FORM 2 THE PATENTS ACT, 1970 (39 of 1970) COMPLETE SPECIFICATION (See section 10) "PRODUCTION OF POLYMER / FOOD GRADE o» SOLVENTS FROM PARAFFIN RICH LOW VALUE STREAMS EMPLOYING HYDROPROCESSING' INDIAN OIL CORPORATION LIMITED, a Public Limited Company, having its Head Office at G-9, All Yavar Jung Marg,-Bandra (East), Mumbai 400 051, India, and ENGINEERS INDIA LIMITED, a Government of India Undertaking, having its registered office at EIL Bhavan, 1, Bhikaji Cama Place, New Delhi 110 066, India. The following specification (particularly) describes the nature of the invention and the manner in which it is to be performed. FIELD OF THE INVENTION The present invention relates to the process for the preparation of Polymer/ Food grade hydrocarbon solvents of naphtha range essentially free from olefins less than 20 ppm of aromatics and less than 1 ppm of sulfur from paraffinic-rich low value streams such as raffinate from the solvent extraction units in crude oil refineries employed for recovery of aromatics from reformate by hydrogenation in the presence of a nickel based catalyst. BACKGROUND OF THE INVENTION Petroleum naphtha is a widely used solvent both in industry and laboratories. Different grades of naphtha solvents are available for various applications. These solvents are normally used as a) an ingredient in the finished product, b) solvents for extraction of vegetable oil (from oil seeds), minerals, pharmaceuticals, c) solvents for reactants e.g. in polymerization reactor and d) solvents for cleanup and maintenance operations. These solvents are manufactured from low boiling refinery streams like naphtha and find their applications in agriculture, food and pharmaceuticals, petrochemicals, printing, paint and coating, chemical industries etc. Though, there does not exist any universally accepted nomenclature for hydrocarbon solvents, most manufacturers group their solvents according to boiling point and composition. Distillation range is also one of the main criteria. Solvent grades with narrow and wide boiling ranges, also referred to as " Special Boiling Point Spirits " (SBPS) are generally found in the boiling range of 45-160° C. White spirit is the name given to solvents in the boiling range-of 150 - 220° C. Composition is another important parameter for characterizing the solvents. Depending upon the type of compounds present in the solvents, they are named such as aromatic solvents, Isoparaffinic solvents. For most of the polymers and resins, solvent power is found in the following order: Aromatics > Naphthenes > n-Paraffins > i-Paraffins Aromatics are known to have maximum solubilizing power. However, from toxicological and performance point of view, benzene-free and very low aromatic content solvents are required in applications e.g. Oil seed extraction, polyolefins manufacture, printing inks and adhesives. 2 Evaporation rate, appearance, color and odor are some of the other important characteristics of solvents. When the solvents consist mainly of a single hydrocarbon, they are named after the same e.g. Hexane, Isohexane, Heptane etc. In an extraction processes, the function of the solvent is to extract selectively some particular ingredient from a mixed product or raw material e.g. vegetable oils, pharmaceuticals, cleaning & degreasing etc. For food processing and pharmaceutical applications, criteria must be strictly qualified with respect to present of compounds having toxicological properties. In chemical process, the function of the solvent is to act as an inert reaction medium or catalyst carrier e.g. in polyolefins manufacture. In these and other process, apart from benzene and other aromatics content, strict specifications are maintained with respect to olefins, Sulfur also. The product form Hydrocracking unit are very low in aromatics. Therefore, fractionation of hydrocracker naphtha into suitable boiling ranges is often done to produce dearomatised solvents. But hydrocracker naphtha is a premium product and has other preferred uses e.g. feedstock for reforming, ethylene cracking etc. The 'S' content in Hydrocracker naphtha quite often exceeds 1 ppm due to recombination of H2S and olefins into mercaptans. This amount of sulfur is unacceptable in Food Grade and Polymer Grade solvents. Catalytic hydrogenation of Straight Run Naphtha streams is another option. However, the sulfur present in these streams require prior desulfurisation and the higher pressure process employing sulfided catalysts are uneconomical. Patents describe processes for hydrogenation of benzene in heterogeneous reactor either in liquid phase (US 4327234 : Hydrogenation process using supported nickel catalyst) or in gas phase (US 577186 : Process for hydrogenating benzene in hydrocarbon oils) as well as in homogeneous reactor (US 5668293 : Catalyst and a benzene hydrogenation process using said catalyst). Patents also describe proceses for benzene hydrogenation through catalytic distillation (US 6084141 : Hydrogenation process comprising a catalytic distillation zone comprising a reaction zone with distribution of hydrogen, US 6048450 : Process for the selective reduction to the content of benzene and light unsaturated compounds in a hydrocarbon cut). However, all these processes are meant for meeting benzene specifications in MS where benzene is required to be reduced to 1 vol % only to meet the most stringent Euro-IV standard. In addition to hydrogenation of benzene in fuels, patents describe processes for production of cyclohexane also through hydrogenation of Benzene (US 5589600 : Preparation of cyclohexene by partial hydrogenation of benzene). 3 Solvent extraction is another method used for the production of dearomatised solvents. A patent (IN 168536) describes such a process of Separation of benzene and C5-6 non-aromatic hydrocarbons from naphtha fractions by countercurrent extraction for recovery of food-grade n- hexane. However, for very low aromatics concentration in solvents, solvent extraction would be prohibitively expensive. Moreover, solvent extracted paraffins have relatively lower solvent power in comparison to hydrogenated solvents, as the naphthenes from saturation partly compensate for the solvent power of the aromatics. Moreover, solvent extracted solvents may not be able to meet the specifications for Bromine member (olefins). Adsorptive dearomatisation using silica gel, alumina and activated carbon has not so far been attractive for large scale commercial production due to requirements for continuous regeneration. Acidic clays also been used for production of solvents. But they only remove olefins, not aromatics. Two patents (IN 184574 & IN 179409) describe methods to produce Food Grade Hexane through adsorption and clay treatment. SUMMARY OF THE INVENTION The present invention generally relates to a catalytic hydrogenation process for the preparation of polymer / food grade hydrocarbon solvents in naphtha range essentially free from olefins and having aromatics less than 20 ppm and sulfur less than 1 ppm nil olefins from naphtha range petroleum stock and more preferably from raffinate obtained from BTX extraction unit, e.g. Udex, using a Nickel based catalyst. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a catalytic hydrogenation process for the preparation of polymer / food grade hydrocarbon solvents of naphtha range containing very low aromatics, especially benzene less than 20ppm from naphtha range petroleum stock. More particularly, the present invention relates to a catalytic hydrogenation process for the preparation of polymer / food grade hydrocarbon solvents of naphtha range essentially free of olefins, containing aromatics less than 10 ppm and containing sulfur less than 1 ppm from paraffinic rich solvents which are substantially free from Sulfur, chlorides. The present invention provides a process for preparing polymer/food grade hydrocarbon solvents of naphtha range containing very low aromatics, especially bezene less than 20ppm from naphtha range petroleum stock, said process comprising: 4 a. heating the naphtha range petroleum feed to a temperature in the range of 70° -180° C; b. adding stoichiometric amount of hydrogen to the naphtha range petroleum feed at a pressure between about 5 to 30 bar; c. passing the mixture of feed and hydrogen through a reactor having a nickel based catalyst, and d. removing any excess hydrogen to obtain polymer/food grade hydrocarbon solvents of naphtha range containing very low aromatics. In an embodiment of the present invention, the naphtha range petroleum feed is preferably raffinate from the solvent extraction units employed for recovery of aromatics from reformate. In another embodiment of the present invention, the raffinate feed has sulfur < 50ppm, preferably < 5ppm and most preferably