FIELD OF THE INVENTION

The present invention relates to a method and a device for treating hydrolyzed biomass in which thermally treated biomass is discharged from a pressurized hydrolysis reactor into a blow tank, wherein the absolute pressure is maintained below atmospheric pressure. The hydrolyzed biomass is cooled and detoxified by flash evaporation to a temperature suitable for further treatment with for example enzymes.

BACKGROUND ART

It is known in the art to pre-hydrolyse biomass sugars in a reactor in which hemicellulose sugars are dissolved from biomass at elevated temperature and pressure, optionally in the presence of an acidic catalyst. With biomass is here meant any type of lignocellulosic material which contains hydrolysable hemicellulose or cellulose sugars. The biomass is heated with saturated or superheated steam and the temperature in the reactor is typically around 200° C. and the pressure is in the range 15 to 20 bar. There is no free liquid level in the reactor, but the biomass comprises a certain amount of moisture. The biomass is discharged from the reactor after completed sugar hydrolysis to an atmospheric receiving container, in the following called a “blow tank”. The temperature of the discharge, which contains mainly biomass, hydrolyzed sugar and water, drops rapidly to about 100° C. when water and other volatile compounds evaporate to equilibrium at the pressure prevailing in the blow tank. Said pre-hydrolyzed biomass is then enzymatically hydrolyzed to release cellulosic sugars. The released sugars are fermented to ethanol or used as feed stock for various chemicals.

The pre-hydrolyzed biomass must be cooled to low temperature and it must be diluted typically to about 40-60° C. and to 10-20% insoluble material content to secure a good enzymatic activity in downstream hydrolysis steps. It is known in the art to cool and dilute hydrolyzed biomass by mixing it with a cold or cooled coolant for example water, recycled process condensate or recycle process filtrate. Additionally, the pH of the pre-hydrolyzed slurry must be adjusted to a pH range 4.0 to 6.5 for optimum enzymatic activity.

Said method of cooling by mixing coolant to the pre-hydrolyzed biomass may result in dilution of the slurry below 15%, which has the drawback of suboptimal conditions with high operation cost because the dilution liquid will be removed in downstream process stages. In addition, the diluted slurry contains carboxylic acids and furfural, which are toxic and may reduce activity of enzymes and yeast in the enzymatic hydrolysis and fermentation steps.

EP 18 208 026.7 discloses a method for treating biomass in which thermally treated biomass is discharged from a pressurized pre-hydrolysis reactor unit and dewatered followed by cooling of the separated liquid phase. Said invention teaches to install a dewatering press or alike to separate hot hydrolysate before cooling it with a heat exchanger. The cooled hydrolysate is then back-mixed with the dewatered biomass which is pumped to enzymatic hydrolysis. The inventors claim that problems with scaling and plugging of cooling heat-exchangers may be reduced with their invention.

US 2015/0232902 A1 describes a method of cooling a thermally hydrolyzed biomass prior to mixing it with enzymes. Said patent teach to split the enzymatic hydrolysis in two steps such that the first enzymatic hydrolysis step operates with a shortened duration (small reactor volume) and the second enzymatic step(s) operates with long duration (large reaction volume) to provide completed saccharification. The duration of the first reactor is such that it saccharifies (liquefies) cellulose enough to provide a pumpable water (hydrolysate) solution through a recycle cooler (heat exchanger). This way it is no longer necessary to recycle coolant (hydrolysate) over the second enzymatic step. The inventors claim that the total (first and second) reaction volume is much smaller in comparison to having only one enzymatic reaction volume. However, said application is still proposing cooling of hot hydrolysate by a recycling process.

EP 3212764 B1 discloses a method for treating biomass and organic waste in which the biomass is discharged from a pressurized pre-hydrolysis reactor by steam explosion into one or more pressure relief tank(s), in which the pressure is reduced from 5-35 bar to atmospheric pressure. The discharged biomass has a temperature of 95-110° C. and is processed to ethanol, hydrogen, lactic acid, methane organic acids or other desired produced by fermentation at a pH of 7-8.5.

There is still a need for an improved method and device for cooling thermally treating biomass to a temperature range of optimum enzymatic activity, which is typically about 40 to 60° C., which avoids diluting the thermally treating biomass with a cooling liquid. Additionally, the problems of adjusting the pH to the pH range of optimum enzymatic activity of 4.0 to 6.5, without further diluting the thermally treating biomass and the presence of toxic components like carboxylic acids and furfural in the pretreated biomass slurry are not solved.

SUMMARY OF THE INVENTION

The present invention relates to a method for treating biomass comprising the steps of(a) feeding lignocellulosic biomass to a pressurized reactor;(b) supplying steam to the pressurized reactor and at least partially condensing said steam in the pressurized reactor;(c) hydrolysing hemicellulose sugars of said biomass with said at least partially condensed steam in the pressurized reactor;(d) discharging said hydrolyzed biomass from the pressurized reactor into a blow tank by steam explosion to produce steam exploded biomass;(e) diluting the steam exploded biomass in the blow tank with an aqueous diluent to a pumpable slurry; discharging the pumpable slurry from the blow tank as discharged pumpable slurry stream;(f) treating the discharged pumpable slurry stream with an enzyme, characterized bywithdrawing a vaporous aqueous stream from the blow tank for maintaining the absolute pressure in said blow tank below atmospheric pressure.

Further, the present invention relates to a device for treating biomass comprisinga biomass feeding system;a pressurized reactor for thermally treating the biomass;a blow tank;a discharging system for discharging the hydrolyzed biomass from the pressurized reactor to the blow tank by steam explosion; anda unit for enzymatically treating a pumpable slurry stream of biomass discharged from the blow tank;whereinthe blow tank comprisesmeans for maintaining the absolute pressure in the blow tank below atmospheric pressure, preferably below 0.5 bar (a), still more preferably below 0.3 bar(a) and most preferably from 0.01 to 0.25 bar (a);at least one injection port for injecting an aqueous diluent;at least one discharge port for discharging said pumpable slurry stream of biomass;at least one discharge port for discharging a vaporous aqueous stream.

Definitions

Biomass is any source of plant material suitable for converting into pulp and paper material, cellulose-based construction material or biofuel. Suitable sources of biomass are lignocellulosic biomass such as virgin biomass, waste biomass and energy crops. Virgin biomass includes all naturally occurring terrestrial plants such as trees, bushes and grass. Waste biomass is produced as a low-value byproduct of various industrial sectors such as agriculture, e.g. selected from corn stover, sugarcane bagasse, straw etc., and forestry, e.g. selected from saw mill and paper mill discards. Energy crops are crops with high yield of lignocellulosic biomass produced to serve as a raw material for production of second generation (2G) biofuel. Examples include switch grass (Panicum virgatum) and elephant grass. In the present invention lignocellulosic biomass is used. The terms “lignocellulosic biomass” and “biomass” are used exchangeably.

