BACKGROUND

The present disclosure relates to oxidation of white liquor in wood processing industry, and more particularly to an arrangement and a method in white liquor oxidation.

In wood processing industry, oxidation of white liquor (WL) has been used since the commercialization of oxygen delignification, also known as oxygen bleaching. The use of white liquor as an alkali source in oxygen bleaching has required oxidizing various sulphur forms to sulphates in an external vessel. This is done because if non-oxidized sulphurs are taken into the bleaching reactor, the oxygen atmosphere in the reactor would react to oxidize the non-oxidized sulphurs, which would consume the oxygen to undesired reactions and liberate heat, and thereby make temperature control in the reactor difficult.

It is also known that poorly oxidized white liquor (OWL) cannot be used in bleaching stages which utilize oxygen or peroxide because the partially oxidized sulphur compounds consume additional bleaching chemicals in a given stage or in subsequent stages, thus rendering the use of oxidized white liquor uneconomical in such applications. Additionally, the amount of thiosulphate should be kept as low as possible in oxidized white liquor (OWL) because it is known that thiosulfate causes cellulose degradation, and it is also corrosive to many of bleaching reactor materials.

Typically, this process is followed by the laboratory measurements, such as sulphide titration, or Ion Chromatography (IC) of oxidized white liquor. However, sulphide titration is not a good enough method for evaluation of the process, and IC analysis from OWL is very time consuming. Furthermore, in general, laboratory analysis requires taking samples from the process flow and the feedback loop is slow.

BRIEF DESCRIPTION OF THE DISCLOSURE

An object of the present disclosure is to provide a new arrangement and method in white liquor oxidation.

The object of the disclosure is achieved by a method and an arrangement which are characterized by what is stated in the independent claims. Some embodiments of the disclosure are disclosed in the dependent claims.

The disclosure is based on the idea of determining an amount of at least one substance in the process flow by means of spectroscopy and use it for controlling white liquor oxidation process. In other words, the measurement is made using spectroscopy and online from the process flow without a need for taking samples and taking samples to a laboratory for detailed analysis.

An advantage of the disclosure is that the measurements can be accurate and real time. The feedback time may be reduced considerably, and the process control improved.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the disclosure will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which

Figure 1 illustrates schematically some features related to a white liquor oxidation process according to an example;

Figure 2 illustrates an arrangement in white liquor oxidation; and

Figure 3 illustrates a method in white liquor oxidation.

DETAILED DESCRIPTION OF THE DISCLOSURE

The disclosure relates to an arrangement and a method in white liquor oxidation.

Figure 1 illustrates schematically some features related to a white liquor oxidation process 100 according to an example. Different ways of implementing white liquor oxidation process are known, and it is known that related arrangements and details may vary. Therefore, these different examples are not discussed here in more detail and only one arbitrary example is disclosed in Figure 1 to illustrate some features and concepts.

In wood processing industry, oxidation of white liquor is widely used in connection with oxygen delignification, also known as oxygen bleaching. The use of white liquor as an alkali source in oxygen bleaching precludes that various sulphur forms, usually mostly sodium sulphide, are oxidized to sulphates in an external vessel 110. This is important, because if non-oxidized sodium sulphide is taken into a bleaching reactor, oxygen atmosphere in the reactor would oxidize the Na2S, which in turn would liberate heat and thereby make the temperature control in the reactor difficult.

In addition, poorly oxidized white liquor cannot be used in bleaching stages which utilize oxygen or peroxide because the partially oxidized sulphur compounds consume additional bleaching chemicals in a given stage or in subsequent stages, thus rendering the use of oxidized white liquor uneconomical in such applications.

Additionally, it is known that thiosulphate causes cellulose degradation, and it is also corrosive to many of bleaching reactor materials, whereby the amount of thiosulphate should be kept as low as possible in oxidized white liquor (OWL). Figure 2 illustrates an arrangement in white liquor oxidation.

An arrangement 1 in white liquor oxidation, in other words an arrangement in connection with white liquor oxidation, such as the arrangement 1 in white liquor oxidation of Figure 2, comprises a detector 2 arranged inside an oxidized white liquor process flow, in other words inside a white liquor oxidation process flow, and configured to measure at least one characteristic of the process flow by means of spectroscopy. This enables providing an inline measurement, in other words a measurement directly from the process flow by a detector 2 provided in the process flow, which is a real time measurement, as compared to samples taken to a laboratory for measurements.

The arrangement 1 in white liquor oxidation, such as the arrangement 1 of Figure 2, also comprises a controller 3 connected to the detector 2 to receive measurement data from the detector 2. The controller 3 is also configured to determine an amount of at least one substance in the process flow based on the measurement data. The controller 3 is further configured to control the white liquor oxidation process based on the determined amount of the at least one substance. According to an embodiment, the controller 3 may comprise a control system of the arrangement 1 or a part of a control system of the arrangement 1 . According to an embodiment, the controller 3 may comprise a control unit and/or a regulator.

According to an embodiment, the detector 2 may comprise an infrared spectrometer. More particularly, infrared spectrometer may be used to measure the concentration of compounds, for instance in one or more measurement points of the process, using infrared spectroscopy. According to an embodiment, the detector 2 more particularly may comprise a Fourier Transform Infrared (FTIR) Spectroscopy spectrometer. Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. An advantage of such embodiments is that an FTIR spectrometer can simultaneously collect high-resolution spectral data over a wide spectral range. FTIR spectrometers are known as such and are, thus, not explained here in more detail.

Detectors 2 based on spectroscopy typically have a wavelength range, within which the detector 2 can be used and/or is configured to be used. In arrangements 1 and methods disclosed in this description, the detector 2 may be configured to use the whole wavelength range of the detector 2, when making the measurements disclosed in this description and accompanying claims and figures. According to an embodiment, the detector 2 may be a spectrometer configured to use midinfrared wavelength range in the measurement. According to an embodiment, the detector 2 may be a spectrometer configured to use a wavelength range of 400-4000 cm ¹ . According to an embodiment, the detector 2 may be a spectrometer configured to use a wavelength range of 648 - 4000 cm ¹ . According to an embodiment, the detector 2 may be a spectrometer configured to use a wavelength range of 800 - 1220 cm ¹ , and according to a further embodiment, the detector 2 may be a spectrometer configured to use a wavelength range of 1000 - 1168 cm ¹ . In some embodiments, it may be beneficial to use a wider range of wavelengths. This may be true for instance in embodiments, in which multiple characteristics of the process flow, for instance amount, such as concentrations, of multiple substances a measured. In some other embodiments, especially in embodiments, in which only one substance or only a few substances are measured, it may be beneficial to select a narrower range, within which the strongest correlation between the infrared spectroscopy measurements and amounts, such as the concentrations, of the substances of interest are found. For instance, in the embodiments of this disclosure, the strongest correlations for sulphur forms, such as sulphite, sulphate and thiosulphate, may be found within the wavelength range of 800 - 1220 cm ¹ or 1000 - 1168 cm ¹ ., for example, whereby it may be beneficial to use one of these ranges.

According to an embodiment, the detector 2 may be arranged inside the oxidized white liquor process flow by mounting the detector to a line and/or a vessel of the process flow. According to an embodiment, the detector 2 may be arranged to the line and/or vessel of the process flow by means of a mounting flange, directly to the line or vessel and/or in another manner suitable for the type of the detector 2 in question. A vessel may comprise any type of a vessel, container, receptacle or similar suitable for receiving the material or process flow in question.

According to an embodiment, the amount of at least one substance in the process flow may comprise at least one of the following: an amount of sulphite in the process flow material, an amount of sulphate in the process flow material and an amount of thiosulphate in the process flow material. According to an embodiment, the amount of at least one substance in the process flow may comprise the amount of sulphite, sulphate and thiosulphate in the process flow material measured by means of spectroscopy.

According to an embodiment, the controller 3 may be configured to adjust at least one of the white liquor and oxygen process flow in the white liquor oxidation process based on the determined amount of at least one substance in the process flow to ensure sufficient oxidization of the oxidized white liquor for the purpose of oxygen bleaching. According to a further embodiment, the oxidized white liquor may comprise at least one of the following: partly oxidized white liquor and totally oxidized white liquor (TOWL).

According to an embodiment, the process flow, in which the amount of at least one substance is measured, comprises oxidized white liquor, more particularly totally or partly oxidized white liquor.

According to an embodiment, the controller 3 may be further configured to adjust process temperature in the white liquor oxidation process to an optimal reaction temperature based on the determined amount of at least one substance in the process flow to ensure sufficient oxidization of the white liquor for the purpose of oxygen bleaching. According to an embodiment, process temperature may be adjusted to improve oxygen solubility and to optimize oxygen consumption to desired reactions. Oxidation reactions are exothermic, whereby lowering the temperature may enhance at least some of the oxidation reactions. The process temperature may also affect the delays in the oxidation reactions.

Figure 3 illustrates a method in white liquor oxidation.

A method in white liquor oxidation, such as the method of Figure 3, may comprise the steps of: measuring 31 by a detector 2 arranged inside an oxidized white liquor process flow at least one characteristic of the process flow by means of spectroscopy; receiving 33 in a controller 3 connected to the detector 2 measurement data from the detector 2; determining 35 by the controller 3 an amount of at least one substance in the process flow based on the measurement data received from the detector; and controlling 37 the white liquor oxidation process based on the determined amount of the at least one substance.

According to an embodiment, the method in white liquor oxidation, such as the method of Figure 3, may be implemented using an arrangement in white liquor oxidation according to an embodiment or a combination of embodiments disclosed in this description and/or accompanying claims and drawings.

According to an embodiment, the detector 2 may be arranged inside the oxidized white liquor process flow by mounting the detector to a line or a vessel of the process flow.

According to an embodiment, the amount of at least one substance in the process flow may comprise at least one of the following: an amount of sulphite in the process flow material, an amount of sulphate in the process flow material and an amount of thiosulphate in the process flow material.

According to an embodiment, the method further comprise adjusting by the controller at least one of the white liquor and oxygen process flow in the white liquor oxidation process based on the determined amount of at least one substance in the process flow to ensure sufficient oxidization of the white liquor for the purpose of oxygen bleaching.

An advantage of the disclosure is that measuring and controlling quality of oxidized white liquor using on-line spectroscopy, preferably an FTIR measurement system, enables fulfilling required quality aspects in a more economical manner than in the known solutions.

Measuring the quality of oxidized white liquor may, then, comprise measuring the content of sulphite, sulphate, and thiosulphate ions in the process flow. These measurements also enable adjusting both white liquor and oxygen process flows on-line in an effective and efficient manner.