Transcript
University of South Florida
Investigation of Methods and Processes to Increase Efficiency for Carbon Activation Processes
Joshua Dang
ENC 3246
Dr. Dianne Donnelly
June 20, 2014
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Abstract: The demand for activated carbon keeps growing while supply will dwindle. As a result economic feasibility of such an important material will become nonexistent. Alternative sources of carbon rich raw material need to be used, along with innovative methods of activating carbon, which currently is a high energy and high cost endeavor. As a result, possible improvements could be made to increase the overall efficiency of the activation process, increase effectiveness of activated carbon desired characteristics, and decrease the detrimental impacts to the environment. These current improvements when applied together within the process chain will allow for greater stability of raw resources, make activated carbon pound for pound more effective, and preserve the environment.
INTRODUCTION The material known as activated carbon affects countless lives. The application of this material can be found in products throughout household items, including soft drinks and shampoos to large-‐scale industries including removal of mercury from natural gas, and carbon dioxide from fermentation processes [8]. The reason to the wide application of activated carbon is due to molecular carbons special structure characteristics, which has a great capacity and affinity for impurities. Activated carbon is not found naturally with these characteristics but needs to be created through an extended process. It is said that any carbon rich raw material can be used as a precursor to activated carbon [1]; however current raw materials are mainly sourced from coal and wood [9], and are activated through physical means, which require a high-‐energy input. These current sources and methods present several issues both economically and environmentally. Activation of carbon is a high energy and cost endeavor; however as research continues for more sustainable sources such as agricultural byproducts, viable processing methods such as chemical activation, and recycling techniques such as regeneration of used activated carbon, the cost will decrease as well as an increased impact to preserve the environment. An investigation of the activated carbon process will result in measures to increase the overall efficiency of the activation process, increase effectiveness of activated carbon desired characteristics, and decrease detrimental impacts to the environment. BACKGROUND History The earliest use of activated carbon has been lost in history [10]. It is believed that the earliest application dates back to 3750 B.C. where activated carbon was used by ancient Hindus in India as a process for water filtration [11]. The first documented
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use of activated carbon was found on Egyptian papyrus dating back to 1500 B.C. as a method to absorb unpleasant odors [10]. The desirable characteristic of activated carbon have been known for more than one and a half millennia, however its main application today are still targeted at fundamentally similar organic impurities. As late as the 18th century sources of raw carbon were derived from blood and animals, which were then used to purify liquids [11]. Documented uses of activated carbon, which became noted in medical journals, were a treatment for ingested poisons [10]. Early uses were also for medicinal purposes, and most widely accepted in the 19th century were uses found in the treatment of poultices, sloughing ulcers, and gangrenous sores [10]. Some noticeable improvement pertains to the manufacturing process the produces a different shape and size of activated carbon. These different shapes allow for longevity of the carbon purification performance as well as improved shipping and handling durability. At the beginning of the twentieth century activated carbon was only available in a powder form. During the First World War granular activated carbon was used in gas mask to capture deadly organic gases [11], this granular processing eventually lead to the widespread manufacturing of granular activated for other applications such as water treatment, and gas purification. The wide application and available sources of activated carbon throughout history and until this day is a testament to the imperative usefulness of this material. Through activated carbons intrinsic physical and chemical properties the usefulness has been experienced and applied to a vast array of situations. Very prominent to this day is the application of activated carbon to purify both gaseous and aqueous phases of substances to prevent environmental harm. Current Application Widespread uses of activated carbon can be found in industrial, pharmaceutical, water treatment processes. Great focus is put on the protection of the environment and the health effects from emission of gases and waste products in industrial and manufacturing processes. These emissions include volatile organic compounds (VOCs) and are known to cause cancer in tested animals [12]. Activated carbon is vital in the many processes that involve VOC’s to stay within EPA regulation of emissions for health concerns. Impurities in an aqueous phase are also an important consideration in many products. These organic impurities are in the form of chemical solvents used in production processes for products such as paints and household cleaners [12]. Activated carbon due to its ability to conduct electricity can also be found as a catalyst in many vital electronic components including batteries, supercapacitors, and fuel cells [7]. A major use of activated carbon is in the purification of water for human consumption. When using this absorbent in the water purification process it is layer after sand and before chlorination [11]. The use of activated carbon not only decreases bad odors and taste but also removes harmful contaminants found in the water sources, such as synthetic organic
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compounds (SOC). These current applications are a constant issue, as nations economical and industrial development demands more activated carbon to continually produce quality products. Molecular Characteristics Activated carbon has a desired physical and chemical structure due to the porosity and surface area different to non-‐activated carbon. The porosity describes the amount of microscopic cavity between carbon molecules and affects the total surface area per unit mass. Another important aspect is the pore size, which is due to the process of activation as well as the raw source for carbon. Impurities can range from one one thousand of a micron to ten thousand microns. To effectively capture these contaminants a correct pore size must be used to allow for proper mechanical fit, which is then preceded my chemical interactions. Surface area is directly related to the capacity to hold impurities. To achieve a high surface area the pore structure must be extensive, in that many channels are present [2]. It is evident that the level of desired molecular characteristics can be altered. This provides a variable in the effort to increase the overall efficiency of the production of activated carbon. PROCCESSING RAW CARBON Current and Alternative Sources Current raw carbon sources as a precursor are from coal and wood. In the recent pass, the selected source must meet several of the following requirements. It must have the potential to produce high quality activated carbon, which is a function of the porosity and resulting surface area. It must have large available supplies; this will in effect lower the cost. And finally it must have the ability to be stored for extended periods of time [3]. Both coal and wood have passed these conditions as leading sources for the production of activated carbon, however an important requirement have been dismissed in past decisions. This important neglected requirement is how does sourcing of this material effect the environment? With 130,000 tons per year of wood and 100,000 tons per year of raw material being harvested, the impact to the environment is one of great magnitude. Wood and coal are currently the leading source for the production of carbon. Coal comes from surface and underground mines, with the majority from surface mines at sixty percent [13]. Coal is considered a non-‐renewable source due to the time it takes to create it. Estimates have said supplies of coal will last only until 2035 [14]. This estimates does not take into account the coal that is too deep and costly to mine. This presents an issue to the economics of using coal as a source. Alternative sources must be researched and tested to keep supplies of raw carbon stable. Without proper preparation a sudden decrease in supply will trigger
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staggering price hikes, which will effect how companies operate, prices of products, and could even shut down industrial processes and slow the economy. Alternative sources need to meet all constraints previously set as well as an additional constraint of sustainability. This will be the basis to analyzing the potential of new sources. A study on the use of corncobs proves the economic feasibility and sustainability of this agricultural waste byproduct as a potential source. In a published article from American Chemical Science Journal the use of corncobs have two main advantages, the first is the wide availability, and second is the intrinsic thermodynamics properties of corncobs [15]. There is a vast amount of corncobs that are wasted in the production of food and ethanol. A 51% portion of total U.S. grown corn is dedicated to the production of food and ethanol; within these productions only the kernels are used [16]. These waste products can be recycled and processed to into a high value material of activated carbon. The thermodynamic characteristics of corncobs allows for “a low carbonization temperature compared to other biomass residues” [15]. This allows for a lower temperature during the activation stage where all the undesired components existing within the raw material are vaporized. Vaporization of any material takes a great amount of energy; this is due to how heat is distributed within a substance. The energy input is converted into thermal energy, which then flows down a gradient of temperature differences. Only when the gradient is at equilibrium at the boiling point of the substance does vaporization initiate. Therefore, corncobs with low carbonization temperature will allow for a lower input of thermal energy. This material has potential as an alternative source. The source of municipal refuse is numerous in supply. This refers to solid waste consisting of everyday trash and garbage. The process to which the raw refuse originates is through the sorting out of glass and metal leaving a source full of carbonaceous material ready to be activated [5]. The desired characteristic from the municipal refuse was on the same standard as those that are from coal and wood [5]. Pass considerations for using refuse have been disregarded due to the cheap and highly available supplies of wood. The economics and profit margins were the key driving force to choosing less sustainable sources. However as supplies of coal and the ever-‐increasing cost to produce lumber these readily available waste sources will become comparatively economical. Activation Methods Without an improvement to chemical and physical characteristics of the carbon precursor, the effectiveness of carbon as an absorbent could be 17 to 25 times less absorbent. The possible range is due to the source of raw materials used, which is a factor in the pore structure and surface area. Activation is also crucial in creating certain structure, which have a greater affinity for targeted impurities. Activation is done either physically or chemically; however, both techniques have been used
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simultaneously to yield even higher absorption and adsorption capacities [5] at the expense of higher cost.
Figure 1: Processes for chemical and physical activation [5]. In figure 1 both physical and chemical process are outlined. A physical activation is also referred to as a thermal activation, due to required high temperature conditions. Physical means of activation generally required two steps, as seen in above figure. The first step is carbonization. This involves pyrolysis in the absence of oxygen, which is the breakdown of the raw carbon rich organic matter [6]. This is done with a high-‐energy input to raise temperatures to a level in which a precession of vaporization of volatile components are possible. To achieve a condition without the presence of oxygen, inert gases are pumped into the system. Inert gases are non-‐reactive agents; this prevents side reactions, which is desired for the conservation of pure carbon. The result of pyrolysis is a reduction of raw material, but also an increase in the quality and purity of carbon atoms [3]. The second stage is activation; this process is carried out with oxygen or steam. The purpose of activation is to increase the porosity of the structure as well as increase the surface area. High-‐energy cost due to high temperature processes is associated predominately with physical means of activation. In contrast to a high temperature process, chemical means of activation is a one step process and allows for carbonization at a significantly lower temperature, as a result
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there is a greater porous structure [7]. However, chemical precursors are needed, which are typically an acid, or strong base [4]. Raw materials are impregnated with the chemicals, which begins the process of removing the impurities through dehydration which effects pyrolytic decomposition of impurities. Chemical activation allow for less thermal energy to be expended, however washing to remove the impregnated chemicthals and drying are required [5]. These chemicals are a hazard to the environment if not recycled and reused. The potential to recycle pyrolytic chemical present an advantage over physical means of activation. The economics associated with recycle stream within industrial application saves fresh material, which is directly associated with lower cost of purchasing these chemicals. For every unit of mass activated carbon is created there is small portion of chemicals that are required. The ability to use the fraction of the activated carbon product to remove the impurities of the chemical for reuse of the chemical presents a possible solution to lower energy consumption within the activated carbon process. Taking into account the large surface area that is created due to activation, only a small portion of the total product needs to be used in the recycling process of the pyrolytic chemicals; Overall, chemical activation if a more viable method than physical activation. ENERGY & ENVIROMENT Energy Consumption Energy considerations for processing will be discussed starting from the sourcing of raw materials. The mining of coal in itself is a high-‐energy process. The large machinery and transportation of raw carbon accounts for most of the energy expenses in the extraction of coal. Its estimated that 15% of the production cost is due to transportation and mining of coal alone [14]. Energy consumption of wood as precursors also account for a significant portion of the production. Shipping of wood from less rural area and overseas present significant fuel usage. Temporary bridges must be built over small rivers and streams to gain access to depleting supplies of hard woods. This takes large equipment to get to these areas. After the wood is harvested it has to be dried, this requires a kiln, which is a thermally insulated chamber where heat can be added to evaporate water [18]. High-‐energy inputs are required with vaporization processes. A physical activation technique requires great amount of expended energy to raise temperature to pyrolyze all the undesired substances. The needed temperature of physical activation is on a scale of magnitude twice that required of a chemical activation. High temperatures are needed in the two-‐step process of carbonization and activation, which is approximately 1000˚C and 700˚ C for carbonization and activation processes, respectively. It’s intuitively known and explained by the 2nd law of thermodynamics that heat flows from high temperature to low temperatures. If a substance at a lower temperature needs to be at a state of higher temperature, energy from the surroundings must be inputted in to the system. This is the main
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reason to experienced high operational cost associated with a physical activation process. Chemical activation is a more economical and energy efficient method to physical activation. This method also requires energy inputs to raise temperature in order to start reactions, however these temperature are well below at approximately 500 ˚C depending on certain raw materials. Another advantage of a chemical means is the one step process, which lowers cost of special equipment and larger facilities. For every action there is a reaction, and with energy consumption there is a environmental impact. Environmental Effects of Activation Methods Through a physical means of activation high temperatures are required for extended periods to vaporize impurities within the raw material. The implication of this method is a high-‐energy consumption, and therefore high environmental effects. The method to heating of the carbon precursor is done though resistance heat coil or through the burning of natural gas [5]. Both of these heating techniques have detrimental impacts on the environment. With heat coils, electricity is used. Electricity is produce mainly from nonrenewable resources, with 39% coal, 27% natural gas, and 19% nuclear. These sources of energy pollute the environment with the harmful emissions such as nitrogen oxide (NOx), which is known to cause cancer in animals, destroy natural environments, and affect the health of the ecosystem [17]. A chemical means of activation requires far less energy than with thermal activation. With 300,000 tons a year of activated carbon produce solely for water treatment processes, a huge impact can be clearly seen in small energy saving [5]. A potential detrimental effect to the environment can be seen in the use of chemicals to pyrolyze raw carbon. Possible contamination can happen due to waste chemicals not being handled properly, however with EPA regulations it would be rare. This case would also be unlikely because it is uneconomically to waste high value solvents; a common practice would be to recycle and reuse. Lower energy and use of recyclable solvents, chemical activation is a better for the environment. DISSCUSSION & SUMMARY A Cost Effective and Sustainable Model As a result of the investigation possible improvements could be made to increase the overall efficiency of the activation process. A sustainable model dictates the use of a recycling method. In figure 2 below, a comparison of the main aspects of the process is depicted. It all starts with sustainable sources those that would have otherwise been wasted such as corncob and municipal refuse to list just the few possibilities. With supplies of current carbon precursors becoming scarce, this will provide the shift in economical feasibility of waste sources. The benefits of using
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waste byproducts will positively impact emission levels, natural habitat preservation, and save natural resources. The energy saved from initial production cost of equipment, labor, and transportation will then be decreased due to recycle of the material from waste byproducts. Next the activation of carbon will be processed with a chemical pyrolysis method. This will provide significant lower thermal energy cost than current physical methods due to chemical reactions instead of vaporizations. The ability to recycle chemical activating solvent will provide yet another advantage in economically feasibility and sustainability. With chemical reactions the ability to control precise characteristics in pore structures will help to increase the effectiveness of activated carbon for specific materials and situations.
Figure 2: A comparison of the process chain in the production of activated carbon. On the left is the how the majority of activated carbon is currently produced, and on the right is a more efficient process with sustainable sources and efficient activation methods.
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Conclusion
The gap is widening and the rising cost of current sources is soaring. With resources becoming scarce and difficult to access the economics of the production will not allow current prices and profits. Following this standard sustainable and efficient model, supply shortages and environmental harm will be avoided. The ecosystem as a whole is being affected, but with new highly efficient activation techniques and sustainable sources, the detrimental effect of the destruction to forest, and natural reserves will be limited. With the described cost effective and sustainable model, this guideline provides a path to increase the overall efficiency of the activation process, increase effectiveness of activated carbon, and decrease detrimental impacts to the environment.
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