Lube Oil Block

Introduction

Solvent Dewaxing Unit

After the furfural treatment of oil streams, the streams enter the Solvent Dewaxing Unit. The purpose of this unit is to lower the pour point of the oil streams by removing the paraffinic components which form waxy solids at low temperatures. Thus this unit makes oil suitable to be used at low temperatures. The major processes involved are extraction, crystallisation and filtration.

THEORY:

First we add a solvent which has affinity for the non-paraffinic substances. This will extract and dissolve the non paraffinic part of the oil stream. This solvent is called the Oil Solvent. In this case, Toluene is the oil solvent. But along with non-paraffinic components some of the waxy components may also dissolve. To reduce this dissolving of waxy components, we add another chemical called Wax-Rejecter. Methyl Ethyl Ketone (MEK) acts as the Wax-Rejecter over here. After addition of these solvents there are 2 phases, an oil-in-solvent phase & a wax-rich phase. But to achieve this condition, temperature has to be maintained higher than a minimum temperature called Oil Miscibility temperature. Below this temperature, the solvent starts losing its dissolving power and the oil starts separating out producing a 3rd phase of original composition (wax & oil). An increase in concentration of MEK causes a rise in Oil Miscibility Temperature and therefore it is added in limited quantities. Then the solution is crystallised by chilling it to suitable temperature by different effluents and then by Ammonia-chilling. Cooling of the streams causes the waxy hydrocarbons to solidify and form wax crystals. These are then filtered in the Vacuum Rotary Drum Filter. The filter separates the wax crystals and oil solution. These streams are then sent to their respective solvent recovery sections. The solvents are recycled back.

PROCESS:

The solvent present in the dewaxed oil mix filtrate is recovered in the solvent recovery section. The DWO mix is heated and vaporised by the overhead vapours from the DWO First Flash column, Operating at 0.6 kg/cm2 pressure and DWO second Flash Column operating at 2.06 kg/cm2 pressure, condensing in shell sides of Filtrate Solvent Exchangers. The two phase DWO mix feed enters the DWO First Flash Column having 4 valve trays. The overhead vapours are used for feed preheating. The bottom liquid is fed by DWO First Flash Column Bottom pump to DWO second Flash column having 4 valve trays again. The feed to this tower is heated and vaporised by medium pressure steam in Filtrate/Steam preheater. The overhead vapours go for first flash feed preheating. The bottoms are fed directly to DWO Third Flash Column which also has 4 valve trays and operates at 0.4 kg/cm2. A provision is kept to heat this column feed further using steam. The overhead solvent vapours go for solvent drying whereas bottom liquid flows by gravity as feed to 16th tray of DWO stripper having 19 valve trays. In DWO Stripper, superheated low pressure steam MP Stream is introduced at the bottom to remove the remaining solvent from the dewaxed oil. The solvent and water vapours from overhead go to settler drum. The product dewaxed oil from column bottom is pumped by DWO Product Pump for product cooling and routing to storage.

Lube Oil Block

The solvent from the settler is pumped into the Solvent Drying Column. In this column it is dried with other vapour streams from the flash towers. The dry solvent from pump is added as reflux to DWO second flash.

The slack wax mix from the filtration section is sent to slack wax recovery section in order to recover Solvent from Wax mix. It is heated in wax mix preheater by MP steam at about 64 0C. The wax mix then goes under back pressure control to settling tank which is blanked with inert gas. The slack wax mix is then pumped by wax mix charge pumps. Before being fed to wax mix first flash column, the wax mix discharge first exchanges heat with vapours, then it passes through 2 heat exchangers where it exchanges heat with hot slack wax product bottom and hot vapours and enters flash column at a temperature of 103-105 0C. Hot vapours from the flash tower top go to the solvent drying column where other streams are dried too.Liquid from bottom is pumped and further heated in the heat exchangers (where it exchanges heat with DWO product stream going to storage and by MP steam under temperature control) and enters another column. Overhead vapours from this go to solvent drying column after exchanging heat or directly. Operating pressure of this column is 1.5 kg/cm2. Liquid from bottom of this column is fed to 3rd column at a temperature of about 165-1700 C, after getting heated with MP steam. Overhead vapours from here go to solvent drying column. Bottom Liquid is fed by gravity to slack wax stripper. In the stripper, superheated steam is introduced below the bottom tray in order to remove the remaining solvent from the slack wax. The solvent and water vapour leaving the top (along with solvent and water vapour from DWO stripper) goes to heat exchanger to preheat 1st flash column feed. From the bottom of the stripper, the product slack wax is sent to storage after preheating the column feed.