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The Development of Stirred-Tank Heat Flow Calorimetry As a Tool For SAFETY AND ENVIRONMENTAL PROTECTION IN CHEMISTRY 189 CHIMIA 5/ (1997) Nr. 5 (Mail Chimia 51 (1997) 189-200 drugs, crop-protection agents, coat-pro- © Neue SchlVeizerische Chemische Gesellschaft tecting additives, dyes or pigments) in a ISSN 0009-4293 form (or formulation) which the consumer needs or particularly likes. It must be eco- nomic but also safe and ecologically ac- The Development of Stirred- ceptable. The task of chemical process develop- Tank Heat Flow Calorimetry as ment is to elaborate such processes effec- tively and efficiently. Fast implementa- a Tool for Process Optimization tion of new processes is becoming in- creasingly important. Ideally, in a well- and Process Safetya) developed process, we have a basic under- standing of all reaction steps (kinetics, influence of process parameters on selec- Willy Regenass* tivity, potential of thermally hazardous conditions) and a quantitative, model- based understanding of separation stages. Abstract. Calorimetry based on the measurement of heat release rates has found The tools of process development have widespread use in chemical process work, particularly for aspects of thermal process improved in recent decades to an impres- hazards. However, it has not yet found the breadth of application it deserves. This sive extent. The most important of these contribution discusses the tasks and problems of chemical process development and the tools are: role of heat flow calorimetry in this context. It reviews the historical development of - methods of analysis to follow the com- heat flow calorimetry and analyzes the prerequisites for a successful development of the position of reaction mixtures and method in the future. streams in separation processes, - methods for physical property estima- tion, 1. Stirred- Tank Heat Flow Calorimetry - temperature may be held constant, or methods of steady state and dynamic may be increased or decreased at con- process simulation. Heat flow calorimeters measure the in- stant rate, or follow any imposed vari- Consequently, we might expect a sim- stantaneous rate of heat release (or heat ation over time, ilarly impressive improvement in process consumption) at specified temperatures. - pressure may be varied within limits development effectiveness. Unfortunate- Fig. ], a shows the general scheme, Fig. ], restricted by the strength of the vessel, ly, this has largely not happened. The state b the type where heat flow is controlled - components may be added during of the art is widely different between var- and measured by adjusting the tempera- measurement, in portions, at constant ious process development groups. Some ture of a fluid in ajacket around the sample rate, or following a time program, are indeed much faster and produce far vessel. - stirring and mixing conditions may be better results than what was standard in- Fig. 2 compares heat flow calorimetry varied. with the better known heat accumulation calorimetry, where heat release causes tem- perature changes which are used to meas- 2. Process Development and Its a) ha.t t'ansfe, and ure heat effects. Problems helt flow m••• urlng device Stirred-tank heat flow calorimeters have all essential features of a laboratory When we want to asses the role of heat reactor (and a few more). They are ideally flow calorimetry, we must have a look at suited to do experiments under the condi- chemical process development as a disci- q tions of an industrial process and to deter- pline, at its tasks, and at trends with respect mine in the course of such runs: to requirements and tools. he I Ink rates of transformation (kinetics of re- In the life cycle of a chemical product, sampl v •• I actions, crystallizations, etc.), the quality of process development affects - heats of transformation. the resource consumption to provide Modern instruments provide a wide manufacturing facilities, b) T cemperllturw control variety of operating conditions: - the resources (raw materials, utilities, f"od on<! he.t now m .unng labour) consumed in its manufactur- clev.e mg, - the emissions into the environment, the hazards caused by the process, and thus has a great influence not only on economics but also on environmental and *Correspondence: Prof. Dr. W. Regenass social acceptability. Ciba Specialty Chemicals A chemical process (Fig. 3) is a se- R 1233.2.23 CH-4002 Basel quence of reaction steps and physical sep- aration steps followed by 'finishing' (the Fig. 1. Stirred-tank heat flow calorimeter; a) a) Extended version of a lecture given at the 8th elaboration of the final application form) general scheme; b) with control ofjacket temper- Mettler RC User Forum in October 1996. designed to produce chemicals (such as ature SAFETY AND ENVIRONMENTAL PROTECTION IN CHEMISTRY 190 CHIMIA 5/ (1997) Nr. 5 (Mai) dustry practice 20 years ago. Others have neering'), process developmentre- 3. Range of Applications made little progress. The latter is particu- mains a second-class activity, be- larly true for the interface of process engi- cause short term tasks of high pri- Stirred-tank heat flow calorimeters (the neering and project engineering (i.e., the ority absorb most of the available Mettler RCI [1] being the best-known preparation ofinvestment projects for new management attention. example) provide the following types of manufacturing facilities). With the present trend to short term information: Likely causes of this situation are: 'business focus', there is a lot of pressure - by direct measurement, the instanta- - The fact that sophisticated methods to outsource the process engineering end neous rate of heat release (or con- require highly skilled people who ex- of process development, making the com- sumption) q(t); pect to work as creative and relatively munication problems even more serious - by integrating q over time, we obtain independent partners in a team, com- than they were in the recent past. Conse- Q(t), the total heat removed from bined with the fact that human commu- quently, in the real world, results of pro- (or absorbed by) the sample up to time nication skills have evolved much slow- cess development are often mere recipes t; er than technical skills. and equipment specifications, which de- - if we can attribute this heat to a specific - The appreciation and organizational fine operating procedures and process reaction, we obtain its heat of reaction attachment of process development as plants which work (in most cases), but (-Mf) = Q(t)ln a discipline in the triangle research! miss the opportunity to provide the basic (n = moles converted at time t) and the eng ineeri ng/manufacturing: understanding required to assess the con- instantaneous reaction rate ret) = q(t)1 - sometimes claimed as their domain sequence of deviations from specified con- Qe (Qe = heat released for total conver- by R+D directors with little affinity ditions, or to assure that the chosen condi- sion); for process technology, tions are really the best. Heat flow calor- - the temperature difference between - sometimes considered plainly su- imetry is a tool which can help to narrow sample and wall to create a specified perfluous by research chemists, the gap between chemical research and heat flux provides information about - even with the organizationally most process engineering. It could become even the heat transfer properties of the sam- appropriate attachment (to 'manu- more important in this function in the ple; facturing' or 'production and engi- future. - the heat input (or removal) rate re- quired to create a specified tempera- ture ramp in the sample allows the calculation of the heat capacity of the The limiting cases of calorimetry: a) ideal accumulation b) ideal heat flow sample and its specific heat cp. qf= 0 qf= qs Such information has many applica- tions, not confined to reactions. Heat re- lease may mean very different things: - a hazard, - a chemical engineering problem (like design of heat transfer equipment, of TOt=:Tob matching heat release with available o time 0 time heat transfer capacity), - an opportunity of tracing physicochem- ical transformations (reactions or phase transitions). TEnvironment Applications dealing with the three mentioned aspects are now discussed. r, : rates of conversion qs: heat release within sample time time qf : heat flow from sample o o 3.1. Thermal Process Safety Exothermic reactions may lead to run- Fig. 2. Comparison of heat flow calorimetry with heat accumulation calorimetry away and thermal explosion, when the potential temperature rise is high and heat removal is insufficient, because reaction rate (and consequently heat release rate) increases very rapidly with increasing tem- recycling of perature, according to theArrhenius equa- excess rectants additives tion k(T) = k(To) . exp«EIR)· (lITo-liT») (with E = activation energy; R = gas con- rectants stant) reaction separation finishing The reason for high potential tempera- auxiliary products materials ture rise may be: - too high concentration of reactants of a highly exothermic desired reaction (this recycling of waste may happen in the batch, in the fed solvents, etc. n stages batch, or in the continuous mode); - potential highly exothermic decompo- sition (mostly of reaction masses). Fig. 3. The chemical process Moderate temperature excursions of SAFETY AND ENVIRONMENTAL PROTECTION IN CHEMISTRY 191 CHI MIA 5/ (1997) Nr. 5 (Mai) a) b) 80 140 140% 120 Q" (t), heat released when no accumulation 70 120 120% 100 G> 60 ;;? III • 100 . 100% 'i: CD I ~ G> •• G> I: 80 III •. :> 50 G> == ••G> iT ~ .. 80 '" •• 80% •.... ~ ~ > 60 &..!:.. '" 10 40 ~ .,. G> G>•• '= 60% e- J: 60 J: ..!. S 8 .•. '0 0 30 40 -.. .." ~ 40% !l 40 lD ~!G> •• E 20 0 20 Q. 20% 10 20 0% 0 0 0 0 4 8 10 0 0.5 1.5 2 2.5 3.5 4 hours time (hours) Fig. 4. Determination of reactant accumulation in a fed batch reaction; a) heat flow and heat released; b) effect offeed rate on accumulation a) b) various times of cooling failure 250 175 failure of cooling 225 when 0.5 equivalent fed 200 150 E 175 ~ ~ :> ~ 150 l!! 125 G> .a Q.
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