178,264 research outputs found
Ziegler-Natta catalyzed polymerization kinetics : origin of the molecular weight distribution
No full textTypescript (photocopy).The molecular weight distributions generated by Ziegler-Natta catalyzed polymerizations are examined. Only those catalysts with one type of catalytic species are considered. From the rate laws for propagation and elimination, a series of coupled differential equations result which define the rate of formation of polymer chains composed of i monomer units. Solving these differential equations yields a time-dependent molecular weight distribution function with the observed rate constants for propagation function with the observed rate constants for propagation and elimination as parameters. Careful analysis of this distribution function shows that it predicts a transition from a Poisson to Schulz-Flory distribution with time. This transition is known for both the Ziegler-Natta catalyzed polymerization of olefins and the Fischer-Tropsch catalyzed hydrogenation of carbon monoxide. Hence, this time-dependent distribution function may apply to coordination catalyzed polymerizations in general. Analysis of this distribution function shows that the weight fraction distribution becomes bimodal just before the limiting Schulz-Flory form of the distribution is reached. This bimodal behavior provides an insight into the effect of propagation and elimination rates on the molecular weight distribution. Qualitatively, this behavior may be explained as follows. Before many eliminations occur, the molecular weight distribution obeys the Poisson distribution function, shifting to higher molecular weights with time. At longer times, the catalyst increasingly eliminates chains and starts to grow new chains. This results in a decrease in the number of surviving chains representative of the Poisson distribution and an increase in the lower molecular weight material. These new chains are generated at different times and represent a broad, truncated Schulz-Flory distribution. Eventually, the Poisson peak passes the time-independent peak of the truncated Schulz-Flory distribution and a bimodal distribution is obtained. Since the Poisson distribution decreases with time, it eventually disappears into the high molecular weight tail of the Schulz-Flory distribution. This predicted behavior is confirmed experimentally by analyzing the molecular weight distributions of polypropylene prepared at low temperatures with a vanadium (III) trisacetylacetonate/diethyl aluminum chloride catalyst. In addition, functions describing the time-dependence of the number-average and weight-average degrees of polymerization are derived from the rate laws. These functions are in excellent agreement with the available experimental data
Simulation of thermal plant optimization and hydraulic aspects of thermal distribution loops for large campuses
No full textFollowing an introduction, the author describes Texas A&M University and its utilities system. After that, the author presents how to construct simulation models for chilled water and heating hot water distribution systems. The simulation model was used in a $2.3 million Ross Street chilled water pipe replacement project at Texas A&M University. A second project conducted at the University of Texas at San Antonio was used as an example to demonstrate how to identify and design an optimal distribution system by using a simulation model. The author found that the minor losses of these closed loop thermal distribution systems are significantly higher than potable water distribution systems. In the second part of the report, the author presents the latest development of software called the Plant Optimization Program, which can simulate cogeneration plant operation, estimate its operation cost and provide optimized operation suggestions. The author also developed detailed simulation models for a gas turbine and heat recovery steam generator and identified significant potential savings. Finally, the author also used a steam turbine as an example to present a multi-regression method on constructing simulation models by using basic statistics and optimization algorithms. This report presents a survey of the author??s working experience at the Energy Systems Laboratory (ESL) at Texas A&M University during the period of January 2002 through March 2004. The purpose of the above work was to allow the author to become familiar with the practice of engineering. The result is that the author knows how to complete a project from start to finish and understands how both technical and nontechnical aspects of a project need to be considered in order to ensure a quality deliverable and bring a project to successful completion. This report concludes that the objectives of the internship were successfully accomplished and that the requirements for the degree of Degree of Engineering have been satisfied
Origin and effects of anomalous dynamics on unbiased polymer translocation
No full textIn this paper, we investigate the microscopic dynamics of a polymer of length translocating through a narrow pore. Characterization of its purportedly anomalous dynamics has so far remained incomplete. We show that the polymer dynamics is anomalous until the Rouse time , with a mean square displacement through the pore consistent with , with the Flory exponent. This is shown to be directly related to a decay in time of the excess monomer density near the pore as . Beyond the Rouse time translocation becomes diffusive. In consequence of this, the dwell-time , the time a translocating polymer typically spends within the pore, scales as , in contrast to previous claims
Numerical test of the generalized Flory and generalized Flory dimer theories
No full textWe report results of a numerical test of the generalized Flory and generalized Flory dimer theories. Using continuous space Rosenbluth sampling, we have tested the basic assumptions of these theories. We find that the discrepancy between the predictions of the generalized Flory theory and simulation data is caused by the use of the Carnahan–Starling equation of state to estimate the incremental chemical potential of a chain molecule. Although the assumptions made in deriving the generalized Flory dimer theory are similarly flawed, the predictions of the latter theory are in excellent agreement with simulation results. This is explained by the occurrence of a cancellation of errors. Therefore, it is not to be expected a priori that increasingly higher order extensions of these theories will be improvements. In particular, the disappointing performance of the recently developed generalized Flory trimer theory may be due to the fact that this theory lacks the cancellation of errors that is present in the generalized Flory dimer theory
Intern experience at CH���M Hill, Inc.: an internship report
No full textIncludes author's vita"Submitted to the College of Engineering of Texas A&M University in partial
fulfillment of the requirement for the degree of Doctor of Engineering."Includes bibliographical referencesA review of the author's internship experience with CH���M HILL, Inc.
during the period September 1975 through May 1976 is presented. During this nine month
internship the author worked as an Engineer II in the Industrial Processes discipline of this
large consulting engineering firm... The author's prime responsibility was as one of three
lead design engineers on the design of a large wastewater treatment facility for a pulp mill
in Hoquiam, Washington owned by ITT Rayonier Inc. The work generally consisted of the design
of individual treatment units and associated piping and pumping. The purpose of the project
was to provide wastewater treatment capabilities that would satisfy the effluent limitations
(standards) imposed upon the mill by the State of Washington Department of Ecology and the
U.S. Environmental Protection Agency. The author's assignment also entailed necessary
interaction with the project manager and other CH���M HILL design engineers and support staff
members, the client's representatives, and representatives of two other consulting engineering
firms working on the project. Thus, the internship position at CH���M HILL provided considerable
experience coordinating the author's work with the work of other engineers, guiding the design
and administrative efforts of a support staff, and interacting regularly with the client and
other consulting firms. This broad exposure to a variety of engineering and organizational
problems provided a valuable educational experience
Flory theory for polymers
No full textWe review various simple analytical theories for homopolymers within a unified framework. The common guideline of our approach is the Flory theory, and its various avatars, with the attempt at being reasonably self-contained. We expect this review to be useful as an introduction to the topic at the graduate student level
Intern experience at the Texas Transportation Institute: an internship report
No full text"Submitted to the College of Engineering of Texas A&M University in partial
fulfillment of the requirements for the degree of Doctor of Engineering."Includes vita (leaf 91)Includes bibliographical references (leaf 57)This report discusses the author's engineering experience at the Texas
Transportation Institute (TTI), Texas A&M University from March 1974 through July 1977. A
report of this experience plus twelve additional hours of academic course work were
substituted for an internship according to requirements established by the College of
Engineering. Although the author could not retroactively establish objectives of the type
associated with a typical internship, the work experience gained could be related to the two
general objectives of an intership: 1. To demonstrate an identifiable contribution to the
organization in which the intern served, and 2. To enable the intern to become aware of the
non-technical aspects of working as an engineer in a non-academic environment. While a member
of the Texas Transportation Institute staff, the author worked on a number of projects in the
areas of highway traffic engineering research and the preparation of continuing education
courses for transportation engineering officials. These assignments were contained within the
Urban Transportation Systems Division of TTI. This report delineates the author's contribution
to four particular projects and discusses the technical as well as non-technical experience
gained from each. Having been exposed to more engineering situations than many of the other
Doctor of Engineering students, the author has already formulated some rather specific
milestones for the future. The Doctor of Engineering program is discussed in conjunction with
its potential for helping the author attain these career goals and objectives. Also, some
thoughts are presented regarding the Doctor of Engineering as a viable alternative to the
Ph.D. in preparing for a career in engineering higher education
Kinetic modeling of the Fischer-Tropsch synthesis
No full textTypescript (photocopy)VitaMajor subject: Chemical engineeringThe objective of this work was to develop a kinetic model to describe the conversions and selectivity typical of iron based Fischer-Tropsch catalysts, and to apply the model in simulations of typical laboratory reactors. The model accounts for the formation of paraffins and olefins up to an arbitrary carbon number, and disappearance rates can be calculated from known stoichiometry. Equilibrium adsorption was assumed for CO, CO2, H2, and H2O, and accounts for product and reactant inhibition on rates. There was no significant difference in model results using a steady-state adsorption model and an equilibrium model, but the equilibrium model has the advantage of requiring fewer kinetic parameters. The final model requires 8 constants for Fischer-Tropsch, plus additional constants for water-gas shift kinetics (one in this study). Three types of reactors were considered: gas phase stirred tank and fixed bed (plug flow) reactors, and a stirred tank slurry reactor. The gas phase stirred tank is a simple, well defined reactor used in the model development, while the fixed bed reactor was chosen to represent the common laboratory fixed bed. The slurry reactor is another common laboratory reactor, but the reaction occurs on catalyst in the liquid phase formed by condensing the high molecular weight products of the reaction. Vapor-liquid equilibrium with simultaneous chemical reaction was used to account for the condensation. Non-Schulz-Flory distributions were obtained in the slurry reactor simulations due to olefin readsorption followed by chain growth. Higher concentrations of the high molecular weight olefins in the liquid phase cause the non-Schulz-Flory behavior. The adsorption also gives the trend of decreasing olefin content with carbon number expected from experiment. Both of the gas phase reactors (fixed bed and stirred tank) exhibited nearly classical Schulz-Flory distributions under most conditions. In the fixed bed, conversion increases along the length of the reactor, which leads to changes in selectivity with position. The integral effect of the selectivity profiles causes some carbon number dependence on ���� and olefin content, but the deviations from classical distributions are significant only at low carbon numbers..
The long-wavelength view of GG Tau A: rocks in the ring world
No full textWe present the first detection of GG Tau A at centimetre wavelengths, made with the Arcminute Microkelvin Imager Large Array at a frequency of 16 GHz (λ = 1.8 cm). The source is detected at >6 σrms with an integrated flux density of S16GHz = 249 ± 45 µJy. We use these new centimetre-wave data, in conjunction with additional measurements compiled from the literature, to investigate the long-wavelength tail of the dust emission from this unusual protoplanetary system. We use an MCMC-based method to determine maximum likelihood parameters for a simple parametric spectral model and consider the opacity and mass of the dust contributing to the microwave emission. We derive a dust mass of Md ~ 0.1 Msun, constrain the dimensions of the emitting region and find that the opacity index at λ > 7 mm is less than unity, implying a contribution to the dust population from grains exceeding ~4 cm in size. We suggest that this indicates coagulation within the GG Tau A system has proceeded to the point where dust grains have grown to the size of small rocks with dimensions of a few centimetres. Considering the relatively young age of the GG Tau association in combination with the low derived disc mass, we suggest that this system may provide a useful test case for rapid core accretion planet formation models
Towards a taxonomy of reusable CRM requirements for the Not for Profit sector
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Traditional (or commercial) CRM is a well-defined domain but there is currently no generally accepted definition of what constitutes CRM in the not for profit (NfP) sector. Not for profit organisations are organisations which exist for a social purpose, are independent of the State, and which re-invest all of their financial surpluses in the services they offer or in the organisation itself. This research aims to answer the question “What exactly is CRM as applied to the NfP sector, what are its boundaries and what functions should an NfP CRM information system perform?”
Grounded Theory Method (GTM) within a Design Science framework was used to collect, analyse, categorise, generalise and structure data from a number of NfP organisations and NfP information systems suppliers. An NfP CRM model was constructed from this data in the form of three multi-level taxonomies. The main taxonomy relates to generic and reusable information system requirements both functional and non-functional. Within this taxonomy the high-level categorisations of commercial CRM, namely “Marketing, “Sales” and “Service”, are greatly extended to reflect the special needs of the NfP sector and in particular a much broader definition of “customer”. The two minor taxonomies relate to issues of CRM strategy and CRM systems architecture which need to be considered alongside the system requirements. In addition to and resulting from the taxonomies, an over-arching definition of NfP CRM was developed.
NfP organisations now have a framework that will enable them to know what to expect of CRM systems and from which they can select requirements to build their own specification of information system needs. Using the requirements taxonomy for this task will make the process of requirements analysis and specification easier, quicker, cheaper and more complete than using traditional methods. The framework will also allow NfP system suppliers to know what NfP organisations expect of their systems and will assist them with the specification of new system features. The minor taxonomies will provide NfP organisations with a series of strategic issues and systems architecture options that should be considered when implementing a CRM system.
This research also demonstrates how GTM can be utilised: as the development phase of Design Research, as a general method of domain analysis, and as a tool to develop a taxonomy of reusable information system requirements
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