Microfiltration of thin stillage: Process simulation and economic analyses

In plant scale operations, multistage membrane systems have been adopted for cost minimization. We considered design optimization and operation of a continuous microfiltration (MF) system for the corn dry grind process. The objectives were to develop a model to simulate a multistage MF system, optimize area requirements and stages required for a multistage system and perform economic analysis of a multistage MF system for a 40 million gal/yr ethanol plant. Total area requirement decreased with number of stages but there was tradeoff between higher capital costs involved at higher number of stages. To achieve thin stillage total solids concentration from 7 to 35%, a 5 stage membrane system was found to be optimum with area requirement of 655 m² for minimum cost. Increase in the input stream flow rate from 1.54 × 10⁶ to 2.89 × 10⁶ L/day significantly increased the total capital cost of the system by 47%. Compared to a single stage system, an optimal system had a 50% reduction in operating costs. Optimal system also showed potential to process more than twice the amount of thin stillage compared to a 4 effect evaporator system for given conditions.     

Chemical process simulation for minimizing energy consumption in pulp mills

Chemical process simulation has proven to be an effective tool for performing a systematic and global analysis of energy systems to identify routes for maximizing the process efficiency concerning to the heat recovery. This paper shows an application of computer simulations in a Brazilian pulp mill, using two strategies for minimizing the mill energy consumption. In the first one, the overall heat transfer coefficient has been predicted for each body of the multiple effect evaporators by using continuous on-line data from the industrial plant in the black liquor recover unit. By monitoring oscillations of this heat transfer coefficient, the suitable time for washing the evaporator heat transfer surfaces can be well determined, reducing the energy loss during black liquor evaporation. In the second strategy, the liquor combustion has been simulated as function of the black liquor solids concentration to analyze its effect on the recovery boiler efficiency improvement.     

Deposit Formation in Evaporation of a Pulp Mill Effluent

A preliminary study was undertaken of the fouling of clarified wastewater from a “zero”-liquid discharge bleached chemi-thermo-mechanical pulp mill. This work concerned the fouling process that occurs in mechanical vapor recompression evaporators, which comprise the first stage of the evaporation and concentration system. The clarified wastewater contains dissolved and suspended solids, both of which increase as the liquor is concentrated. Solids contain roughly 20% mineral matter and the remainder is organic material. Thermal fouling measurements were made in a countercurrent double-pipe exchanger in which liquors of three different concentrations were heated by condensing steam. The decline in heat transfer coefficient was tracked over periods of several days at fixed steam temperature and fluid velocity. Fouling increased with both liquor concentration and steam temperature. Soft, sludge-like deposits that contained mainly organic material were found. The inorganic fraction was about 25% and consisted primarily of calcium, magnesium, and silicon. The sources of the inorganic fraction in the deposits are discussed, and deposit analyses are compared with those of the solids present in the liquor.     

Impact of Tube Surface Properties on Crystallization Fouling in Falling Film Evaporators for Seawater Desalination

ABSTRACT

Crystallization fouling on heat transfer surfaces is a severe problem and a complex phenomenon in multiple-effect distillation plants with horizontal tube falling film evaporators for seawater desalination. The choice of tube material affects the wettability, the adhesion forces between surface and deposit, and the induction time of crystallization fouling. The effects of surface properties on crystallization fouling from seawater have been investigated in a horizontal tube falling film evaporator in pilot plant scale. Experiments were performed with artificial seawater and various tube materials. The tube surfaces were characterized by measuring surface roughness and contact angles and by determining surface free energies. The tube materials show qualitative and quantitative differences with respect to scale formation. The interfacial defect model was applied to the system. Spreading coefficients of CaCO₃ scale on the aluminum alloys 5052 and 6060 and stainless steel grade 1.4565 were calculated to be higher than those on copper–nickel 90/10 and aluminum brass, but the quantities of CaCO₃ scale measured on the tube surfaces were much lower compared to CuNi 90/10 and aluminum brass. The application of advanced approaches such as the interfacial defect model depends on the precise knowledge of interfacial free energies, which are very difficult to find. However, results suggest that more similar values of the interfacial free energies of heat transfer surface and deposit lead to increased scale formation.     

自转清洗式平带传热强化自然循环蒸发器的研究

为了解决传统蒸发器结垢需要频繁的停车清洗和强制循环泵高电耗两大难题,研究的新型蒸发器结构特点主要是加热室比较短、采用具有高效传热强化和较低流速下自转清洗功能的平带,沸腾室是大横截面的中心循环管,而且出口段截面为渐扩形。试验研究表明:这种平带的对流传热强化幅度可以达到92.3%,并且能够在传热管内流速大于0.35 m/s时开始自转;这种蒸发器能够为可靠自动清洗要求的自然循环设计流速0.70 m/s提供所需的推动力,并且对各种蒸发工艺条件具有良好的适应性。     

Optimization of biogas production from wheat straw stillage in UASB reactor

In the present study, thermophilic anaerobic digestion of wheat straw stillage was investigated. Methane potential of stillage was determined in batch experiments at two different substrate concentrations. Results showed that higher methane yields of 324 ml/g-(volatile solids) VS_added were obtained at stillage concentrations of 12.8 g-VS/L than at 25.6 g-VS/l. Continuous anaerobic digestion of stillage was performed in an up-flow anaerobic sludge blanket (UASB) reactor at 55 °C with 2 days hydraulic retention time. Results showed that both substrate concentration and organic loading rate (OLR) influenced process performance and methane yields. Maximum methane yield of 155 ml CH₄/g-COD was obtained at stillage mixtures with water of 25% (v/v) in the feed and at an OLR of 17.1 g-COD/(l.d). Soluble chemical oxygen demand (SCOD) removal at this OLR was 76% (w/w). Increase in OLR to 41.2 g-COD/(l.d) and/or stillage concentration in the feed to 33–50% (v/v) resulted in low methane yields or complete process failure. The results showed that thermophilic anaerobic digestion of wheat straw stillage alone for methane production is feasible in UASB reactor at an OLR of 17.1 g-COD/(l.d) and at substrate concentration of 25% in the feed. The produced methane could improve the process energy and economics of a bioethanol plant and also enable to utilize the stillage in a sustainable manner.     

Experimental Investigations on the Performance Enhancement Using Minichannel Evaporator with Integrated Receiver-Dryer Condenser in an Automotive Air Conditioning System

The performance comparison of two automotive air conditioning systems was experimentally studied under two typical cabin conditions viz; 27°C dry bulb temperature / 40% relative humidity and 40°C dry bulb temperature /40% relative humidity in a bench test rig for different blower speeds of the evaporator. Two mobile air conditioning systems viz; enhanced system having minichannel evaporator with integrated receiver-dryer condenser, and baseline system having conventional serpentine evaporator with parallel flow condenser were considered for the study. The compressor was the same for both the systems. The charge quantity, compressor speed, condenser air flow rate were suitably modified to maintain similar suction / discharge pressures, suction super heat and sub-cooling at all test conditions. The percentage of condensate retention during the dehumidification for both the evaporators were also compared. The comparison indicates that the average coefficient of performance of the enhanced system is higher in both low and high cabin temperature condition by about 15% and 8% respectively. The percentage of condensate retention in the evaporator of the enhanced system is lesser in the range of about 17–31% as compared to the base line system evaporator. It is also expected that a specific design of compressor for the enhanced system can yield better performance at all conditions.     

Ammonia–water absorption refrigeration systems with flooded evaporators

The harmful effects of water accumulation in the evaporator in ammonia–water absorption refrigeration systems (AARS) with flooded evaporators are a crucial issue. In this paper, the effects of the ammonia purification and the liquid entrainment and blow-down from the evaporator in AARS are analyzed. A mathematical model based on a single stage system with complete condensation has been developed. The ammonia purification is evaluated by means of the Murphree efficiencies of the stripping and rectifying sections of the distillation column. The entrainment and blow-down are taking into account considering the corresponding flow rates as a fraction of the dry vapour at the evaporator outlet. The influence of the distillation column components efficiency on the attainable distillate concentration and the effects of the distillate concentration and the liquid entrainment and blow-down on the system operating conditions and performance are analyzed and quantified. If no liquid entrainment or blow-down is considered, very high efficiencies in the distillation column are required. Small values of liquid entrainment or blow-down fractions increase significantly the operating range of the absorption system. If high values of the blow-down fraction are required, then a heat exchanger should be added to the system in order to recover the refrigeration capacity of the blow-down by additional subcooling of the liquid from the condenser. For a fixed value of the distillation column efficiency, an optimum value of the liquid blow-down fraction exists. Moreover, an optimum combination of generation temperature, reflux ratio and blow-down fraction can be found, which should be considered in designing and controlling an AARS.     

Composite Fouling on Heat Exchange Surface in Australian Sugar Mill Evaporator

Composite scale of amorphous silica and calcium oxalate is very intractable; it imposes a significant fraction of scale management cost in sugar mill evaporators. This work evaluates silica and calcium oxalate composite fouling using dynamic fouling-loop experiments under different operating conditions with and without sugar. A novel closed-loop setup simulating in a single run the effect of feed concentration in successive stages of evaporation cycle was used. Experiments under constant composition and varying thermal hydraulic effect were conducted. Results indicated synergistic effects for initial silica and calcium oxalate supersaturation values of 2.6 and 1.7. Sugar effects became significant for concentrations in the third effect and above. The fouling mechanism was particulate deposition of silica and calcium oxalate colloidal species strengthened by consolidation; fouling rate increased with decreasing interfacial energy barrier between the surface and foulant, assessed by zeta-potential analysis. Deposit characteristics and deposition rates were concentration and thermal hydraulics dependent. Results confirmed the need that the 5 × 5 (types × stages of fouling) matrix of Epstein needs to be expanded to a 6 × 6 matrix to include composite fouling as a type of fouling. In this study the composite fouling was a surface-controlled process for which the activation energy was similar to typical chemical bond strengths, explaining the role of consolidation in composite fouling. The results contribute to advancement of fundamental understanding of heat exchanger fouling and to development of scale control strategies for composite fouling minimization in sugar mill evaporators.     

Heat Exchanger Fouling in Phosphoric Acid Evaporators

Multistage shell and tube evaporators are frequently used in phosphoric acid plants to increase the concentration of dilute phosphoric acid to 52–55 wt% P₂O_5. The concentrated phosphoric acid solution is supersaturated with respect to calcium sulfate. As a result, part of the calcium sulfate in the liquor deposits on the heat exchanger tube walls. Because the thermal conductivity of these scales is very low, thin deposits can create a significant resistance to heat transfer. Therefore, regular cleaning of heat exchangers is required, frequently at shorter than biweekly intervals. As the major costs in modern phosphoric acid plants are the cost of energy, a thorough understanding of the fouling kinetics and of the effects of various operational parameters on the behavior of calcium sulfate is required to improve operation and design of the shell and tube heat exchangers, which are extensively used. In this investigation, a large set of heat exchanger data was collected from shell and tube heat exchangers of the phosphoric acid plant of the Razi Petrochemical Complex in Iran, and the fouling deposits were analyzed with respect to appearance and composition. The overall heat transfer coefficients and fouling resistances were evaluated at different times, and a kinetic model for the crystallization fouling was developed. It is shown that the crystallization rate constant obeys an Arrhenius relationship with an activation energy of 57 kJ/mol. The average absolute error of 12.4% shows that the predictions of the suggested model are in good agreement with the original plant data.     

Understanding the reductions in US corn ethanol production costs: An experience curve approach

The US is currently the world's largest ethanol producer. An increasing percentage is used as transportation fuel, but debates continue on its costs competitiveness and energy balance. In this study, technological development of ethanol production and resulting cost reductions are investigated by using the experience curve approach, scrutinizing costs of dry grind ethanol production over the timeframe 1980–2005. Cost reductions are differentiated between feedstock (corn) production and industrial (ethanol) processing. Corn production costs in the US have declined by 62% over 30 years, down to 100$₂₀₀₅/tonne in 2005, while corn production volumes almost doubled since 1975. A progress ratio (PR) of 0.55 is calculated indicating a 45% cost decline over each doubling in cumulative production. Higher corn yields and increasing farm sizes are the most important drivers behind this cost decline. Industrial processing costs of ethanol have declined by 45% since 1983, to below 130$₂₀₀₅/m³ in 2005 (excluding costs for corn and capital), equivalent to a PR of 0.87. Total ethanol production costs (including capital and net corn costs) have declined approximately 60% from 800$₂₀₀₅/m³ in the early 1980s, to 300$₂₀₀₅/m³ in 2005. Higher ethanol yields, lower energy use and the replacement of beverage alcohol-based production technologies have mostly contributed to this substantial cost decline. In addition, the average size of dry grind ethanol plants increased by 235% since 1990. For the future it is estimated that solely due to technological learning, production costs of ethanol may decline 28–44%, though this excludes effects of the current rising corn and fossil fuel costs. It is also concluded that experience curves are a valuable tool to describe both past and potential future cost reductions in US corn-based ethanol production.     

稻壳与玉米秆高温厌氧发酵制备生物燃气潜力研究

研究了玉米秆和稻壳在固体浓度为6%时的高温（50℃）发酵性能,并分析了发酵过程中氨氮浓度、碱度及挥发性脂肪酸等参数的变化情况。结果表明,玉米秆和稻壳的挥发性物质产甲烷率接近,分别为（157.67 ± 3.00）mL/g VS和（155.83 ± 6.25）mL/g VS,挥发性物质去除率分别为（53.38 ± 0.81）% 和（42.67 ± 0.3）%。但稻壳相比于玉米秆无需粉碎,降低了输入能耗。发酵过程中氨氮浓度及挥发性脂肪酸数值低于抑制浓度,且碱度对发酵系统酸浓度变化具有很好的缓冲能力,可见玉米秆和稻壳适宜作为沼气工程的原料,并可在6%的固体浓度及高温条件下稳定发酵。     

Fouling of a Heated Rod in a Stirred Tank System

A batch stirred tank device has been developed for measuring fouling from oil samples. The unit consists of a baffled tank equipped with a centrally mounted long blade stirrer, and an electrically heated rod located at 40% of the radius of the tank. Heat transfer from the rod was first characterized. The velocity field was measured, from which the approach velocity to the probe was determined, which allowed the wall shear on the heating probe to be calculated from a literature equation. Fouling of a heavy oil fraction was studied in 1- to 2-day experiments with bulk oil temperatures typically at 320°C, initial probe surface temperatures to 536°C, and stirrer speeds of 100–900 rpm. Micrometer-sized iron oxide particles were added to the oil, such that fouling was due to a combination of particle deposition and coke formation. Deposition rates were measured thermally from the change in heat transfer coefficient when fouling was relatively heavy, and by thickness and mass accumulation when fouling was light. Effects of oil type, film temperature, stirrer rotation speed (or probe wall shear stress), and concentration of suspended particles on deposition rate and deposit composition are presented.     

Evaluation of Existing Heat Transfer Correlations in Designing Helical Coil Evaporators for Low-Temperature Organic Rankine Cycles via Inverse Design Approach

For evaluating the heat transfer correlations from literature in designing helical coil evaporators for low-temperature organic Rankine cycles (ORC), an inverse evaporator design methodology is used. This is done by taking four already performed measurements with different working fluid mass flow rates (0.21–0.23 kg/s) and saturation pressures (1.93–3.05 MPa, reduced pressures of 0.51–0.82) at the helical coil evaporator (66 m long) in an already operational experimental solar/thermal ORC system as real-case references for the evaporator inverse design's boundary conditions. R-404A is considered as the working fluid. The heating water inlet temperature is changed between 353 K and 373 K. The total length is inversely calculated via 15 helical coil two-phase heat transfer correlations for each case. Their end designs are compared with the actual length. Results show that the correlations can be used interchangeably for designing helical coil evaporators for low-temperature ORCs since the heat transfer resistance is dominant on the shell-side. For evaluating the sensitivity of these results, a secondary analysis was made by means of changing the shell-side correlation's accuracy from its initial value of 10% to more accurate 7%. Predictions with several correlations design shorter heat exchangers at reduced pressures less than 0.7.     

A study of the air-side heat transfer and pressure drop characteristics of tube-fin ‘no-frost’ evaporators

A study is presented on the influence of the air flow rate and surface geometry on the thermal-hydraulic performance of commercial tube-fin ‘no-frost’ evaporators. A specially constructed wind-tunnel calorimeter was used in the experiments from which data on the overall thermal conductance, pressure drop, Colburn j-factor and Darcy friction factor, f, were extracted. Eight different evaporator samples with distinct geometric characteristics, such as number of tube rows, number of fins and fin pitch were tested. Semi-empirical correlations for j and f are proposed in terms of the air-side Reynolds number and the finning factor. A discussion is presented on the performance of the evaporators with respect to specific criteria such as the pumping power as a function of heat transfer capacity and the volume of material in each evaporator.     

Experimental and theoretical performance analysis of air-cooled plate-finned-tube evaporators

The determination of the cooling capacity and overall heat transfer coefficient of an evaporator is of great importance in refrigeration industry, so that, an investigation of the theoretical and experimental performance analysis of evaporators was carried out in this study. The experimental evaporator was analyzed in account with the most common and widely used correlations together with the parameters of air velocity, fin spacing, tube diameter, evaporator temperature, refrigerant type and frost height. After a good agreement between the experimental and theoretical results was obtained, the parameters which had not been able to investigated experimentally were analyzed theoretically. It is concluded that when the experimental and theoretical overall heat transfer coefficients were compared with those from the manufacturing catalogues (for the same working conditions), the latter was found to be 15–30% higher than the former one.     

Investigating the Impact of Boiling Conditions on the Fouling of a Crude Oil

ABSTRACT

The impacts of nucleate boiling and pressure on crude oil fouling are factors that have not been heavily investigated in previous research. Variables such as wall temperature and fluid velocity/shear are often a main focus, as they are key variables for predictive fouling models, which provide insight to fouling thresholds. Research detailed in this report shows that nucleate boiling and pressure greatly impact the measured fouling rate of a crude oil tested using the Heat Transfer Research, Inc., rotating fouling unit. When nucleate boiling is occurring, the use of fouling resistance plots to measure fouling rates is not a reliable method due to the impact boiling has on the heat transfer coefficient. Visual inspection of fouling deposits to validate fouling resistance data has also been found to be critical. Images of fouling deposits are included. Fouling under nonboiling conditions was shown to increase with increasing pressure.     

垃圾渗滤液浓缩用MVR蒸发管内CaCO3结垢过程数值分析

基于圆管立式降膜蒸发传热传质的相关理论,建立了其管内CaCO3结垢过程的数学模型,并应用于垃圾渗滤液浓缩用机械蒸汽再压缩（MVR）立式降膜蒸发管内CaCO3结垢过程研究,得到了各结垢参数在不同结垢阶段的变化规律。结果表明：结垢初期,蒸发管内CaCO3的沉积速率远大于剥蚀速率,净存速率较大,污垢层厚度、污垢热阻快速增加,使得蒸发管总传热系数快速减小,进而引起蒸发器的蒸发量、浓缩比快速减小;结垢中期,CaCO3的净存速率变小,污垢增加变缓,各结垢参数变化趋势由急变缓;结垢末期,CaCO3的净存速率趋近于零,污垢不再增加,各结垢参数趋于稳定;相比蒸发管入口,出口液膜溶液的流量小、CaCO3浓度高,结垢更严重,且受整个蒸发管结垢的影响,液膜溶液流量、CaCO3浓度变化较大,各结垢参数变化更迅速,更早趋于稳定。     

Fouling in a Centritherm Evaporator With Whey Solutions

Fouling caused by sweet cheese whey and reconstituted whey powder solutions was studied in a Centritherm evaporator, with the behavior being estimated by measuring the change in the overall heat transfer coefficients with the time. It was found that there was no fouling detectable for the reconstituted whey powder solution. For sweet cheese whey, the fouling rate was strongly linked with the evaporating temperature and temperature difference. Visual observation showed that the deposit layer on the evaporator surface was very thin and soft, and like the type A milk deposit described by other investigators. A high velocity of liquid film delayed the formation of deposit on the surface. Some interactions between the evaporation temperature, feed flow rate, rotating speed, and temperature difference are discussed. The fouling is caused by denaturation of bovine serum albumin following a second-order rate reaction.     

Heat Transfer Characteristics of Flood-type Evaporator Using R22 and R134a for Fishing Vessels

ABSTRACT

Most of fishing vessels pour plain ice into fish storages to maintain freshness of catches, which causes varying storage temperatures and changes in the salinity, and damages in catches. In contrast, seawater cooling systems that directly chill the seawater in the storage provide seawater with a constant temperature and salinity. Furthermore, the application of a flood-type evaporator enables the size of the system to be reduced due to the enhanced heat transfer performance of the evaporator. This study conducts experimental research on the boiling heat transfer characteristics of flood-type evaporators under various operating conditions. The test section of the experiment consists of the flood-type evaporator including tubes made of three different materials and having two different shapes, and R134a and R22 are used. As a result, low-fin tubes were found to present a higher heat transfer coefficient than that of the plain tube. Aluminum-brass low-fin tubes exhibited relatively 4.5% higher heat transfer coefficient than that of copper-nickel low-fin tubes when R22 was used, and relatively 5.3% higher value when the R134a was used. When R22 was used with an aluminum-brass low-fin tube, the heat transfer coefficient was relatively 6.9% higher than that when using R134a.