MINUTES OF THE 11TH HEXAG MEETING HELD AT BP, SUNBURY ON THAMES, ON THURSDAY 26TH NOVEMBER, 1998.
Over 60 attended the 11th HEXAG meeting, held at the BP Engineering & Research Centre, Sunbury on Thames at the invitation of Murtaza Khakoo of BP Exploration. The meeting had the theme of heat exchanger-reactors, linking the interests of ETSU, which is promoting both heat exchanger technology and process intensification, and a number of other organisations with an interest in one or both of these technologies. It was also an opportunity to meet members of the Dutch Heat Exchanger Users' Group and the Process Intensification Group, and also representatives of GRETh, the French heat transfer and heat exchanger group based in Grenoble. It is anticipated that pan-European collaborations involving HEXAG, the new UK Process Intensification Network (PIN) funded by the EPSRC, and these groups will increase in the future. (See accompanying letter and reply form).
Members were welcomed by Terry Lazenby, Chief Engineer of BP. After describing the development of the Company, which commenced operations at Sunbury in 1917, he highlighted several major breakthroughs in technology which had been brought to fruition there. Turning to activities of relevance to the HEXAG meeting, he stressed the strong involvement of BP personnel in networks such as HEXAG, and detailed some of the specific areas of heat exchanger technology where improvements could still be made. Fouling still cost $200 million per annum, and Terry Lazenby pointed out that the mechanisms still needed proper understanding. He also mentioned problems which can arise when compact heat exchangers (CHEs) are used off-shore, in conditions where a high operating envelope with respect to dynamic behaviour has led to unit failure, potentially decreasing credibility in the plant. He concluded by echoing the theme of the meeting, using his variant of a well-known film title: "Honey, I shrank the Chemical Plant'!
After outlining the aims of HEXAG and the motivation behind this particular meeting, David Reay introduced Mike Butcher of BHR Group, (fax: 01234 750074) who gave a talk on design techniques for compact integrated reactor-heat exchangers (HEX-reactors) for process intensification. Mike stressed that the crucial benefit of HEX-reactors was lower capital cost, the reduced volume having strong implications for the building cost. He said that most reactions can be fast and can be intensified, thus the range of potential uses was great. Two large projects had been carried out by BHR in this context, one being an EU JOULE project which concentrated on laboratory procedures for, e.g. macro-mixing and flow structures, and the other being an ETSU-supported project, involving the characterisation of CHEs as HEX-reactors. Organisations involved were ICI Acrylics, Cal Gavin, Heatric, Alfa Laval Thermal and Chart Marston. One aspect of the R&D showed that feed distribution was critical to performance. Extrapolation of performance data showed that 60,000 tpa of product could be processed in a single loop diffusion-bonded HEX-reactor. Another interesting aspect was the ways in which different CHE types developed turbulence - visible in mixing performances. The new Chart Marston Marbond diffusion-bonded CHE was shown, forming the basis of one of the HEX-reactors tested.
A number of industrial examples of studies of the use of HEX-reactors were described by Mike. These included Sulzer-Sumitomo (clear polystyrene), Linpac (polystyrene) and ExChem (continuous nitration). In the last case, a current batch process made continuous could give production rates to 800 tpa. Batch time could be reduced to seconds. The HEX-reactor would cost £40k, £0.3-0.5M installed. Using multiple injection points in the HEX-reactor, a 50% greater productivity could be achieved.
Bernard Thonon of the French GRETh organisation (fax: +33 476 88 51 72) gave a talk not unrelated to the mixing effects mentioned by Mike Butcher. Bernard pointed out that mixing efficiency was a function of flow structure, citing work on plate, plate-fin and ceramic plate-fin heat exchangers. Local and plate width analyses were carried out on single channels. Thermographic analyses were also employed in the work. For the PHE, it was found that mixing was a function of several geometric variables, the plate geometry, corrugation angle and pitch/height ratio. For the PFHE perforated fins were not as effective for good mixing as offset strip fin and louvred fin geometries.
In corrugated heat exchangers, the corrugation angle effect was most marked when it was 600. Here little entrance effect was visible except when the Re was low (<40). By the time Re reaches 500, plug flow is almost achieved, with homogeneous mixing. However, with a 300 angle the flow passed 7-8 corrugations before full mixing was visible.
In the PHE, with Re's of the order of 10,000, vortex flows were observed, the vortices being about 50% of the channel depth. This gives very good mixing. Corrugations were much more effective than straight channels for mixing. IKE at Stuttgart is preparing a flow map for mixing efficiency in PHEs, as a function of corrugation angle/size, pitch/height ratio etc.
Another application of CHEs and intensification was described by David Reay (fax: 0191 252 2229). The 'gas turbine reactor' concept, the subject of a study by David and Colin Ramshaw for BP Exploration, is directed at using GTs to generate power and to perform useful chemical reactions, leading to chemical products. David first described techniques for turbine blade cooling, including the possibility of carrying out endothermic reactions within blades, and then assessed the various other components which could be used as reactors. These included recuperators (exothermic reactors), compressor intercoolers (endothermic reactors) and the combustion chamber. Work on HEX-reactors at Newcastle University has shown that it would be feasible to use a methane plate reformer to give 1000 t/d synthesis gas, with combustion gases being expanded through the turbine, giving 9 MW power and low NOx.
A second concept, the 'turbo-cracker', first studied at ICI and recently by Arthur Gough at Newcastle, uses a GT in place of a cracking furnace for ethylene production. The concept should result in improved yield and a substantial cost reduction in downstream plant, as well as energy savings and NOx reductions. In conclusion, David stated that effective heat transfer in CHEs is essential if higher GT efficiencies are to be realised. This heat transfer can provide energy for the production of chemicals, and at least 4 major components of intercooled, recuperated GTs could be used as reactors.
Continuing on the reactor theme, John Burns of Newcastle University (fax: 0191 222 5292) detailed project work funded by BNFL on micro-reactors, made possible by developments in micro-fabrication. John stated that diffusive mixing was the basis of effective heat & mass transfer. He listed characteristic reactor channel lengths for good mixing, showing that reductions in diameter to as low as 10 microns could lead to residence times as low as 0.1 s. The reactor concept involved injection into micro-channels, rapid diffusion within the channels, and extraction of product and unreacted components, the latter being recycled.
Faster reactions have been achieved than in some typical nitrations within the chemical industry. One channel could handle 0.4 ml/h, while a cubic metre of appropriate plates could yield 100,000 ml/h, or 875 tpa. As one cannot physically stir the vessel, the heat exchanger must be built in to the reactor, so that hot spots do not occur.
A second presentation from Newcastle, by Roshan Jachuck, (fax: 0191 222 5292) updated us on work on the spinning disc reactor. The flow across the disc can be described as a highly sheared, mixed thin film, and the device has initially been used for polymerisation reactions. Work on measuring local and instantaneous mass transfer on inclined planes shows that peaks in mass transfer occur where there is a surface wave. High heat transfer coefficients can also be obtained, to 15 kW/m2K. material flowing over the disc has a one-pass residence time of 2-3 s, but even in this short period some conversion does occur during the reaction. A realistic conversion rate of 80% was achieved in slightly over 40 min (a few seconds of the process being on the disc), compared to a period of 4 hours in a conventional reactor stream. This latter time is due to the drop in effectiveness of the initiator after 40 min, when used in the conventional process. Other features of the spinning disc reactor include good control, the ability to operate continuously, and small size (500 mm diameter discs).
The reactions which have been implemented on the discs show that micro-mixing does take place. The discs can be used for fast exothermic reactions which do not use solvents, such as polymerisation. In response to a query concerning capacity, Roshan said that the 500 mm diameter disc could handle 1 t/h throughput, but of course multiple discs could be used.
Heat exchanger materials are of great interest, particularly as one moves up the temperature range. Fred Starr, (fax: 01372 817 606) who has many years of experience in the heat exchanger materials field, gave us a wide-ranging talk on modern metallic materials, effectively putting them into context in terms of the needs of heat exchangers. He started by highlighting the need to work out what an advanced heat exchanger is, selecting three main criteria - firstly, it is one of conventional design, operating under more demanding conditions; secondly, it is of novel design, but used in a near conventional process, and thirdly, it is of novel design and incorporated in a novel process.
Fred discussed the need to design for high temperature mechanical properties. These include strength, acceptable long term ductility, the avoidance of embrittlement, and weldability. A most revealing curve shown was a plot of log (stress) against linear temperature. This showed that all modern high temperature alloys fall almost on a single line. He then went on to explain why we use different alloys for different temperatures, stressing the 6 significant mechanisms for high temperature corrosion and the properties of alloys to counter this. An application area which was particularly demanding was waste incineration. In Germany there are many such plants operating, about one per million inhabitants, while in the UK there are only 5 plants.
Concluding, Fred said that advanced heat exchangers require both advanced materials and advances in materials. One needs to accelerate Code approval of new materials, and sophisticated life prediction methods are critical to their successful use.
A common feature of HEXAG meetings is an update on Government and EU programmes relevant to members. Fiona Porter of ETSU (fax: 01235 433727) briefed us on the strategy of the DETR with respect to her areas of heat exchangers and process intensification. The main aim of the programme is to improve energy efficiency, by dissemination and R&D etc. A new initiative is the Energy and Environmental Helpline, operating from 1 December 1998. This offers free assistance to companies to save money and energy. The issue of sector energy consumption guides continues, as do the other outputs of the Best Practice Programme. There is an increasing emphasis on R&D, and Fiona said that Government would pay typically 35% of R&D costs of a successful proposal. SMEs were welcome to participate. Current areas of interest for proposals were: High and low temperature heat exchangers, process technologies, process intensification, and heat transfer/waste heat recovery. David Reay then updated us on the fifth Framework Programme of the EU (FP5). The first call for proposals is provisionally scheduled for mid-February 1999, and process intensification should feature strongly in the industrial energy efficiency R&D component. This may well include topics such as heat exchangers combined with other intensified unit operations, and opportunities for heat exchangers should also exist in demonstration and industrial materials and technology programmes. (See letter for proposed further HEXAG initiatives in this area).
Henk Akse of NOVEM, (see end of minutes for Dutch HX Users' Group contact point) perhaps best described as the Dutch equivalent to ETSU, with an annual budget of £140 million, first gave a talk to ETSU at the beginning of 1998. The HEXAG co-ordinator then visited DSM to give a talk to the Dutch Heat Exchanger Users' Group, and Henk returned to update us on the opportunities for co-operation between UK companies, in particular equipment suppliers, and Dutch users. Henk said that co-operation was required in both heat exchangers and process intensification (PI), and Dutch companies needed new ideas and technologies. Those with strong interests in heat exchangers included Shell, DSM, Exxon, Kemira, Nerefco and ABB Lummus, while the PI interests were currently centred on Shell, Methanor, Akzo, Arco, Gastec, KTI and DSM. In this area, two projects had been identified in 1998 and one project had been completed. Henk said that there was an emphasis on 'breakthrough' technologies - those which led to a step change in cost, efficiency, weight, volume etc.
Later, Henk van den Berg, (see end of minutes for contact point) who leads the PI activity on behalf of NOVEM, stated that the challenge for the Dutch PI task force was to prepare for the next step in energy reduction. The methods used to put across the message would include a number of activities: Communication of PI concepts and success stories; studies to trace new technologies for applications; promote PI projects in industry, and support pilot demonstrations. Potential topics included reactors/heat exchangers/separations; logistics and process engineering; process design and process control, and transient reactors. There were also opportunities for work on membrane technologies.
The impromptu presentation slot included four talks. Grant Cuthbertson of Heriot-Watt University (fax: 0131 451 3129) described progress on developing power plant steam condensers with enhancement by surface treatments designed to promote drop-wise condensation. Experimental data are being obtained for both film-wise and drop-wise condensation on bundles of tubes of identical geometry, to provide a direct comparison. Data so far interestingly revealed that while the promoting treatment gave a modest increase in overall heat transfer coefficient (20%) on the first row of condenser tubes, 15 rows downstream the 'U' value for drop-wise condensation remained identical to that of the first row, while the 'U' for the film-wise mode had dropped to 40% of that of the treated tubes. (Note that the drop-wise coefficient is of course much higher than the film-wise value, but the U value takes into account the unchanged coefficient on the water side).
A presentation which has subsequently aroused much interest was that by Paul Isherwood of Applied Coolant Technologies Ltd., (fax: 01527 457151) on 'the micro-mechanical approach to fouling. Paul and his brother Adam have been using micro-fibres to inhibit fouling in vehicle cooling systems. Paul cited work in the former USSR on using soluble polymers to reduce drag in a vehicle engine, which was not very successful. The approach of Applied Coolant Technologies is to use insoluble polymers of 12 microns diameter, 250 microns in length, added to the coolant circuit, (water-side). Tests in a Ford Fiesta have shown that as well as the coolant circuit exhibiting no fouling on any surfaces after a 60,000 mile tour, cavitation was also reduced. The company is keen to apply the technology to process (and other) heat exchangers to assess the effectiveness of the method in other applications.
Song Lin of Heriot-Watt University (fax: 0131 451 3129), presented progress on his EPSRC-funded research on boiling in narrow channels. In particular, he is concentrating on two-phase flow and heat transfer in small (mm diameter) tubes. The data which he will obtain over his three year research project will be of considerable use in the design of highly compact two-phase heat exchangers.
Warwick University is involved in a JOULE project on vortex generators in compact heat exchangers. Martin Fisher (fax: 01203 418922) described how the enhancement is implemented, and listed partners in the project, which include Britannia Heat Transfer, another HEXAG member. One of the outputs of the project will be a map showing heat transfer variations in a heat exchanger employing such vortex generators. To date, the experimental results had showed that vortex generators could be effective in positively influencing the heat transfer coefficient a long way downstream of the projections.
While only a relatively short time remained for the workshop, Frank Zhu of UMIST (fax: 0161 236 7439) introduced a short discussion by querying how one applies new technologies to the process industries, and whether one has to develop new software for integration and optimisation of such systems. His experience showed that integration techniques could be used to solve a particular heat transfer enhancement problem, showing the optimum placement of enhanced heat exchangers. Subsequently it was found that similar opportunities existed in a vast number of other places on sites. There was discussion on the role of process intensification, the marketing issues involved, economics and the push for new products by industry which opened up new opportunities for radical process plant.
David Reay then closed the meeting by thanking all those who had contributed, in particular Murtaza Khakoo and his BP colleagues.
Contact points for Dutch HX and PI Groups:
Heat exchangers: Frans Vreugdenhil. Tel: 00 31 76 5032677. Email: firstname.lastname@example.org
Process intensification: Henk van den Berg. Tel: 00 31 115 648066. Email: email@example.com
(Minutes written by David Reay, HEXAG Co-ordinator, 18 December 1998 - please communicate any inaccuracies or additional points to him by 31 January 1999, for inclusion in HEXAG News).