Study On The Utilization Of Bean Dregs Resource-Subcritical Water Hydrolysis Process And Pyrolysis Process

Biomass is vastly available and considered as a valuable renewable resource. The rational utilization of biomass wastes is important not only for the prevention of environmental issues, but also for the effective utilization of natural resources. As biomass wastes, bean dregs are the main by-product of soybean processing industry. The output of bean dregs is huge. More than 800,000 tons wastes of bean dregs are produced annually. The main components of bean dregs are crude cellulose and protein. Effective utilizing these two components and turning bean dregs waste into valuable material are of great interesting.Amino acids are the basic"building blocks"that combine to form proteins. They play an important physiological role in all life-forms. However, not all of the 20 standard amino acids can be metabolically synthesized by all creatures. Some of them (so-called essential amino acids) cannot be produced by the human organisms. These amino acids, therefore, have to be digested in sufficient amounts from foods. Amino acids are also important for several medical, cosmetic, and other industrial applications. Therefore, production of amino acids from bean dregs protein by hydrolysis has great significance.With the development of modern society, more and more energy are consumed. The use of fossil resources, such as petroleum and coal, produces a huge amount of carbon dioxide that exacerbates global warming. Reserves of petroleum and coal decrease. The dwindling supply of fossil resources calls for the immediate development of new energy and chemical resources. Among the many new energy resources developed, biomass energy is a strong candidate because biomass is a renewable resource and carbon dioxide is fixed by photosynthesis through regeneration. They do not cause additional increase in the carbon dioxide level. Reducing sugar, as a biomass energy precursor, can be further transformed to fuel alcohol in a fermentation process by means of micro-organisms or yeast. Therefore, production of reducing sugars from bean dregs cellulose by hydrolysis also has great significance.In conventional methods, the raw materials are split by acid, alkaline, or enzymatic hydrolysis, whereas the addition of further acidic or alkaline chemicals is necessary for the two former methods. These chemicals not only cause equipment corrosion but also lead to pollution and generation of a large amount of waste acid and waste alkali. Enzymatic process takes a long time (8-20 h) and leads to incomplete hydrolyses . The high cost of enzymes makes the process uneconomical.In recent years, sub-critical water has been gaining increasing attention as both an environmentally friendly solvent and attractive reaction medium for a variety of applications. It is cheap, non-toxic, non-flammable, and non-explosive. Its distinctly different behavior compared to water at ambient conditions is due to the dramatic changes in physical properties, namely dielectric strength and ionic product, which in turn can easily be altered by changing temperature and pressure. The ionic product of subcritical water is as much as three orders of magnitude higher than under ambient conditions. Under these conditions, there is a high H₃O⁺ and OH^- ion concentration (equivalent to weak acid or weak base). Itself has the function of acid-base catalysis. In this paper, subcritical water hydrolysis was employed as a method for producing high value-added products (amino acids and reducing sugars) from bean dregs waste.The feasibility of amino acid production from bean dregs protein by hydrolysis in subcritical water was studied. It was investigated that the effect of reaction temperature, reaction time and carbon dioxide on amino acid composition and on total amino acid yield. The optimum hydrolysis technology conditions for amino acid production were obtained. According to the characteristics of reaction, the kinetic model for amino acid production from bean dregs was proposed. The kinetic parameters were obtained by fitting them to experimental results. The results show that a variety of amino acids are produced. The concentrations of arginine, lysine and alanine are relatively high. Temperature and time have a great influence on the hydrolysis reaction. The effects of reaction temperature and time on yields of different amino acids vary. The concentration of arginine increases with increase of temperature. The addition of carbon dioxide can promote the hydrolysis of bean dregs. The highest yield of total amino acids is 22% at 330℃and 30 min. Two main reactions, hydrolysis of bean dregs protein to amino acids and decomposition of amino acids to other products, were observed in the protein hydrolysis. According to the characteristics of reaction, a kinetic model to describe the hydrolysis of bean dregs protein was proposed: a single consecutive reaction. The rate constants for arginine, alanine and total amino acids for different temperatures were determined based on the kinetic model. When the reaction temperatures are 200, 220 and 240℃, the formation rate constants for arginine are 0.0003, 0.0007 and 0.0011 s^-1 respectively and the decomposition rate constants for arginine are 0.0011, 0.0022 and 0.0036 s^-1 respectively; the formation rate constants for alanine are 0.0006, 0.0008 and 0.0011 s^-1 respectively and the decomposition rate constants for alanine are 0.0015, 0.0033 and 0.0097 s^-1 respectively; the formation rate constants for total amino acids are 0.00127, 0.0014 and 0.0017 s^-1 respectively and the decomposition rate constants for total amino acids are 0.0005, 0.0011 and 0.0024 s^-1 respectively. The formation activation energies and pre-exponential factors for arginine are 65.8KJ mol^-1 and 5.831×10³ s^-1 respectively; the decomposition activation energies and pre-exponential factors for arginine are 59.9KJ mol^-1 and 4.669×10³ s^-1 respectively. The formation activation energies and pre-exponential factors for alanine are 30.6KJ mol^-1 and 1.404s^-1 respectively; the decomposition activation energies and pre-exponential factors for alanine are 94.0KJ mol^-1 and 3.338×10⁷ s^-1 respectively. The formation activation energies and pre-exponential factors for total amino acids are 14.6KJ mol^-1 and 0.0516s^-1 respectively; the decomposition activation energies and pre-exponential factors for total amino acids are 79.1KJ mol^-1 and 2.696×10⁵ s^-1 respectively. The feasibility of reducing sugar (a biomass energy precursor) production from bean dregs cellulose by hydrolysis was studied. It was investigated that the effect of reaction temperature, reaction time and carbon dioxide on reducing sugar yield. The optimum hydrolysis technology conditions for reducing sugar production were obtained. According to the characteristics of reaction, the kinetic model for reducing sugar production from bean dregs was proposed. The kinetic parameters were obtained by fitting them to experimental results. Temperature and time have a great influence on the hydrolysis reaction. Under the experimental conditions, the yield of reducing sugar increases with increase of temperature. The addition of carbon dioxide can promote the hydrolysis of bean dregs and leads to an increase in reducing sugar yield. The highest yield of total reducing sugars is 65.7% at 300℃, 360 s and 3MPa (CO₂). The crude cellulose is converted into reducing sugars in the hydrolysis of bean dregs. Two main reactions, hydrolysis of crude cellulose to reducing sugars and decomposition of reducing sugars to other products, were also observed in the hydrolysis. According to the characteristics of reaction, a kinetic model to describe the hydrolysis of bean dregs cellulose was proposed: a parallel and consecutive reaction. The rate constants for total reducing sugars for different temperatures were determined based on the kinetic model. When the reaction temperatures are 260, 280 and 300℃, the formation rate constants for total reducing sugars are 0.00128, 0.00221 and 0.00450 s^-1 respectively and the decomposition rate constants for total reducing sugars are 0.00104, 0.0022 5and 0.00273 s^-1 respectively. The formation activation energies and pre-exponential factors for total reducing sugars are 79.7KJ mol^-1 and 7.95×10³s^-1 respectively; the decomposition activation energies and pre-exponential factors for total amino acids are 61.7KJ mol^-1 and 1.26×10³ s^-1 respectively.Bean dregs pyrolysis reaction was studied using an in-situ visualization capillary technique and thermogravimetric analysis. Pyrolysis characteristics and kinetics were investigated. The in-situ technique enables us to observe directly the processes and temperature of bean dregs transformation during pyrolysis reaction. Reaction phenomena in real-time can be recorded by a CCD camera monitoring system. In-situ visualization of reaction revealed that how oil is generated and expulsed concurrently from bean dregs during pyrolysis. Pyrolysis characteristics were investigated under a highly purified N₂ atmosphere using a thermogravimetric analyzer from room temperature to 800℃at different heating rates of 10, 30, and 50℃/min. The results show that three stages can be distinguished during the heating process. The moisture is removed in the first stage; the second stage is the main pyrolysis process and most of the organic materials are decomposed in this stage (the mass loss of this stage is more than 60% of total volatiles); and the solid residual slowly decomposed in the third stage. The initial temperature of pyrolysis and the temperature at which the pyrolysis rate reaches the peak value shift to the higher temperature as the heating rate increasing. The kinetic parameters (activation energy, pre-exponential factor) were obtained by Coats-Redfern method, Kissinger-Akahira-Sunose method and Flynn-Wall-Ozawa method.The innovation of this paper is as follows: High value-added products (amino acids and reducing sugars) were produced from bean dregs waste by hydrolysis in subcritical water. This method is simple, efficient, renewable, sustainable, and safe for environment. This technological process provides a new solution for the disposal of waste pollution and for the rational utilization of resources. The kinetic models for amino acid and reducing sugar production from bean dregs waste by hydrolysis were established. The kinetic parameters were obtained. The kinetic results may provide a new method for study on kinetics of biomass hydrolysis in subcritical water. The in-situ visualization capillary technique was developed. The technique enable us to observe directly the processes of bean dregs transformation during pyrolysis reaction. The technique is useful in helping us understand and study mechanisms.