AN INVESTIGATION OF STABILITY AND MULTIPLICITY OF STEADY STATES IN A LABORATORY BIOLOGICAL REACTOR

Methanol utilizing organisms have long been considered as potential sources of single cell protein. Since the growth rate of these organisms is a nonmonotonic function of the growth limiting substrate, multiple steady states, of which one is unstable, may exist for pure culture in a continuous stirred tank reactor. The existence of multiplicities and instabilities cannot be ignored in understanding microbial growth, and coping with such situations in practical applications like SCP production. This type of behavior has not previously been investigated experimentally. Thus, a laboratory demonstration of the existence of unstable and multiple steady states would provide confirmation of theoretical predictions as well as important information relative to modelling, optimization, and control. Classical feedback control was used to demonstrate the existence of unstable and multiple steady states in an isothermal biological reactor for the first time. Unstable steady states were stabilized through use of a turbidostat. The natural instability of the steady state was demonstrated by removing the feedback control and leaving the feed rate constant at the steady state value. Subsequent reactor transients were always away from the steady state. Using the open-loop reactor to obtain stable steady states, and the closed-loop reactor to obtain unstable steady states, multiple steady states were also demonstrated. The data, taken over a wide range of methanol concentrations, were used to fit an empirical growth model that incorporated variable yield. Phase-plane analysis of this model showed that when proportional-integral control was used to stabilize an otherwise unstable steady state, and the manipulated variable was sufficiently constrained, five steady states were possible. Cascade control could be applied to this system to show the existence of the two new unstable steady states. The fate of the reactor in terms of the stable steady state at the constraints, was shown to be a function of initial conditions. With the exception of the stable steady state at the upper constraint, this behavior was demonstrated experimentally for the first time. Control at an unstable steady state was shown for a PI controller with constraints. Cascade control was used to operate the laboratory reactor at both the upper and lower constraints. Washout of the closed-loop system was also demonstrated. Failure to reach the stable steady state at the upper constraint was attributed to inadequacies inherent in the perturbation methods used. In general, steady state predictions of stabilization of unstable steady states with simple, constrained, or cascaded control were relatively good. The dynamic behavior was not well predicted in many cases, and indicated that the idealized model was inadequate for such predictions. This was due to its lack of specific biochemical structure. The methanol, formaldehyde, and yield data taken at stable and unstable steady states and which were not previously available, were interpreted via a conceptual model for methanol metabolism proposed by other workers. This supported the proposed important role of formaldehyde in the kinetics and energetics of growth, and thus the stability behavior. The results, which showed relatively high formaldehyde concentrations and drastic yield reductions at the unstable steady states were consistent with ideas set forth in the model. The need for quantitative consideration of formaldehyde, at the least, was indicated

Undergraduate Research and the Difference It Makes for LGBTQ+ Students

Published Versio

Biochemical engineering : annals of the New York academy of sciences, VI Vol. 589/ Edit.: Walter E. Goldstein

xi, p. 715.: ill.; 23 c

Achieving Learning Outcomes through Project-Based Education

The WPI Plan is predicated on the concept that project work provides an environment in which students “learn by doing.” In addition to requiring disciplinary competence, WPI’s undergraduate programs feature broad learning outcomes such as the ability to address open-ended problems, to communicate effectively, to function well in teams, and to understand the societal and cultural contexts within which science and technology function. A set of required projects is central to the achievement and demonstration of such learning outcomes. As a practical matter, we have found that problems drawn from the “real world” provide very effective learning experiences for students working in small teams with guidance from faculty advisors and sponsor liaisons. WPI students complete two such projects--an interactive technology/society project done in multidisciplinary teams, and a capstone design or research project in the major area of study. Both of these projects are degree requirements for every WPI student. Assessment indicates that these experiences are especially effective contexts for motivating high levels of student achievement. Because of WPI’s well-established project-based approach to global learning, the Global Perspective Program, more than 500 WPI students participated last year in semester-long study-abroad programs culminating in a major team-based project. The WPI Global Perspective Program presently provides opportunities at over twenty sites for students to complete disciplinary and interdisciplinary projects — all of which are advised by faculty in residence with the students at the global site. In this paper, we will discuss the educational objectives of these two types of student projects in terms of outcomes and their assessment. In particular, we will focus on the challenges and benefits of achieving and measuring broad learning outcomes in open-ended project settings. Some of these learning outcomes are particularly well-suited to demonstrating fulfillment of ABET criteria, including those criteria that are mostdifficult to achieve in a conventional, course-based curriculum. We will conclude by describing how the evidence provided by assessing these projects has been used in two highly successful ABET EC 2000 reviews