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 oil stream after furfural/NMP treatment is pumped from storage to the Solvent Dewaxing Unit (SDU) as two parallel streams at about 75 C for IN to 85 C for BS stocks. The ̊ ̊primary dilution solvent at 35 C is added to both the streams by solvent pumps. Two feed ̊mix streams are then heated in Feed Mix/steam heater to a temperature well above the slack wax congealing point in order to ensure that all the wax is in solution and no nucleus exists to avoid abnormal crystals formation subsequently. Feed mix streams are then cooled to a temperature just above the cloud point to ensure that crystallization will start only in DP exchangers and then the streams are joined. The feed is divided into eight parallel trains. Each train passes through 2 double Pipe (DP) scraped surface exchangers where it is cooled by the cold filtrate. Then it passes through three DP chillers in series where liquid ammonia is used as a coolant in the outer pipe. The chilling rate through 1st DP chiller is controlled by adjusting the back pressure controller on the refrigerant (ammonia) being vaporised in the outer pipe. The temperature at the outlet of 2nd and 3rd DP chiller is controlled by regulating flow of ammonia vaporising. Ammonia vapours from chillers go to the respective knock-out drums located above each chiller. Any liquid ammonia present separates out and is sent back to the chiller. The vapours are routed to the ammonia compressor where ammonia liquid is produced again.

Lube Oil Block

The secondary dilution solvent, after being chilled by cold filtrate and then liquid ammonia, is added to each train at the outlet of 1st or 2nd DP chiller. All the feed mix trains join after further chilling in 3rd DP chiller and are routed to the Filter Feed Drum which ensures a constant feed to the filter. The tertiary dilution solvent (which is cooler than the secondary dilution solvent) is added to the combined chilled feed mix, before it goes to Filter Feed Drum, as required for different feed stocks. The chilled feed mix, in the form of slurry of wax crystals in oil and solvent, flows by gravity from the Filter Feed Drum which is blanketed with inert gas, and is distributed to eleven parallel filtration trains. The filter feed flows to the Vat section of each of the eleven rotary vacuums filters which are also under inert gas blanketing.The filter drum carrying filter media is submerged and rotates in filter vat filled with the chilled slurry of dewaxed oil solvent mixture containing suspended wax crystals. Vacuum produced by the inert gas vacuum compressors draws the filtrate from the cake pick-up zone of filter drum and deposits the wax crystals on the filter cloth. As the wax cake formed on the filter cloth emerges from the liquid slurry, it is washed continuously by the cold solvent at the filtering temperature. The cold wash solvent displaces the oil-solvent solution which was originally present in the cake. After the cold wash zone, inert gas is drawn through the filter cloth in order to dry the cake. The wax cake is then dislodged by blowing the chilled inert gas from inside the filter cloth. A doctor blade gently removes the cake over to a rotating scroll conveyor, which conveys it to the filter boot. A steam coil in the filter boot heats up the wax mix so that it can be pumped in a liquid condition. It is sent to the slack wax Solvent Recovery Section. The DWO mix filtrate and the inert gas after passing through filter cloth are separated in Filtrate Receiver. The inert gas from filtrate receiver flows to the inert gas knock out drum to eliminate any entertainment before it enters the inert gas vacuum compressor.

The DWO mix stream gains some heat by heating the secondary dilution solvent first, then in the DP exchangers (with feed) and finally with the dilution solvent again after which it goes to DWO Solvent Recovery Section. Solvent Heating SystemThe Solvent, after exchanging heat with filtrate in heat exchangers, is chilled in Solvent/Ammonia coolers. A part of this cold solvent is diverted as secondary dilution solvent. Remaining solvent is further chilled to filtration temperature as per requirement. A part of this is routed as tertiary solvent. The remaining solvent, after passing through cold solvent Filters, in order to prevent choking of spray nozzles, goes for cold washing to eleven rotary drum vacuum filters.

Solvent RecoveryThe 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

Lube Oil Block

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.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 C. 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 C. 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.

The products slack wax and dewaxed oil are then sent to the Wax Hydro Finishing Unit and Lube Hydro Finishing Unit for further processing.

Solvent Drying Section:

The solvent recovered from the flash column and strippers overhead from the DWO, slack wax and foots oil solvent recovery sections are sent for solvent drying in order to reduce the water content of the circulating solvent to about 0.2 wt% and also to ensure a constant solvent composition. The DWO stripper, slack wax stripper and foots oil stripper overhead vapour having substantial water vapour is first cooled by cooling water. Solvent drying column and Azeotrope column overhead vapours are cooled in34E15 A/B/C and then joins cooled solvent/water vapours Ex-34E17b andenters 34E15d from which the solvent/water is routed to solvent/waterseparater (34B11) having inert gas blanketing.

Lube Oil Block

The water-rich phase and the solvent-rich phase seperation take place in34-B-11 having a partition baffle. The solvent rich phase is withdrawn bypumps 34P20 A/B and level control (of 34B11 solvent-rich phase) and is fedto solvent drying column (34C10) on top tray as reflux. The drying column isoperated at @ 0.4 Kg/cm2g.The heat required for solvent drying is supplied by overhead vapours DWO,slack wax and foots oil recovery section flash columns enterig 34C-10 atdifferent points.For operational flexbility a thermosyphon type reboiler (34E30) using LPsteam as heating medium is also provided as additional heat source forsolvent drying.The dry solvent is withdrawn from 34C10 bottom with pumps 34-P 21 A/Bunder level control (of 34-C10). It is cooled to 30-400C finally in heat exchanger34E17E (where it exchanges heat with DWO mix feed to 34C1) and then in coolers 34E17 A/C and is then sent to solvent receivers 34B8and 34B9. Both these receivers are connected to provide adequate hold-upvolume.The dry solvent from solvent receivers is circulated by pumps 34 P4/5/5Rfor various dilutions, cake washing, reflux to flash columns, etc. A provisionis also kept to rout the hot solvent from 34P21 A/B for use at hot wash forthe filters after passing through hot solvent filters 34B18 A/B.The water-rich phase in 34B11 is withdrawn by pumps 34-P-111/111R underlevel control (of water-rich phase of 34B11) and is fed o azeoprope column34C8 on top tray. The stripping steam (VB) enters below the bottom tray.The overhead vapours from 34C8 go to 34E15 A/B/C/D. The solvent freewater leave the column from bottom under level control (of 34C8).The fresh solvent make-up is continuelly added to 34B11 at controlled rateby pump 34P22 which is a metering pump with stroke adjustment facilityand take suction from solvent receiver 34B10. The solvent is pumped fromoffsite to 34B10 intermittently. A provision is also kept for continuous makeup to 34B11 from the offsite under flow control, if required,or fresh Solventmake-up can be taken directly in B9/B8.