CHEMICAL ENGINEERING TRANSACTIONS VOL. 52, 2016 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Peng-Yen Liew, Jun-Yow Yong, Jiří Jaromír Klemeš, Hon Loong Lam Copyright © 2016, AIDIC Servizi S.r.l., ISBN 978-88-95608-42-6; ISSN 2283-9216 The Potential for Digestate Thickening in Biogas Plants and Evaluation of Possible Evaporation Methods Marek Vondra*, Vítězslav Máša, Petr Bobák Brno University of Technology, Institute of Process Engineering & NETME Centre, Technická 2896/2, 616 69 Brno, Czech republic vondra@upei.fme.vutbr.cz Rising number of biogas plants (BGP) is associated with a rising amount of digestate, which is a by-product of biogas production. Liquid digestate (LD) is produced in mechanical separation of digestate, and contains valuable nutrients as well as large amounts of water. Treatment of the LD is thus rather expensive for the plant owners. One of the ways to drain the LD and obtain valuable concentrated nutrients is vacuum evaporation. Vacuum evaporation allows to utilize low-potential waste heat which occurs in excessive amounts in BGP. The potential for digestate thickening is further discussed. This paper also compares specific heat consumption and other selected parameters of three types of industrial evaporators, which may be suitable for the LD thickening. Presented results shows that the proposed model of MSF evaporator has the best thermal performance. 1. Digestate thickening potential in BGP One of the biggest challenges of modern society these days is to decrease impact of human activities on the environment while maintaining potential for sustainable future development and wellbeing of future generations. The European Union has been a world proponent and advocate of environmentally-friendly initiatives and measures. The EU has adopted the so called 20-20-20 Strategy which aims to reduce EU carbon dioxide emissions by 20 % (in comparison to 1990 levels), increase a share of renewable energy sources in the energy mix to 20 %, and increase efficiency of power plants by 20 % (compared to 2007 outlooks) (EC, 2010). One of the consequences of this EU policy has been an immense increase in number of biogas plants (BGP). There are more than 17 thousand of them in Europe in 2015, total installed capacity exceeds 8.3 GWel (EBA, 2015). The BGP boom was, among others, promoted by significant financial aid that makes BGP an attractive investment opportunity and facilitates an easy expansion of this environmentally-friendly technology. Thanks to power generation from biogas, BGP contribute to reduce consumption of primary sources of energy and decrease CO2 emissions. In contrast to conventional fossil fuels, BGP preserves natural carbon cycle in nature. However, BGP also has several disadvantages. One of the most significant ones is an ineffective use of heat that is produced in cogeneration units in biogas combustion. BGP commonly uses only 20–40 % of the heat and the rest is considered waste heat, and is usually not utilized at all. Yet, there are theoretically several ways to utilize the waste heat. The produced heat is mandatorily used for heating of the fermentor (main consumer of the heat). In addition to that, the heat from BGP could be used in a district heating system, greenhouse heating, drying of digestate, sludge and wood sawdust, cooling, and additional power production using ORC and Kalina cycle (Rutz et al., 2012). One of the reasons that none of the possible methods for utilization of the heat is ever explored and executed is the distance of most of the BGPs from industrial zones and populated areas; other reasons include quantity and quality of the heat, which ranges from 80–450 °C; seasonal fluctuations in heat demands (most heat is produced in summer); high costs of certain utilization methods (ORC) and low prices of fossil fuels (Rutz et al., 2012). DOI: 10.3303/CET1652132 Please cite this article as: Vondra M., Máša V., Bobák P., 2016, The potential for digestate thickening in biogas plants and evaluation of possible evaporation methods, Chemical Engineering Transactions, 52, 787-792 DOI:10.3303/CET1652132 787 BGP owners further have to face problems related to management of large quantities of digestate. There are no accurate statistics available, but on average, 15 thousand m3 of digestate per 1 MWe of installed capacity is quoted to be produced annually (CzBA, 2010). If current BGP capacity in the EU 27, Switzerland, Croatia and Serbia amounts to 8.3 GWel, digestate production in these countries reaches more than 120 million m3 of digestate. Digestate is a by-product of anaerobic digestion, described in detail in Pontoni et al. (2015), and it preserves minerals from the original materials. Therefore, it may serve as a fertilizer. However, nutrient concentrations in the digestate are rather low. Digestate leaving the fermentor commonly contains only ca. 8 % of dry matter, the rest is water. There are several reasons why the plant owners aim to reduce water from the digestate, and thus reduce its volume and increase its nutrient concentrations. One of the reasons is transport and storage costs. For small and medium-size BGPs, transport costs may amount up to 40 % of all costs incurred, and their minimization is obviously very much desired (Bojesen et al., 2015). Other motivation may come from legislation; legislators have limited use of agricultural fertilizers in order to protect quality of surface and groundwater (Drosg et al., 2015). Certain areas with intensive animal and plant production generate surplus of digestate; owners are then forced to transport it to distant lands or have it processed in waste water treatment plants. Basic digestate processing method is separation of digestate into solid and liquid fractions. Processing is done using belt filters and screw press, or decanter centrifuge. Solid fraction in the digestate contain more than 18 % of dry matter (depending on the technology); LD contains 2–4 % of dry matter and takes up most of the original digestate volume. Solid components contain notable amounts of phosphor and ammonia, but in a stable form that is not available to plants. They are therefore mostly used for aeration of soil (Heviánková et al., 2014). Most nutrients remain in the LD; its subsequent treatment may include several steps which aim to obtain concentrated nutrients. Membrane technologies (micro- and ultrafiltration, reverse osmosis), ammonia stripping, and evaporation are commonly employed here. Costs of these particular technologies (in relation to distance of fertilized lands and transport costs) are compared in Drosg et al. (2015). If waste heat is part of the technology, evaporation then seems very promising. Thickening of LD using evaporation is in many ways similar to thickening of common types of waste water and process water. But evaporation of LD is affected by higher share of dry matter and high viscosity of the suspension. There are many industrial evaporator manufacturers operating on the market, and they offer many product models for different kinds of wastewater. Heat for the evaporation process may be supplied from an external heat source in the form of steam or hot water. A heat pump and mechanical vapour compression are other types of heat source for the process. If there is waste heat available, last two evaporator options mentioned earlier are not that economical. Evaporators commonly have 1 to 3 evaporation chambers; the more evaporation stages are included, the higher is the efficiency of the process. A survey among manufacturers’ marketing materials suggests that specific thermal energy consumption of industrial evaporators with external heat source is in the range of 0.1 to 0.87 kWh kg-1, and specific electricity consumption varies between 0.008 and 0.062 kWh per kg of distillate. The particular consumption is strongly dependent on the unit capacity, number of evaporation stages and wastewater thermo-physical properties. 2. Comparison of different evaporation technologies Design of industrial evaporators using waste heat may differ in various aspects. Their purchase prices and energy demands which affect total payback period differ, too. A list comparing all available evaporators using waste heat is basically impossible to develop, and this paper therefore focuses on comparison of only 3 types of evaporators. These are two commonly available technologies with a basic design: single effect evaporator and multiple effect evaporator, and one less common multi-stage flash evaporator. In order to compare the technologies, we selected common parameters that reflect normal conditions in BGP (𝑇𝑑𝑖𝑔=20 °C, 𝑇ℎ𝑤 𝑖𝑛 =90 °C), average liquid digestate properties (𝑐𝑝𝑑𝑖𝑔=3.9 kJ kg -1 K-1, 𝑥𝑑𝑖𝑔=0.04 kg kg -1) and limitations of evaporation technologies and their components (𝑇𝑇𝐷ℎ=10 °C, 𝑇𝑇𝐷𝑐=5 °C, 𝑇𝑇𝐷𝑒𝑣=10 °C). Overall coefficient of heat transfer for water/digestate exchanger was determined, using our own experience, as 𝑈ℎ=0.5 kW m -2 K-1. Approximation (Eq(1) and (22)) was used for calculation of properties of the evaporator and condenser (El-Dessouky and Ettouney, 2002), and was compensated with a coefficient of 𝐹𝑈=0.6. The coefficient makes up for higher viscosity of the digestate compared to water which also worsens heat and mass transfer in the liquid digestate. The calculation was further limited by maximum attainable external pressure (𝑝𝑚𝑎𝑥=-0.95 barg) and highest concentration of dry matter in the digestate that still allows for transport of the digestate via regular pumps, that is 𝑥𝑚𝑎𝑥=0.16 kg kg -1. Influence of dry matter on the evaporation was neglected, and we assumed properties of pure water in the calculation. It is assumed that the distillate is pure water only. Specific heat capacity of the water was assumed to be constant (𝑐𝑝𝑤=4.18 kJ kg -1 K-1). Other thermo-physical properties of the water (𝐿𝑣,𝑇𝑠𝑎𝑡) were determined using IAPWS IF-97. Calculations are idealized in the sense that heat and hydraulic pressure drop, and concrete device geometry have not been 788 considered in the calculations. Negative pressure in chambers was determined using Generalized Reduced Gradient Algorithm in MS Excel 2013 solver. Specific heat consumption (𝑠𝑇ℎ) was an objective function. Liquid digestate flow rate (𝑀𝑑𝑖𝑔) is 1 kg/s. Following equations were used for the comparison; they apply for all types of the evaporators: 𝑈𝑐 𝑤 = 1 ∙ 10−3 ∙ (1617.5 + 0.1537 ∙ 𝑇𝑠𝑎𝑡 + 0.1825 ∙ 𝑇𝑠𝑎𝑡 2 − 0.00008026 ∙ 𝑇𝑠𝑎𝑡 3 ) (1) 𝑈𝑐 𝑑𝑖𝑔 = 𝑈𝑐 𝑤 ∙ 𝐹𝑈 (2), 𝑠𝑇ℎ = 𝐻𝐷/(3600 ∙ 𝑀𝑑) (3) 𝐿𝑀𝑇𝐷 = (∆𝑇𝑚𝑎𝑥 − ∆𝑇𝑚𝑖𝑛) ln⁡(∆𝑇𝑚𝑎𝑥/∆𝑇𝑚𝑖𝑛) (4), 𝑠𝐴 = 𝑀𝑑 −1 ∙ ∑ 𝐴𝑖 𝑛 𝑖=1 (5) 2.1 Single effect evaporator Single effect evaporators have the most simple design but they also have the lowest energy efficiency. Commonly, the evaporation chamber is supplemented with a circuit for waste water heating, and with a condensation section. We have compared the basic evaporator design with forced external hot water heating that uses cooling water for distillate condensation (Figure 1). Performance of single effect evaporator was determined using the following equations: 𝑀𝑑𝑖𝑔 ∙ 𝑐𝑝𝑑𝑖𝑔 ∙ (𝑇𝑠𝑎𝑡(𝑝𝑒) − 𝑇𝑑𝑖𝑔 𝑖𝑛 ) + 𝑀𝑑 ∙ 𝐿𝑣(𝑝𝑒) = 𝑀𝑟𝑒𝑐 ∙ 𝑐𝑝𝑑𝑖𝑔 ∙ (𝑇𝑟𝑒𝑐 𝑖𝑛 − 𝑇𝑟𝑒𝑐 𝑜𝑢𝑡) (6) 𝑀𝑑 ∙ 𝐿𝑣(𝑝𝑒) = 𝑀𝑐𝑤 ∙ 𝑐𝑝𝑤 ∙ (𝑇𝑐𝑤 𝑜𝑢𝑡 − 𝑇𝑐𝑤 𝑖𝑛) = 𝐴𝑐𝑤 ∙ 𝑈𝑐𝑤 𝑤 ∙ 𝐿𝑀𝑇𝐷𝑐𝑤 (7) 𝑀𝑟𝑒𝑐 ∙ 𝑐𝑝𝑑𝑖𝑔 ∙ (𝑇𝑟𝑒𝑐 𝑖𝑛 − 𝑇𝑟𝑒𝑐 𝑜𝑢𝑡) = 𝑀ℎ𝑤 ∙ 𝑐𝑝𝑤 ∙ (𝑇ℎ𝑤 𝑖𝑛 − 𝑇ℎ𝑤 𝑜𝑢𝑡) = 𝐴ℎ𝑤 ∙ 𝑈ℎ𝑤 ∙ 𝐿𝑀𝑇𝐷ℎ𝑤 = 𝐻𝐷 (8) 𝑇𝑟𝑒𝑐 𝑖𝑛 = 𝑇ℎ𝑤 𝑖𝑛 − 𝑇𝑇𝐷ℎ (9), 𝑇ℎ𝑤 𝑜𝑢𝑡 = 𝑇𝑟𝑒𝑐 𝑜𝑢𝑡 + 𝑇𝑇𝐷ℎ (10) 𝑇𝑐𝑤 𝑜𝑢𝑡 = 𝑇𝑠𝑎𝑡(𝑝𝑒) − 𝑇𝑇𝐷𝑐 (11), 𝑀𝑑𝑖𝑔 ∙ 𝑥𝑑𝑖𝑔 = 𝑀𝑐𝑜𝑛𝑐 ∙ 𝑥𝑐𝑜𝑛𝑐 (12) 𝑀𝑑𝑖𝑔 = 𝑀𝑐𝑜𝑛𝑐 + 𝑀𝑑 (13), 𝑠𝑀𝑐𝑤 = 𝑀𝑐𝑤/𝑀𝑑 (14) Figure 1: Design of single effect evaporator, used in the calculations Figure 2: Design of multiple effect evaporator, used in calculations 789 2.2 Multiple effect evaporator Multiple effect evaporators (MEE) have overcome major operational deficiencies in latter half of 20th century thanks to boom of desalination industry (El-Dessouky and Ettouney, 2002). In addition to desalination, they are frequently used in food industry, paper industry and petrochemical industry. Prado et al. (2015) have researched application of MEE in ethanol production. For purposes of this paper, we opted for a 3-stage parallel flow arrangement with a horizontal falling film evaporator and hot water heating (Figure 2). The LD is preheated using the condensing distillate from the previous chamber prior to entering the chamber. Certain manufacturers do offer this design. This design type has, compared to other MEE evaporators, a more simple construction that may be suitable for thickening of the LD. LD flow rate and evaporator heat transfer surface are assumed to be similar in every chamber. Performance of this type of MEE was determined using the following equations: This applies only to evaporation chamber no 1: 𝑀𝑑𝑖𝑔,1 ∙ 𝑐𝑝𝑑𝑖𝑔 ∙ (𝑇𝑠𝑎𝑡(𝑝𝑒,1) − 𝑇𝑑𝑖𝑔,1) + 𝑀𝑑,1 ∙ 𝐿𝑣(𝑝𝑒,1) = 𝑀ℎ𝑤 ∙ 𝑐𝑝𝑤 ∙ (𝑇ℎ𝑤 𝑖𝑛 − 𝑇ℎ𝑤 𝑜𝑢𝑡) (15) 𝑀ℎ𝑤 ∙ 𝑐𝑝𝑤 ∙ (𝑇ℎ𝑤 𝑖𝑛 − 𝑇ℎ𝑤 𝑜𝑢𝑡) = 𝐴𝑒𝑣,1 ∙ 𝑈𝑒𝑣,1 𝑑𝑖𝑔 ∙ 𝐿𝑀𝑇𝐷𝑒𝑣,1 = 𝐻𝐷 (16) 𝑇ℎ𝑤 𝑜𝑢𝑡 = 𝑇𝑠𝑎𝑡(𝑝𝑒,1) + 𝑇𝑇𝐷ℎ (17) This applies only to evaporation chamber no 3: 𝑀𝑑𝑖𝑔,3 ∙ 𝑐𝑝𝑑𝑖𝑔 ∙ (𝑇𝑑𝑖𝑔,3 − 𝑇𝑑𝑖𝑔) = 𝑀𝑑,3 ∙ 𝐿𝑣(𝑝𝑒,3) = 𝐴𝑐,3 ∙ 𝑈𝑐,3 𝑑𝑖𝑔 ∙ 𝐿𝑀𝑇𝐷𝑐,3 (18) For evaporation chamber 𝑖, the following generally applies: 𝑀𝑑𝑖𝑔,𝑖 ∙ 𝑐𝑝𝑑𝑖𝑔 ∙ (𝑇𝑑𝑖𝑔,𝑖 − 𝑇𝑑𝑖𝑔) = 𝑀𝑑,𝑖 ∙ 𝑐𝑝𝑤 ∙ (𝑇𝑠𝑎𝑡(𝑝𝑒,𝑖) − 𝑇𝑑,𝑖) (19) 𝑀𝑑𝑖𝑔,𝑖 ∙ 𝑐𝑝𝑑𝑖𝑔 ∙ (𝑇𝑠𝑎𝑡(𝑝𝑒,𝑖) − 𝑇𝑑𝑖𝑔,𝑖) + 𝑀𝑑,𝑖 ∙ 𝐿𝑣(𝑝𝑒,𝑖) = 𝑀𝑑,𝑖−1 ∙ 𝐿𝑣(𝑝𝑒,𝑖−1) (20) 𝑀𝑑𝑖𝑔,𝑖 ∙ 𝑐𝑝𝑑𝑖𝑔 ∙ (𝑇𝑑𝑖𝑔,𝑖 − 𝑇𝑑𝑖𝑔) = 𝐴ℎ,𝑖 ∙ 𝑈ℎ,𝑖 ∙ 𝐿𝑀𝑇𝐷ℎ,𝑖 (21) 𝑈𝑒𝑣,𝑖 𝑤 = 1 ∙ 10−3 ∙ (1939.4 + 1.40562 ∙ 𝑇𝑠𝑎𝑡(𝑝𝑒,𝑖) − 0.020752 ∙ (𝑇𝑠𝑎𝑡(𝑝𝑒,𝑖)) 2 − 0.0023186 ∙ (𝑇𝑠𝑎𝑡(𝑝𝑒,𝑖)) 3) (22) 𝑈𝑒𝑣 𝑑𝑖𝑔 = 𝑈𝑒𝑣,𝑖 𝑤 ∙ 𝐹𝑈 (23), 𝑀𝑑,𝑖−1 ∙ 𝐿𝑣(𝑝𝑒,𝑖−1) = 𝐴𝑒𝑣,𝑖 ∙ 𝑈𝑒𝑣,𝑖 𝑑𝑖𝑔 ∙ 𝐿𝑀𝑇𝐷𝑒𝑣,𝑖 (24) 𝑀𝑑𝑖𝑔 ∙ 𝑥𝑑𝑖𝑔 = 𝑀𝑐𝑜𝑛𝑐,𝑖 ∙ 𝑥𝑐𝑜𝑛𝑐,𝑖 (25), 𝑀𝑑𝑖𝑔,𝑖 = 𝑀𝑐𝑜𝑛𝑐,𝑖 + 𝑀𝑑,𝑖 = 𝑀𝑑𝑖𝑔/3 (26) And finally: 𝑀𝑑 = ∑ 𝑀𝑑 𝑖 3 𝑖=1 (27), 𝑠𝑀𝑐𝑤 = 𝑀𝑑𝑖𝑔/𝑀𝑑 (28) Figure 3 Design of multiple effect evaporator, used in calculations 2.3 Multistage flash evaporator Multistage flash evaporators (MSF) are commonly employed in desalination industry. Although the technology has been around for a while, its role in large-scale desalination is irreplaceable, and MSF is still a subject of 790 many R&D (Sowgath and Mujtaba, 2015). Manufacturers basically don’t offer the technology for thickening of waste water and process water. However, MSF evaporators have a distinct advantage over MEE in the sense that evaporation does not occur on heat transfer surface but directly in the liquid bath thanks to decreased pressure. 3-stage “once through” type (Figure 3), with a simple construction, was selected for the comparison. Hot water is the heat source. Similar heat transfer surface area is assumed for all condenser calculations. This allows to decrease manufacturing costs of the unit. Performance of this type of MSF was determined using the following equations: This applies only to evaporation chamber no 1 and heater: 𝑀ℎ𝑤 ∙ 𝑐𝑝𝑤 ∙ (𝑇ℎ𝑤 𝑖𝑛 − 𝑇ℎ𝑤 𝑜𝑢𝑡) = 𝑀𝑑𝑖𝑔 ∙ 𝑐𝑝𝑑𝑖𝑔 ∙ (𝑇𝑑𝑖𝑔,0 − 𝑇𝑐𝑤,1) = 𝐴ℎ ∙ 𝑈ℎ ∙ 𝐿𝑀𝑇𝐷ℎ = 𝐻𝐷 (29) 𝑇𝑑𝑖𝑔,0 = 𝑇ℎ𝑤 𝑖𝑛 − 𝑇𝑇𝐷ℎ (30) , 𝑀𝑑𝑖𝑔,0 = 𝑀𝑑𝑖𝑔 (31) For evaporation chamber 𝑖, the following generally applies: 𝑀𝑑𝑖𝑔,𝑖−1 ∙ 𝑐𝑝𝑑𝑖𝑔 ∙ (𝑇𝑑𝑖𝑔,𝑖−1 − 𝑇𝑠𝑎𝑡(𝑝𝑒,𝑖)) = 𝑀𝑑,𝑖 ∙ 𝐿𝑣(𝑝𝑒,𝑖) (32) 𝑀𝑑𝑖𝑔 ∙ 𝑐𝑝𝑑𝑖𝑔 ∙ (𝑇𝑐𝑤,𝑖+1 − 𝑇𝑐𝑤,𝑖) = 𝐴𝑐,𝑖 ∙ 𝑈𝑐,𝑖 ∙ 𝐿𝑀𝑇𝐷𝑐,𝑖 = 𝑀𝑑,𝑖 ∙ 𝐿𝑣(𝑝𝑒,𝑖) (33) 𝑇𝑐𝑤,𝑖 = 𝑇𝑠𝑎𝑡(𝑝𝑒,𝑖) − 𝑇𝑇𝐷𝑐 (34), 𝑇𝑑𝑖𝑔,𝑖 = 𝑇𝑠𝑎𝑡(𝑝𝑒,𝑖) (35) 𝑀𝑑𝑖𝑔,𝑖 = 𝑀𝑑𝑖𝑔,𝑖−1 − 𝑀𝑑,𝑖 (36), 𝑀𝑑𝑖𝑔 ∙ 𝑥𝑑𝑖𝑔 = 𝑀𝑐𝑜𝑛𝑐 ∙ 𝑥𝑐𝑜𝑛𝑐 (37) This applies only to evaporation chamber no 3: 𝑀𝑑𝑖𝑔 ∙ 𝑐𝑝𝑑𝑖𝑔 ∙ (𝑇𝑐𝑤,3 − 𝑇𝑑𝑖𝑔) = 𝑀𝑑,3 ∙ 𝐿𝑣(𝑝𝑒,3) (38), 𝑀𝑐𝑜𝑛𝑐 = 𝑀𝑑𝑖𝑔,3 (39) And finally we can use Eq. (27) and (28) to determine 𝑀𝑑 and 𝑠𝑀𝑐𝑤. 3. Results and discussion Table 1 presents the results. As expected, SEE with specific heat consumption of 0.69 kWh/kg seems to be the least effective type of evaporator. However, MEE also has rather low efficiency with sTh equal to 0.62 kWh/kg. For this reason, parallel/cross flow configuration could be considered for further research. On the other hand, MSF unit has the lowest 𝑠𝑇ℎ of 0.33 kWh/kg. Obtained results are comparable with performance of industrial evaporators for other kinds of waste water. Although the heat consumption of selected evaporators is rather high, it is not surprising. The reasons lie in thermal-hydraulic properties of LD which affected low 𝑈 values. SEE unit has the highest requirements on heat transfer area (982.9 m2kg-1s-1), which greatly increases manufacturing costs. MEE unit has low sA values; compared to MSF, it has higher temperature differences on heat transfer area (up to 38 °C). Other disadvantages of SEE include high requirements on external cooling water 𝑠𝑀𝑐𝑤, consumption is distinctly higher than for the other two technologies (cooling is secured by preheating of the LD). On the other hand, advantages include utilization of most of the temperature gradient (cooling up to 43 °C) and theoretical ability to preserve high amounts of dry matter in the concentrate (even if there is little dry matter entering the unit). Levels of liquid digestate thickening in MEE and MSF are rather low. If 𝑥𝑚𝑎𝑥=0.16 kg kg -1 should be achieved, partial recirculation of the concentrate would have to be introduced. MEE technology has the lowest requirements on vacuum quality, negative pressure of -0.88 barg is enough for optimum performance. Table 1: Results of comparison of three types of evaporators for liquid digestate thickening 𝑠𝑇ℎ(𝑘𝑊ℎ/𝑘𝑔) 𝑠𝐴(𝑚2/𝑘𝑔/𝑠) 𝑠𝑀𝑐𝑤(𝑘𝑔/𝑘𝑔) 𝑥𝑐𝑜𝑛𝑐(𝑘𝑔/𝑘𝑔) 𝑝𝑚𝑎𝑥(𝑏𝑎𝑟𝑔) SEE 0.69 982.9 73.6 0.16 -0.95 MEE 0.62 141.9 15.5 0.043 -0.88 MSF 0.33 426.4 14.5 0.043 -0.94 4. Conclusion As the number of BGP rises, there is an eminent need for a technology, which can effectively thicken LD from anaerobic digestion. According to presented assumptions, more than 120 million m3 of digestate is produced yearly in Europe. Thanks to the permanent excess of waste heat from cogeneration of biogas, vacuum evaporation seems to be the promising technology. Our ambition was to compare specific heat consumption and other relevant parameters of three types of industrial evaporators that we have deemed sufficient for thickening of LD in biogas plants. Evaporators were selected for their simple construction which might make 791 the liquid digestate processing easy. Our calculations prove that MSF is most efficient in use of the heat (0.33 kWh/kg of the distillate); MSF is followed by MEE (0.62 kWh/kg) and SEE (0.69 kWh/kg). Performance of selected evaporators are comparable with different types of industrial evaporators although they are affected by worse thermal-hydraulic properties of the LD. MEE unit has the lowest requirements on heat transfer area (141.9 m2kg-1s-1). LD thickening to 16 % dry matter concentrations, without any further technology adjustments, may be achieved only with SEE technology. Nomenclature: 𝐴 Heat surface area, m2 𝑇𝑇𝐷 Terminal temperature difference, °C 𝑐𝑝 Specific heat capacity, kJ kg-1 K-1 𝑈 Overall heat transfer coeff., kW m-2 K-1 𝐻𝐷 Heat duty of heating water, kW 𝑥 Dry matter content, kg kg-1 𝐹𝑈 correction coefficient for liquid digestate, - Indices 𝐿𝑀𝑇𝐷 Logarithmic mean temperature difference, °C ℎ Water/Liquid digestate heat exchanger 𝐿𝑣 Heat of vaporization, kJ kg -1 ℎ𝑤 Heating water 𝑀 Mass flow rate, kg s-1 𝑐 Condenser 𝑀ℎ Mass flow rate, kg h-1 𝑐𝑜𝑛𝑐 Concentrate 𝑛 Number of heat exchangers in the system 𝑐𝑤 Cooling water 𝑝 Pressure in an evaporation chamber, barg 𝑑 Distillate 𝑠𝐴 Specific heat surface area, m2 kg-1 𝑑𝑖𝑔 Liquid digestate 𝑠𝐸𝑙 Specific electricity consumption, kWh kg-1 𝑒 Equilibrium 𝑠𝐸𝑛 Specific overall energy consumption, kWh kg-1 𝑒𝑣 Evaporator 𝑠𝑀 Specific water consumption, kg kg-1 𝑖 Effect number 𝑆𝑡 Number of evaporation effects (stages) 𝑟𝑒𝑐 Recirculation 𝑠𝑇ℎ Specific thermal energy consumption, kWh kg-1 𝑠𝑎𝑡 Saturation 𝑇 Temperature, °C 𝑤 Water Acknowledgments The research leading to these results has received funding from the Ministry of Education, Youth and Sports of the Czech Republic under the National Sustainability Programme I (Project LO1202). 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