Most of the available tools and methods applied in the design of chemical processes are not effective at the critical early stages of design when the process data is very limited. Businesses are often under pressure to deliver products in shorter times and this in turn prevents the evaluation of options. Early identification of options will allow for the development of an experimental program that will support the design process. The main objective of this work is to apply the Process Synthesis approach to develop a structured method of designing a process using mostly qualitative information based on limited experimental data, prior experience, literature and assumptions. Fischer-Tropsch (FT) synthesis of hydrocarbons from syngas generated by reforming natural gas and/or coal has been used as a case study to illustrate this method. Simple calculations based on experimental data and basic thermodynamics have been used to generate some FT Synthesis flowsheet models. The evaluation of different flowsheet models was done using carbon efficiency as a measure of process efficiency. It was established that when choosing the optimal region for the operation and design of an FT Synthesis process, the influence of the system parameters must be well understood. This is only possible if the kinetics, reactor, and process design are done iteratively. It was recommend not to optimize the reactor independent of the process in which it is going to be used without understanding the impact of its operating conditions on the entire process. Operating an FT Synthesis process at low CO per-pass conversions was found to be more beneficial as this will avoid the generation of high amounts of methane which normally results in large recycles and compression costs. Whether the process is run as a once-through or recycle process, the trend should be to minimize the formation of lighter gases by obtaining high Alpha values because carbon efficiency increases with the increase in value. Experiments should be performed to obtain process operating conditions that will yield high values. However, if the aim is to maximize diesel production by hydrocracking long chain hydrocarbons (waxes), then an optimal value should be targeted to avoid the cost of hydrocracking these very heavy waxes. The choice of the syngas generation technology has a direct impact on the carbon efficiency of an FT synthesis plant. This study has established that running an FT synthesis process with syngas obtained by steam reforming of natural gas with CO2 addition can yield high carbon efficiencies especially in situations were CO2 is readily available. In FT synthesis, CO2 is normally produced during energy generation and its emission into the environment can be minimized by using it as feed during the steam reforming of natural gas to produce syngas.

Thesis (M.Eng. (Development and Management Engineering)--North-West University, Potchefstroom Campus, 2008.

Thesis (M.Eng. (Development and Management Engineering)--North-West University, Potchefstroom Campus, 2008.

Thesis (M.Eng. (Development and Management Engineering)--North-West University, Potchefstroom Campus, 2008.

<div>Thermochemical processing using fast-pyrolysis technology has been used to upgrade feedstocks like biomass and natural gas and more recently studied for plastic recycling. This work aims to improve the selectivity to desired products from a pyrolysis process through better catalysts and reactor design.</div><div>Fast-pyrolysis of biomass to fuels is considered a promising technology due to the higher yields to liquid fuel products. However, the process suffers from low carbon efficiency to hydrocarbon products due to carbon losses to biochar, accounting for 25-40 wt.% of the product stream depending on the biomass type. Using a combination of inorganic free-model compounds, biomass pretreatments and mass spectrometric analyses coupled with lab-scale reactor experiments, the char contribution from the lignocellulosic components (cellulose, hemicellulose, and lignin) and mineral content was investigated. The lignocellulosic components were found to follow the order: Lignin > Hemicellulose > Cellulose. Addition of inorganic salts (K, Na and Ca) to cellobiose, a model compound for cellulose, was found to catalyze additional dehydration reactions on primary pyrolysis products (e.g., levoglucosan) to yield secondary products (e.g., 5-HMF), and produce more char. This knowledge of char formation contributors can enable optimization of the bio-refining process sequencing using process system engineering tools and thus achieve higher carbon efficiency for biomass conversion.</div><div>While biomass has been viewed as a future energy source, there is a need for a transition fuel with the lowest possible greenhouse gas (GHG) footprint. Shale gas, consisting primarily of methane, is a potential candidate due to its large availability and high hydrogen to carbon ratio. Recently, single-atom catalysts have been studied as stable and non-coking catalysts for the non-oxidative coupling of methane (NOCM) to higher hydrocarbons (like ethylene). However, lack of post reaction catalyst characterization and rigorous kinetic testing have raised questions on the stability of these materials. This work combines homogenous (Chemkin simulations, gas phase kinetics) and heterogeneous reaction kinetic studies (reaction orders, steady state kinetics), coupled with microscopy (Scanning and Transmission Electron Microscopy (SEM, TEM)) and surface characterization tools (BET, TGA, Raman spectroscopy, CO-IR spectroscopy) to understand the role of the solid materials during NOCM. Post reaction catalyst characterization using transmission electron microscopy (TEM) analysis on the spent samples (CH4 treated at 975 deg C for 3 hours) reveals that the materials containing Pt single atoms (SA) and Pt nanoparticles (NP) are found to sinter to particles approximately 5-7 nm in size. Ethylene hydrogenation experiments, a kinetic probe for surface Pt, shows initial ethane formation rates that are four orders of magnitude lower on the isolated Pt+2 sites, found on Pt SAs, when compared to the rates obtained if all the surface Pt were assumed to be metallic. These results suggest that single atoms are not the active sites. However, under same reaction conditions (50 mL min-1 CH4 flow and 975 deg C), the ethylene formation rates (in mol h-1) on the solid materials are 2-7 times higher than the empty tube rates, indicating that the surface plays a role during NOCM. Addition of incremental amounts of the solid material increases methane conversion, extrapolating to the bare tube conversion at zero loading. This indicates that the solid materials improve the NOCM performance.</div><div>Experiments with pure methane feeds indicate that the solid materials are found to deactivate due to coking on the surface, evidenced by the coke buildup observed using thermogravimetric analysis (TGA) and the initial time-on-stream kinetic results showing rapid methane deactivation. Raman spectroscopy on the spent catalysts indicate at the development of a similar graphite-like surface intermediate under steady state conditions on all the materials. When compared under the same reaction conditions (975 deg C, 60 mL min-1 Pure CH4 with 10% UHP N2 feed, space velocity = 39.6 L h-1 gcat-1), these coked surfaces show a linear dependence for the ethylene formation rate (in mol h-1 gcat-1) with the spent surface area of the material (in m2 gcat-1). This observation is irrespective of the type of the material studied (alpha Al2O3, Davisil SiO2, 1 wt.% Pt/CeO2, Graphene, Graphite, etc.). In conclusion, these results prove that the spent surface area is critical for NOCM.</div><div>Similar experimental setup was used to study the dehydrogenation of methane, ethane, and propane mixture in the gas phase. Initial experiments at 1 bar pressure and reaction temperatures ranging from 650-850 deg C revealed that ethylene and hydrogen are the main gas phase products, with methane acting as a diluting agent under these reaction conditions. These results could enable direct processing of the shale gas without the use of a conventional ethane/propane separation step. These results were further studied by the system engineers using ANSYS ChemkinPro. For practical applications, these experiments were suggested to be performed at much higher operating pressures (~30 bar) and low residence time (~0.2 s), with a quick quenching step added after the reactor to prevent change in the exit stream compositions. A new reaction system was built to experimentally validate these recommendations.</div>

都市CO2的過量排放，是造成全球氣候暖化與環境變異的主因之一。低碳都市的發展成為目前永續發展的核心議題，亦是抑制氣候暖化的關鍵。低碳都市的概念，包括空間和非空間維度。空間維度探討了都市型態、土地利用與運輸系統，非空間維度則著眼於經濟、能源使用和節能產品。然而過去低碳方面的研究，多聚焦於非空間維度之探討上，甚少關注空間維度面向，但面對全球氣候暖化與低碳發展，都市不僅可落實低碳發展政策，其規劃更在低碳都市的建設與發展過程中扮演關鍵角色。因此，本研究依循「全球考量，在地行動」之概念及方式，以低碳都市為主軸，並以空間規劃及環境經濟之角度切入，針對低碳都市衡量指標建構及分析、縣市低碳效率評估及低碳政策影響效果之探討、都市發展及空間結構與碳量間關係之探討等三項研究主題，進行實證分析與探討，企圖補強低碳都市在空間維度之論述，使都市的低碳化有所依循，亦使都市能藉此更達低碳與永續之目標。 在「低碳都市衡量指標系統之建構及探討」主題上，以台灣為實證案例，根據低碳都市的內涵、理念與目標，研撰適宜之衡量指標系統，透過因子分析定權法進行衡量指標的擇取，藉此評估台灣各縣市的低碳發展狀態。根據實證結果，顯示台灣的低碳都市評估指標系統，包括「環境品質及資源消耗」、「經濟及建設」、「財政及資源循環」、「社會狀態」等4構面，共22個指標；而台灣各縣市在低碳發展的綜合表現上，以台北市的狀態最佳，而以彰化縣表現最差。 而在「縣市低碳發展效率評估及低碳政策影響效果之探討」主題上，以台灣各縣市作為實證案例，在考量非意欲產出下(碳排放威脅、環境污染衝擊)，透過DEA中的不良產出模型(Bad Outputs Model)評估縣市低碳發展效率，並藉由Tobit模型，釐清低碳政策對縣市低碳發展效率之影響效果。依據不良產出模型之效率評估結果，顯示臺北市、新竹市、臺東縣、花蓮縣在低碳發展效率上相對具有效率，且已達最適規模水準；而高雄縣是目前亟待改善的首要縣市。此外，藉由Tobit模式的實證分析，其結果顯示工廠家數、資源回收量、市區公車系統路線總長、具綠建築標章的建物數、綠地面積等變數，對縣市低碳發展效率有顯著影響，其中，除工廠家數對縣市低碳發展效率有負向影響外，其餘變數皆呈正向影響效果。而由此亦得知產業結構、資源循環利用、低碳運輸、低碳建築、碳匯規劃等低碳政策，對於縣市低碳發展效率是顯著有效的。 在「都市發展及空間結構與碳量關係之探討」主題上，以台灣各縣市作為實證案例，透過偏最小平方法的分析，實證都市發展及空間結構對碳量之影響關係與影響程度，並找出低碳都市發展中關鍵的空間規劃要素。而根據偏最小平方法的實證結果，顯示都市混合度、都市發展強度及密度與都市綠化對碳淨存量產生負向影響，亦即都市混合度、都市緊密度及綠化程度愈高，愈能減少碳淨存量；而都市工業機能、都市交通狀態對碳淨存量有正向影響，亦即都市工業機能與都市交通狀態愈高，愈會增加碳淨存量，其中又以都市交通狀態的影響效果最大。 綜合上述，本研究之研究成果包括：「釐清低碳都市模糊且紛雜的概念及內涵」、「建構適宜台灣本土的低碳都市衡量指標系統」、「評估台灣各縣市低碳發展之整體狀態」、「評估台灣各縣市之低碳發展效率」、「檢視台灣現行低碳政策對於縣市低碳發展效率之影響效果」及「找出低碳都市發展中關鍵的空間規劃要素」，期待此些成果，能作為地方政府或規劃單位於低碳目標下，進行低碳都市發展及規劃開發時之參考。 / Excessive CO2 emission is one of the main causes of causing global warming and climate change. Therefore, low-carbon urban development is currently the core issue in sustainable development, and it is also the key to curb climate warming. The concept of the low-carbon city includes spatial dimension and non-spatial dimension. The former contains the issues of urban form, transportation system, and land use planning, the latter contains the issues of economy, energy consuming, and energy saving product. The spatial dimension plays the important role in the construction and development of the low-carbon city, but only the few studies focused on. Therefore, this paper takes the spatial dimension of the low-carbon city as the main scope, and discusses the topics of “research on low-carbon city indicator system”, “evaluating the low-carbon efficiency of city and analyzing its influential policies”, and “the effects of urban development and spatial structure on carbon emissions”. In the topics of “research on low-carbon city indicator system”, this paper takes cities and counties of Taiwan as the empirical cases, and uses factor analysis to establish the appropriate low-carbon city indicator system for Taiwan. Furthermore, this paper evaluates the cities'/counties' status of low-carbon developments by this low-carbon city indicator system. According to the empirical results, there are four evaluating aspects of the low-carbon city: 1) environmental quality and resource consumption, 2) economy and construction, 3) finance and resource recycling and 4) social status. Based on the comprehensive performance of the low-carbon city indicator system, Taipei City shows the best performance, whereas Changhua County shows the worst performance. In the topics of “evaluating the low-carbon efficiency of the city and analyzing its influential policies”, this paper takes cities and counties of Taiwan as the empirical cases, and assesses low-carbon efficiency of the city by Bad Outputs Model. Moreover, this paper uses Tobit Model to analyze the influence of low-carbon policies on the low-carbon efficiency of the city. According to the result of Bad Outputs Model, Taipei City, Hsinchu City, Taitung County, and Hualien County are efficiency, and achieve the optimal scale level. Moreover, Kaohsiung County is the county in the most urgent need of improvement. Another result of Tobit model indicates that the industrial unit has the negative impact on the low-carbon efficiency of the city. Moreover, resource recovery, the total length of the urban bus route, the number of green buildings, and green area all have the positive impact on the low-carbon efficiency of the city. In the topics of “the effects of urban development and spatial structure on carbon emissions”, this paper takes cities and counties of Taiwan as the empirical cases, and uses partial least squares method (PLS) to identify the impact of urban development and spatial structure on carbon emissions. The empirical results show that the mixed land use, urban intensity and density, and urban greening all have the negative impact on carbon emissions. Moreover, industrial function and urban transportation all have a positive impact on carbon emissions. Finally, these results can be used as reference for future low-carbon urban planning and policy.