книги / Переработка нефти и газа

When mixing oil with solvent at constant temperature, oil is completely solved in solvent. Under further dilution, the disperse system is formed. Such disperse system has two phases: dispersing medium (solvent with dissolved components) and dispersed phase (undissolved components with solvent). Under high solution ratio, oil is completely dissolved.

If solvent ratio is not changed and temperature increased, content of dissolved oil components becomes higher, and, reaching definite temperature, termed Critical Solution Temperature, and higher, oil is completely mixed with solvent, thus, forming homogenous system. Oil solubility curve depends on oil quality

and solvent type.

Extraction takes place if the disperse

Fig. 3. Critical Solution Temperature

system is formed. It can be provided by

Curve for “Paraffin Oil-Furfurol” system selecting dilution temperature and degree (fig. 3).

Theoretical Background of Extraction

The extraction regularities are interpreted from the point of molecular solution theory. According to it, the phase state of chemical substances depends on two opposite factors: intermolecular interaction and thermal motion of molecules.

Intermolecular interaction force nature:

Orientation interaction. Solvent and oil molecules have polarity. i.e. dipolar moment. Heterorganic compounds of oil are subjected to orientation interaction in polar solvent medium.

Induction interaction. Solvents characterized by high dipolar moment are capable of inducing the dipolar moment in non-polar structure. Polycyclic hydrocarbons are subjected to polarization. Induction forces depend on non-polar molecule capability of being polarized.

Dispersed interaction. Under electron motion, distribution of charges can become non-symmetric. Molecule obtains the instantaneous dipolar moment which creates electric field around the molecule. Such electric field induces the dipolar moments in

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the neighboring non-polar molecules. The major part of attractive power of non-polar and polar molecules accounts for dispersed interaction.

Hydrogen bond. Hydrogen atom in compounds with oxygen, nitrogen, fluorine and chlorine is capable of bonding not one but two atoms of such elements. Hydrogen bonds are formed at low temperature, because hydrogen bond is broken at high temperature.

Solvent Characteristic

Different intermolecular interaction forces act for dissolving oil cut components in polar and non-polar solvents. Non-polar solvents include low-molecular liquid or liquefied hydrocarbons, carbon tetrachloride or compounds low dipolar moment (chloroform and ethyl hydroxide). Dissolution occurs due to dispersion forces.

For lube oil production, polar solvents are widely used. Oil cuts are dissolved in such solvents due to polarization of non-polar molecules under action of solvent molecule electric field. Solubility in solvents depends on solvent ratio and temperature. Solubility of all components in solvents becomes lower as temperature declines. Solubility of oil cut components in solvents depends on solvent structure, which can be characterized by two properties: dissolving capacity and selectivity.

Dissolving capacity means the absolute solubility of oil cut components in definite amount of solvent.

Selectivity means solvent capability of dissolving substances of definite structure. It makes it possible to separate one component from another, i.e. to obtain components which differ in properties.

There is a number of techniques which make it possible to compare solvents by dissolving capacity:

1.yield of dissolved component under similar solvent ratio;

2.Critical Solution Temperature under similar solvent ratio; and

3.amount of solvent required for extracting one and the same component of oil cut.

Selectivity of solvent can be determined by difference between magnitudes such as : density, viscosity index or refractive index of extract and raffinate. Sharing ratio K can be used as criteria for determining solvent selectivity,

К = С ex/ С raf,

where С ex is volume concentration of components in extract; С raf is volume concentration of components in raffinate.

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Propane Deasphaltizing

Purpose – to remove resinous-asphaltenic substances and polycyclic aromatic hydrocarbons, which cause coke ability increase and viscosity index decrease, from oil residues.

Feedstock – oil tar, vacuum column AVT (atmospheric-vacuum pipe still) residue. End product – asphalt-free oil (residual oil component).

By-product – asphaltum which is used for bitumen production, or as boiler fuel oil component.

Solvent – 95–96 % propane. Solvent may contain ethane – up to 2 %, and butane – not more than 4 %. If ethane content exceeds 2 %, solvent selectivity becomes higher, but, at the same time, it is necessary to increase pressure in extracting column and solvent recovery system. In butane content exceeds 4 %, dissolving capacity becomes higher but asphalt-free oil quality becomes lower.

Relationship between Process Variables and Deasphaltizing Efficiency

Deasphaltizing process is impacted by: oil tar quality, temperature, pressure and solvent ratio.

Oil tar quality. If vacuum column fractionation is incomplete, oil tar contains a great amount of cuts boiled out at temperature lower than 500 ºС. Solubility of lowmolecular hydrocarbons of residues in propane is higher than that of high-molecular hydrocarbons. They act as intermediate solvent by enhancing solvent ability in relation to undesirable components. Selection of fractional content of oil tar depends on chemical composition of residual cuts. If resinous-asphaltenic substance content in oil tar is high, low-boiling cuts can be retained until defined limit. For low-resin oil refining, it is advisable that tar concentration is high.

Extraction temperature. At 50–70 ºС propane has high dissolving capability and low selectivity, and is, mainly, asphaltene precipitating agent. At high extraction temperature (85 ºС and higher) propane dissolving capability is low and selectivity is high. Differential temperature is maintained along the entire height of the extracting column, which is termed Extraction Temperature Gradient. High temperature at the top of the extracting column, and low temperature at the bottom of the extracting

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column. Top temperature characterizes asphalt-free oil quality, and bottom temperature characterizes the required End product yield.

Pressure. Deasphaltizing process is a liquid phase one. To prevent solvent evaporating, the extracting column pressure shall be maintained within the range from 3.6 to 4.2 MPa. Propane density is increased, as pressure becomes higher, and it causes asphalt-free oil yield scaling up but its quality lowering.

Propane – oil tar ratio. The higher oil tar coke-forming compound content, the lower solvent ratio is required for producing asphalt-free oil of required quality (coking ability is about 1 %). The optimal ratio for resin oil tars is 4.5–5.5 : 1, and 7:1 for low-resin oil tars.

At commercial plants, extraction is conducted in countercurrent columns, 18–22 m height, with louver or perforated trays with packing (fig. 4).

Propane is supplied to the bottom of the column, and tar, heated to 120–150 ºС, to the upper part. Process pressure is 3.7 MPa. For complete removal of hydrocarbons from tar, the column bottom temperature is maintained at 50–65 ºС. The column top temperature is 75–85 ºС. Pressure difference along the height is created by transferring heat to asphalt-free oil solution in the upper heater.

There are single-stage and two-stage commercial deasphaltizing units. Under two-stage process, first stage column asphalt solution is supplied to the second stage column. Propane is supplied to the second stage column.

The second column temperature and pressure are lower than those of the first stage column. The second stage asphalt-free oils are characterized by higher viscosity, higher aromatic hydrocarbon content and lower viscosity index than the first stage asphalt-free oils.

Fig. 4. Deasphaltizing Column with three tar and propane inlets:

1 – pressure safety valve nozzle; 2 – louver tray; 3 – heater; 4 – tar inlet; 5 – propane inlet.

Process lines: I – tar supply; II – propane supply; III – asphalt-free oil removal; IV – asphalt removal

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Solvent Refining of Oil Distillates and Asphalt-Free Oils

Purpose – to remove resinous substances and polycyclic hydrocarbons from oil cuts to enhance viscosity index and oxidation resistance.

Feedstock– vacuum distillates and asphalt-free oils. End product – raffinate (distillate and residual).

By-products – extracts used for producing bitumen, black carbon, coke, reclaiming oil as for rubber and tire industry.

Relationship between Process Variables and Solvent Refining Efficiency

Solvent refining efficiency depends on: feedstock quality (chemical anкачество сырья (химиче fractional composition), nature and amount of solvent, temperature and extracting column performance.

Feedstock quality. Oil quality and process efficiency greatly depend on fractional content of feedstock. From feedstock with wide fractional content it is also possible to extract valuable low-boiling hydrocarbons with relevant solubility. The smaller range of boiling point, the more efficient refining. In asphalt-free oil refining, the great role plays the depth (level) of deasphaltizing which is expressed with coke ability.

Solvent structure. Phenol (C6 H5 OH), furfurol (C5H4O2), and N-methylpyrrolidone (C4H9CON) are used for solvent refining.

Phenol (hydroxybenzene or carbolic acid) with boiling point 181оС and melting point 41оС is characterized by high dissolving capacity if compared to furfurol and low selectivity, so the extraction is conducted at lower temperature and solvent ratio. Together with polycyclic hydrocarbons, sulfur-, nitrogenand oxygen-containing compounds are removed. The bottom temperature under phenol refining is restricted to phenol melting point.

Furfurol is a heterocyclic furaldehyde with boiling point 161.7 оС and melting point minus 38.7оС. It is characterized by high selectivity and low dissolving capacity. Furfurol refining provides clear extracting of low-index components and high raffinate yield. However, furfurol refining requires high solvent consumption. Advantage of furfurol is low melting point, and it makes it possible to use a wide range of operating temperature in the extracting column. Disadvantage of furfurol is high oxidation capacity, and that is why the process has special stage – feedstock de-

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aeration stage. At such stage, air and moisture are removed by superheated steam under vacuum.

N– methylpyrrolidone – boiling point is 204оС, melting point is minus 24 оС. It is characterized by higher dissolving capacity, if compared to furfurol, but lower than phenol. It differs from phenol by higher selectivity, nontoxicity and lower melting point. Under N– methylpyrrolidone refining, raffinate yield is 5–7 % phenol refining, and solvent consumption is 50 % lower. N – methylpyrrolidone is not capable of forming azeotropic mixture with air, and it reduces energy consumption for solvent recovery on 25–30 %.

Solvent ratio. The higher the content of low-index components, the higher solvent ratio required. The more stringent raffinate quality requirements, the higher solvent ratio required. Solvent refining of feedstock with wide fractional content requires high solvent ratio. It was established that phenol consumption for distillates should be 1.5–2: 1, and 3–4: 1 for residues. If viscosity index requirements are more stringent, it is necessary to refine raffinate once more and introduce antioxidant additives.

Extraction temperature depends on Critical Solution Temperature and required raffinate quality. The higher feedstock boiling point, the higher the Critical Solution Temperature, and higher temperature is required for refining. High content of resinous-asphaltenic substances and polycyclic aromatic hydrocarbons makes the Critical Solution Temperature lower and requires lower extraction temperature. Refining temperature shall be 10–15 ºС lower than the Critical Solution Temperature. For phenol refining, distillate temperature should be 55–70 ºС, and – 75–95 ºС for residues. For furfurol refining, 60–90 ºС and 95–115 ºС, respectively. Higher temperature causes raffinate quality and yield lowering. Raffinate quality and yield are controlled by selecting the extraction temperature gradient. At phenol refining units it is 10–20 ºС, and –30–40 ºС at furfurol refining units.

Cooled extraction solution. One of the effective ways to provide sharpness of fractionation is cooled extraction solution loading to the extracting column. For phenol refining, it is provided by adding phenolic water to the settling zone or extracting phase in amount of up to 7 %, and for furfurol refining, cooled extraction solution is loaded in the bottom part of the column in amount of from 30 up to 70 % of initial feedstock.

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Solvent Refining of Oil. Basic Process Flow Diagram

Solvent refining includes extraction in continuous operation process vessels to form two phases: raffinate and extract solvent, solvent recovery from extract solution and solvent recovery from aqueous solutions.

Extraction section. There packed extracting towers and tray extracting columns (fig. 5).

Fig. 5. Packed Column:		Fig. 6. Rotor-Disk Contactor:
1	– solvent and feedstock manifold;	1 – shaft with fixed discs;
2	– portioning plate;	2 – circular weirs;
3	– packing tray;	3 – sieve deck tray;
Process lines :I – raw;		Process lines: I – raw;
II – interim raffinate inlet;		II – furfurol;
III – solvent inlet;		III – raffinate solution;
IV – raffinate solution outlet;		IV – extract solution
V – extract solution outlet

Rotor-disk contactors are usually used for furfurol refining of light oil (fig. 6). Specific surface and rotor-disk performance depend on the agitators mounted on the vertical shaft.

It is not used for phenol refining. Viscosity of phenol solutions is high, and it causes phase emulsifying and raffinate solution solvent content increasing.

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Solvent Recovery from Raffinate and Extract Solutions

The major portion of solvent is recovered in flash towers. Solvent residues are stripped with steam in stripper (fig. 7).

Fig 7. Solvent Refining Unit

1 – extracting column; 2, 5 – raffinate and extract solution pipe still;

3, 4 – raffinate solution solvent stripper; 6, 7 – extract solution solvent stripper; 8 – water solution solvent stripper; 9 – solvent tank.

Process lines: I – feedstock; II – raffinate; III – extract; IV – dry solvent; V – water and solvent mixture; VI – water; VII – steam

Lecture 3

OIL DEWAXING

Purpose – to produce oils with low pour point. Feedstock – solvent refined raffinates.

End products – distillate and residual dewaxed raffinates.

By-products – slack wax and petrolatum which can be used after sweating for paraffin and ceresin production.

Theoretical Background

Under temperature decrease, first of all, crystals of the most high-melting hydrocarbons are formed. Hydrocarbons with lower melting point and number of atoms in molecule are in secession crystallized on the crystal lattice. Crystal structure is dictated by resin and asphaltene under dendritical and aggregate crystallization.

Factors Affecting Dewaxing

Dewaxing is affected by feedstock quality, solvent structure and composition, solvent-feedstock ratio, solvent loading method and cooling rate.

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Feedstock quality. The higher boiling point, the less solid hydrocarbon recovery rate, the high pour point. Resins remained in oil after solvent refining, significantly affect the solid hydrocarbon structure, and form irregular crystals with it, thus, complicating solid phase removal from liquid phase.

Solvent structure and composition. Solvents are used for making feedstock viscosity lower to prevent fine crystal forming. Both polar and non-polar solvents can be used for dewaxing. Propane is a non-polar solvent. Dewaxing is characterized by Dewaxing Temperature Gradient which means a difference between final feedstock cooling temperature and pour point of produced dewaxed oil. Disadvantage of propane is high Dewaxing Temperature Gradient ranged 15–25 оС. Moreover, oil content in solid phase is increased, and it is necessary to increase process vessel pressure. Polar solvents (ketone) are most often used in solvent refining. Solid hydrocarbons can be dissolved in ketone only at high temperature. At low temperature solid hydrocarbons are precipitated from solution, i.e. ketone is paraffin precipitating agent. In addition, liquid component solubility in polar solvent is low. To enhance dissolving capacity, toluene is added to ketone. Dewaxing Temperature Gradient under applying methyl-ethyl ketone mixture with toluene ranges from 0 up to 5 ºС. Thus, ketone is precipitating agent for solid hydrocarbons, and toluene is solvent for the rest oil components. Sometimes, mixture of chlororganic compounds, for instance dichloroethane and dichloromethane mixture, is used for dewaxing. Dewaxing Temperature Gradient for such solvents is low (0–1 ºС). Disadvantage is high corrosiveness, toxicity and low thermal stability.

Solvent – feedstock ratio. It depends on feedstock composition and properties. The higher boiling point of cut, the higher solvent – feedstock ratio required (due to high viscosity of feedstock). For distillate raffinate, solvent – feedstock ratio is 2–3: 1, and 3.5–4.5: 1 for other raffinates.

Solvent loading. Crystallization is a phased process, so it is necessary to create optimal conditions for each stage. For this purpose, solvent batching and temperature decrease are applied.

Cooling rate. Crystal size depends on cooling rate. Cooling rate for some non-polar solvents is 4–5 ºС per hour, and for polar solvents it is increased to 100–200 ºС per hour, because solid hydrocarbon solubility in non-polar solvents is rather higher.

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Fig. 8 Solvent Refining Unit. Basic Process Flow Diagram: Process lines: I – feedstock; II – solvent; III – feedstock solution;

IV– solid hydrocarbon slurry; V – dewaxed oil solution; VI – slack wax or petrolatum solution; VII – dewaxed oil;VIII – solid hydrocarbons (slack wax or petrolatum)

Feedstock to be dewaxed and solvent (fig. 8) are mixed in agitator 1. Mixture is thermally treated in steam heater 2 at 60ºС. Feedstock is cooled in water cooler 3, and, then, in regenerative crystallizers 4, which use filtrate as refrigerant, and in ammonia (propane) crystallizes to final temperature (–20–25 ºС). If it is necessary to produce dewaxed oil with pour point lower than –30 ºС, ethane should be used as refrigerant at the final stage. Cooled solid hydrocarbon slurry comes to filters 6 for filtration. Filter cake is washed with cooled solvent and pumped to screw, and some solvent is fed to it. After filtration, dewaxed oil solution, which contain 75–80 % of solvent, and solid hydrocarbon solution (slack wax or petrolatum) with low oil content. Both solutions are directed to the solvent recovery sections 7 and 8.

Recovered solution is recycled to agitator, filter and screw. The unit is equipped with inert gas system. It is designed for:

1.preventing forming explosive mixtures in vacuum filter drum and solvent tanks;

2.reducing volatile solvent loss;

3.preventing ketone oxidizing;

4.preventing icing in equipment.

Urea Dewaxing

Purpose – to reduce pour point of oil fuels and low-viscosity oils by forming ureaparaffin hydrocarbon complex of normal structure.

Feedstock – diesel fuel and light oil components.

End products – winter diesel fuel and low-viscosity oils.

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