн книгу.
a supported metal catalyst; gasifying the mixture; and producing the high-quality liquid fuel.
An exemplary embodiment of the present disclosure includes a high-quality fuel prepared by a process that includes melting a solid plastic waste; adding a metal hydride and a supported metal catalyst; and producing the high-quality liquid fuel.
1.15.3.5 Decomposition in a Hot Oil Medium
The decomposition of waste plastic has been investigated at a relatively low temperature (95, 96). The process can decompose a mixed stream of waste plastic at a temperature generally less than 375°C in a hot oil medium. The process converts the polymeric structure of the waste plastic or plastics to smaller chemical molecules such as the monomeric units and related chemical structures at a relatively lower temperature. It also serves the market for such products.
The low-molecular weight distillate from the waste plastic may help reduce the demand for imported petroleum products and help decrease our dependence on foreign crude oil.
Before mixed plastic wastes were studied, each individual component plastic was thermally decomposed to have a basis for the difference between mixed and individual resource recovery, and whether the mixed plastic wastes when thermally decomposed had an unexpected interaction.
A laboratory setup was used for preliminary experimentation which consisted of a thermally regulated flask with water condenser. The flask temperature was regulated to within 5°C and the overhead distillate condensed with 16°C water while the amount of uncondensed overhead gas was measured. The SAE 50 oil and the appropriate plastic resin were placed into the flask and the flask was purged with nitrogen before heating began. After the proper time at temperature, the flask was quenched.
The above tests indicated that the apparent optimum temperature for the hot oil decomposition of PE and PP was about 425°C, about 400°C for PS, about 375°C for poly(ethylene terephthalate) (PET), and about 325°C for PVC. Thus a temperature staging process was employed with mixed waste plastics. PET was not employed in this experiment since its solid decomposition product tended to clog the apparatus. A further aspect in omitting PET was that recent trends in recycling of waste plastic have been to separate out the bottles made of PET and recycle them directly to the bottle manufacturer.
Then, a three-stage temperature experiment was performed using 270°C for 20 min, 410°C for 30 min, and 450°C for 45 min.
1.15.3.6 Catalytic Cracking
It has been proposed to pyrolize or catalytically crack the waste plastic so as to convert high molecular weight polymers into volatile compounds having a much lower molecular weight (97). The volatile compounds, depending on the process employed, can be either relatively high boiling liquid hydrocarbons useful as fuel oils or fuel oil supplements or light to medium boiling hydrocarbons useful as gasoline-type fuels or as other chemicals.
Catalytic cracking of mixed waste plastic is a well-known process. For example, a method for controlling the pyrolysis of a complex waste stream of plastics to convert such a stream into useful high-value monomers or other chemicals has been presented (98). Here the catalyst and the temperature conditions were identified that permit the decomposition of a given polymer in the presence of others, without substantial decomposition of the other polymers (98).
During catalytic cracking, five main families of products are produced (97):
1 Gases,
2 Gasoline fraction,
3 Kerosene fraction,
4 Diesel fraction, and
5 Waxy compounds.
While the last four families may all find existing market applications, the valorization of the gases is more complicated. This often leads to the utilization of such an off-gas for energy recovery directly on the production site.
In general, it is advantageous to increase the kerosene and diesel fraction, since these are valuable products. Furthermore, the quality of diesel fraction obtained should be high, i.e., it should contain a low amount of aromatics.
So, in the catalytic cracking of waste plastic, high molecular weight polymers should be cracked into smaller molecules of lighter weight. In theory, the end product of such cracking reaction mainly consists of gases comprising lightweight hydrocarbons. Therefore, a priori, low-molecular-weight hydrocarbons should not readily react under cracking conditions. If at all, lightweight hydrocarbons should undergo reactions which would transform them into even lighter compounds.
It has been found that contrary to these expectations, lightweight hydrocarbons being introduced within a reactor for cracking waste plastic are transformed into heavier products, thereby increasing the amount of valuable kerosene and diesel fractions (97). Furthermore, it was found that even the undesired gases in the product stream obtained from such cracking reactor can be recycled into the process, thereby even further increasing the kerosene and diesel fractions. Additionally, it was found that the additional introduction of lightweight hydrocarbons within the cracking reactor yields diesel fractions of higher quality with respect to a decreased amount of aromatics obtained.
The plastics used in the process are preferably polyolefins such as HDPE, low-density poly(ethylene), PP and also PS. Other polymers, such as PVC, PET poly(urethane), ABS, nylon, thermosetting polymers and fluorinated polymers are less desirable. The catalyst used in the process is preferably a zeolite-type catalyst.
Specific examples for suitable zeolite-type catalysts are ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-50, TS-1, TS-2, SSZ-46, MCM-22, MCM-49, FU-9, PSH-3, ITQ-1, EU-1, NU-10, silicalite-1, silicalite-2, boralite-C, boralite-D, BCA, and mixtures there of.
The degradation process was carried out as follows (97):
Degradation Process 1–1: In a catalytic run in the semibatch mode, 30 g of plastic (20% PP, 80% PE) was loaded inside the reactor and a defined amount of catalyst was stored in the catalyst storage tank. The reactor was closed and heated from room temperature to 200°C during 20 min, while simultaneously purging with a 150 ml min–1 nitrogen flow. When the internal temperature reached the melting point of the plastic, stirring was started and slowly increased to 690 rpm. The temperature was held at 200°C for 25–30 min. During this heating process, the nitrogen coming out from the reactor was not collected. Meanwhile, the catalyst storage tank containing the catalyst was purged with nitrogen several times.
After this first pretreatment step and only in those experiments where propylene was fed, the flow of nitrogen was decreased from 150 to 103 ml min–1 and 38 ml min–1 of propylene was introduced inside the reactor. The flows were allowed to stabilize for several minutes, after which the mixture of gases was collected in a gas sampling bag for 3 min. After this collection of gases, the temperature control setpoint was changed from external to internal, the internal temperature was increased to the reaction temperature at a heating rate of 10°C/min, and the collection of gases and nitrogen in the corresponding gas sampling bag was started.
When the internal temperature reached the reaction temperature, the catalyst was introduced inside the reactor, and the circulation of the gaseous products was commuted to another pair of glass traps and corresponding gas sampling bag. This was considered as the zero reaction time. During the selected time periods, liquid and gaseous products were collected in a pair of glass traps and their associated gas sampling bag, respectively. At the end of the experiment, the flow of propylene was stopped and the reactor was cooled to room temperature. During this cooling step, liquids and gases were also collected.
References
1. D.V. Rosato, N.R. Schott, and M.G. Rosato, Plastics Engineering, Manufacturing & Data Handbook, Vol. 1 of Plastics Engineering, Manufacturing & Data Handbook, Springer US, 2001.
2. A.A. Fagade and D.O. Kazmer, Journal