CONVERSION OF WASTE POLYTHENE TO ETHANOL FUEL
FIELD OF INVENTION
This invention deals with the management of waste polythene by its conversion into a clean fuel
5 (ethanol) by a systematic reaction sequence. Huge amounts of non biodegradable wastes have
been a menace for the society for some time now. The biggest contributor in this is polythene
and related plastic waste. Our project focuses on solving this long stagnant problem by
converting this waste polythene into other useful products in a way that could prove to be much
more efficient than the existing conventional recycling methods in terms of energy input to
10 output ratio. Hence this invention goes a long way solving two very complicated ecological
problems viz. ecological degradation by increasing amounts of non-biodegradable pollutants and
the lack of efficient methods for production of clean fuel. These problems do seem to be quite
different from each other but a deeper and a more rational approach reduces them to two faces of
the same problem.
15
BACKGROUND OF THE INVENTION
Polythene is used at a very large scale in India Most of the families throw polythene in the
nearby areas, unaware of the harmful effects of polythene. Many people are unaware of the
increasing soil, air and water pollution caused by polythene waste.
20 Cheap and abundantly available Fuel is the major demand of the people in today's population.
Ethanol is a very clean fuel and also has a high calorific value.
Use of ethanol would reduce stress on non renewable fossil fuels and also reduce pollution
caused by the use of fossil fuels like coal, diesel and petroleum.
Once the identification of these issues was done it looked evident that production of ethanol and
25 other clean fuels from waste polythene could be a path breaking initiative.
The next step was to weigh this idea on scientific standards and come out with the most efficient
way for achieving the above mentioned conversion.
SUMMARY
We propose the conversion of polythene into ethanol (major product), other higher alcohols
(minor products and solid hydrocarbons (in very small amounts). The conversion includes a two
35 step process.
In the first step solid polythene waste after being cleansed in suitable medium (probably a weak
mineral acid) will go through thermal cracking (pyrolysis or cracking in absence of air) in an
inert environment at about 350°C-750°C. During this process it will break into different smaller
hydrocarbons. Since the cracking follows a free radical mechanism therefore it is almost
40 impossible to completely negate the formation of by products that are hydrocarbons other than
ethene. Though ethene is the major product of the cracking process and other compounds will be
formed only in smaller amounts.
In the second step the formed ethene gas will be hydrated in the presence of an acid catalyst like
phosphoric acid. The formed gases will be passed through a multilayered mesh coated with solid
45 phosphoric acid catalyst inside an environment of water vapour. This will provide large area for
catalytic action and lead to efficient hydration of the hydrocarbons (ethene) into respective
alcohols (in gaseous state). The major product again would be ethanol.
The gaseous components would then will be cooled and dissolved into water by spraying water
over them inside a scrubber. The lower alcohols will get dissolved into water but the higher ones
50 and other non reactive hydrocarbons will get isolated. Hence we would get a mixture of lower
alcohols with ethanol being the major component. Now this fuel may be used as it is or it may be
distilled using fractional distillation in order to isolate ethanol completely.
DETAILED DISCRIPTION OF THE INVENTION
55 The basic idea behind this invention is the clubbing together of two well known reactions which
are:
1) Thermal Cracking of Polymers (poly-ethene)
2) Hydration of alkenes (ethene) into alcohols (ethanol)
3.1) Thermal cracking of polythene
60 W H - CH-) , - n CHz----CH2
The cracking of polymers can be achieved by strong heating under high pressure and in the
absence of air. Many variations of this process are available where certain catalysts are used to
increase the rate of the reaction (catalytic cracking). At the laboratory level thermal cracking is
sufficient to produce appreciable amounts of ethene from polyethylene but at the industrial scale
65 methods like catalytic cracking would be suitable.
Cracking of polyethylene occurs through a free radical mechanism where there is a random
scission of bonds and formation of free radicals which further react to form a variety of
compounds though prolonged strong heating breaks the compounds into smaller and smaller
groups. Smaller the groups get lesser is their tendency to polymerize back to longer chain
70 compounds. Hence through prolonged heating a high yield of ethene gas is achieved. The
formation of byproducts cannot be ruled out but they can be further utilized by conversion into
desirable forms which is discussed in the next section.
Some major products obtained are:
1) Ethene
75 2) Lower alkenes and dienes
3) Lower alkanes
Various isomers of above mentioned compounds are also obtained through 1,3 methyl shifts and
other mechanisms. The conversion of all these compounds into alcohols is discussed in the next
section.
80 3.1.1) Reaction Conditions: High temperatures (typically in the range of 450°C to 750°C) and
pressures (up to about 70 atmospheres) are used to break the large hydrocarbons into smaller
ones. Thermal cracking gives mixtures of products containing high proportions of hydrocarbons
with double bonds - alkenes.
3.1.2) Reaction mechanism (for formation of common bv products):
Initiation:
13-n - zn
Intra~uolecitlsrt ransfer follo\ved by t~ decomposition ~.eactian:
R . -c R-CII-CIT~-R~ It. + CII?=CH-CIG-R' (3)
4 R-Cl'I=C112 + It'. (4)
I~ltermolcculart ransfer followed by n decomposition reaction:
R-CH*-R + R. - R-CII-R + rcn
R-GI!-R 4 R-CH=CHZ + R.
Termination :
R is the macromolecule and R' smaller n-alkane group.
3.2) Hydration of ethene
dration of Ethene
90 Ethanol is manufactured by reacting ethene with steam
The formation of the ethanol is exothermic
C2H4 (GAS) + H20 (GAS) C~HSOH(G AS) AH= -45 KJIMOL (Ethene)
(Steam) (Ethanol)
Only 5% of the ethene is converted into ethanol at each pass through the reactor.
95 By removing the ethanol from the equilibrium mixture and recycling the ethene, it is possible to
achieve an overall 95% conversion
3.2.1) Catalyst and reaction conditions
Solid silicon dioxide coated with H3PO4 catalyst is used as a catalyst in hydration of ethene.
Generally it is applied on a layered mesh through which the reacting gases viz. steam and ethene
100 are passed.
The reaction is an exothermic reaction hence equilibrium is shifted to the right at low
temperatures but at very low temperature the reaction becomes kinetically unfeasible and hence a
temperature of 3 0 0 ' ~ is considered optimum.
The reaction leads to decrease in gaseous number of moles and hence high pressure is preferred.
105 A pressure of around 60-70 atmosphere is considered suitable.
Some of the lower alkenes are also converted into alcohols or di-01s via above hydration process.
Lower alkanes can be directly used as fuel as they do not readily react with water due to lack of
.n- electrons. Various products obtained can be easily separated by using difference in solubility
in water and boiling points. The processes involved are discussed later.
110 4) Thermodvnamic analysis of the combined process of thermal cracking followed by
hvdration of ethene
.... (i)
Energy given by us in the process of making ethanol
1) (-CH2-CHy) . (solid) n C2H4 (gas)
(POLYTHENE) (ETHENE)
115 AH (depolymerization) = - (AH polymerization)
= - (-25 Kcallmol)
= 25 Kcallmol
-- 103 KJImol.. ...........................................................
2) H20 (liquid) H20 (gas)
120 AH (vaporization) = 43.9 KJMOL.. ..........................................................
(ii)
IDEAL CONVERSION NEGLECTING EQUILIBRIUM
3) C2H4 (GAS) + H20 (GAS) C2H50H (GAS)
(Ethene) (Steam) (Ethanol)
125 AH = -45 KJ/MOL .......................................................( iii)
AH process = [-CH2-CH2-] n (solid) C2H50H (gas)
= (i) + (ii) + (iii)
= [lo3 KJ/mol+ 43.9 KJ/MOL]-[45 KJIMOL]
= 146.9 KJMOL - 45 KJIMOL
130 = 101.9 KJ/MOL
Total energy input = 101.9 KJMOL
Calorific value of ethanol
AH (combustion C~HSOH( G ~=~ 2)8) M JIkg
= 1300 KJMOL
135 Hence thermodynamically a net energy gains of around 1200 KJ. This energy is not being
produced out of thin air but is actually a judicious conversion of non usable form of energy
(waste polythene) into a usable form of energy (ethanol).
Though thermodynamic calculations are based of 100% efficiency assumptions but due to
reversible nature of reactions, mechanical inefficiencies at the industrial level and production of
140 by products we must practically assume an efficiency level of around 40%. That still gives us an
energy surplus of around 600 KJImol.
Generally recycling processes for polythene give an energy input is to output ratio of 40% but
the above reaction sequence via the conversion of non usable form of energy into usable form
effectively give us an efficiency of greater than 1, again this should be not be considered a
145 violation of the conservation of energy by the above stated logic.
Keeping in mind that the major objective is the management of waste polythene the extra energy
produced should be literally acknowledged as a bonus!
Here we are converting a complex non-biodegradable organic substance i.e. (Polythene) which
was of no use to the environment rather it was polluting the environment, into a biodegradable
150 and clean organic product (ethanol) which is a great demand of the future as a clean fuel. This
will greatly reduce stress on common non renewable energy sources like fossil fuels.
So in this way we conclude that energy is getting converted from non-usable form to usable
form. Thus is "equivalent to the production of energy"
Hence summing up the above process is a novel combination of two reactions and is a viable
155 solution to the complex problems related to plastic waste.
DETAILED DESCRIPTION OF DRAWINGS
In one chamber under high pressure and high pressure conditions (mentioned in section 3.1.1 and
figure 2) thermal cracking of polythene takes place. Steam is produced in another chamber.
160 Both the reactants then go to the reactor which is a layered mesh coated with H3PO4 catalyst over
solid Si02. The reaction conditions are mentioned in section 3.2.1.
The products are then passed on to a scrubber where water is sprayed over them. Lower alcohols
being soluble are separated out and unreacted ethene stays undissolved in the scrubber which is
then cycled back to the reaction chamber. A representative diagram showing this cyclic process
165 is figure 3.
The final products are sent to the fractional distillation chamber where various alcohols and
alkanes are separated on the basis of difference in boiling points.
f
We Claim:
1. A process combining two reactions viz. thermal or catalytic cracking of waste
polythene and hydration of the subsequent products into an efficient and clean fuel
which has ethanol as its major component. Other lower alcohols are present in small
amounts in this fuel. The byproducts which are lower alkanes are also potential fbels.
2. As claimed in claim 1 the sequential conjugation of two processes will convert waste
polythene into a clean alcoholic fuel primarily containing ethanol. Earlier attempts of
converting polythene waste to fuel were limited to the conversion of polythene into
various hydrocarbons which have very low calorific values as compared to ethanol.
3. The cracking of polythene and subsequent hydration in a multi-cycle process as
claimed in claim 1 and 2 to get a high yield of ethanol is a unique combination of two
well known processes to solve a complicated environmental problem.
4. Thermodynamic and practical feasibility of the conversion of waste polythene to
ethanol as claimed in above claims had not been identified earlier.
5. The process as claimed in claim 1 has the double faceted benefit of management of
polythene waste and production of a clean and efficient fuel.