NL1018275C2 - Continuous removal of polycyclic aromatic hydrocarbons from mud by contact with ozone and catalyst comprises continuously controlling the ozone supply - Google Patents

Abstract

Continuous process for removing polycyclic aromatic hydrocarbons (PAH) from mud by contacting the mud with ozone and a catalyst in a reactor to produce a PAH-depleted effluent containing PAH decomposition products comprises continuously monitoring the amount of unreacted ozone in the effluent and using the result to control the amount of ozone supplied to the reactor. An independent claim is also included for apparatus for removing polycyclic aromatic hydrocarbons from mud, comprising a reactor equipped with a stirrer and/or radial diffusion-promoting means; an inlet for contaminated mud; one or more inlets for ozone and optionally catalyst; an ozone generator; a control unit for continuously monitoring the ozone level in the reactor and controlling the ozone generator; and an effluent outlet.

Description

Continuous process for the removal of polycyclic aromatic hydrocarbons from contaminated dredging spoil and equipment for it

The invention relates to a continuous process for removing polycyclic aromatic hydrocarbons from contaminated dredging sludge, a device for removing polycyclic aromatic hydrocarbons from contaminated dredging sludge, and to the use of a catalyst for removing polycyclic aromatic hydrocarbons from contaminated dredging sludge. .

10 Contaminated dredging spoil is one of the largest waste streams in the Netherlands.

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According to a recent inventory, approximately 1,000,000,000 m will have to be removed until 2015. The majority of the dredging spoil is still being dumped. However, Dutch regulations ("Building Materials Decree") and in development ("Active Soil Management") have been developed for nautical and environmental hygiene reasons to combat this.

There is also an increasing demand for building materials in the civil engineering sector. Primary materials such as surface minerals were normally used for this, but the Dutch government is increasingly discouraging the use of these primary materials. Therefore, purified dredging spoil could be a valuable secondary material for providing a building material.

Known methods for purifying contaminating dredging spoil have a number of disadvantages. For example, dewatering and maturing in transit depots, a process in which organic contaminants, for example polycyclic aromatic hydrocarbons, are degraded aerobically, is very time-consuming and slow. Moreover, it appears that a residual fraction remains which, because of the relatively high content of organic impurities, represents a high burden on the environment. If this technique is used, it will generally be necessary to subject the dredging sludge to a pre-treatment, in particular if the dredging sludge contains a high content of polycyclic aromatic hydrocarbons (PAHs). It is estimated that at least 10% of the contaminated dredging spoil in the Netherlands contains more than 200 mg of PAHs / kg of dredging sludge, so that this dredging sludge is unsuitable for reuse.

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In WO 98/54098 a method is described for removing polycyclic aromatic hydrocarbons from contaminated material such as contaminated soil (a list of examples is described for example on pages 9, lines 28 - 34 and page 33, lines 1 - 5 of WO 98/54098, however, dredging spoil is not explicitly mentioned). According to this method, the contaminated material is brought into contact with a catalyst, ozone and a first oxidant other than ozone to form a mixture, after which ozone is supplied to the mixture. The first oxidant can be hydrogen peroxide, a perchlorate, a permanganate or a persulfate. The catalyst can be iron, gold, titanium, copper, silver, manganese, cobalt, nickel, stainless steel, steel wool, or oxides, an alloy or a composite thereof, or a combination of the oxide, alloy or composite. Preferably the catalyst is metallic iron. The process can be carried out in one or more stirred reactors. A disadvantage of this method is that the contaminated material must be transported to the location of the device, which is economically unfavorable and which makes a high and continuous production of purified dredging spoil impossible.

The invention provides a solution to the above-described problems and therefore relates to a continuous process for the removal of polycyclic aromatic hydrocarbons from contaminated dredging spoil, wherein: (a) contaminated dredging sludge, gaseous ozone and a catalyst in contact with each other in a reactor are introduced and converted to form an effluent, which mainly comprises dredging spoil purified from polycyclic aromatic hydrocarbons and decomposition products from polycyclic aromatic hydrocarbons, (b) the amount of unreacted ozone in the effluent is continuously determined, and (c) the amount of ozone supplied to the reactor is continuously controlled by the amount of unreacted ozone as determined in step (b).

According to the invention, the contaminated dredging spoil preferably contains 200 mg of PAHs / kg of dredging sludge, calculated on the total amount of dredging sludge. It is further preferred that the solid: water ratio in the dredging sludge is as constant as possible. This ratio is preferably between 1: 5 and 1: 15 and in particular between 1: 8 and 1: 12.

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According to the invention, contaminated dredging spoil is therefore supplied directly from a dredger via one or more steel tubes to one or more reactors, the reactors preferably being connected in series and preferably slurry reactors. In this reactor or reactors, the dredging spoil is brought into contact with ozone (which originates from an external ozone generator in which ozone is produced from oxygen, air or a mixture thereof with the help of electrical energy) and a catalyst, whereby PAHs are converted. The ozone concentration in the effluent is continuously measured with the aid of a control unit coupled to the ozone generator and as more unreacted ozone is present in the effluent, the ozone generator will supply less ozone to the reactor or reactors. A further advantage of the method according to the invention is that odor nuisance is reduced to a minimum. After all, it is known that contaminated spoil generally emits a lot of odor (for example a strong tar and / or rotting air) which has virtually disappeared after treatment according to the method of the invention.

According to the invention, it is preferred that the ozone be supplied to two or more places in the reactor, so that a homogeneous distribution of the ozone through the reaction mixture is achieved. Moreover, it is preferred that the reactor be provided with one or more radial diffusion promoting agents, this to promote the homogeneity of the reaction mixture. The reactor is also preferably provided with a stirrer.

The catalyst is preferably supplied to the reactor in a finely divided state, for example as a powder, or is it present in the reactor in solid form, which is preferably achieved by applying the catalyst to agents with a high specific surface area, preferably in in the form of a (ceramic) coating on the stirring blades of the agitator of the reactor or as a coating on the walls of the reactor.

The catalyst is preferably a compound of iron, gold, titanium, copper, silver, manganese, cobalt, nickel, stainless steel, steel wool, or oxides, an alloy or a composite thereof or a combination of the oxide, the alloy or the composite. The catalyst more preferably comprises metallic iron or titanium oxide and the catalyst is in particular a combination of metallic iron and titanium oxide.

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The invention also relates to a device for the continuous removal of polycyclic aromatic hydrocarbons from contaminated dredging sludge comprising: (a) a reactor provided with a stirring device and / or radial diffusion-promoting means, (b) a feed line for contaminating dredging sludge, a or more supply lines for ozone and optionally a catalyst, (c) an ozone generator, (d) a control unit that continuously records the concentration of ozone in the mixture of contaminated dredge spoil, polycyclic aromatic hydrocarbons and catalyst present in the reactor, and which ozone ozone through the ozone generator, and (e) an effluent discharge conduit comprising dredging sludge essentially purified from polycyclic aromatic hydrocarbons and decomposition products from polycyclic aromatic hydrocarbons.

The radial diffusion promoting agents can be static or dynamic.

The invention also relates to the use of a catalyst comprising a compound of iron, gold, titanium, copper, silver, manganese, cobalt, nickel, stainless steel, steel wool, or oxides, an alloy or a composite thereof or a combination of 20 the oxide, alloy or composite comprises for removing polycyclic aromatic hydrocarbons from contaminated dredging spoil. The catalyst more preferably comprises metallic iron or titanium oxide and the catalyst is in particular a combination of metallic iron and titanium oxide.

Example

A dredging sludge containing 850 mg of PAHs / kg of dredging sludge was treated with ozone in a batch reactor for 90 minutes, whether or not in the presence of a catalyst (combination of metallic iron and titanium oxide). The decrease in PAHs in the dredging spoil was determined at different times: 30

Time (minutes) with catalyst Ö ~ 85Ö _ __ 's · * / ƒ if ^ I' * * *. «··· '/ 5 _ Γ620 25 74Ö" 55 62Ö "85 ~ Ï8Ö

Claims (17)

A continuous process for removing polycyclic aromatic hydrocarbons from contaminated dredging sludge, wherein: (a) contaminated dredging sludge, gaseous ozone and a catalyst are brought into contact with each other in a reactor and are converted to form an effluent, which is essentially dredging spoil purified from polycyclic aromatic hydrocarbons and decomposition products from polycyclic aromatic hydrocarbons, (b) the amount of unreacted ozone in the effluent is continuously determined, and (c) the amount of ozone supplied to the reactor is continuously controlled by the amount of unreacted ozone as determined in step (b). 2. Method according to claim 1, wherein the dredging spoil comes directly from a dredger ship. The process according to claim 1 or claim 2, wherein the gaseous ozone is supplied at two or more locations in the reactor. Method according to one or more of the preceding claims, wherein the reactor is provided with one or more radial diffusion-promoting means. Method according to one or more of the preceding claims, wherein the reactor is provided with a stirrer. Process according to one or more of the preceding claims, wherein the catalyst is supplied to the reactor in a finely divided state. 7. Process according to one or more of the preceding claims, wherein the catalyst is present in the reactor as a coating. The method of claim 7, wherein the catalyst is applied to agents with a high specific surface area. 9. Method according to one or more of the preceding claims, wherein the catalyst is a compound of iron, gold, titanium, copper, silver, manganese, cobalt, nickel, stainless steel, steel wool, or oxides, an alloy or a composite thereof or a combination of the oxide, the alloy or the composite The method of claim 9, wherein the catalyst comprises metallic iron. The method of claim 9, wherein the catalyst comprises titanium oxide. · V. 1 ï '> The method of claim 10 or 11, wherein the catalyst comprises metallic iron and titanium oxide. 13. Device for the continuous removal of polycyclic aromatic hydrocarbons from contaminated dredging sludge comprising: (a) a reactor provided with a stirring device and / or radial diffusion-promoting means, (b) a supply line for contaminated dredging sludge, one or more supply lines for ozone and optionally a catalyst, (c) an ozone generator, (d) a control unit that continuously records the concentration of ozone in the mixture of contaminated dredging sludge, polycyclic aromatic hydrocarbons and catalyst present in the reactor, and which orders the production of ozone by the ozone generator and (e) a discharge line for an effluent, which comprises dredging sludge essentially purified from polycyclic aromatic hydrocarbons and decomposition products from polycyclic aromatic hydrocarbons. 14. Use of a catalyst comprising a compound of iron, gold, titanium, copper, silver, manganese, cobalt, nickel, stainless steel, steel wool, or oxides, an alloy or a composite thereof or a combination of the oxide, the alloy or the composite for removing polycyclic aromatic hydrocarbons from contaminated dredging spoil. The use of claim 14, wherein the catalyst is metallic iron The use of claim 14, wherein the catalyst comprises titanium oxide. The use according to claim 15 or 16, wherein the catalyst comprises metallic iron and titanium oxide.