ORIGINAL ARTICLE

Energetic and Environmental Integration in Factories of Sugar Cane Byproducts

MSc. Dania Alonso-Estrada^{I  *}  , Dr.C. Manuel Díaz-de los Ríos^I  , Mr. Dongrui Zhan^II  , Ms. Jinghai Jian^II  , Ms. Qi Zhang^II  , Ms. Saihua Wang^II 

^ICuban Research Institute of Sugar Cane By-products, Havana, Cuba

^IITsinghua Innovation Center in Dongguan Biorefinery Engineering Research Center, Dongguan China

ABSTRACT

Anaerobic treatment is an alternative to treat residual of the ethanol production process to produce biogas as renewable energy. The purpose of this study is the use of Microsoft Excel, Solver and SolverTable add-ins for the simulation of processes in an agroindustrial complex. It is formed by a torula yeast plant that operates with vinasse as a carbon source and the production of biogas using digesters of the UASB type for the management of some vinasse and the residual of the process of torula yeast production. The study considers an ethanol production capacity of 90,000 L/d and a maximum yeast production of 30 t/d. The results show that the energy demand of a production of 17 t/d of yeast can be satisfied by the generation of biogas and a waste out COD concentration between 11 and 17 kg/m³.

Key words: bioethanol waste; torula yeast waste; biogas

INTRODUCTION

The use of sugar cane industry wastes has been widely researched by many authors. As sugar cane, bagasse is mainly used in energy cogeneration and the filter cake has a wider use in composting; vinasse from distilleries have found an excellent opportunity for biogas production, although their most widespread use in Cuba and Brazil continues being the cane fertilization and irrigation.

A comparative study on biogas production potential of various wastes resulting from the sugar cane industry in Brazil was presented by ^{Janke et al. (2015)}. In their work different types of wastes, such as vinasse, bagasse and filter cake are evaluated from the kinetic point of view. In the case of vinasse, they conclude that the methane potential is in the order of 246-302 ml CH₄ / g COD, depending on the distillery (independent or attached to a sugar mill) and the results fit to a first order kinetic model.

Mathematical modeling of the anaerobic digestion process is a very useful tool to predict the potential of biogas production and the behavior of the biogas reactor. Several types of models have been reported in the literature depending on the research objective. An excellent review of existing kinetic models for anaerobic digestion systems was presented by ^{Lyberatos & Skiadas (1999)}. Although kinetic modelling is beyond this study, it is necessary to evaluate the advances in vinasse and yeast waste in this field and biogas potential forecasting in those wastes.

Basic research on kinetic of biogas production from vinasse, using modified Gomperts model with variation of COD:N relation in batch anaerobic digestion was presented by ^{Budiyono et al. (2013)} and ^{Syaichurrozi & Sumardiono (2013)}. They found that variation of COD:N ratio affected parameters of kinetic model for biogas production; COD:N relation of 600:7, was the best ratio with values of both biogas production potential and minimum time to produce biogas of 109-132.6 mL biogas/gCOD and 0.803 day, respectively. Another studies (^{Budiyono & Sumardiono, 2014a}), were directed to evaluate the influence of initial pH in batch anaerobic digestion and the total solid content on biogas production rate (^{Budiyono and Sumardiono, 2014b}). Best results were showed at pH 7 with a biogas potential of 6,49 mL biogas/g VS, while vinasse: water ratio of 1:3 produced the maximum total biogas (37.409 mL/g COD); although vinasse: water ratio of 1:2 had the biggest COD removal.

Temperature effects on the methane genesis and sulfate reduction of distilleries vinasse was investigated in up-flow anaerobic sludge blanket reactors (UASB) at 35, 45, and 55 ºC by ^{Castro and Durán (2002)}. Their work concluded that temperature has a positive effect on the generation of methane by the methanogenic bacteria in the range studied, whereas H₂S generation remains constant, indicating that sulfate-reducing bacteria activity is neither increased nor diminished due to temperature effects. First order kinetics model was fitted to all runs.

Research on the kinetics of biogas from various substrates has shown that the most important variables in both, the process control and the levels of organic matter removal, are the pH of the medium, temperature, organic loading rate (OLR), hydraulic retention time (HRT), the nutrient content and effluent: inoculum ratio. Therefore, researches have been focused to these aspects and toward the design and evaluation of different types of reactors (^{Sosa et al., 2014}).

UASB reactor is the most popular anaerobic digester tested for this kind of wastewater, although there are other reactors configuration and combined treatment methods which have been also considered (^{Sosa et al., 2014}).

Several studies on the biogas production from vinasse have been developed in Cuba. ^{Cabrera and Díaz (2013)} investigated the production of biogas in an anaerobic up-flow packaging filter of 3.4 L of capacity using organic loads between 1.9 to 19.9 g COD / L.d and different hydraulic retention times. The COD removal percentages were shown to be inversely proportional to the load applied and about 70 % COD of removal can be obtained if either loads of 5 g COD/L.d and TRH of 1 day or 16 g COD/L.d for 2 days of TRH are applied. Nevertheless, the biogas potential from Candida utilis yeast wastewater has received poor attention due its low DBO content (7400 mg/L) as result of the aerobic process of yeast production itself. Some researchers have evaluated different alternatives to treat wastewater coming from the baker’s yeast production (^{Zub et al., 2008}), including anaerobic digestion at industrial scale (^{Sirbu and Begea, 2011}). Wastewaters from baker’s yeast production are sulfate-containing waste, so many studies on anaerobic digestion of these waste have led to the sulfate removal in them (^{Krapivina et al., 2007}; ^{Zub et al., 2008}).

For the purpose of this work, models focused on evaluating the potential of biogas production are recommended. Simple ways to calculate the biogas production from organic matter are the models of ^{Buswell & Mueller (1952)}, ^{Boyle (1977)} or ^{Chernicharo (2007)}, because those are time independent models based on data about organic matter composition. All those models have been used for steady state simulation of biogas production from different substrates.

This study, by means of process simulation, considers the incorporation of a biogas plant for processing the waste in a yeast plant attached to a distillery, which utilizes vinasse as carbon source for yeast production.

METHODS

The study considers an ethanol production capacity of 90,000 L/d and a maximum yeast production of 30 t/d. Microsoft excel, Solver add-in and Solver table add-in were used for process simulation and sensitivity studies. Table 1 shows the process conditions considered during the simulation.

TABLE 1 Characteristics of process plants considered in the simulation 

Parameters	Industry
	Distillery	Yeast plant	Biogas plant
Plant capacity	90000 L/d	6-30 t/d	According waste volume
Waste water flow	16 L/L of ethanol	100 -200 m^3/t	120-130 L/m^3
Waste COD (kg/m^3)	50-70	14-20	Simulated
Percent of COD removed		60	Simulated 30-55

The Mathematical model considered both the mass and organic solid balances in each process step for the three plants. These are expressed by the following equations:

Mass balance

$\sum _{k=1}^2\left\{\sum _{j=1}^M\left[\sum _{i=1}^{N1}{F}_{i,j}^{in}-\sum _{i=1}^{N2}{F}_{i,j}^{out}\right]\right\}=0$

$F_{vt}-F_{vb}-F_{vy}=0$

Carbon balance

$\sum _{k=1}^2\left\{\sum _{j=1}^M\left[\sum _{i=1}^{N1}{F}_{i,j}^{in}.C_{i,j}^{in}/d_i-\sum _{i=1}^{N2}{F}_{i,j}^{out}.C_{i,j}^{out}/d_i\right]\right\}=0$

Energy balance can be expressed through the following equation:

$E_p-E_d-E_e-E_{g }\ge 0$

Methane production is estimated according to (^{Chernicharo, 2007}), while biogas generation considered 60% of methane content.

$V_{CH4}=\frac{{COD}_{CH4}}{K\left(t\right)}$

$K\left(t\right)=\frac{P.K}{R.\left(273+T\right)}$

Where:

k	- Plant number (yeast and biogas)
N1	- Number of inlet stream in each process stage
N2	- Number of outlet stream in each process stage
M	- Number of process stage in each plant
$F_{i,j}^{in}, F_{i,j}^{out}$	- Inlet and outlet streams (i) in the js process stages (kg/h)
Fvt	- Total vinasse flow (kg/h)
$C_{i,j}^{in}, C_{i,j}^{out}$	- Inlet and outlet concentrations related with different streams in js process stages (carbon concentrations). (kg/m^3)
di	- Density of flow (kg/m^3)
Fvt, Fvb, Fvy	- Vinasse total flow, flow used in biogas generation and flow used in yeast production, respectively.
Ep	- Energy generated by biogas plant
Es	- Energy used in yeast drying
Ee	- Electric Energy used in the yeast plant
Eg	- Electricity exported to the network.
VCH4	- Volume of methane production (L)
CODCH4	- Load of COD removed from the reactor and converted into methane (gCOD)
K(t)	- Correction factor for the operational temperature of the reactor (gCOD/L)
R	- gas constant (0,08206 atm.L/mol.^oK)
P	- atmospheric pressure (1 atm)
T	- operational temperature of the reactor (^oC)
K	- COD corresponding to 1mol of CH4 (64gCOD/mol)

RESULTS AND DISCUSSIONS

Substrates Characterization

The properties of the two wastewater considered in the analysis are showed in Table 2. It can be seen that there is a great dispersion of values reported for the vinasse as a result of the operation at the distillery with molasses from different sugar mills throughout all the harvest period. COD: BOD ratio for vinasse is 2,34±1,68 and 2,41±1,60 for yeast wastewater.

The use of distilleries vinasse as carbon source in the production of fodder yeast has a positive impact on the reduction of yeast production costs (replacing molasses), and helps to reduce the vinasse organic load about 60%, so this alternative was set up as a strategy for vinasse treatment.

However, even with the use of vinasse as carbon source, the production of fodder yeast, or torula yeast, as it is also known in Cuba, has competitive disadvantages with soybean meal for animal feeding, given its high production cost due the high levels of energy consumption in the process. The electricity consumption of this production amounts 1300 kWh per ton of yeast, while fuel oil consumption for yeast drying is of 420-430 kg/t of yeast.

TABLE 2 Composition of wastewater used in the simulation 

Parameters	Vinasse		Yeast wastewater
	Mean	SD	Mean	SD
COD (mg/L)	53738	13548	15824	5477
BOD (mg/L)	22999	8050	6542	3425
Total Nitrogen (mg/L)	272	274	140	90
Total Phosphorus (mg/L)	245	242	589	274
ST (mg/L)	39619	18359	13525	5121
STF (mg/L)	10743	5759	5825	3621
STV (mg/L)	31624	11952	7700	2905
SDT (mg/L)	25218	25563	12390	4620
SDF (mg/L)	6613	7047	5430	3298
SDV (mg/L)	18635	18795	6960	2484
SST (mg/L)	18561	18098	1135	622
SSF (mg/L)	9913	29222	395	402
SSV (mg/L)	14259	13268	740	457
pH	4,22	0,34	5,84	1,94
CE (mS/cm)	11,05	6,76	9,68	1,92

The wastewater resulting from yeast production is about 120-130 m³/t yeast with an organic load of about 15 kg/m³ of COD. Preliminary methanogenic test of this effluent reported that it still has a biogas potential equivalent to 30 % of COD, which could contribute to reduce the energy costs of such production.

Methanogenic test of vinasse is showed in figure 1, where methane production values agree with those obtaining by (^{Janke et al., 2015}).

FIGURE 1 Methanogenic test of vinasse from ethanol distillery from molasses. 

The interrelation among the three industrial installations it is represented in the block diagram of Figure 2, where the distillery’s vinasse it is used in the yeast production, even a fraction of it can be used for biogas generation together with the wastewater resulting from the yeast factory. Higher deviation of vinasse flow to biogas production will raise the percent of COD removal in the factory, with the ensuing increased biogas production, due a higher BOD of vinasse, but the production capacity of the yeast plant will be reduced. The biogas produced can optionally be either employed for drying the yeast or to generate electricity for the process. The problem it is to determine the capacity of yeast that makes it competitive with soybean meal.

FIGURE 2 Interaction between distillery, yeast plant and biogas plant. 

Steady state simulation results

The maximum feasible yeast production depends on the amount of vinasse generated by the distillery and its organic composition. For a vinasse concentration of 53,7 kg/m³ the maximum capacity of yeast production is about 16.4 t/d at the production cost of 718 cuban pesos (CUP)/t, regardless the existing yeast plant capacity is higher. Therefore, in this case, to further increasing of yeast production is essential an additional carbon source, such as molasses, raising production costs due to its market price.

Preliminary evaluation shows that biogas production with yeasts waste only can satisfy the 38% of the energy required by the process to any percent of yeast capacity utilization, because the volume of yeast waste and its use in biogas production also depends on the production capacity.

An alternative is to shift a fraction of vinasse flow to produce biogas, so that energy contribution from waste (vinasse and yeast waste mixed) is increased and the production cost reduced. The variation of energy contribution from biogas production for various levels of yeast capacity utilization using vinasse of 53,7kg/m³ COD is illustrated in Figure 3.

FIGURE 3 Energy level can be satisfied in yeast production with the capacity increased. 

It is evident that while the capacity does not exceed 9,6 t/d of yeast, energy produced may meet 100 % of the energy required with biogas resulting from a mixture of vinasse - yeast waste, and even there it is a surplus of electricity for sale to the grid. Above 9.6 t/d of yeast the satisfaction of energy level decrease, first affecting demand in electricity and subsequently the demand in yeast drying consumption when production capacity exceeds 14t/d.

During the increase of yeast production capacity, the vinasse flow toward biogas production decrease with the consequently decrease of surplus electricity to the grid. The yeast production capacity of 9,6 t/d may be satisfied with the 51,8% of the vinasse available, while an additional flow of 683,7 m³/d mixed with yeast wastewater is in off to satisfies the 100% of the yeast energy requirement by biogas production, as it is showed in figure 2.

The impact of this approach on yeast production cost is illustrated in Figure 4, where the existence of an optimal cost is observed for the capacity of 9,6 t/d.

FIGURE 4 Variation of the yeast production cost with increased production capacity. 

From the environmental point of view, it should be noted that the level of COD removal depends on the mixture vinasse: yeast waste used as it is showed in Figure 5. For low production capacities of yeast, where a significant volume of vinasse can be used for biogas production, the percent COD removal exceeds 50%; however, this drops dramatically to 30 % when the maximum capacity available is reached.

The Input organic concentration to biogas plant will depend on the mix vinasse: yeast waste used, while the output concentration depends on both the input conditions and COD removal percent.

COD concentration at the outlet will be between 11 and 17 kg/m³, values that do not meet the dumping standards, in spite of be low, but allow considers the waste as irrigation water.

Of course, the model evaluated is sensitive to changes in the COD vinasse input, so that if it is raised, to a fixed COD reduction level in the yeast plant, the production potential of yeast and biogas will increase and therefore the energy satisfaction of yeast production.

The inflexion point indicating a minimum cost in figure 5 for the use of vinasse of 53,7 kg/m³ of COD, takes values of 10,51 and 11,9 t/d for use of vinasse of 60 and 70 kg/m³ of COD respectively.

FIGURE 5 Variation of COD removal percent and the outlet of yeast plant (inlet of biogas plant) with increased production capacity. 

CONCLUSIONS

• The mathematical model allowed to evaluate through the balance sheets of mass and organic solids the energetic and environmental interrelation of the production plants of ethanol, torula yeast and the biogas plant.

• For a real capacity of distillery and production process for the capacity of 9,6 installed yeast plants, the existence of an achieved and 100% of the required energy can be substituted with biogas.

• From the environmental point of view, a level of COD elimination is obtained, which ranges between 11 and 17 kg/m³, allow reach a waste making it possible to be considered as irrigation water.

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Received: December 15, 2019; Accepted: March 13, 2020

^*Author for correspondence: Dania Alonso-Estrada, e-mail: [e-mail]

Dania Alonso-Estrada; Researcher, Cuban Research Institute of Sugar Cane By-products, Havana, Cuba, e-mail: [e-mail]

Manuel Díaz-de los Ríos; Researcher, Cuban Research Institute of Sugar Cane By-products, Havana, Cuba, e-mail: [e-mail]

Dongrui Zhan; Researcher, Tsinghua Innovation Center in Dongguan Biorefinery Engineering Research Center, Dongguan China, e-mail: [e-mail]

Jinghai Jian; Researcher, Tsinghua Innovation Center in Dongguan Biorefinery Engineering Research Center, Dongguan China, e-mail: [e-mail]

Qi Zhang, Researcher, Tsinghua Innovation Center in Dongguan Biorefinery Engineering Research Center, Dongguan China, e-mail: [e-mail]

Saihua Wang, Researcher, Tsinghua Innovation Center in Dongguan Biorefinery Engineering Research Center, Dongguan China, e-mail:

The authors of this work declare no conflict of interests.