D. Tandon, G. S. Dang* & M. O. Garg

Transcription

1 Paper No 557 Visbreaking: A flexible process to reduce the pour point of heavy crude oils D Tandon, G S Dang* & M O Garg Indian Institute of Petroleum, Dehradun (India) dtandon12@rediffmailcom ABSTRACT With continuous depression in world oil reserves and increasing demand of petroleum products, the refiners are forced to process more and more heavy crude On the other hand heavy crude reserves are increasing day by day whereas petroleum industry and refinery are facing problems to drill and to transport the heavy crude from drilling site to refinery to provide a new source of feed for secondary conversion process and upgrading For upgrading the heavy feedstocks the visbreaking process emerges as a cheapest refining tool This process mainly reduces the viscosity of the feedstock resulting in lower diluents requirement to meet fuel oil viscosity In this process some reduction in pour point is also achieved Here the soaker mode visbreaking process has been applied to upgrade the indigenous available heavy crude oil having high wax contents available in the indigenous (Rajasthan) oil fields with the main objective of reducing the flow properties of the crude oil making it easy flowable in the pipe Line The crude oil was subjected to soaker mode visbreaking at different operating conditions keeping pressure constant The operating conditions were optimized at which the produced treated charge was having maximum reduction in pollr point limiting to the stable product It is found that with hollow soaker pour point was considerably reduced and it could further be lowered 3 C more using advanced soaker visbreaking, technology ie soaker with internals KEYWORDS: Crude oil, Visbreaking, cracking, soaker INTRODUCTION The refining industry has entered the new1 millennium and the challenges confronting refiners have become more complex The availability of light and sweet crude oils is becoming limited and in its place heavier crude oils with high impurities of sulfur, nitrogen, metals etc are being produced / supplied more and more in the world oil market Some known heavier crude oif(s) sources are Venezuela, Mexic02 and some middle east countries In India also a re~ativejy heavier crude oil has been found in Rajasthan fields This particular Crude o~ lias poor flowing ch~racteristics mainly wrt pour point Transportation of this crude oil or similar crude oils through pipe lines is a big problem There are different ways3 to improve flow characteristics of the heavier crude oils like blending with some lighter crude oils (if available at production site), addition of pour point depressant, transportation as emulsion with water etc Additionally use of an alternate approach was explor~d at lip and that is application of vis breaking process to lower the pour point/viscosity of the given heavier crude oil by thermal cracking the wax/heavier molecules present in it For this study, soaker mode visbreaking with hollow soaker as well as soaker with internals4 has been used The main aim of the study has been to establish the applicability of soaker visbreaking technology for improving the flow characteristics of heavier crude oil and further what additional benefits are obtained when the soaker drum is equipped with suitable internals EXPERIMENTAL Material In the present study the heavy crude oil has been procured indigenously (Rajasthan field) and is used as feedstock The feedstock characteristics are given in Table - 1 * Corresponding address: Indian IllStitute of Petroleum, P 0 lip MohkamplIr, Dehmdlln-! (II/{liilj

2 1TIJ Experimental set~up and procedure TABLE -1 Feedstock Characteristics SNo Characteristics Heavy crude oil 1 Specific gravity, 60/60DF Pour Point, DC +42/+45 3 Kin Viscosity at 100DC, cst CCR, %wt 48 5 Wax Content, % wt Penetration at 25DC at 25 C, 01 mm, g, 5 sec 7 Softening point, DC Metals, ppm Fe 180 Ni 380 V 10 Cu 038 lip Visbreaking pilot plant has been used for this study It consists of a feed section, six helical5 coils arranged in series followed by a soaker drum (reactor section), followed by flasher condenser and cooler The schematic diagram of the pilot plant is shown in figure 1 The heating media is salt bath furnace(s) heated electrically The 6thcoil outlet temperature was varied in the range of DC,and soaker inlet temperature was studied in the range of DCwhile the soaker outlet pressure was maintained at 12 kg/cm2 in all the experiments The feed rate was ranging between 7 to 11 Kg/hr so as to achieve a residence time of 17 to 30 minutes in the respective experiments (on cold oil velocity basis) in the soaker The set point conditions for four experimental runs are given in Table-2 FEED SECTION FIIRNArF ~FCTlnN PROD REC WASTE ~ METER l - LIGHT F D m8 ~~~n'" FIr - 1 VI<O:RRF AKINr PHoT PIA NT

3 TABLE - 2 Experimental conditions, Fractionation and Characterization Run No Hollow Soaker Soaker with internals Experimental Details Experimental conditions Coil outlet, Uc Soaker inlet, Uc Feed rate, Kg/hr (on output basis) Pressure, Kg/cm Water injection, %vol Fractionation, %wt Conversion; Gas + (IBP- 150uC) -- '\ S = Stable, US=Unstable A4 I 1777 I 21A5 l 2314 Cond R 150uC CharClcterizationof treated charge & R 150uC+ Pour Point, Uc - Treated charge R 150oC Kin Viscosity at 100uC,cSt - Treated cnarge R 150oC+" 638 5A A8 Stability (p-value) S US S US Viscosity reduction (150u+) 058 OA5 050 OAO At first the feedstock was homogenized in an electrically heated vessel maintained at 75-90oC The temperature of feed tank and metering tank was so maintained that it always remains below the initial boiling point of the crude oil so as to minimize the loss of vapours during its heating and circulating After attaining the required bath temperatures, the platform on which the salt bath furnaces are kept was raised hydraulically and the coils were immersed in molten salt baths Initially the preheated gas oil was fed to the furnace section using a metering pump, adjusted for a desired flow rate to attain the reaction temperatures Once the temperatures were attained the gas oil supply was switched off and actual feed was fed in the furnace section The water was injected (approx 1% vol of feed flow rate) in the coil-1 during each run in order to create turbulence and prevent coking in the coils The variations in the temperature, pressure and flow rates were monitored/controlled and the system was allowed to attain the steady state Once the steady state is reached, the sampling/product collection is started and the operating conditions on feedstock and water flow rates, coil/soaker temperatures, pressure and cumulative volume of generated gases were recorded at every ten minutes interval, during sampling time In all experiments, gas samples were collected and analyzed (Table-3) The visbroken charge from the tar vessel and the distillate from the lighter receiver were collected in pre-weighed containers over a time span of 30/40 minutes for each run

4 S No Run Number TABLE - 3 GAS ANALYSIS Gas sample from Hollow Soaker Soaker with internals Gas Constituents, % C C2' C C3' C ic C4' nc C4' Contd 10 cisc4' Cs Cs' ncs nce The injected water during experiment was separated from the lighters, collected during run, and mixed with the visbroken charge The prepared composite vis broken product then was subjected to dehydration using older-shaw/packed column About one liter dehydrated treated charge was withdrawn and its physico chemical characterization and stability test were done Further the atmospheric distillation was done upto 150 C using the same distillation unit and residue 150oC+, obtained after fractionation, was subjected to physico chemical characterization and stability test Experiments have also been performed using soaker with internals in the same manner as above The experimental conditions, fractionation and characterization details are given In Table-2 DATA AND DISCUSSION ~ From the data reported in Table 1-3 it can be interpreted that: The given feedstock/crude oil is heavier in nature having high wax content (252 %wt) The viscosity (11 cst) of crude oil at 100 C is relatively lower compared to other heavier crudes but the pour point (+42/+45 C) is higher due to its high wax content In the present study it has been found (Table-2) that as the temperature of the furnace and soaker drum increases the cracking increases and pour point of the treated charge decreases It is also observed that at maximum temperature (Run conditions of 02, Table-2) the pour point of the treated charge/crude oil could be reduced to +24oC but the treated charge becomes unstable as per the stability test As per experiment No 01

5 (Table-2), with lower temcrerature but with increased severity wrt residence time the pour point reduces to +27 C with the production of stable treated charge/crude oil The Viscosity reduction (ratio of the viscosity of the R150oC+ to the viscosity of feedstock, at 100 C) of R150oC+ at 440 C soaker inlet temperature is maximum (045) in case of hollow soaker and 040 in case of soaker with internals The stable treated charge was however obtained at the viscosity reduction of 058 (Run No 01) The treated charge/crude oil becomes unstable beyond 430 C soaker inlet temperature irrespective of residence time used in the study This is most likely due to disturbance of equilibrium between Ascrhaltenes & maltenes present in the feedstock/crude oil at temperature beyond 430 C The conversion ie Fraction 150 C- increases with the increase in soaker inlet temperature as per Table-2 and the maximum conversion is achieved about 21 %wt with the production of stable treated charge/crude oil having minimum achievable pour point as +24 C using soaker with internals The soaker internals are proving effective and are providing increased viscosity/pour reduction compared to hollow soaker, under identical conditions of soaker inlet temperatures and residence time CONCLUSION The use of visbreaking process for improving the flow characteristics of the given heavier crude oil is found effective Through use of hollow soaker and soaker with internals along with maintaining of stability of the treateq charge, the ~our point of feedstock/crude +42/+45OC to +27 C and +24 C respectively oil could be reduced from The viscosity of the treated charge/crude oil was reduced to half the value of feedstock viscosity Use of internals in soaker further helped to reduce it to one-fourth Pour point & viscosity reduction was found to be better when soaker with internals was used in the visbreaking process Any further decrease in pour point or viscosity of treated charge/crude oil can be achieved through blending of some suitable lighter crude oil, if available in the region or by addition of lighter stock or by use of a suitable pour point depressant REFERENCES 1 Edward Houde, Gregory Thompson & Roger Marzin, Pedro Pereira, Mike McGrath & Howard Feintuch "The Aqua Conversion process - A new approach to residue processing", NPRA March 15-17, 1998, San francisco, California 2 Weiss H, Schmalfeld J & Bhandarkar P G, The LR Coker - a novel process for vacuum residue conversion, 10th RTM, February 11-13, 1998 in Mumbai 3 Rhoe A, and Blignieres C de, "Visbreaking a flexible process Hydrocarbon Processing January' 1979, p Studies on the effect of soaker geometrics and internals on Visbreaking process lip Report no CPA: 69:97 December Visbreaking studies on Bombay High Short Residue, lip Technical Report CPD:1210:2004, June' 2004 ACKNOWLEDGEMENT Authors gratefully acknowledge the active participation of technical supporting staff of lip who worked for successful completion of this study