Method for direct disposal of plastic waste by chemical recycling in a partial oxidation process

The present invention relates to direct disposal of plastic wast by means of a method for chemical recycling of plastic material using the material as feed stock for petrochemical processes More specificly the invention concerns recycling of plastic wast as feed stock into the reactor unit of a partial oxidatio process operating at high temperatures and pressures in th presence of oxygen and with steam as a moderating agent. Th partial oxidation process comprises gasification, waste hea recovery and carbon removal in a scrubber unit during formatio of soot water slurry containing the unburned carbon and ash.

So far recycling development activities have mainly bee concentrated on remelting and reusing of the plastics i competition with virgin plastic material. However, the handlin and reprocessing costs for such uses are very high and th resulting plastic materials cannot compete with virgin plasti materials. A more recent trend of development takes recyclin back upstream of chemical processes. The material is the reprocessed at or before the polymerization stage, to regenerat hydrocarbon feedstock. However, economic viability is hard t achieve also by such routes, so that the necessity to brea through the cost barriers is also growing for such recyclin routes.

It has therefore been proposed to develop a feedstock preparatio unit which can be added to the front of an existing, commercia oil processing plant or petrochemical production plant. Th recycled plastic materials are then reprocessed by a chemical o thermal treatment to depolymerize the long chain molecules an to make a liquid feed suitable to be processed in a petrochemica plant.

Furthermore, and according to EP patent application 0.474.62 recycled plastic waste is proposed to be used as feedstock fo hydrocarbon gasification processes making synthesis gas hydrogen and carbon monoxide. After purification of such a ra gas, which will be heavily loaded with impurities, it may be use as a raw material and feed for the production of basic chemical such as ammonia and ethanol.

It is a prerequisite for a successful operation that the plasti waste is gasified under high pressure and that such process ca be carried out continuously. According to the above paten application heavy oil residue is heated to 150-250°C and i transported to an extruder/mixer where it is intimately mixe with plastic granules or particles, which has already been wette by the oil. This results in the preparation of a fluid mixtur which is kept under a pressure which is higher than the existin pressure in a subsequent reactor. The plastics granule/oi mixture is pressed through a die and into the reactor in th presence of steam and oxygen and gasified into synthesis gas.

At the introduction into the reactor the temperature of the flui feedstock is between 50-150°C and the viscosity at least 300°cSt preferably 500°cSt and the pressure in the reactor is 15-35 bar

However, such method is not applicable for use in a continuous partial oxidation gasification process. There is given n solution to the problem of handling the unburned carbon/ash

residue which will be of substantial proportions. According t present process design, the unburned carbon is to be recovere and continuously recycled to the feed. But with the recirculatio of a carbon/ash residue resulting from the gasification o substantial amounts of plastic waste, the contamination of th feedstock will be so high that the process will ultimately com to a stop unless a large bleed is applied.

Furthermore, the extra step involved with use of an extruder mixer as described in the above EP-patent application represent an undesired addition and complication for a partial oxidatio plant.

The viscosity of the oil/plastic mixture depends on the amoun and type of plastics and the initial viscosity of the base oil Accordingly, for avoiding that the viscosity of the mixture get too high, any increase in plastic waste concentration must b balanced by a less viscous - more expensive base oil. Viscosit below 250-300°cSt is usually desired.

The idea to reduce the impact of plastics on fuel viscosity b thermal treatment has also been proposed. According to previou experiences those operations cause severe fouling of the hea exchangers.

The main object of the present invention was to provide a metho for chemical recycling of plastic waste where the above drawback are eliminated. Primarily it is an object of the presen invention to provide a continuous once-through productio process, thereby eliminating the need for recirculation of th unburned carbon/ash residue. This will give greater versatilit regarding the permissible contaminants which can be allowed t pass through the reactor.

Furthermore, it is an object of the invention to provide process where the feedstock comprising recycled plastic waste ca be passed directly into the reactor without running int viscosity problems and resulting economic disadvantages.

Finally it is an object of the present invention to provide once-through continuous gasification process for recycled plasti waste, eliminating costly and time-consuming plastic handling an pretreatment by devicing a more direct route allowing the plasti material to be used directly without any special pretreatment.

During performance of simulated tests with mixtures of heavy oi and plastic (polyolefin) in solutions under conditions whic prevail within a partial oxidation gasification reactor, it wa found that the viscosity increased dramatically with plasti material concentrations from about 10% and higher. When th concentration was increased from 15-22% (by weight) , th viscosity increased by a factor of 40.

Thus it will not be feasible to introduce highly concentrate solutions of polymer waste into a gasification reactor withou exceeding the max. viscosity for proper atomization.

Partial oxidation processes for hydrocarbon feedstocks wer developed and commercialized during the 1950's. The best know processes, such as the Shell gasification process and the Texac gasification process, have been utilized in a number of commer cial plants.

Gasification processes utilizing recycled plastics and othe hydrocarbon feedstock normally comprise three principal steps:

- gasification, in which the feedstock is converted into ra synthesis gas in the presence of oxygen and a moderating agent

- waste heat recovery in which high pressure steam is generat from the hot gases leaving the reactor, and

- carbon removal, in which residual carbon contained in t reactor outlet gas, is removed in a mul istep water wash.

Hereby the unburned carbon from the gasifier will be made int a carbon slurry, an aqueous suspension containing soot and significant amount of ash, depending on the feedstock, which ha to be further processed and recycled.

A serious drawback of such processes is already that a certai percentage of the feedstock is not gasified and remains in th form of soot mixed with appreciable amounts of ash.

The soot/ash slurry from the carbon separation step will at leas contain 0.5-3% unburned carbon and 0.1-2% ash. The ash ma contain appreciable amounts of Ni, Fe and V as well as Cl pigments and other inorganic substances used in the productio of plastic materials.

Without recirculation of unburned carbon and ash the overal efficiency of the gasification process is improved and the amoun of contaminants in the feedstock may be appreciably increase because there is no gradual build up of impurities due t recirculation of such matter.

Such recirculation is avoided with an improved filtration step the soot/water slurry from the carbon/ash removal step i initially passing through a sedimentation step after flocculatin agents have been added to the solution. Filtration is carried ou at low temperatures below 80-75°C and preferably between 20-60" while confining the soot water slurry containing impurities fro the plastic material combustion between movable filter belt exerting a constant line pressure until the filtercake contains

less than 80% water and is no longer soapy or pasty but i tranferred into flakes or agglomerates which are easily separate and well suited for subsequent transport and processing.

Now according to the present invention there is provided a metho for chemical recycling of plastic waste by means of the abov once-through process route. Oil and plastic material are directl fed to the gasification reactor without any chemical or therma pretreatment breaking down the molecular chains.

The oil feedstock may comprice various types of waste residue and oil, like highly aromatic oils, PCB and other waste product from petrochemical industries which to day are disposed of b high temperature incineration.

Seperate feed of oil and plastic materials directly to th reactor burner avoids the above mentioned problems which wil arise when oil and plastic materials are mixed.

Essential features of the invention are as defined in th accompanying claims 1-6 and further important aspects will als be apparent from the detailed description of the invention below

Fig. 1 shows a general flow sheet of the gasification process comprising application of oil/plastic feedstock.

Plastic waste material 15 and oil 12 are directly fed into th reactor 13 of the gasification unit. Gasification and partia oxidation is carried out at 40-60 bar and at a temperature o 1300-1500°C in the presence of oxygen 16 using steam 17 as moderating agent. After passing through a waste heat recover unit 14 the gases are passed through a carbon removal step 1 i which unburned carbon and ash are removed in a water wash. The

resulting soot/ash water slurry emerging from the carbon/as separation stage 1 is initially passed through a cooler 2 t lower the temperature from approximately 95°C to 20-60°C.

The slurry is then passed to a soot slurry tank 3 and thereafte through a mixer 4 to administer flocculation agents from a sourc 5. Two flocculants are used, one positively charged polyelectro lyte sold under the trade mark Praestol 611BC (cationic) and on negatively charged polyelectrolyte sold under the trade mar Praestol 2440 (anionic) , both being in the form of whit polyacrylamide granulates. The ratio between the two types o flocculants can be varied within wide limits, but shoul preferably be in the range of 1-5 : 0.5-2 cationic : anioni flocculant.

The total amount of flocculants added was adjusted to betwee 60-100 ppm and the ratio of cationic : anion polyelectrolyte wa 1.5 : 1. The flocculants were added to the slurry in the form o a 0.1-0.3% solution in water.

Initially the slurry was dewatered while passing a sedimentatio tank 6. Thereafter it was passed through the filter unit 7 wher it was pressed between two vertically moving filterbands an finally compressed by means of rollers pressing the filterband together to exert a constant pressure until the water content o the filtercake was reduced to <80%. Filtrate from the filtratio stage 7 was returned to the soot separation unit 1 by means o a pump and pipe 10.

Thereafter the filtercake was released on to a movable conveyo belt in the form of dry flakes or plates with an averag thickness of approx. 2 mm and fed into the combustion unit 8 The resulting product 9 could be handled and further processe without any problems. The proportion of oil and recyclin plastics can be varied within wide limits and the upper limit of

plastics can be 40-50% (by weight) or higher. Also the conten of impurities in the plastic waste is less critical than in know recycle processes. Even chlorine containing plastic materia (PVC) can be tolerated to a certain extent. Due to the hig temperatures, any type of organic chlorine compound will b desintegrated. The resulting hydrochloric acid is disposed o within the process according to the invention by neutralization.