GB2370004A - Reducing solids deposition from supercooled solutions using ultrasound - Google Patents

Abstract

Solids deposition, such as of waxes, asphaltenes or gas hydrates, from a supercooled solution flowing in a pipeline, is reduced by applying ultrasonic vibration of 10-200kHz so as to cause cavitation. The process results in the formation of many small crystals. Seed crystals may also be added to assist the process and the whole flow or a side stream thereof may be processed. The vibrations may be applied at several positions or temperatures in the flow process.

Description

Field of the Invention.

This invention relates to methods of reducing deposition of materials from supercooled solutions, and especially concerns the deposition of wax on cooling waxy oils, such as diesel fuel, residual and crude oils; the deposition of asphaltenes on cooling crude oil with a high asphaltenic content; and the deposition of gas hydrates, on cooling in pipelines carrying gas and water, and optionally crude oil, such as gas pipelines and live crude oil pipelines. Deposited solids tend to adhere to the inside walls of pipes, in valves, in tanks and on heat exchangers. If this material is allowed to build up, it causes increased pressure losses, reduction in flow rate in pipes or heat exchange capacity and ultimately pipe blockage. Fouling of pipelines by waxes, asphaltenes and gas hydrates is a major problem in the petrochemical industry, particularly for offshore fields.

State of Art Relating to the Invention.

Crystallisation is used herein to mean the change of state from a liquid with no bulk molecular ordering to a solid with a bulk crystalline molecular structure.

Crystallisation for any system is dependent on both temperature and pressure.

Solidification of liquids involves two processes-nucleation of crystals, followed by growth of these crystals to form an ordered solid. Many liquids have a strong tendency to cool below their melting point before initial nucleation occurs, this is termed supercooling. The crystal structure of a solidifying melt is dependent on factors such as the density of nucleation sites, rate of cooling, composition of phases present for polymorphic materials, and the presence of other materials.

Waxes are essentially crystalline mixtures of long-chain hydrocarbons (n-paraffins) with carbon chain lengths ranging from CIS to C75+. With pure n-paraffins, as the chain length increases the melting point, as determined by standard analytical techniques such as differential scanning calorimetry (DSC), increases. On cooling a mixture of n-paraffins those molecular species with the highest melting point will nucleate, as the temperature is reduced crystals grow by the addition of further molecules of the initial paraffin species and also when their melting point is attained by the co-crystallisation of shorter chain paraffins. In pipelines, such as sub-sea pipes containing hot waxy oils, the temperature at the wall will be the coolest and the flow velocity lowest and it is in this region that initial nucleation of waxes will be expected to occur and in particular wax crystals will grow on the wall of the pipe. Subsequent crystal growth will then be expected to occur on these pre-formed crystals, little spontaneous nucleation will be expected within the bulk of the flow where the temperature is greatest and the flow velocity highest.

The chemical structure of asphaltines is difficult to define. Asphaltines from crude oils comprise condensed aromatic and naphthenic molecules of molar masses ranging from several hundred to several thousands grams per mole and it is generally thought that they may be partly dissolved and partly suspended by resins in crude oil and their stability is dependent on various factors including composition, pressure and temperature of the crude oil. On cooling in pipelines asphaltine deposition will be expected to occur at the wall for reasons described for waxes above.

Gas hydrates are particularly hydrates of light hydrocarbons such as natural gas, petroleum gas, or other gases with a fluid. These hydrates may form when water is present alongside light hydrocarbons, either in gaseous phase or in the dissolved state in a liquid phase, such as liquid hydrocarbon, when the mixture reaches a temperature that is lower than the thermodynamic temperature of hydrate formation, this being a given temperature for a composition of gases at a given pressure.

The most common method employed to control wax, asphaltine and hydrate deposits in the petrochemical industry is the addition of specific chemical inhibitors to the crude oil. But these are chemicals are expensive, they contaminate the water and are difficult to apply in deep water wells. Once deposits have been formed on the wall they may be removed by conventional pigging. Apparatus for cleaning walls with ultrasonic devices have also been described (for example US005727628 and US005595243), these devices remove solids from the wall and are not used to process the crude oil in any manner. The problems associated with pigging, conventional or ultrasonic, are that the pipeline needs to be closed down, this is inconvenient and disrupts production schedules.

Alternative approaches have been examined for example ultrasonic energy has been applied (US004945937 and US004982756), in combination with a wax crystal modifiers, to reduce the viscosity and the gel strength in the transfer of waxy crude oil. The ultrasound is also considered to solubilise the wax crystal modifier into the hydrocarbon. In Russian patent no 571657 ultrasound is applied to heat oil to enhance the pumping rate. In UK patent 2294886 physical disturbances, including ultrasound are applied to prevent the formation of hydrogen bonds, and to prevent the formation of hydrates within a fluid. It is also claimed that wax and asphaltene formation may be prevented. Patent SU 442287 disclose the use of an ultrasonic wave to break up hydrate crystals and release the trapped gas. Patent HU 186511 discloses the idea of transmitting an electromagnetic wave of selected frequency values and propagation methods to melt any hydrates formed In general, the prior art devices are all flawed and none are used extensively. We have devised an improved device whereby the problems associated with existing devices are mitigated or overcome.

Statement of Invention.

According to the present invention there is provided a method of reducing solids deposition from supercooled solutions, comprising the step of inducing crystallisation of many small crystals in the supercooled or partially solidified solution by the application of a physical disturbance. These small crystals are carried in the flow and are not likely to settle or cause blockages. Also the rate of crystallisation is increased, i. e. the solution is supercooled for a shorter period and consequently this reduces the potential of crystal growth on solid surfaces.

The physical disturbance may be generated in many different ways, for example by mechanical vibration or stirring, oscillation or rotation, enhancement of natural convection, electric fields, magnetic or electromagnetic stirring, detonation and shock waves, vortices (however produced), agitating with gas bubbles, release of pressure, etc. Generation of cavitation in the solution is preferred. This cavitation may also be achieved using hydrodynamic means such as a propeller or lifting surface so as to create vortices in the liquid. The preferred form of the disturbance is sonic or ultrasonic vibration, most preferably ultrasonic.

Ultrasound can be applied to the solution as soon as physical conditions allow it to become supercooled and preferably before any spontaneous nucleation may occur. If desired more than one method of effecting the disturbance may be used, especially ultrasonic vibration and a mechanical method e. g. hydrodynamic or mechanical agitation. Nucleation of the supercooled fluid may be effected at a position within the process stream at which the temperature has cooled to below that of the highest melting point component. Alternatively the process stream may be forced cooled to reduce the temperature to below that of the highest melting point component and the physical disturbance applied during cooling or immediately following cooling.

In some systems of interest, for example waxes, there will be present molecular species with a wide range of melting points. Nucleation of the highest melting point fraction will generally produce crystals which are isomorphous for lower melting point components and co-crystallisation of the lower melting point species will be expected to occur during subsequent cooling. However it is possible that the mass concentration of the larger molecular species will be low resulting in to a low crystal number density, this may be insufficient to accelerate significantly the crystallisation of other molecular species. It may also occur that the higher melting point crystals are not isomorphous for all other components. Therefore it may be preferable to apply the physical disturbance at several positions in the process flow to ensure that all molecular species are nucleated and that a high density of nucleation sites are achieved. This will also be beneficial in that grain refinement of existing crystals will occur, further increasing the crystal number density.

Small bubbles of the vapour phase or dissolved gas form out of a liquid as a wave of physical disturbance passes through the solution. These bubbles collapse after the wave has passed, causing a large pressure change which in turn induces nucleation in the liquid.

The preferred frequency of the ultrasound is 10-200kHz. Above 200kHz little nucleation can be obtained except at extremely high power. The particularly preferred frequency is between 10 and 60kHz. Energy densities of 0. 1- 1000 J/cm3. for example 1-100 J/cm3 are preferred, especially at the latter frequencies.

The duration of applied ultrasound is usually 0.05-360 sec at each treatment position.

The application of ultrasound to a supercooled solution can result in the formation of many nucleation sites, resulting in small crystals which are desired in the invention.

In liquids where crystals exist, either formed'spontaneously'or by applied physical disturbance, new crystals may be formed by fragmentation either due to cavitation or other effects induced by the ultrasound, such as acoustic streaming. This phenomenon is referred to as grain refinement or grain multiplication. Crystals formed in an ultrasonic field may also be found to have different surface properties (charge etc.) which modify their subsequent adhesion properties.

For many applications the use of ultrasound will modify simultaneously both nucleation and crystal growth. However, these two phenomena may be controlled independently in the invention by applying the disturbance at appropriate different stages of the solidification process, especially from supercooled solutions from which solid deposits slowly. Generally, the ultrasound may be applied to the solution either before or during the solidification process or following storage.

Ultrasonic vibrations may be generated by a convenient means, in particular using electromagnetic, electrostrictive or magnetostrictive devices, magnetostrictive devices giving low power per unit volume of solution contacted and piezoelectric devices connected to acoustic horns giving high power per unit volume. The ultrasonic power and frequency required will, in part, be determined by the viscosity, temperature, pressure, presence of solids, immiscible liquids and gas bubbles, dissolved gas content etc. of the fluid to be treated. In general, the desired ultrasonic conditions are those which result in cavitation in the bulk supercooled liquid to induce nucleation or to cause grain refinement. High intensity ultrasound may be generated most readily at the lower ultrasound frequencies, such as 10-40 kHz.

The efficiency of nucleation in a fluid will be dependent on the extent of supercooling and the density and distribution of the cavitation sites within the fluid. The degree of supercooling may be > 2 C especially for slow crystallizers such as waxes or hydrates.

The sources of ultrasound may be coupled directly to the solution but may also be connected indirectly via coupling mechanisms such as horns, waveguides and/or through the walls of the container holding the liquid, or walls of the pipe line through which it passes. The flow may be processed by modification to existing pipeline or by passage through specialised crystal generators.

The whole flow may be processed or alternatively a proportion of the total flow may be processed, which is then used as a source of seed crystals to treat the remaining unprocessed flow. In a preferred embodiment, seeds are injected into the central region of the unprocessed flow, to ensure that crystal growth occurs onto the seeds in the bulk of the fluid, removing any potential of crystal growth at the walls.

Preferably, the apparatus of the present invention includes a means for monitoring the size of the crystals. Alternatively, a separate apparatus may be provided to monitor the size of the crystals. This allows any failure or malfunction of the apparatus of the present invention to be detected before mineral salt can build up on the solid surface. Experimental Proof Nucleation of Wax in a batch system.

The test system employed was: 10% w/v C25-C35 wax cut 94% v/v decane 1% toluene.

The solution was mixed with heating to a temperature of 40 C. Samples (100 ml) were then cooled, with mixing in a cooling bath, programmed to cool at a rate of 0. 5 C min-1. One set of samples were removed at 23 oc and insonified with an ultrasonic horn (20kHz) and then replaced in the cooling bath. These samples was reexamined when the cooling apparatus attained 22OC, the solutions were obviously cloudy due to crystallisation. In contrast, the control (non-sonicated samples) showed

no obvious crystallisation at 22OC, spontaneous nucleation occurred in these samples over several degrees (12 C to 20oye).

Tube blocking with Wax With the solution described in (1) above, samples were cooled with mixing to 22OC.

This solution was then was then pumped at a rate of 1 ml min-1 through a stainless steel coil (1/16 inch i. d, length of coil 1 metre) submerged in a low temperature bath cooled to 4 C. In the control, non sonicated sample, tube blocking occurred within 20 minutes, the solution collected from the tube exit was clear. With an ultrasonic probe placed in a flow through cell before the steel coil no blocking occurred with 60 minutes and the solution issuing from the stainless tube was cloudy.

Nucleation of a hydrate in a batch system.

The test system was a eutectic mixture oftetrahydafuran in de-ionised water (melting point SOC) The solution was mixed with heating to a temperature of 10 C. Samples (100 ml) were then cooled, with mixing in a cooling bath, programmed to cool at a rate of 0. 5 C min-1. One set of samples were removed at 3 C and insonified with an ultrasonic horn (20kHz) and then replaced in the cooling bath. These samples was reexamined when the cooling apparatus attained 2 C, the solutions were obviously cloudy due to crystallisation. In contrast, the control (non-sonicated samples) showed

no obvious crystallisation at 2 C, spontaneous nucleation occurred in these samples over several degrees (-SaC to 0 C).

Tube blocking with hydrate With the solution described in (3) above, samples were cooled with mixing to 5 C. This solution was then was then pumped at a rate of 1 ml min-l through a stainless

steel coil (1/16 inch i. d, length of coil 1 metre) submerged in a low temperature bath cooled to-10 C. In the control, non sonicated sample, tube blocking occurred within 2 minutes, the solution collected from the tube exit was clear. With an ultrasonic probe placed in a flow through cell before the steel coil no blocking occurred with 10 minutes and the solution issuing from the stainless tube was cloudy.

Claims (8)

1. Claims Claim 1. A method of reducing solids deposition from supercooled solutions, comprising the step of inducing crystallisation of many small crystals in the supercooled or partially solidified solution by the application of a physical disturbance.
2. Claim 2. A method as claimed in claim 1 in which the physical disturbance generates cavitation in the supercooled fluid.
3. Claim 3. A method as claimed in claim 2 in which the physical disturbance is sonic or ultrasonic energy.
4. Claim 4. A method as claimed in claim 3 in which the frequency of ultrasound is between 10 and 40 kHz.
5. Claim 5. A method as claimed in any of claims 1 to 4 in which the physical disturbance is applied at several temperatures during cooling of the process flow.
6. Claim 6. A method as claimed in any of claims 1 to 5 in which the whole of the process flow is treated.
7. Claim 7. A method as claimed in any of claims 1 to 5 in which seed crystals are generated for the reduce solids deposition when added to a bulk flow.
8. Claim 8. A method as claimed in any previous claim in which the solids deposited are waxes, asphaltenes or gas hydrates as encountered in the petrochemical industry.