Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1999-12-02
2001-02-27
Hofsass, Jeffery A. (Department: 2736)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S589000, C340S591000, C340S870010
Reexamination Certificate
active
06195010
ABSTRACT:
The maintenance of safe operation conditions in chemical reactors is of paramount importance to avoid personal and installation damages, and environmental pollution. In general, severe accidents due to runaway reactions can be reduced to one single scenario: if, for some reason, the rate of heat generation by chemical reaction exceeds the rate of heat removal of the cooling system, the temperature of the reacting mass will begin to rise. This in turn will cause an increase of the heat generation rate. This positive feedback mechanism results in an auto-acceleration behaviour of the heat generation rate, producing a large amount of heat in a very short time with the possibility of triggering off-side and chain reactions that eventually can lead to explosion and the destruction or inoperability of the plant.
Major safety advancements of a particular chemical process can only be achieved through the long process of research and engineering experience. Nevertheless, despite conventional fall-back mechanisms, switches and multi-layer control circuitry available today, there is always the possibility of undetected runaway events. A safe reactor is therefore not only characterized by the degree of the complexity of its safety measures but also by the rate with which unexpected and unavoidable potentially dangerous situations can be handled. Early warning devices are therefore indispensable, irrespective of the detailed mechanisms of the reaction and of other safety measures.
Safe operation of chemical reactors in which strongly exothermic reactions take place requires the availability of measurement devices able of detecting hazardous states already in the early stages to allow plant operators to adopt the necessary counter-measures to return to safe conditions. The principle bottleneck for an on-line detection system is the criterion which distinguishes between dangerous and non-dangerous situations. Especially for batch reactors, due to the variety of the process, the on-line safety criterion has to be as independent as possible of the actual process carried out in the plant.
The methods for early on-line detection can be divided into two categories depending upon the quantities being used:
Measurable signals.
Non-measurable state variables, process parameters or characteristic quantities.
In the first case, measurable information about the status of the process is used to detect a malfunction. In the second case it is necessary to develop estimation methods and process models to calculate the non-measurable quantities that will be used afterwards in the criteria for the detection system.
Measurable input and output variables can be directly used to monitor changes in the process. The most common method consists in temperature supervision, but detection procedures based on pressure could be more suitable for certain types of reactions in which an undesired decomposition reaction produces a non-condensable gas. A number of other variables: pH, viscosity, thermal conductivity, etc. may also be easily measurable and can be used for some processes. For example, oxidation can be a dangerous secondary reaction; therefore the oxidation-reduction potential (Redox) of the reacting medium is a good measure to detect initiation of these reactions at its earliest stage. However, for practical applications the sensitivity and reliability of the sensor play an important role to determine the choice of the measurement chosen for the detection system, temperature measurement being often preferred.
The most common procedure consists in measuring online the chosen variable and checking it against preselected limit values, i.e. maximum and minimum. This is referred to an absolute value check. If the measured variable exceeds the set limit an alarm signal is produced. Early warning detection systems, with a criterion based on an absolute value check, are easy and cheap to install and can have good predictive capacity, but they are completely dependent on knowledge of the process, i.e. the limits have to be established beforehand, and are unsuitable for detection of unexpected dangers.
The loss of control in an exothermic batch or semibatch process is characterized by thermal and pressure excursions of the reacting mass due to the large amounts of heat released in a very short time. That means that the derivatives of temperature and pressure or derivatives of the rate increase can be used to predict the runaway excursion. The method of supervising the rate of temperature or pressure rise is not as cheap or as simple as temperature or pressure measurement, because amplification and filtering are necessary before reliable derivative can be calculated. Since the “safe” temperature or pressure need not be specified, the independence and selectivity of this method is higher, but depends also on specific knowledge of the system in order to define the limits, that in this case will be the maximum and/or minimum rate increase of the measured variable. The method of monitoring the acceleration suffers from the same drawbacks as the previous method.
A first improvement in this direction was the safety criterion developed by L. Hub, see his article “On-line hazards identification during chemical processes” published in Proc. Loss Prevention and Safety Promotion Proc.Ind. DECHEMA (1978), Frankfurt, pages 265-272. This criterion which has been used industrially considers the state of the reactor as dangerous when both the first derivative of the temperature difference between the reactor and the jacket and the second derivative of the reactor temperature with respect to time are positive as follows:
d
2
T/dt
2
>0 and
d
(
T−T
e
)/
dt>
0 (1)
The difficulty in using a double differentiation of the measured temperature with respect to time is that noisy parts in the signal are greatly amplified and affect considerably the result of the evaluation, false alarms being the principal problem. Furthermore, in case of an autocatalytic reaction taking place in the reactor, due to the autoacceleration of the rate of reaction, and, hence, the rate of heat generation, this criterion will give always an alarm despite the hazardousness of the process, if no sufficient positive limits are defined for the values of the first and second derivatives.
On the other side, a considerable amount of work has been done in studying the parametric sensitivity of chemical reactors; such studies had in mind the definition of simple criteria that allowed to establish in which regions of the parameters—that characterize the algebraic-differential equations that describe mass and energy balances of these reactors—the operation would be safe. The question that arises at this point is if it is possible from these studies to extract a safety criterion that may be used on-line as early warning detection criterion.
If a mathematical model of the process exists, the state of the reactor can be reconstructed from measurable variables, which will allow predictive calculation of the future status, or at least, evaluation of new criteria based on these non-measurable quantities. That means that highest predictive power and selectivity can be reached and hence better early detection of hazardous states can be obtained.
Firstly, all the known information about the process is put in the form of a mathematical model that normally consists of a set of algebraic and differential equations. This model is solved on-line by numerical procedures in order to obtain the whole state of the batch reactor.
In addition to the available measured variables, the model-based detection system must be supplied with all the control variables, and the initial and operating conditions. The simulated variables must be compared with the measured variables; a non-zero difference will indicate an incorrect calculation in the model that can be due to unknown disturbances, unknown initial conditions, erroneous parameters, etc. Consequently, the model must be corrected from process measurements. The method to correct these deviations is by minimizing the error usi
Strozzi Fernanda
Zaldivar José Manuel
European Atomic Energy Community
Hofsass Jeffery A.
Previl Daniel
Sughrue Mion Zinn Macpeak & Seas, PLLC
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