Field

The invention relates to a stripping column arrangement of and a stripping method of acid filtrate from hydrolysis of cellulosic feedstock.

Background

The hemicellulose syrup obtained from the lignin separation is fed to the hemicellulose concentration subprocess, where an aim is to extract a proportion of acids from the syrup while energy consumption should be minimized.

The steam consumption in the traditional steam stripping of the hemicellulose syrup is quite high when roughly 99 m-% of the acids is required to be recycled back to process. One of the problems is a low relative volatility of formic acid compared to water in aqueous phase.

Steam is used extensively as a stripping agent in a stripping column, and the water proportion of the concentrate may bear problems since the viscosity of the liquid phase may not be appropriate for the stage column. All in all, the minimization of energy consumption while keeping purity of the end product in a desired range, meeting the recycling requirements and keeping a size of the stripping column arrangement compact is a technical challenge.

Brief description

The present invention seeks to provide an improvement in stripping.

The invention is defined by the independent claims. Embodiments are defined in the dependent claims.

If one or more of the embodiments is considered not to fall under the scope of the independent claims, such an embodiment is or such embodiments are still useful for understanding features of the invention. List of drawings

Example embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which

Figure 1 illustrates an example of a stripping column arrangement;

Figure 2 illustrates an example of composition profiles of a stripping column with no added acetic acid;

Figure 3 illustrates an example of relative volatilities of conventional stripping arrangement with no added acetic acid;

Figure 4 illustrates an example of composition profiles of acetic acid addition to stages 1, 6 and 10;

Figure 5 illustrates an example of acetic acid feed to the first stage, to the sixth stage and the tenth stage;

Figure 6 illustrates an example of a controller; and

Figure 7 illustrates of an example of a flow chart of a stripping method.

Description of embodiments

The following embodiments are only examples. Although the specification may refer to “an” embodiment in several locations, this does not necessarily mean that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment. Single features of different embodiments may also be combined to provide other embodiments. Furthermore, words "comprising" and "including" should be understood as not limiting the described embodiments to consist of only those features that have been mentioned and such embodiments may also contain features/structures that have not been specifically mentioned. All combinations of the embodiments are considered possible if their combination does not lead to structural or logical contradiction.

It should be noted that while Figures illustrate various embodiments, they are simplified diagrams that only show some structures and/or functional entities. The connections shown in the Figures may refer to logical or physical connections. It is apparent to a person skilled in the art that the described apparatus may also comprise other functions and structures than those described in Figures and text. It should be appreciated that details of some functions, structures, and the signalling used for measurement and/or controlling are irrelevant to the actual invention. Therefore, they need not be discussed in more detail here.

The solution described herein is related to production of hemicellulose rich syrup from a cellulosic or lignocellulosic feedstock. To be more precise, the solution is based on preprocessing stages of the hemicellulose production. According to what is taught in this document, the recovery rate of the bio-solvent components which may be present in the feed material used to concentrate hemicellulose in order to obtain purified hemicellulose syrup can be optimized. During the concentration process the impurities, namely organic acids and potentially furfural, are evaporated from the liquid phase in order to obtain a hemicellulose syrup containing only a minor fraction of impurities such as organic acids. One of the major difficulties in this hemicellulose purification process is related to the steam consumption and hence energy consumption. Energy consumption is an important issue environmentally.

Stripping may be performed in towers that has stages. Even a centrifugal stripping apparatus can be considered such a tower although in addition to gravitation it utilizes a centrifugal force. In this document the stripping tower is called a stripping column. A stripping column arrangement may comprise one or more stripping columns. The stripping column is typically a tubular, upright construction wherein substances fed to the stripping column become completely mixed up together. The stripping column may be used for separating various fluids and solid material from each another by means of a flow of the fluids. A typical field of application is chemical industry. The stripping column, perse, is known to those skilled in the art.

Examine now such a stripping column arrangement which receives acid rich filtrate from hydrolysis of a cellulosic feedstock processing system, and a corresponding stripping method in the stripping column arrangement. Fig. 1 illustrates an example of a stripping column arrangement, which may comprise one or more stripping columns 102, 102’, 102”. In the explanation, the stripping column 102 is used as an example but all the stripping columns 102, 102’, 102” may be structurally and operationally similar. The stripping column 102 receives residual part of the cellulosic feedstock which is hydrolysated and processed earlier in the process. The feedstock fed to the stripping column 102 composes of dissolved hemicellulose sugars, residual fragments of lignin and organic acids used in the upstream processing of the biomass. The feedstock fed to the stripping column 102 may have been processed from wood, non-wood material, or both. The feedstock may additionally or alternatively include material processed from grass-stemmed plants. The feedstock may be recycled or fresh feedstock.

The stripping column 102 receives and hence contains acetic acid and formic acid in a desired proportion, and acetic acid and formic acid are utilized as bio-solvents for the process. In an embodiment, the relative amount of acetic acid in the organic acids may be about 15 % or more. In an embodiment, the relative amount of acetic acid in the organic acids may be about 85 % or less. In an embodiment, the relative amount of acetic acid in the organic acids may be about 30 % or more. In an embodiment, the relative amount of acetic acid in the organic acids may be about 70 % or less.

The stripping column 102 may operate for instance in a pressure range of 0.2 bar to 1.5 bar. In an embodiment, the pressure is 1 bar. Heavy and/or solid substances may be removed from a bottom 101 of the stripping column 102. The substances removed from the bottom 101 may be directed to further processing, for instance. The substances removed from one stripping column 102 (102’) may be fed to a next stripping column 102’ (102”). Vaporized substance may be removed from the top 103 of the stripping column 102.

The stripping column arrangement comprises a feed arrangement 104 that feeds acetic acid to the at least one stripping column in response to variation of concentration of formic acid within the at least one stripping column 102 and/or removal of formic acid from the at least one stripping column 102. The feed arrangement 104 inputs acetic acid to the upper parts 103 of the stripping column 102 in order to enhance the separation efficiency of the acids, namely formic acid has a low tendency to evaporate and it may have among the lowest tendency to evaporate in the mixture considering the vaporizable components (lower than acetic acid and furfural, for example).

The feed arrangement 104 may also be considered to feed the feedstock and formic acid and other substances to the at least one stripping column 102, but in general the feed arrangement 104 feeds at least acetic acid to the at least one stripping column 102, and in the following the feed of acetic acid is mainly described. The feed arrangement 104 may comprise one or more adjustable orifices or nozzles. In general, the feed arrangement 104 may comprise at least one tube or duct through with acetic acid may flow, and each of the tubes or ducts comprise an openable and closable aperture for controlling the input to the at least one stripping column 102. The feed arrangement 104 may be an actuator, which may be controlled by a controller 105, which may be a structural and/or operational part of the feed arrangement 104. The control of the feed arrangement 104 may directly be based on a signal or signals from the at least on sensor 200. The control of the feed arrangement 104 performed by the controller 105 may be based on a speed of flowing matter, the size of the opened aperture and duration how long the aperture is open. Namely, these can be used to define how much flowing matter, such as acetic acid is input to the stripping column 102.

This kind of solution is applicable to be used in a stripping column 102 and also in multistage evaporation train (columns 102, 102’, 102”) since the main principle is same in both cases. In the stripping column arrangement 100, the hemicellulose rich or hemicellulose including bio-solvent is fed to the top of the stripping column 102. Steam may be used as a stripping agent and can be fed to the bottom 101 of the stripping column 102. By introducing the vapor phase to the stripping column counter currently, desorption of desired components is achieved in the liquid-vapor interface. The bio-solvent components compromising mainly of acetic acid, formic acid and potentially furfural are classified as volatile components in the process. Thus, the desired components are transferred through liquid-vapor interface from the liquid phase to the vapor phase.

Examine now in more detail the operation of the stripping column 102. The desorption taking place in the stripping column 102 in the molecular level includes a desorption of the simple polarous molecules into the vapor phase. As the desorption phenomenon is examined more thoroughly it is noticed that the concentration of the vapor phase determines the effectiveness of the separation taking place in the stripping column 102. Also, the proportion of the bio-solvent components taking place in the desorption phenomenon is a critical variable considering the effectiveness of the separation and the consumption of the stripping agent (steam).

By investigating stage wise composition profile of the stripping column 102 and by calculating the relative volatility values for the components it is seen from a simulation, for example, that by increasing a concentration of acetic acid in the stripping column 102 the desorption of formic acid is enhanced. The effect observed in the process may be explained by considering the relative volatilities in the liquid phase. It is noticed that introducing a minor proportion of acetic acid to the multiple stages of the stripping column 102, the relative volatility of formic acid is increased. Thus, the separation effectiveness is increased compared to the case where similar product quality including an acetic acid and formic acid fraction in the product is considered. Correspondingly, a different extractive compound can be used in the stripping column 102 in a similar manner as described above. Similarly, the effect can be seen in the multistage evaporation train when a slight amount of acetic acid is introduced to the first stage of the evaporation.

Basically, the solution described herein covers the usage of acetic acid as a separation solvent in the stripping column 102. By increasing the concentration of acetic acid on a plate, the relative volatility of formic acid and water is increased. As a result, more formic acid is transferred from a liquid phase to the vapor phase. Thus, the hemicellulose syrup which is transferred through each stage from the upper part of the stripping column 102 to the bottom 101 contains less formic acid. The concentration of formic acid is often seen as a bottleneck in the purification process since other volatile components have higher relative volatility compared to water. The addition of acetic acid does not require excessive purification procedures since acetic acid is already present in the biosolvent feed. According to a simulation, the phenomenon is visible when acetic acid mass fraction compared to formic acid is higher. Optimal dosage of acetic acid needs to be calculated in a manner that an acetic acid mass fraction is approximately 50 % to approximately 400 % higher compared with that of formic acid. An optimal or more accurate proportion or proportion range for acetic acid depends on many factors and/or parameters such as materials processed in the stripping column 102, substances used in the process, processing temperature, processing pressure. In an embodiment, acetic acid mass fraction is approximately 100 % to approximately 200 % higher compared with that of formic acid.

Table 1. Enhancement of formic acid recovery into evaporate fraction by addition of acetic acid.

The proportion of acetic acid in each stage may be set considerably higher compared to a formic acid mass fraction. By increasing acetic acid concentration in upper stages, the relative volatility of formic acid can be increased resulting a higher purity for the hemicellulose syrup. Also, formic acid has been noticed to be the limiting factor in the process considering the required acid losses in the process to the concentrate. Acid proportion of acetic acid may be about 55 m-% to about 75 m-% in the liquid phase to enhance formic acid separation in a desired manner. For example, if the feed flow of bio-solvent contains 1 mol of formic acid, acetic acid concentration may be about 1.2 to about 3 moles in the mixture in order to enhance the separation efficiency in an optimal manner. In the laboratory experiment the acid proportion between formic acid and acetic acid were about 66 m-% to about 75 m-% in favor of formic acid, see Table 1.

Two batch-wise laboratory experiments were carried out in order to verify the phenomena observed during computational process simulations. The experiments were carried out as reproducible as possible. The first multistage evaporation was carried out using pure water injections during evaporation stages The latter evaporation experiment was carried out using diluted acetic acid injection between the evaporation stages. In both cases the hemifiltrate was diluted between each evaporation. Concentrate then was fed to next evaporation stage.

The results of the experiments are seen in Table 1. Added amount of water injections and diluted acetic acid injection concentrations is shown in fist two left-hand side columns. In the right hand side the results of the formic acid recovery is shown between each evaporation stage. As it can be seen the diluted acetic acid experiment yielded higher formic acid recovery rate.

The evaporation setup was executed in multi stage evaporation to mimic the behaviour of the stripping column.

In an embodiment, the stripping column arrangement may comprise at least one sensor 200 that measures the variation of concentration of formic acid within the at least one stripping column 102 and/or the removal of formic acid from the at least one stripping column 102. The at least one sensor 200 also sends information on the variation of concentration of formic acid within the at least one stripping column 102 and/or the removal of formic acid from the at least one stripping column 102 to the feed arrangement 104 for enabling the feed arrangement 104 to control the feed of acetic acid to the at least on stripping column 102.

In an embodiment, the at least one sensor 200 may measure the variation of a relative concentration of formic acid and acetic acid within the at least one stripping column 102. Alternatively or additionally, the at least one sensor 200 may measure the variation of a relative concentration of formic acid and acetic acid removed from the at least one stripping column 102 for enabling the feed arrangement 104 to control the feed of acetic acid to the at least on stripping column 102. The controller 105 is configured to keep the relative concentration of formic and acetic acid in a predetermined range, or guide the relative concentration of formic and acetic acid to a predetermined range if it is outside the predetermined range. The predetermined range may be adaptive as a function of time and/or stage of the stripping column 102, for example.

In an embodiment, the feed arrangement 100 may have, or be configured to receive, estimated data on the variation of concentration of formic acid within the at least one stripping column 102 and / or the removal of formic acid from the at least one stripping column 102 as a function of time and/or as a function of stages of the at least one stripping column 102 for controlling the input of acetic acid into the at least one stripping column 102. The estimated data may be formed in a test process, simulation or calculated based on a theory. The estimated data may be stored in one or more memory of the controller 105.

In an embodiment, the feed arrangement 104 may feed acetic acid to one or more stages in an upper half of the at least one stripping column 102. The term upper half refers to a part of the stripping column 102 that is between the top 103 from where vapor is removed and the middle of the stripping column 102. The top 103 is typically higher than the bottom 101 from where the hemicellulose concentrate product is removed.

In an embodiment, the feed arrangement 104 may feed steam to the at least one stripping column 102 in response to the variation of concentration of formic acid within the at least one stripping column 102 and/or the removal of formic acid from the at least one stripping column 102.

In an embodiment, the feed arrangement 104 may feed furfural in addition to acetic acid to the at least one stripping column 102 in response to the variation of concentration of formic acid within the at least one stripping column 102 and/or the removal of formic acid from the at least one stripping column 102.

In an embodiment, the feed arrangement 104 may feed acetic acid to the at least one stripping column 102 based on the relative percentage of acetic acid and formic acid within the at least one stripping column 102, or in vapor or liquid removed from the at least one stripping column 102.

In an example of Fig. 2, composition profiles of conventional stripping arrangement with no added acetic acid is shown. As it can be seen from Fig. 2, acetic acid fraction decreases drastically during the first 5 stages. Correspondingly the decrease of the formic acid proportion is remarkably decelerated.

In an example of Fig. 3, relative volatilities of conventional stripping arrangement with no added acetic acid is shown. The relative volatilities are compared to that of water. Furfural possesses a highest relative volatility thus it evaporates from the mixture first. Steam is used as a stripping agent thus water constantly evaporates and condensates to the liquid phase. As it was noted in association with Fig. 2, acetic acid concentration decreased drastically during the first 5 stages. The effect of acetic acid concentration diminishing can be seen from the relative volatility of formic acid which decreases also when there are less acetic acid present in the stages 5 to 30. The relative volatility of formic acid decreases below 0.4 at the stage 4. Next, we shall inspect the profiles from a case where acetic acid is injected to the stripping column 102.

In an example of Fig. 4, it can be noticed that the acetic acid fraction keeps high during the first 15 stages. This results in effective vaporization of formic acid. The steam feed is kept constant in the example of Fig. 4. Comparing the relative volatility of formic acid in Fig. 3 and Fig. 5, it can be seen that resulting from an acetic acid injection the relative volatility of formic acid is maintained in a higher level longer. Previously the relative volatility of formic acid decreased below 0.4 in the stage 4. In the acetic acid injection case in the example of Fig. 4, the relative volatility of formic acid decreases below 0.4 as late as in the stage 9.

The example of Fig. 5 illustrates the same results as the example of Fig. 4, but in the example of Fig. 5, the relative volatilities are compared to water.

The feed material in the example of Figs. 2, 3, 4, and 5 consisted of 19.5 m-% of water, 20.6 m-% of acetic acid, 13.9 m-% of formic acid and 2.5 m-% of furfural, the rest being hemicellulose in the liquid phase. The feed flow was 27 t/h which means there were 5.6 tons of acetic acid in the feed. Similarly, there were 3.7 tons of formic acid.

The example of Fig. 6 illustrates an example of the controller 105, which may comprise one or more processors 600, and one or more memories 602 including computer program code. The one or more memories 602 and the computer program code are configured to, with the one or more processors 600, cause the feed arrangement 104 at least to feed acetic acid to the at least one stripping column 100 in response to variation of concentration of formic acid within the at least one stripping column 102 and/or in formic acid removed from the at least one stripping column 102.

Figure 7 is a flow chart of the stripping method. In step 700, acetic acid is fed to the at least one stripping column 102 in response to variation of concentration of formic acid within the at least one stripping column 102 and/or in formic acid removed from the at least one stripping column 102.

As a conclusion, the steam consumption can be lowered when acetic acid is injected to the upper stages of the stripping column 102. What is described in this document is related to existing Formicopure™ technology since the condensate obtained from the stripping column consists mainly of formic acid, acetic acid, water and potentially furfural. However, process these components are individually separated in the Formicopure™ technology and the Formicopure™ technology suffers from problems of the prior art. This means that proportion of acetic acid obtained from the chemical recovery process may be to be recycled back to hemicellulose concentration process.

By following what is taught in this document, the steam consumption in the stripping column 102 may be lowered (and/or end product quality may be heightened). As the desired purity for the hemicellulose syrup is typically set to be constant, by introducing extractive component i.e. acetic acid to the stripping column 102, the separation efficiency is enhanced. This can be seen from a simulation where the relative volatility of formic acid is increased. By increasing the relative volatility of formic acid compared to water, steam consumption can be lowered compared to the case where acetic acid injections were not made. The described solution provides advantages. There is no compulsory need for a more complicated and additional purification equipment when acetic acid is added. The size of the stripping column (arrangement) may be smaller than in the prior art if the steam consumption i.e. the energy requirement of the stripping column (arrangement) is maintained at the same level. Energy consumption may be lowered as formic acid becomes more volatile in the system when acetic acid is added in the stripping column arrangement. As a result of addition of acetic acid, formic acid evaporates more easily. Hemicellulose rich or hemicellulose including syrup becomes purer with less energy when acetic acid is fed to the at least one stripping column in response to variation of concentration of formic acid within the at least one stripping column and/or removal (desorption) of formic acid from the at least one stripping column.

The method shown in Figure 7 may be based on control implemented as a logic circuit solution or computer program. The computer program may be placed on a computer program distribution means for the distribution thereof. The computer program distribution means is readable by a data processing device, and it encodes the computer program commands, carries out the measurements and optionally controls the processes on the basis of the measurements.

The computer program may be distributed using a distribution medium which may be any medium readable by the controller. The medium may be a program storage medium, a memory, a software distribution package, or a compressed software package. In some cases, the distribution may be performed using at least one of the following: a near field communication signal, a short distance signal, and a telecommunications signal.

It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the example embodiments described above but may vary within the scope of the claims.