Intensification of Regenerative Heat Exchange in Chemical Reactors

Generations of chemical reaction engineers have devoted considerable time and effort to addressing the challenge of regulating the reactor temperature...
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Ind. Eng. Chem. Res. 2004, 43, 4773-4779

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Intensification of Regenerative Heat Exchange in Chemical Reactors Using Desorptive Cooling Marcus Gru 1 newald and David W. Agar* Department of Biochemical and Chemical Engineering, University of Dortmund, 44221 Dortmund, Germany

The desorption of an inert material from a mixed fixed-bed of absorbent and catalyst can be exploited to provide an efficient means of cooling for heterogeneously catalyzed exothermic reactions without the need for extensive heat exchange surface within an adiabatically operated reactor. The use of desorptive heat effects enables one to extend the regenerative cycle times by roughly an order of magnitude over simple, sensible heat uptake and thus attain operating periods long enough to be of technical interest. 1. Introduction Generations of chemical reaction engineers have devoted considerable time and effort to addressing the challenge of regulating the reactor temperature and suppressing the hot spots that arise in exothermic reactions. The reason for the attention lavished on this question is not hard to identify: the temperature is the critical parameter dictating reactor performance, particularly, as is commonly the case, if a thermally sensitive catalyst is being used. Often a temperature window of only a few degrees is acceptable for optimal reactor performance. The combination of the strongly nonlinear dependence of the reaction rate, and thus heat generation, on temperature with the linear relationships describing the heat removal process together with the two-dimensional and often unsteady-state nature of the temperature profiles of interest renders the modeling of reactor behavior both interesting and complex. The models developed are seldom amenable to analytical solution and usually necessitate the utilization of expedient numerical techniques. It is above all in this field, that Gerhard Eigenberger has made numerous seminal contributions over the course of more than 30 years.1 2. Methods of Heat Exchange in Chemical Reactors The most widespread piece of equipment for carrying out exothermic reactions on an industrial scale is the multitubular reactor, in which the heat is removed, or in the case of an endothermic reaction supplied, in a recuperative process via heat conduction through the reactor wall that spatially separates reaction medium from coolant. The shortcomings of such reactors are apparent in the dramatic local hot spots of up to 100 °C or more that often arise. The temperatures attained in the vicinity of the hot spot are decisive for reactor performance, in terms of such disparate factors as selectivity, catalyst lifetime, safety, and materials of construction. Much skill and imagination has been invested in measures for thermally debottlenecking multitubular reactors, but a panacea remains elusive. Fluidized beds * To whom correspondence should be addressed. E-mail: [email protected].

offer excellent temperature control but exhibit several inherent reaction engineering disadvantages, are notoriously difficult to scale-up, and are unsuitable for many catalyst systems.2 Microreactors, a technology that has received considerable attention over the past decade,3,4 remains economically out of reach for most industrial purposes. The initial euphoria has given way to a more sober assessment of its potential together with an appreciation that it often entails technological overkill and high development costs and that the transfer to large-scale application is by no means as trivial as originally envisaged. It is thus instructive to examine if fundamentally different approaches might offer a competitive alternative to the multitubular reactor and, if so, under what circumstances. Excluding more exotic methods of heat transfer, such as microwave heating, for which there are anyway usually no analogous cooling mechanisms, one can differentiate between convective, recuperative, reactive, and regenerative heating-cooling processes (Figure 1). The convective processes, the best known example of which are provided by cold-shot reactors, usually offer only a limited cooling capacity and comparatively coarse temperature profile regulation. These drawbacks can be overcome to some extent by employing inert coolant streams, exploiting latent heat effects by using liquid injection or by distributing the coolant flow continuously along the reactor using a porous membrane, which may also permit advantageous manipulation of the concentration profiles within the reactor.5 Nevertheless, none of these performance-enhancing measures enjoys widespread acceptance, probably due to the additional limitations that they themselves impose. In addition to the use of fluidized beds or microreactors, recuperative heat transfer can be enhanced either by increasing the specific heat exchange surface within the reactor, as with the so-called Linde isothermal reactor,6 or by simply diluting the catalyst with inert material in the region in which hot-spot formation takes place.7 These techniques both result in better harmonization between heat generation and heat removal, and since the basic reactor technology remains the same, the additional costs to be met can be estimated reliably. The scope of such measures is however restricted and the modest benefits accrued do not generally justify the extra expense involved.

10.1021/ie034126r CCC: $27.50 © 2004 American Chemical Society Published on Web 07/10/2004

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Figure 2. Temperature profile control for endothermic hightemperature processes. Figure 1. Methods of heat transfer to control reaction temperature.

A thermal coupling of endo- and exothermic reactions has long exerted a powerful fascination for reaction engineers and is in fact used industrially more commonly than is usually appreciated, in oxydehydrogenations, such as the Andrussow process for hydrogen cyanide manufacture, for instance.8 However, a direct coupling of an endothermic and exothermic reaction within the reactor itself often results in more problems than it solves: unwanted side-reactions, additional downstream separations, poor kinetic or thermodynamic compatibility, and so on. Indirect coupling through a heat exchange surface only makes sense if the resistances on the coolant side are comparable to or greater than those on the reaction medium side, which is seldom the case for low or intermediate reaction temperatures with heterogeneously catalyzed reactions below 600 °C. Furthermore, it proves difficult to tune the behavior of the two reaction systems to localize the temperatures as the reactor designer would wish.9,10 Measures to achieve this, such as structured activity profiles or distributed feed, detract from the appealing elegance of the original concept. The use of heat regeneration in chemical reactors has enjoyed a renaissance since the 1980s largely due to the pioneering work of Matros and Boreskov,11 even though heat regenerators were by no means an unknown quantity, being widely employed in power stations, glass manufacture, coke manufacture, and blast furnaces. The insight of Matros and associates was to couple the principle of heat regeneration and the catalytic fixedbed in the reverse flow reactor and to recognize that the efficacy of this technique was tailor-made for dilute gaseous reaction systems exhibiting adiabatic temperature rises in the range 10-100 °C. In an exemplary piece of work, Nieken et al.12 demonstrated the asymptotic equivalence of a reverse flow reactor to countercurrent recuperative cooling at short switching times and developed an important refinementsthe withdrawal of a hot sidestream from the middle of the reactorsto facilitate temperature control. It is therefore at first glance somewhat surprising that regenerative heat exchange has made few major inroads into chemical reactor engineering beyond the niche of oxidative catalytic waste gas treatment. A cursory analysis8 of three high-temperature endothermic reactions, ethylbenzene dehydrogenation, steam re-forming,

and hydrogen cyanide synthesis, which should especially lend themselves to regenerative heat transfer, reveals that both the dominant industrial technologies and even the less well-represented alternatives are exclusively convective, recuperative, or reactive (Figure 2). A more detailed analysis of the hydrogen cyanide synthesis even suggests that a reactor with regenerative heat exchange could overcome the serious defects of the reactive Andrussow and recuperative BMA processes, namely, poor yields and a fragile reactor construction, respectively. More detailed modeling casts some light on this enigma: the cycle times calculated, which are primarily determined by the amount of heat that can be stored in the catalyst bed, are on the order of just a few minutes and thus, although higher than the typical reactor residence times, too low to be of practical relevance. Short cycle times entail rapid and, probably as a consequence, inefficient heating-cooling periods without reaction to provide reasonable productivities, and frequent flushing between the reactive and nonreactive cycle phases both complicates reactor operation and increases product losses.13 The question therefore becomes: is it possible to extend the cycle time of a regenerative process to a level of 1 h, say, where one might still profit from the advantages of regenerative heat exchange while the complications mentioned above become much less acute? 3. Concept of Desorptive Cooling Taking a cue from reactors with convective and recuperative heat transfer, which exploit evaporative cooling phenomena, leads one to the idea that latent heat effects might also be used to attenuate the simple storage of sensible heat within a fixed bed and so lengthen the attainable cycle duration. Evaporation is preferable to fusion as a phase change by virtue of the greater enthalpies involved. The direct evaporation of an inert coolant within the reactor is, however, fraught with difficulties, such as undesirable condensation causing catalyst deactivation, pressure fluctuations arising from irregular boiling, and the constraints imposed on the operating pressure. These flaws can be circumvented by substituting desorption in place of evaporation, which offers similar or higher enthalpies while the complicating presence of a liquid phase is excluded and, through the choice of suitable adsorbent and inert adsorbate, the range of feasible reactor operating temperatures and pressures is expanded.

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must be set against the simplified reactor construction and superior performance resulting from the more uniform temperature profile. The inert material employed should be selected to facilitate separation from the product in a simple downstream processing step. 4. Feasibility Studies Rough calculations base on typical adsorbent loadings and heats of adsorption for water vapor on zeolite (eq 1)

∆T [c F (1 - ν) + cp,adsFadsν] ) [(-∆HR)rFk(1 ∆ t p,k k ∆X ν)] - ∆Hdes Fν (1) ∆t

[

Figure 3. Concept of heat removal using an integrated desorption.

The concept of a desorptive cooling process (Figure 3) envisages a mixed fixed-bed of adsorbent and catalyst, the proportions of which may be varied along the reactor. Initially, in a nonreactive cycle phase, the adsorbent is loaded uniformly with an adsorbate that is inert as far as the reaction and catalyst system of interest are concerned. The heat liberated in the adsorption process must naturally be removed from the reactor, for example, by use of a heat exchanger or adsorbate evaporation in an external recycle loop. In the absence of the reaction system, temperature excursions can be tolerated insofar as the catalyst can withstand them. Following completion of the adsorption and the attainment of a uniform temperature profile, the reactive cycle phase begins. The heat liberated in the catalyst particles by the exothermic reaction is taken up by the neighboring adsorbent particles through desorption of the inert material into the gas phase. The close proximity of the particles means that the heat transfer is very efficient and that temperature gradients arising within the bed are minimal. The kinetics of the desorption process is determined by the adsorbentadsorbate system chosen, the local adsorbate loading at any point in time, the level of the inert material in the bulk gas, the temperature of the adsorbent particle, and the mass transfer resistances influencing the diffusion of adsorbate into the gas stream and can be adjusted to more or less correspond to the local rate of heat generation through the catalytic reaction. After a certain, hopefully extended, reaction period, the adsorbate levels on the adsorbent will have been exhausted or diminished to the point where the cooling effect through desorption is inadequate to maintain the reaction temperature within the desired window. The adsorbent is then reloaded with inert adsorbate as described above. The actual reactor operation is thus adiabatic, requiring no heat exchange surface within the reactor. The criteria dictating the selection of the adsorbent and adsorbate are inert behavior with respect to the catalyst and reaction system, a high adsorbate loading under the desired reaction conditions, and a large heat of desorption. The inherently unsteady-state operation, the lack of production during the nonreactive cycle phase, and the increased fixed-bed volume due to the additional adsorbent fraction, which cuts space-time yield, are the major operational disadvantages that

]

indicate that the cycle periods can be extended by a factor of 10 compared to the base case with inert pellets in place of the adsorbent particles to around 20 min, a value that can be considered operationally realistic. More detailed modeling work was carried out for a specific reaction-catalyst-adsorbent system comprising CO oxidation on a supported palladium shell catalyst with water vapor adsorption on 3A zeolite at 125 °C and atmospheric pressure.14 This system was chosen for minimal interaction between the adsorptive and reactive components because it involves a well-documented adsorbent-adsorbate system. The kinetics for a specific catalyst (R0-20, BASF AG) was determined by conventional techniques based on the conversions and temperature profiles in a thermally well-characterized integral reactor using expressions developed for similar catalysis elsewhere.

r)

kcCO (1 + KcCO)2

(2)

The equations for component i in the catalyst, adsorbent, and gas phase were developed initially for a onedimensional axial dispersion model and later extended to include the effect of heat losses. Gas velocity u was assumed to be constant, even though a slight change will result from the effects of temperature rise (∆Tmax ) 20 K), pressure drop (400 mbar/m), and variation in the number of moles due to reaction and desorption.

∂ci,g ∂2ci,g u∂ci,g 6 1- )+ Dax 2 -βK (1 - v)(ci,g ∂t  ∂z d ∂z p  6 1- ci,K) - βads v(c/i - ci,ads) (3a) dp  ∂ci,K 6 1 F ) βK (ci,g - ci,K) - ri ∂t dp (1 - ) K

(3b)

The desorption kinetics were described using the linear driving force with experimentally determined parameters found for the stripping of water vapor from a previously loaded 3A zeolite (Grace Davidson) under well-defined temperature conditions and similar flow rates of an inert sweep gas. The adsorption isotherms

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Figure 4. Simulation results for the startup of reaction with and without integrated desorption (full lines, temperature profile without desorption; long dashed lines, temperature profile with desorption; short dashed lines, adsorbent loading). Table 1. Model Parameter Set Used for Simulation Studies Operating Parameters pressure feed temperature feed flow rate feed CO concentration

1 × 105 Pa 373 K 5 N m3/h 0.5 mol/m3

Reactor Geometry reactor length reactor diameter particle diameter voidage

1m 0.05 m 2 mm 0.4

and heats of desorption were taken from data supplied by the manufacturer.

∂X Deff ) 2 (X* - X) ∂t d

(3c)

p

The thermal behavior of the reactor was described by the following heat balance (eq 4).

∂Tg ∂Tg ∂2Tg 6 1- u Fgcp,g ) - Fgcp,g + λg 2 -RK (1 - v) ∂t  ∂z d  ∂z p 6 1- 4 v(Tg - Tads) - kW (Tg - TU) (Tg - TK) - Rads dp  D (4) The first simulations considered a 50% admixture of adsorbent and catalyst, a constant initial axial temperature profile, and an adsorbent loaded to the saturation value of 0.12 kg/kg. The boundary conditions for eqs 3 and 4 are those usually employed for a closed vessel. A space time of 6000 h/L and an inlet concentration of 0.2 mol CO/m3 were employed. The parameters employed in the simulations are listed in Table 1.

After 350 s, the benchmarking case with inert pellets in place of adsorbent had effectively achieved the steadystate profile with an adiabatic temperature rise of 47 °C, whereas the reactor with desorptive cooling was still some way from attaining this condition, even after 2000 s. The control calculations surpassed the threshold of 20 °C above the inlet temperature after 100 s, while the same point was reached only after 1000 s with the desorptive cooling. The underlying efficacy of desorptive cooling thus lay in the range originally predicted. As a consequence of the simple constant ratio between catalyst and adsorbent chosen, an unwanted slight cooling below the desired temperature arose in the initial period of reactor operation. A further unsatisfactory feature was the skewed nature of the desorption process, with the front end of the reactor, as expected, losing water vapor more rapidly than the downstream sections. This meant that the reactive cycle had to be stopped, although desorptive cooling capacity was still available in the second half of the reactor. A nonuniform distribution of adsorbent and catalyst favoring adsorbent in the inlet section of the reactor, where the reaction mainly takes place, would thus seem to be a prerequisite for attaining maximal cycle times. Following the promising results from the orientating simulations (Figure 4), a bench-scale reactor (diameter, 5 cm; length, 35 cm) was construction to validate the desorptive reactor concept.15 Although the reactor was insulated with 15 cm of glass wool, the behavior was polytropic rather than adiabatic and it was necessary to characterize the heat losses of the reactor system in control experiments. The temperature profiles in the reactor were determined using a total of 12 thermocouples distributed throughout the catalyst-adsorbent fixed-bed. The conversion within the reactor was monitored using infrared spectroscopic analysis of the outlet stream. A flow chart of the bench-scale reactor system

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5. Discussion

Figure 5. Flow chart of experimental setup and operating parameters.

Figure 6. Experimental results for the temperature profile dynamics of reaction with and without integrated desorption (axial position 1, 0 cm; axial position 12, 35 cm).

is presented in Figure 5. The operating parameters employed in the experimental studies are tabulated within Figure 5. Despite various experimental imperfections, the results clearly demonstrate the slowing effect of desorptive cooling on the development of the steady-state temperature profile: whereas the steady-state was achieved after about 900 s with inert material in place of adsorbent, the desorptive cooling extended the transient period to roughly 4800 s (Figure 6). At first glance, the ratio of five or so between the two times would seem to be disappointing in light of the factor of 10 predicted by the simulation results. Closer attention, however, reveals that the absolute values of transient durations found are also much larger than those calculated. This discrepancy is almost certainly due to the considerable contribution of the thermal inertia of the reactor itself to the development of the temperature profile, a phenomenon that was initially neglected in the model. The heat uptake by the comparatively massive reactor construction exerts an additional cooling effect that retards the dynamics of temperature profile change, even in the absence of desorptive cooling. The net result is to diminish the relative influence of desorptive cooling on the overall unsteady-state behavior. The distortion described is primarily a feature of the small scale at which the experimentation was conducted. At a larger scale the contribution of the thermal inertia of the reactor will become much less important. After making allowance for this experimental shortcoming, the results can be said to back up the simulation studies and to indicate the principle feasibility of the technique of desorptive cooling.

At this point it is useful to consider where else analogous phenomena have been exploited for similar purposes. The principle of desorptive cooling can be considered to correspond to the quasicontinuous injection of liquid coolant along a reactor according to the local rate of heat generation, i.e., an asymptotic case of convective cooling utilizing latent heat effects. The heat of adsorption of the toluene produced by the dehydrogenation of methylcyclohexane has been used to supply roughly half of the heat of reaction needed for this endothermic reaction.16 In this elegant scheme, in situ adsorption serves the dual purpose of displacing the equilibrium position toward the product and covering a large proportion of the heat requirements. Adsorptive heat effects are also utilized in chemical heat pumps, many of which are also based on water vapor-zeolite systems.17 In a low-pressure phase, a heat source at an intermediate temperature level is used to desorb water vapor from zeolite, the former being condensed at a lower sink temperature with the production of waste heat. In the subsequent high-pressure phase the water is evaporated at the intermediate temperature of the heat source and the uptake of the water vapor thus generated on the zeolite releases the heat of adsorption at a higher working temperature. At a more mundane level, microencapsulated phase change material based on wax have been incorporated into fabrics to provide a degree of temperature regulation: when the wearer is active, his or her body heat melts the wax within the microcapsules and this heat is thus stored to be released in more sedentary periods.18 Products exploiting this principle in winter sports attire are already commercially available. The potential of desorptive cooling to exhibit selfstabilizing properties that might help to maintain a constant temperature level in the face of external disturbances is an additional attraction of the concept. The phenomenon can best be visualized by considering a simplified system with a uniform temperature profile in which the adsorbent/catalyst ratio (ν) has been structured so as to compensate exactly the local rate of heat production through reaction. If the level of inert material in the gas phase is low enough to be ignored, neglecting temperature gradients between adsorbent and catalyst particles and assuming a rectangular adsorption isotherm, i.e., the partial pressure of the inert over the adsorbent is independent of its loading yields, the simplified unsteady-state heat balance is given by eq 5.

k0e-Ea/RTcCO ∂T [cp,kFk(1 - ν) + cp,adsFadsν] ) (-∆HR) Fk ∂t (1 + Kc )2 CO

Deff (1 - ν) - ∆Hdes 2 (X* - X)Fν (5) dp The reactive heat generation will be balanced by the desorptive heat consumption maintaining the temperature level at a constant uniform value. Under such conditions, convective and conductive heat transfer within the bed can be discounted. Should a temperature deviation arise, the subsequent behavior depends on the temperature sensitivity of the reactive source and desorptive sink terms. The former is characterized by the activation energy of the reaction Ea. The latter is

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dictated through the linear driving force relationship by the temperature dependency of the adsorption isotherm, which, in analogy to the Clausius-Clapyron relationship (eq 6), reflects the heat of adsorption. / (X* - X) ∼ pH ∼ e2O

This work is supported by the Deutsche Forschungsgemeinschaft. Nomenclature

∆Hv RT

Acknowledgment

(6)

If the rate of desorption increases faster than that of the reaction for a temperature rise, the cooling effect will dominate and tend to restore the original temperature. Similarly for a fall in temperature, if the rate of desorption diminishes more than the reaction rate, the excess heat generated will tend to restore the original temperature value. This somewhat superficial analysis in analogy to the slope condition for the stability of exothermic reaction in a stirred tank reactor suggests that heats of desorption in excess of the activation energy will tend to lead to stable temperature profile behavior without the need for external control measures. Desorptive cooling can also be depicted as being an example of microreactor technology in the broadest sense of the term. The heat removal process takes places at the millimeter scale, at which it is best harmonized with the processes of reaction and diffusion. The large specific surface area available for cooling is that of the adsorbent particles within the fixed-bed and is achieved without any costly fine heat exchanger structures. A similar approach is discernible in the development of membrane-encapsulated catalyst pellets as a form of miniaturized membrane reactor.19 In contrast to reactive coolants utilizing an endothermic reaction to intensify recuperative heat exchange by enhancing heat uptake,9 the desorptive cooling effect can be easily “fixed” at a particular location within the reactor and will not just migrate arbitrarily according to the whims of kinetics and equilibrium. 6. Conclusion Both simulations and experimental studies indicate that the desorptive cooling concept has the potential to extend regenerative cycles by an order of magnitude in comparison to sensible heat uptake and thus achieve the cycle times of 20-30 min, which are feasible for industrial application. To improve the agreement between simulation and experiment, the thermal inertia of the bench-scale reactor system should be reduced and taken into account in the model. Furthermore, the potential available in locally structuring the adsorbent/ catalyst ratio and the stability of the reactor behavior should be investigated in greater depth beyond the intuitive, qualitative analysis presented here. The extension to other temperature windows and further adsorbate-adsorbent systems should also be examined. Finally, criteria for situations where desorptive cooling might be especially beneficial should be developed and a benchmarking against conventional technologies conducted. The relationships between the manipulation of temperature profiles using desorptive cooling and of concentration profiles in adsorptive reactors8 could also prove fertile, as could the exploitation of inert adsorption to enhance mass transport in porous catalysis by inducing a convective flow through periodic variation of inert partial pressure in analogy to the use of pressure oscillations for this purpose.18

c ) concentration cp ) heat capacity dp ) particle diameter D ) reactor diameter Dax ) axial dispersion coefficient Deff ) effective pore diffusion coefficient Ea ) activation energy ∆Hdes ) desorption enthalpy ∆HR ) reaction enthalpy k ) reaction rate constant k0 ) Arrhenius constant kw ) reactor wall heat transfer coefficient K ) adsorption constant in reaction rate equation p ) pressure R ) gas constant r ) reaction rate t ) time T ) temperature u ) superficial velocity X ) adsorbent loading z ) axial coordinate R ) heat transfer coefficient β ) mass transfer coefficient  ) porosity λ ) thermal conductivity ν ) fraction of adsorbent particle F ) density

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Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4779 (14) Franke, M. Bewertung desorptiver Kuehlung von Fettbettreaktoren. Diploma thesis, University of Dortmund, Dortmund, Germany, 2001. (15) Gerlach, I. Desorptive Kuehlverfahren: Untersuchung reaktionstechnischer Einflussgroessen; Research Project Report; University of Dortmund: Dortmund, Germany, 2002. (16) Yongsunthon, I.; Alpay, E. Design of periodic adsorptive reactors for the optimal integration of reaction, separation and heat exchange. Chem. Eng. Sci. 1999, 54, 2647-2657. (17) Wongsuwan, W.; Kumar, S.; Neveu, P.; Meunier, F. A review of chemical heat pump technology and applications. Appl. Thermal Eng. 2001, 21 (15), 1489-1519. (18) Clever stuff. Economist Mar 27, 2003.

(19) Foley, H. C.; Lafyatis, D. S.; Mariwala, R. K.; Sonnichsen, G. D.; Brake, L. D. Shape slective methylamines synthesis: Reaction and diffusion in a CMs-SiO2-Al2O3 composite catalyst. Chem. Eng. Sci. 1994, 49 (24A), 4771-4786. (20) Zirkwa, I.; Gruenewald, M.; Agar, D. W. Intensivierung des Stofftransports bei heterogen katalysierten Reaktionen durch Druckoszillation. Chem.-Ing.-Tech., manuscript submitted.

Received for review September 15, 2003 Revised manuscript received April 6, 2004 Accepted April 7, 2004 IE034126R