EP0146062A2 - Process and device for working up oil sludge for combustion - Google Patents

Abstract

For oil sludge, which can have a water content of up to 80% and arises on board ships, for example from separators and filters for purifying the fuels and lubricants, working-up is provided by reducing the water content and thus increasing the calorific value for combustion. In a vessel which can be heated by waste heat, the oil sludge is exposed to a reduced pressure of between 0.04 and 0.4 bar and heated to a temperature which is 15 to 30 DEG C above the boiling point of water at the same reduced pressure. In addition, the oil sludge is continuously taken off from the bottom of the vessel and returned via a circulation pump to the top of the vessel against an impingement plate. The water vapour thus separating out is taken off via a condenser. In a line through which the oil sludge is circulated, a measurement sensor for measuring the water content then still present is provided. If the water content is less than 30%, the oil sludge is taken away to a stock tank. Approximately at the same time, the upper part of the vessel is replenished with untreated oil sludge from a sludge collection tank, so that the vessel is always filled with a minimum quantity of oil sludge.

Description

The invention relates to a method and a device for treating oil sludge, which is obtained in ship or land operation, for combustion.

The oil sludge, which accumulates, for example, on ships when cleaning fuels and lubricants from separators and filters or from water separators, contains various impurities and in particular has a high water content and low calorific value, so that it cannot be used in this form on board.

The oil sludge can be delivered ashore, which is very expensive due to the necessary lay time at the appropriate acceptance points and requires a correspondingly large sludge storage tank on board.

The oil sludge can be burned with a suitable burner in an auxiliary boiler, so that the energy contained in it can still be used for on-board operation. However, due to the high water content, the sludge can only be burned with the addition of additional fuel, which is to be regarded as uneconomical, since the use of the fuel in a different application normally results in a higher efficiency.

It has been determined that the oil sludge can be burned without additional fuel on board if its water content is 30% or less.

When de-oiling bilge water where possible Oil-free water is desired, it has already been proposed (DE-PS 28 33 106) to separate water and oil by evaporating the water. The oil-containing bilge water is placed under vacuum in a retention tank and heated by supplying waste heat from an internal combustion engine, so that the water evaporates and the oil can flow into a dirty oil tank which is also under pressure. The vacuum applied means that the evaporation takes place even at low temperatures, which can be achieved with waste heat on board. In this method, however, the temperature is at most at the boiling temperature of the water corresponding to the negative pressure.

However, this method is not useful for reducing the water content of oil sludge. The oil sludge has a high viscosity because of the proportions of heavy oil it contains and because of its high water content, which preclude circulation and thus even heating of the mass in a container in an economically reasonable time. In addition, the oil sludge can be in the form of a dispersion of oil and water, from which the water cannot be dissolved to a satisfactory extent when it is heated to its boiling temperature. It is also difficult to determine an average water content in the case of a viscous mass in a container.

The invention has for its object to provide a method and an apparatus with which the water content of the oil sludge obtained, for example, on board a ship can be reduced in an economically justifiable time and with waste heat available on board a ship, so that the calorific value of the oil sludge allows combustion without the addition of other fuels.

The following measures are provided to solve this task. A heated, vacuum-loaded container is partially filled with oil sludge. Oil sludge is continuously removed from the lower part of the tank, pumped through a pipeline running outside the tank and pumped into the upper part of the tank into the pressurized space above the liquid level, whereby the oil sludge fed in is distributed. The water content of the pumped quantity is measured during pumping and outside the container. At least part of the pumped oil sludge is discharged to a tank when the water content reaches less than 30% from the amount removed from the container. At about the same time, treated oil sludge is refilled into the container. In the container, the oil sludge is exposed to a vacuum that is between 0.04 and 0.4 bar. In the container, the oil sludge is heated to a temperature which is 15 to 30 ^° C. above the boiling point of pure water at the same negative pressure.

A negative pressure between o, o5 and o, 2 bar and a temperature of 20 ° C above the boiling point of water at this negative pressure are preferred. These conditions make it possible to heat the container with the waste heat that is present on board, for example with the warm engine cooling water. The oil sludge is preferably treated until a water content of 10 to 15% is reached.

The device according to the invention comprises a container with a level sensor for determining the liquid level and with devices for generating the negative pressure and for removing the steam, a feed pump for supplying the oil sludge from a collecting tank, lines and a sludge circulation pump for pumping the sludge with a capacitive measuring probe for determining the water content and in upper part of the container opposite the outlet opening of the sludge pressure line and above the maximum liquid level, a baffle plate that divides the oil sludge introduced. A feed line from a sludge collecting tank for the untreated sludge and a further line to a sludge storage tank in which the largely dewatered oil sludge is discharged are also provided.

Further details of the invention are explained with reference to the following experimental examples and the drawing described later.

Experiments were carried out with various pressures and the boiling temperatures of pure water corresponding to them, for example 0.2 bar and 60 ° C. However, these showed that under these conditions the removal of water by evaporation takes place only very slowly. Even by pumping in the manner according to the invention, the removal of water could not be significantly improved. These conditions therefore do not result in an economically satisfactory process even when waste heat is used.

However, it has been found that the separation can be accelerated by raising the treatment temperature above the boiling point corresponding to the pressure. Too much an increase in temperature, however, leads to mud droplets with the water vapor to be removed entrained and cannot be separated from the condensate in a conventional oil separator. In addition, the surface of the oil sludge in the tank becomes very unsteady when the temperature is too high, making the level measurement unsafe.

The following experiments were carried out using an oil sludge taken from a ship which no longer contained free water, but which was a polydisperse emulsion in which a water content of 57 to 59% in the form of fine and very fine water drops in the oil was determined .

The container used had a volume of approximately 500 dm ³ . The system was filled with an average of approx. 230 dm ³ oil sludge, in which about 133 dm ³ water was bound. As far as a circulation pump was used in the tests, its delivery rate was about 8 to 10 m ³ per hour.

1. Try:

Without circulation and at pressures from o, o5 to almost 1 bar (absolute), at which boiling temperatures corresponding to the respective pressure were aimed for, only up to about 20 dm ^{3 of} water could be removed per hour. Since a uniform temperature distribution of the mass in the container was difficult to produce, local overheating cannot be ruled out, which sometimes resulted in this separated amount of water. The surface of the oil sludge was calm, with occasional bubbles of steam rising. Contrary to expectations, no significant improvement in the separation of steam or water was observed even at the higher temperatures at which the water-containing oil sludge has a lower viscosity.

2nd attempts:

Different pressures and temperatures were set as in the first experiment, but the oil sludge was additionally circulated, the sludge being fed into the tank via a baffle plate. A significantly greater separation of water or steam than in the first experiments could not be observed. However, the distributions of temperature and water content in the pumped mass were uniform.

In the second experiments it was possible to observe through a sight glass on the container that water vapor was essentially emitted from the oil sludge introduced into the container through a nozzle against the baffle plate and flowing off from the baffle plate. This observation showed that the pumping not only achieved a better temperature distribution in the sludge treated in each case, but the division of the sludge in the head space favors the elimination of the water. The observations of the water vapor emerging from the sludge in different ways in the first and second experiments confirm the assumption that in the first experiments steam bubbles only resulted from local overheating on the heated container wall. Such overheating of the stationary sludge must be avoided, however, since it leads to the formation of incrustations on the container wall.

The results of the second experiments, in which the oil sludge was treated at the appropriate pressure at the respective boiling temperature, were also unsatisfactory, since only a little more than 20 dm ^{3 of} water could be separated off per hour.

3. Attempt:

The temperature was raised above the boiling temperature corresponding to the pressure. Without circulation, there was increased blistering, which resulted in the formation of foam in the upper part of the container. The foam bubbles could get into the condenser with the water vapor, so that there is a risk that the condensate will be enriched with sludge particles.

4th attempt:

As in the third experiment, but with a circulation of the oil sludge. Foam formation no longer occurred or disappeared shortly after the onset of circulation. At a pressure of 0.1 bar (abs.) And an overheating of the oil sludge by about 20 ° C to 65 ° C, 30 dm ³ / h of water were separated. The surface of the oil sludge was relatively calm. There was no danger that sludge particles could get into the condensate.

5th attempt:

As in the 4th experiment with circulation of the oil sludge, at a pressure of 0.1 bar (abs.), But with overheating to 73 ° C. 130 dm ³ / h of water were separated off. However, the sludge bubbled strongly and the surface was eruptive, so that small oil particles could be entrained in the condenser in the test facility.

6th attempt:

The fourth experiment was carried out over a long period of time with the same amount of sludge of 230 dm ³ at about 70 ° C. with a water separation of about 60 dm ³ / h until no more distillate was obtained. A laboratory test found that the sludge was drained to 0.2% water. The distillate contained 133 dm ^{3 of} water and 14 dm ^{3 of} light mineral oil, which settled on the water without difficulty, so that the water can be cleaned in a gravity separator, for example. That as Water obtained from the distillate corresponded to an original water content of 58% in the oil sludge. Mud particles could not be found in the water.

The tests showed that with circulation of the oil sludge and at a temperature which is 15 to 30 ^° C. above the boiling temperature of pure water corresponding to the pressure, thermal breakage of the oil sludge emulsion and separation of the water from the sludge is possible even in an economically reasonable period with sufficient treatment performance and using the waste heat generated on ships.

On a ship with a propulsion power of 10,000 kW, a daily amount of around 1.2 t of oil sludge from the processing of the heavy oil and from other places where oil sludge is generated can be expected. Assuming that the average water content, which cannot be removed by settling free water, is about 50%, i.e. about 0.6 t per day, a system as used in the tests with the conditions of test 4 would have to be approximately Be operated 20 hours a day, with the conditions of Experiment 5 but only less than 5 hours a day. However, the size and shape of the container used in the tests can be changed in order to adapt the performance of a sludge dewatering system to the prevailing conditions on board. The surface available for the escape of steam from the sludge can also be increased by suitable container shape or internals. When dimensioning a plant, it should be noted that the composition of the oil sludge can vary widely, with the proportion of water bound as an emulsion being around 45 to 80% of the sludge.

The experiments showed that dewatering of the sludge is also possible at a pressure above 1 bar if the oil sludge is appropriately overheated to above 115 ^° C. However, sufficient pressure-resistant boilers and other facilities are required for such treatment, which would make the costs of a system for a ship considerably more expensive. A correspondingly hot heating medium is usually not available on board a ship.

The preferred negative pressure of 0.04 to 0.4 bar enables the container to be heated with engine cooling water, e.g. 80 ° C or with steam from an exhaust gas boiler. The heating is controlled so that the temperature of the oil sludge remains below 100 ° C even at a pressure of 0.4 bar. A pressure between 0.05 and 0.2 bar is preferably selected, and the oil sludge is treated between 50 ° C. (at 0.05 bar) and approximately 80 ° C. (at 0.2 bar). Since the water-containing sludge has a very high viscosity, it must be heated to about 50 ° C. in order to be able to be pumped out of the sludge collecting tank. A pressure of 0.05 bar therefore only requires a slight heating of the tank in order to keep the sludge at this temperature. A pressure between 0.1 and 0.2 bar appears to be particularly favorable in order to be able to control the dewatering process by heating with the existing heating media, with slight pressure fluctuations, which should remain in this area, due to the feed of untreated sludge, the evaporation of the Water and the release of dewatered sludge can occur.

The oil sludge used in the experiments described above had a calorific value of approximately 3,000 kcal / kg (= 12,500 kJ / kg) with a water content of 58% and bound in the emulsion before the treatment therefore could not be burned. A heating value of at least 6,500 kcal / kg (= 27,215 kJ / kg) is considered necessary for combustion, which requires a water content of not more than 30%. However, a lower water content of 10%, for example, is more advantageous for combustion since the calorific value rises to about 8,600 kcal / kg (= 36,000 kJ / kg). Such a low content of finely divided water is advantageous for the combustion, so that a further dewatering of the sludge below an average of 10 to 15% is not necessary. The procedure for sludge dewatering is discontinuous, since untreated sludge is fed in as soon as the fill level falls below a predetermined value, and largely dewatered sludge is released when the measuring probe in the line through which the sludge is circulated detects a sufficiently low water content. This allows dewatered sludge to be withdrawn from the bottom of the tank at the same time and untreated sludge to be fed in at the top, the latter mixing with the dewatered sludge. So that the dispensing continues over a sufficient period of time, the measurement values of the probe can be used to control the system in such a way that the valve for dispensing dewatered sludge is only opened at a lower water content and closed at a higher water content, so that in the Sludge storage tank sets a desired average water content. If sludge with a water content of 10% is to be burned, the discharge valve can be opened at 5% and closed at 15%. These deviations of + 5% from the average value are sufficient to compensate for fluctuations occurring during the delivery, without there being a considerably different composition of the dewatered sludge which could interfere with the combustion.

The dewatering of the oil sludge according to the invention not only creates a possibility of being able to burn the sludge on board without the addition of fuel, but also reduces the amount of sludge to be stored on board to less than half, which is also important, if the sludge is not to be burned but delivered to a station on land. The reduction in water content also causes the viscosity of the slurry to decrease, making it easier to pump, and reducing its heating for pumping.

A preferred embodiment of an apparatus for carrying out the method is shown in the accompanying drawing. The drawing shows only a simplified diagram. A device provided for practical use can be provided with further valves and monitoring devices.

Oil sludge heated to pumpability is fed from a sludge collecting tank 1 through the sludge pump 11 and the pressure line 12 into the container 2, a level measuring device 28, 29 switching the pump 11. During the treatment, the sludge is removed from the lower part 22 of the container 2 by a circulating pump 30 via the suction line 27 and pressed through the pressure lines 31, 32 and an outlet opening 25 into the upper part 21 of the container 2, where a baffle plate 26 is arranged . A capacitive measuring probe 33 for measuring the water content is located in the pressure line 31. It controls the valves 34, 35, which can also be combined into a single valve to circulate the sludge or when the water content is low dispense via line 36 into the sludge storage tank 3. From the upper part 21 of the container 2, an exhaust pipe 24 leads to a condenser 4, in which the separated steam is precipitated as a distillate. The distillate is fed through a line 51 to a distillate tank 5 and from this through a distillate pump 53 via a pressure-maintaining valve 54 and optionally a tank into a de-oiler, not shown, in which the light mineral oils contained in the distillate can be easily separated from the water. A vacuum generator 60, for example a water ring pump, is connected to the condenser 4 via line 61. Condensate that forms on the vacuum generator can be discharged into a bilge well. The condenser 4 is provided with cooling water lines 41, 47. The jacket 23 of the container 2 is heated via lines 37, 38 by a suitable heating medium, the supply of the medium being controlled by a temperature sensor 39 on the suction line 27.

The method according to the invention creates the prerequisite for being able to use the energy still contained in the oil sludge by combustion in a boiler without adding valuable fuels. However, it is not limited to the oil sludge generated in normal ship operation, but can also be used with the oil sludge falling off during tank cleaning. In addition, the process can also be carried out with oil sludge from land installations and can be used for the dewatering of other sludges which contain a high water content and preferably also oil, even if the dewatered sludge is not intended to be burned.

Claims (10)

1. A process for preparing oil sludge for combustion by reducing the water content, the oil sludge being heated in a container under reduced pressure and the water vapor emitted from the oil sludge being drawn off, characterized in that oil sludge is introduced into a container until it is partially filled, continuously extracting oil sludge from the amount present in the lower part of the container, pumping it around and pressing it into the upper part of the container into the pressurized space above the liquid level,
the water content of the pumped quantity is measured during pumping and outside the container and
at least part of the oil sludge is removed from the amount removed from the container when the water content is less than 30%,
the oil sludge in the container is subjected to a vacuum which is between 0.04 and 0.4 bar (abs) and is heated to a temperature which is 15 to 30 ° C above the boiling point of pure water at the same vacuum. 2. The method according to claim 1, characterized in that the negative pressure in the container is on average at a value between 0.05 and 0.2 bar. 3. The method according to claim 1 or 2, characterized in that the average temperature of the oil sludge in the container is about 20 to 25 ° C above the boiling point of water, that in the container prevailing negative pressure corresponds. 4. The method according to any one of claims 1 to 3, characterized in that oil sludge is dispensed at a water content of an average of 10 to 15% in the amount pumped. 5. The method according to any one of claims 1 to 4, characterized in that the discharge of dewatered oil sludge begins when the water content in the pumped sludge is about 5% below the desired average value, and ends when the water content is about 5% above that desired value. 6. The method according to any one of claims 1 to 5, characterized in that when the level in the container drops below a predetermined lower value preheated oil sludge is fed from a collecting tank until an upper level is reached again. 7. Device for performing the method according to one of claims 1 to 6, with a heatable and pressurizable container with at least one level sensor for determining the liquid level in the container, with devices for generating the vacuum and for removing the steam and with a feed pump for supplying the oil sludge from a sludge collecting tank, characterized in that
from the lower part of the container (2) leads a sludge suction line (27) to a sludge circulation pump (30) and from there a sludge pressure line (31,32) via a capacitive measuring probe (33) to determine the water content to an outlet opening (25) in the upper part of the container (2) runs,
a baffle plate (26) is arranged opposite the outlet opening (25) of the sludge pressure line (32) in the container (2) and still above the maximum liquid level in the container and the sludge pressure line (31) with a line (36) for discharge of the processed sludge can be connected to a storage tank (3). 8. The device according to claim 7, characterized in that a valve arrangement (34,35) for removing the processed sludge via the line (36) from the probe (33) is controlled and that the sludge feed pump (11) for supplying untreated Sludge in the upper part (21) of the container (2) is regulated by a level sensor (28, 29). 9. Apparatus according to claim 7 or 8, characterized in that the feed line (12) for not yet treated sludge from the sludge feed pump (11) shortly before the confluence with the container (2) is connected to the sludge pressure line (32) is. 10. Device according to one of claims 7 to 9, characterized in that the outlet opening (25) of the pressure line (32) is designed as a nozzle.

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