A process for producing partly renewable light unsaturated hydrocarbons, characterized by comprising: (a) contacting ethanol with an acid catalyst in at least one reactor under conditions suitable to form a reaction effluent stream comprising water, ethene, propene and unsaturated hydrocarbons with four carbon atoms, besides other co-products of secondary reactions; (b) feeding the stream generated in (a) to a direct-contact cooling unit under conditions suitable to form a top stream containing most of the light unsaturated hydrocarbons present in the effluent stream of step (a) and a bottom stream containing most of the water present in the effluent stream of step (a); (c) compressing the top stream rich in light unsaturated hydrocarbons generated in (b); (d) contacting naphtha with steam in a pyrolysis furnace of an existing cracking unit under conditions suitable to form a cracking effluent stream comprising light unsaturated hydrocarbons, fuel oil, gasoil and gasoline; (e) separating the effluent generated in the cracking reaction (d) into a light hydrocarbon stream containing most of the light unsaturated hydrocarbons present in the effluent of step (d) and at least one heavy hydrocarbon stream; (f) feeding the stream rich in light unsaturated hydrocarbons from step (e) to a direct-contact cooling unit under conditions suitable to form a top stream containing most of the light unsaturated hydrocarbons present in the effluent of reaction (e) and a bottom stream containing the condensed compounds and most of the water present in the effluent of reaction (d); (g) compressing the top stream rich in unsaturated hydrocarbons generated in (f); (h) combining the streams (c) and (g) and conducting them to the steps of separating and purifying unsaturated hydrocarbons C2, C3 and C4 of the existing cracking unit, wherein at least one pyrolysis furnace of said existing cracking unit for producing light unsaturated hydrocarbons from hydrocarbons is gradually replaced by at least one reactor for conversion of ethanol to light unsaturated hydrocarbons. (a) contacting ethanol with an acid catalyst in at least one reactor under conditions suitable to form a reaction effluent stream comprising water, ethene, propene and unsaturated hydrocarbons with four carbon atoms, besides other co-products of secondary reactions; (b) feeding the stream generated in (a) to a direct-contact cooling unit under conditions suitable to form a top stream containing most of the light unsaturated hydrocarbons present in the effluent stream of step (a) and a bottom stream containing most of the water present in the effluent stream of step (a); (c) compressing the top stream rich in light unsaturated hydrocarbons generated in (b); (d) contacting naphtha with steam in a pyrolysis furnace of an existing cracking unit under conditions suitable to form a cracking effluent stream comprising light unsaturated hydrocarbons, fuel oil, gasoil and gasoline; (e) separating the effluent generated in the cracking reaction (d) into a light hydrocarbon stream containing most of the light unsaturated hydrocarbons present in the effluent of step (d) and at least one heavy hydrocarbon stream; (f) feeding the stream rich in light unsaturated hydrocarbons from step (e) to a direct-contact cooling unit under conditions suitable to form a top stream containing most of the light unsaturated hydrocarbons present in the effluent of reaction (e) and a bottom stream containing the condensed compounds and most of the water present in the effluent of reaction (d); (g) compressing the top stream rich in unsaturated hydrocarbons generated in (f); (h) combining the streams (c) and (g) and conducting them to the steps of separating and purifying unsaturated hydrocarbons C2, C3 and C4 of the existing cracking unit, wherein at least one pyrolysis furnace of said existing cracking unit for producing light unsaturated hydrocarbons from hydrocarbons is gradually replaced by at least one reactor for conversion of ethanol to light