To be presented at: 1999 Global Symposium on recycling, Waste Treatment and Clean Technology,REWAS, San Sebastian, Spain, 5-9 September 1999

THERMAL RECYCLING OF HETEROGENEOUS CHLORINECONTAINING PLASTIC WASTE STREAMS

J.M.N. van Kasteren and P.P.A.J. van Schijndel

Eindhoven University of Technology (TUE), Faculty of Chemical Process TechnologyCentre for Environmental Technology (CMT),

PO BOX 513, 5600 MB Eindhoven, The Netherlands

AbstractThis study describes a back to feedstock method for heterogeneous chlorine rich containingplastic waste streams, which has been developed at the Technical University of Eindhoven(TUE).In this process plastic waste containing 20 wt% of chlorine is gasified with the aid of steam at1250 K into a gas consisting of H2, CO, CO2, CH4 and HCl. The conversion of the organic partof the waste is complete, without the formation of tars. The inorganic materials are blown outof the reactor and can be separated as solid material. The formed HCl is recovered as a 30wt% muriatic acid or pure HCl gas and can be reused as feedstock for the VC production. Thegasses can function either as a syngas or as a fuel gas for energy recovery with high efficiency.Cogeneration of electricity and heat with high efficiencies (40-50%) via a combined cyclesystem with relatively small scale processes (5-30 kton per year) makes this process interestingfor conversion of complex plastic waste streams. Exergy analyses show that this process ismuch more efficient that the usual waste incineration route.

Introduction

Many studies have been carried out with the aim to develop methods for recycling of plasticwaste [1]. For polyolefins material recycling is relatively easy via melting and extrusion.Problems are encountered when a mixture of different polyolefins has to be recycled, becausethe different polyofenic plastics (e.g. polyethylene, polypropylene and polyvinyl chloride) donot compatibilise easily. This results in poor product qualities and consequently in pooreconomical possibilities. For optimal recycling plastics have to be separated and they require >99% purity. This is often difficult and very costly. In many cases however, this is even notenough to reach good product quality, because of the presence of many different additives anddeterioration of the polymers due to ageing processes. This limits the material recycling ofpolymers to mono streams. The alternative for the remaining plastic fractions can be found inback-to-feedstock processes. In these processes the macromolecules of the plastic waste arecracked at high temperatures resulting in a mixture of gas and liquid compounds. Back-to-feedstock processes make use of thermal process technologies such as: pyrolysis, gasificationand combustion [2].Within these back-to-feedstock processes PVC takes a special position, because itcontaminates the produced oils, gases and char with chlorine and chlorine containinghydrocarbons (e.g. chlorobenzene or dioxines). Therefore the presence of PVC in the wastestream has to be minimised. This results in chlorine rich and chlorine poor waste streams ofwhich the latter can be processed relatively easy. For the chlorine rich waste streams (> 5 wt %PVC) special processes have to be developed, because also incineration in waste incinerators isnot preferred due to the formation of HCl and subsequently the chlorine corrosion, heavymetals present in the PVC-waste and the possible formation of dioxines.For this reason investigations were started at the Eindhoven University of Technology with theaim to develop a process for the back-to-feedstock recycling of PVC containing waste streamswith recovery of the formed HCl and no formation of chlorinated hydrocarbons and/ordioxines. The accompanying gaseous organic product stream can be used as feedstock foreither chemical or petrochemical industry and/or for energy recovery.A good candidate to fulfil these demands is hydrothermal gasification. With the aid of steamPVC-waste is converted in a bubbling fluidised bed reactor containing alumina as catalyst to asyngas and HCl, which can be recovered as muriatic acid solution (30wt% ). The use of steamas oxygen source has the advantage of reducing the formation of chlorinated hydrocarbons andease of gas separation by reducing the gas volumes. The muriatic acid can be normallymarketed or directly used for vinyl chloride (VC) production, thus closing the chlorine cycle.The gas phase can be converted to syngas or used as fuel gas for energy recovery [3].On a lab scale experiments have been conducted [4] which have shown that a stationarybubbling fluidised bed reactor is an excellent reactor for the hydrothermal conversion of PVC-waste into useful products.The aim of this study is to show that the hydrothermal conversion process has much betterexergetic efficiencies than the normal waste incineration route thus proving that gasification isa more preferable route for the chlorine rich containing plastic waste streams. For this reasonan exergy analysis has been carried out based on experimental data and a conceptual processdesign.

Exergy analysis

Exergy analysis resembles the enthalpy or energy analysis. The difference is that in exergyanalysis enthalpy and entropy are applied. An exergy balance can be performed for a whole

plant or for different unit operations. Information about exergy analysis can be found inliterature [5,6]. The following definition for exergy is used:

‘Exergy is the maximum amount of work that can be obtained from a stream of matter, heator work as it comes to equilibrium with a reference environment. It is a measure of thepotential of a stream to cause change, as a consequence of not being completely stablerelative to the reference environment. Exergy is not subject to a conservation law, but it isdestroyed due to irreversibility’s during any process.’

Exergy is a measure for quality of mass (chemicals) and energy streams. By use of an exergyanalysis, processes can be optimised into more sustainable processes since it’s a tool to identifyinefficient parts in processes. In this research exergy analysis was carried out first to comparetwo processes, gasification and incineration. Secondly, the PVC-waste gasification process wasstudied for reasons of exergetic analysis.In this paper exergetic efficiencies of processes are calculated. The definitions of theseefficiencies are given here since there is no consistency in literature about them. The generalFormula (1) for the exergy balance is:

E E Iin out= + (1)

with E being the exergy flows in and out, respectively, and I the so-called irreversibility. Thismeans the process is far away from thermodynamic equilibrium and there is still ample roomfor efficiency improvements..Exergetic process efficiency can be calculated for different processes to rank processes.However, there are many definitions of this exergetic efficiency. A commonly used one is theso called ‘simple’ efficiency but a much better one in terms of reality is the ‘rational’ efficiency[7]:

Simple efficiency: η

simple

out

in

E

E= (2)

Rational efficiency: η

rational

desired output

used

E

E= (3)

In formula 3, E desired output is defined as Eproduct - Eraw materials. And Eused is defined as the differenceof ingoing and outgoing resource flows, e.g. air and exhaust gases. The simple efficiency onlysays something about the irreversibility of a process. Processes with high heat loss but lowirreversibility can still have a high simple efficiency. The rational efficiency only looks at theexergy of the useful products with respect to the total exergetic input. This gives a betteroverall view of the efficiency of a process.

Process description

Figure 1 shows schematically the back–to-feedstock process as it has been developed at the TUEfor the PVC containing waste streams. PVC-containing waste is gasified at temperatures of 800-1000°C with steam in the presence of an alumina catalyst in a bubbling fluidised bed to CO, H2,CO2, CH4 en HCl. The HCl is separated as 30 wt% solution (muriatic acid) en can be used asfeedstock for vinylchloride (VC). The gas formed can be used as feedstock for a gas turbine withwhich it is possible to convert it to electricity with a high energetic efficiency. Part of the gas isneeded as energy supply for the endothermic gasification process.

Figure 1. Schematic view of the back-to-feedstock process for PVC-containing wastestreams.

The overall reaction mechanism for the gasification of virgin PVC has been determined to be[8]:

(C2H3Cl)n + 2.7n H2O → n HCl + 3.4n H2 + 1.0n CO + 0.8n CO2 + 0.15n CH4 (4)

Based on experimental data of the bench-scale gasification unit a conceptual design for a 50kton PVC-waste/year plant has been made with the use of the software package AspenPlus[9]. In this design study the HCl is upgraded via distillation to pure, gaseous HCl (0,7 Mpa)suitable for VC production. The syngas is purified to be suitable for electricity production via agas turbine and steam cycle (combined cycle system).Table 1 shows the mass balance for a 50 kton PVC-waste /year process. The PVC-waste inputis based on an average composition as given by the EVCM [10]. As can be calculated fromtable 1 PVC-waste contains about 40 wt% PVC, plasticizer dioctylphtalate (DOP), filler(CaCO3 ) and other inorganics. The percentage of filler in the form of CaCO3 (21wt%) is aworst case approach since this amount is usually lower. Due to the presence of CaCO3 CaCl2

is formed via reaction with HCl. This means a loss in HCl recovery. The syngas has anaverage low heating value (LHV) of 8,6 MJ/mo

3.The major heat requirements of the process are given in table 2. The hydrothermal process isendothermic and heat must be supplied. This can be done by external heating, for instance withfire rods, or by internal heating by partial combustion with oxygen. The latter one is the easiestfrom a technological perspective. The necessary heat is provided by combustion of part of thesyngas produced. The rest of the syngas can be used for production of electricity. Utilitiesinput is 7,4 MW for the gasifier; 2,8 MW (steam) for the HCl distillation unit and CaCl2

vaporiser (needed for the purification of HCl), so the total input is 10,2 MW. The total outputis 35 MW, which means that about 21% of the thermal output is needed to keep the processgoing. Thus 25 MW thermal is available for external steam and electricity production

Hydrothermal

treatment

Solid

Separation

HCl

Recovery

Energy

Recovery

PVC

Water

Hydrochloric acidSolid waste Electricity

Heat

Water

Table 1 : Input and output flows of the 50kton/year PVC-waste gasification process. (DOP=dioctylphtalate)

In OutStreams [kg/s] PVC-waste Water Syngas CaCl2 waste HCl-gasPVC 0.696DOP 0.522CaCO3 0.348Inorganics 0.174 0.173CaO 0.020CaCl2 0.344H2O 1.280 0.087 0.000HCl 0.000 0.179H2 0.211CO 0.980CO2 1.005CH4 0.000Total 1.740 1.280 2.283 0.537 0.179

Table 2 : Major energy requirements PVC-waste gasification process (50 kton/year)

Energy required [MW] UtilityReactor input 7.4 SyngasReboiler extractive distillation column 0.5 SteamEvaporator CaCl2 2.3 SteamTotal energy required 10.2

Exergetic comparison of PVC waste incineration and gasification

An exergy analysis of the PVC gasification and the PVC incineration process has been carriedout. In this study a simple process lay out has been used to get a first order approach of theefficiency of both processes. In both processes all streams are related to the same amounts of 1kg of pure PVC. The chemical exergy value of 19 MJ per kg of PVC was calculated accordingto Kotas [6] with an estimated Net Combustion Value (NCV) of 18 MJ/kg PVC, which isconsistent with Ayres et al. [11]. Heat losses during processes are estimated to be 10%(enthalpy based).In the incineration process, the heat from PVC combustion is used to produce electricity via asimple steam cycle system. In the gasification process the scrubbed syngas is transferred intoelectricity via a combined cycle system.In the case of the gasification HCl gas is scrubbed and recovered as muriatic acid. In case ofthe waste incinerator the HCl is not recovered, as is the case with the normal wasteincineration processes. No other end-of pipe treatments were considered in this simple case.Calculations were based on the stoichiometric reaction of incineration of PVC:

(C2H3Cl)n + 2,5n O2 → n HCl + nH2O+ 2nCO2 (5)

For the calculations an air surplus has been assumed of 2,2 times the stoichiometric amount.The gasification calculations are based on reaction (4) with subsequently burning of the syngaswith the same air surplus. Results of the comparison are summarised in Table 3.

Table 3. Comparison of PVC incineration and gasification processAll mass values in kg en exergy values in MJ.

PVC-incineration PVC gasificationInput 1 kg PVC

air1 kg PVC and 0,6 kg Steam (2 MJ)1,4 kg water and air

Output 2,3 kg Stack gas (CO2, H20,HCl)

4,8 MJ electricity

2,3 kg Stack gas (CO2, H20)

2 kg HCl 30% solution (1 MJ)10 MJ Electricity

Efficiency,simple

33% 66%

Efficiency,rational

25% 49%

From the results of the first simple comparison it was concluded that the gasification processhas a twice-higher efficiency. This is due to the higher energetic efficiency of a combined cyclesystem. Moreover, the recovery of HCl is an extra advantage over incineration. In figure 2, theinternal exergy losses in the simple process are shown. It becomes clear that the biggest exergylosses occur in the burner and the steam cycle.

Figure 2. Internal exergy losses in simple PVC gasification process

Highest exergy loss (4 MJ/kgPVC) occurs in the burner of the gasturbine system. This loss ismuch lower however, when compared with the exergy loss (8,25 MJ/kg PVC, not shown) inthe combustion chamber of the incineration process. Clearly the use of a gas turbine with steamcycle (combined cycle) improves the efficiencies. Since in a normal waste incinerator thissystem cannot be used (due to material problems) the energetic efficiencies of these systemswill remain low. Gasification systems can overcome these problems and thus make better useof resources.

0

1

2

3

4

MJ

Gas

ifie

r

Scr

ub

ber

Co

mp

ress

or

Mix

ing

Bu

rner

Exp

and

er

Ste

am c

ycle

Exergy loss

Exergetic analysis of PVC gasification process

For the exergetic analysis of the gasification process the input composition of the 50 kton/yearconceptual design has been used and the exergetic values have been determined as shown intable 4. The chemical exergy of 1 kg of PVC waste is about 18,9 MJ, which is comparable withpure PVC.

Table 4. PVC-waste composition for exergy analysis

Component Compositionweight fraction (%)

Chemical ExergyMJ/kg

PVC 40 19DOP 30 37CaCO3 20 0,01Al2O3 10 2

As can be seen from the design data (table 2) about 7.4 MW is necessary to run the gasificationreaction for the 50 kton/year plant. In the simulated process this energy input is achieved byburning 21 vol% of the syngas in a pipe burner system within the gasifier, thus maintaining agood caloric value of the syngas.The syngas is converted with high efficiency into electricity and steam. The efficiency of syngasconversion into electricity in a Combined Cycle system lies between 50-60%. For this study anefficiency of 56% was calculated. Outcome of the simulated process can be found in table 5.Exergy input and output have been visualised in figure 3.

Table 5. Exergetic data for 1 kg of PVC waste (based on simulation of a 50 kton/year PVCwaste gasification process [9]). All MJ are exergy values.

PVC gasification PVC gasification + combinedcycle

Input 0,74 kg Steam, 1 kg PVC waste,1,2 kg air1,3 MJ heat (steam)1,5 MJ heat (for HCl production)

0,74 kg Steam, 1 kg PVC wastecombustion airHeat integration system

Output 1,0 kg Syngas netto, 16 MJ (CO,H2 H2O, CO2)0,10 kg pure HCl gas (0,24 MJ)0,31 kg waste

6,9 MJ Electricity0,10 kg pure HCl gas (0,24 MJ)Stack gas (CO2 and H2O)0,31 kg waste

Efficiency,simple

74% 41%

Efficiency,rational

72% 38%

The 72% rational efficiency for the production of HCl and syngas from PVC-waste is verygood, so only small improvements are expected to be feasible. Based on table 2 it can becalculated that the production of HCl requires about 16 MJ/kg HCl. This is a high energeticinput in comparison to the chemical exergy value of HCl (2.4 MJ/kg HCl). This means that therecovery of HCl has a poor exergetic efficiency, mainly due to the high steam input in reboilerand vaporiser. Availability of membranes for the separation of HCl from the syngas coulddecrease this energy demand. If the amount of CaCO3 in the waste is lower than 20% the

amount of free HCl produced will increase as also its efficiency. The production of electricityout of syngas shows an exergetic efficiency of 56%. This is good compared to the fact that allcombustion processes have exergetic efficiencies lower than about 60%. The combined cycleprocess has one of the highest efficiencies of all incineration processes. Around 40% ofexergetic efficiencies can be reached with this gasification system, which almost doubles that ofconventional waste incinerator systems.

Figure 3. Exergy Input en Output for syngas production from PVC waste

Conclusions

Gasification as back-to-feedstock technology for PVC-waste treatment gives much higherexergetic and energetic efficiencies than conventional waste incinerators.The process of PVC waste gasification is preferred, when compared with traditional wasteincineration due to advantages like higher energetic efficiencies and different possible productslike syngas, electricity and HCl recovery.The exergetic analysis of the process shows clearly where the greatest losses occur and wherethe best improvements can be achieved. In this way exergy analysis is a tool for processanalysis and comparison.

References

1. J. Brandrup, Die Wiederverwertung von Kunststoffe (Munchen: Hauser, 1995).

2. F. Kapteijn, Pyrolyse van gemengd plastic afval (IOP report 91904, Delft University ofTechnology (TUD), 1993).

3. M.J.P. Slapak, J.M.N. van Kasteren and A.A.H. Drinkenburg, Selection of a recyclingroute for heterogeneous PVC-waste, Proceedings first European Workshop on Reuse,Eindhoven, The Netherlands, 11-13 nov. 1996, 267-275.

4. M.J.P. Slapak, J.M.N. van Kasteren and A.A.H. Drinkenburg, Hydrothermal recycling ofPVC-waste in a bubbling fluidised bed reactor, Proceedings seventh European Polymerfederation symposium on polymeric materials, Szczecin, Poland, 20-24 september 1998,277-278.

5. J. Szargut, D.R. Morris and F.R. Stewart, Exergy Analysis of Thermal, Chemical, andMetallurgical Processes, (Berlin: Springer Verlag, 1988).

Exergy Output

Syngas (net.)77%

Exergy Loss20%

HCl1%

Waste2%

Syngas (net.)

HCl

Waste

Exergy Loss

Exergy Input

30%43%

10%8% 1%

3%

5%

PVC

DOP

Al2O3

Steam Gas.

Heat Gas.

Vaporiser

Reboiler

6 T.J. Kotas, The Exergy Method of Thermal Plant Analysis, (Malabar: Krieger publishingCompany, 1995, 2nd edition).

7. R.L. Cornelissen, “Thermodynamics and sustainable development, The use of exergyanalysis and the reduction of irreversibility”, (PhD thesis, Twente University ofTechnology, The Netherlands, 1997).

8. J.M.N. van Kasteren, Energie uit chloorrijk kunststofafval (Novem report,projectnumber 355298/4020, november 1998).

9. A. Van Kraaij, Design of a hydrothermal PVC recycling process (Stan AckermansInstitute, Eindhoven University of Technology, ISBN 90-5282-907-1, 1998)

10. ECVM, European Counsil of Vinyl Manufacturers, Brussels, 1998.

11. R.U. Ayres, L.W. Ayres, and K. Martinas, “Exergy, waste accounting, and life-cycleanalysis”, Energy, 23 (5) (1998), 355-363.

12. J.d. Boer and P.v. Schijndel, “Exergy analysis of the PVC gasification system”, (internalresearch report, Eindhoven University of Technology, The Netherlands, 1997).