WO2013083772A1

WO2013083772A1 - Method for the pretreatment of process wastewater originating from hydrothermal carbonization processes

Info

Publication number: WO2013083772A1
Application number: PCT/EP2012/074777
Authority: WO
Inventors: Ingo Bauer; Ute Raab-Brill
Original assignee: L'Air Liquide, Société Anonyme pour l'Etude et l'Exploitation des Procédés Georges Claude
Priority date: 2011-12-09
Filing date: 2012-12-07
Publication date: 2013-06-13
Also published as: DE102011120631A1

Abstract

Method for the pretreatment of process wastewater originating from hydrothermal carbonization processes to prepare it for the downstream treatment by applying a technology for the separation of phenolic compounds wherein an antioxidant is added to the process wastewater for suppressing flocculent precipitation.

Description

Method for the Pretreatment of Process Wastewater Originating from Hydro- thermal Carbonization Processes

Field of Invention

The invention relates to a method for the pretreatment of process wastewater originating from hydrothermal carbonization processes to prepare it for downstream treatment by applying a technology for the separation of phenolic compounds.

State of the Art

In connection with hydrothermal carbonization processes, also referred to as HTC processes, biomass together with water and a catalyst are exposed to elevated temperatures of, say, between 180 and 350 °C in a pressure-tight reactor, whereby the water in the reactor is held in the liquid phase. The product water obtained as a result of the chemical reactions is taken up by the water fed to the reactor at the beginning of the process. Hereinafter, this combined water phase is described as process water. Part of this water can be used further for the next charge and part has to be discharged from the HTC process for controlling the concentration of byproducts in the process water and must be disposed of as process wastewater.

The byproducts contained in the process water include phenolic substances of which the monovalent phenols like phenol, cresol and guaiacol call for special attention because they are hazardous to health and to the environment since, depending on the nature of the biomass fed and on the process tech- nology applied, their concentration in the process wastewater can be so high that the process wastewater must not be routed directly into a biological wastewater treatment plant.

Hence special measures have to be applied in order to separate the major part of the monovalent phenols from the process water.

Known methods for separating phenols from water are extraction processes such as the Lurgi Phenolsolvan Process, and adsorption processes such as the activated carbon method of BERGBAUFORSCHUNG, cf. Lurgi Manual, issue 1970, Lurgi Gesellschaften, Frankfurt am Main and Ullmann's Encyclopedia of Industrial Chemistry, 4^th edition, volume 14, chapter 6.1 .2.

It has been found that the direct application of the known separation methods to the wastewater of the HTC process causes problems in the form of floccu- lent precipitates in this wastewater on contact with atmospheric oxygen, presumably caused by oxidation of the vegetable secondary substances contained in the HTC process or wastewater, such as tannins and other polyphenols and/or oxygenates. This phenomenon sensibly hampers the process flow of the phenol separation method employed.

When applying an extraction process, the equipment used such as columns and/or mixer-settlers are contaminated with the flocculent precipitates; in the adsorption process the adsorbent is contaminated and blocked. Upstream filtration of the wastewater will not reliably eliminate the flakes because, as a function of the vegetable material used in the HTC process, floc- culation may only set in with a delay which means when the wastewater has already passed through the filter. Consequently, the substances causing floc- culation may go through the filtration stage in a dissolved or colloidal form and trigger the above-described problems in the downstream treatment stage. The contaminations caused by flocculation require thorough cleaning of the equipment employed and/or lead to high losses of adsorbent and hence economic inefficiency of the separation process for phenols. Moreover, the time of availability of the after-treatment stage is reduced by the necessary cleaning work.

The problem to be solved was therefore to make available a method of separating monovalent phenols from HTC wastewater which would avoid the above-described drawbacks.

Description of the Invention

The problem is solved by a technology which comprises the features of Claim 1 . Preferred embodiments of the process according to the invention are specified in the sub-claims.

Furthermore, the invention relates to the use of salts of the sulfurous acid, its anhydride sulfur dioxide (S0₂) and/or sulfurous acid proper as an antioxidant for the pretreatment of the process wastewater originating from hydrothermal carbonization processes, as specified in Claim 6.

Description of the Invention and of its Preferred Embodiments

A preferred embodiment of the invention consists in using an antioxidant whose active component is formed of sulfite ions and/or hydrogen sulfite ions (S0₃ ^2", HS0₃ ^"). Agents of this kind are readily available at a moderate price on the chemicals market. In principle, all salts of sulfurous acid are suitable. Moreover, mixtures of the aforementioned substances may be applied as antioxidants. Another preferred embodiment of the invention consists in the use of a metal sulfite solution as antioxidant, specifically sodium hydrogen sulfite. This embodiment has an advantage in that the addition of the metal sulfite with an antioxidant effect safely suppresses the above-described flocculation where the existent sodium cation is especially unproblematic with a view to the downstream wastewater treatment. In this way, as a function of the predetermined sulfite concentration a chemical stabilization of the HTC process wastewater is attained which lasts up to several weeks. Thanks to this stabilization it was possible to overcome the above-described drawbacks and thus to have in place an economically feasible state-of-the-art separation method for the phenols.

A further preferred embodiment of the invention consists in the use of sulfu- rous acid and/or sulfur dioxide as antioxidant. These substances are chosen whenever the addition of metal ions to the treated water is not desirable.

Advanced modifications, advantages and potential further applications of the invention also result from the description of examples of embodiment and quantities as well as the drawings provided below. All features described and/or illustrated per se or in any discretionary combination constitute the in- vention, irrespective of their summary in the Claims or their back-reference.

Based on the embodiment example below, the process according to the invention is explained in detail. The necessary amount of antioxidant for adequate stabilization of the HTC process wastewater is dependent on the reaction conditions and, in particular, the feedstock composition in the HTC process. Empirically, this amount is in the range of 50 to 200 ppm S0₃ ^" and can easily be determined in advance by an expert with a simple manual assay. A distinct brightening and yellowing of the HTC wastewater after the addition of sulfite may be an indicator for the right sulfite supply. After this sulfite addition, furthermore the color intensifica- tion of the HTC wastewater and/or the flocculent precipitate after a previously defined holding time of the wastewater may be used as an indicator.

The so-called phenol index (DIN EN ISO 1 1885) represents a rough approxi- mate value for the analytical determination of the necessary sulfite addition. This index is analytically determined in the unit mg/liter and is in a positive correlation with the visually observed flocculation and color intensification of the HTC wastewater. Empirically, the necessary sulfite addition is in the range of 20 to 50 % w/w of the previously determined phenol index of the wastewater. As a function of the feedstock characteristics, wastewaters from the HTC process may differ distinctly in their composition. But based on the aforementioned guidance values an expert may easily determine the sulfite amount to be added to the respective wastewater. Example of Embodiment and Quantities

Applying the above-described stabilization method, 200 ppm SO₃ ^2" were added to HTC wastewater with a total phenol concentration of 1000 ppm and the wastewater was subsequently pumped through an adsorption bed. A styrene- divinylbenzene copolymer (Amberlite XAD4^®) was used as adsorbent. The process can be operated troublefree for a period of 8 hours, i.e. without recognizable clogging of the adsorption bed until the capacity of the adsorbent is exhausted.

The volume flow rate of the HTC wastewater through the adsorption bed was 2 times the liquid volume of the adsorption bed per hour.

The table below shows the chronological sequence of the total phenol concentration trend at discharge from the adsorption bed. The breakthrough of the phenols after exhaustion of the adsorbent after an operating period of be- tween 7.5 and 9.5 h can be clearly recognized. Table: Chronological sequence of the total phenol concentration trend (phenol index according to DIN EN ISO 1 1885 in mg/l) up to the outlet of the adsorption bed Time [hi Phenol concentration [ppml

0.0 10

0.5 10

1 .5 10

3.5 10

5.5 13

7.5 13

9.5 140

Reference Test

An analogous test without sulfite addition, but otherwise as illustrated with the embodiment example, had to be stopped after about one hour on account of increasing clogging of the adsorption bed.

Claims

Claims:

Method for the pretreatment of process wastewater originating from hydrothermal carbonization processes to prepare it for downstream treatment by applying a technology for the separation of phenolic compounds, comprising the following process steps:

(a) Making available process wastewater originating from hydrothermal carbonization processes,

(b) Addition of an antioxidant at a rate which prevents the flocculent precipitation,

(c) Submission of the pretreated process wastewater from step (b) to a downstream treatment process for separating phenolic compounds.

The method of Claim 1 wherein an antioxidant is used whose active component is a sulfite ion and/or hydrogen sulfite ion.

The method of Claim 2 wherein a metal sulfite and/or metal hydrogen sulfite solution is used as antioxidant.

The method of Claim 3 wherein a sodium hydrogen sulfite solution used as antioxidant.

The method of Claim 2 wherein sulfurous acid and/or sulfur dioxide is used as antioxidant.

Use of salts of sulfurous acid, its anhydrides and/or sulfurous acid proper as an antioxidant for the pretreatment of process wastewater originating from hydrothermal carbonization processes.