INCORPORATION OF A CONSIDERATION OF FOULING INTO THE DESIGN OF VERTICAL THERMO-SIPHON RE-BOILERS

Two causes of under-performance within the tubes of vertical thermo-siphon re-boilers (VTR’s) are considered: premature dry-out within the unit and chemical reaction fouling at the tube wall. In many cases dryout is itself a cause of fouling. Both of these causes depend on re-boiler geometry. Some geometry performs satisfactorily. Other geometry performs poorly. There are two basic forms of flow boiling: nucleate flow boiling and convective flow boiling. In nucleate flow boiling part of the vaporisation results from the formation of bubbles of vapour on the heated surface. Unfortunately, a thin liquid film (the micro-layer) formed beneath these bubbles can fully evaporate and dissolved solids can deposit on the heated surface. Under convective flow boiling conditions the vapour shear generated by the two-phase flow results in the formation of a thin liquid film on the tube wall. This film moves at high velocity and provides very effective heat transfer. The result is a marked reduction in wall temperature and suppression of the nucleate boiling process. It is clearly desirable to move from a nucleate boiling mechanism to a convective boiling situation over as short a tube length as possible. The length of tube that is subject to nucleate boiling is again a function of re-boiler geometry. Wall shear is important in suppression of chemical reaction type fouling. Shear is function of recirculation rate and vapour mass quality. Again these factors depend on reboiler geometry. In this paper the authors describe how the better geometry can be identified. Dry-out within VTR`s The efficiency of a vaporisation process is affected by how well the liquid wets the hot surface. If only partial wetting occurs then two problems immediately arise. The first is a fall-off in the flow boiling coefficient; the second is increased wall temperature and possibly full evaporation of droplets of liquid that impinge upon the hot surface. The full evaporation of liquid then gives rise to deposition of otherwise dissolved salts or suspended solids on the heat transfer surface. The dry-out process is extremely complex and has more than one fundamental cause. At low vapour mass qualities dry-out only occurs at very high mass fluxes and is due to either excessive vapour flow generated by vigorous nucleate boiling (similar to the form of dry-out that occurs in pool boiling) or to the formation of bubble swarms that flow close to the tube surface and prevent liquid from the bulk contacting the surface. At high vapour mass qualities dry-out is mainly due to the vapour flow exerting a high shear on the liquid film and pulling it away from the tube surface. A group of heat transfer experts (forming ESDU`s Heat Transfer Steering Group) oversaw the development of an comprehensive methodology for the determination of dryout conditions in upward flow through circular tubes (1). This work was an extension of the comprehensive work conducted by Groeneveld (2) at AECL. The predictions of the methodology were tested against a large data base. This methodology has been incorporated into a new computer program that is being developed for the design and simulation of vertical thermo-siphon re-boilers. Chemical Reaction Fouling Chemical reaction fouling has been the subject of much study in recent years. This has mainly been promoted by the problem of fouling in heat exchangers used in pre-heat trains processing crude oil. The work conducted by ESDU (3) and by oil companies such as Total (4) has centred on the use of the EbertPanchal Model for prediction of chemical reaction fouling rates. This model assumes that competing mechanisms ECI Symposium Series, Volume RP5: Proceedings of 7th International Conference on Heat Exchanger Fouling and Cleaning Challenges and Opportunities, Editors Hans Muller-Steinhagen, M. Reza Malayeri, and A. Paul Watkinson, Engineering Conferences International, Tomar, Portugal, July 1 6, 2007 Produced by The Berkeley Electronic Press, 2014 occur. The mechanism that promotes fouling is assumed to be chemical reaction close to the heated surface. The rate at which this occurs is assumed to be given by the product of a “reaction rate” (characterised by an Arrhenius equation) and “reaction volume” (which depends on the “thickness” of the thermal film). The mechanism that suppresses fouling is assumed to be related to the shear stresses developed at the surface of the deposit (and, so far, it has been assumed that the mechanism follows a linear relationship with respect to shear stress). The model published by Ebert & Panchal is only applicable to single phase flow in tubes. However, studies of fouling in tubes with and without tube inserts (5) has indicated that the model can be extended beyond its original scope. This has lead to the development of a new equation (the derivation of which is to be published in separate work (6)) that can be used for both the prediction of fouling in tubes fitted with inserts and prediction of fouling rates under convective boiling conditions. The equation is: w f fc D RT E A R γτ α δθ δ −         − = exp is a function of convective heat transfer coefficient fc α , film temperature Tf and wall shear stress w τ . The fouling process is characterised by Activation Energy E and by a suppression constant γ . This model has been incorporated into the new re-boiler program. It should be noted that is this type of equipment the convective heat transfer coefficient is a function of mass flux and vapour mass quality and varies from point to point in the tube. This coefficient does not necessarily provide a measure of the heat transfer resistance on the cold side of the reboiler. Both nucleate boiling effects and vapour phase mass transfer resistance (where mixtures are being vaporised) also influence wall (and hence film) temperature. Regimes Occurring in VTR`s The prediction of VTR recirculation rates, internal wall temperature and heat transfer distributions have been reported elsewhere (7). On entering the reboiler the recirculating liquid is at higher pressure (due to the head of liquid above it) than the saturated liquid in the feed reservoir. So, relative to the local pressure it is sub-cooled. Consequently, at low temperature driving forces the heat transfer in the lower parts of the tube is single phase convective heat transfer. At high temperature driving forces the wall temperature becomes high enough for nucleation to occur. The result is sub-cooled boiling. In this situation the heat transfer is enhanced. However, this is an undesirable heat transfer regime. For bubbles formed at the wall initially grow but then collapse once they come into contact with cooler liquid within the thermal boundary layer or in the bulk of the flow. The collapse of the bubbles is accompanied by rapid influx of cold liquid which strikes the wall of the tube. The result can be erosion of the tube. This regime is to be avoided. The recirculating liquid increases in temperature as it flows up the tube. The local pressure reduces with tube length. The liquid eventually reaches its saturation temperature. However, this does not necessarily correspond to the formation of vapour. Nucleation is necessary. This nucleation usually occurs at the tube wall and requires a finite superheat. The single phase convective heat transfer regime can extend into the saturated flow region. Once nucleation occurs the superheat present in the bulk of the liquid is immediately extracted by vapour formation. From this point onwards the convective heat transfer is significantly enhanced by the presence of a two phase flow. It can also be enhanced by nucleate boiling at the tube wall. The heat transfer in this saturated flow boiling region is given by: