Improvements in the manufacture of asphalt

Abstract

A modification of the conventional asphalt manufacturing process in which heat present in the dried aggregate as it leaves the drying stage of the process is captured for use in its own right. The cap

Description

Field of the Invention The invention relates to the manufacture of asphalt, a substance consisting essentially of a heated mixture of bitumen and aggregate and which is strictly defined as “asphalt concrete" but more commonly known in the relevant art - and indeed referred to generally - as simply “asphalt”. It will be so described throughout the remainder of this specification. Review of Art known to the Applicant Asphalt is used to form the top load bearing layer of highways, parking lots, airport runways and the like and has been so used since the early 20th century. The method of manufacturing it involves heating the asphalt binder - the bitumen - to decrease its natural viscosity; drying the aggregate to remove moisture from it prior to or in some cases mixing with the heated binder; and delivering the resulting mixture to be laid and compacted into place before it cools and hardens beyond usage. The mixing and drying steps of this process are carried out at relatively high temperatures, and if the resulting asphalt is intended for temporary storage rather than immediate laying and compacting then it is stored in vertical silos which are externally insulated so as to keep their contents still hot. If alternatively the asphalt mixture is intended for immediate use then either the dump trucks into which it is deposited for delivery to site may themselves be insulated (for example, by running the trucks’ engine exhausts around or in proximity to the asphalt-receiving body of the truck) and / or the manufactured asphalt, ideally, is picked up from a manufacturing site as close as possible to the intended usage site. For all these reasons above, it is also noticeable that the majority of asphalt laying and compacting takes place in the warmer months of the year. At colder times, the contractor using the asphalt faces the concurrent problems of the asphalt mixture itself cooling too rapidly as it leaves the truck and the already compacted base ground onto which it is poured cooling, in itself, in the surrounding atmosphere. Asphalt manufacturing sites are typically spread over large ground areas (in order to take advantage of the economies of scale in manufacturing) and will equally typically have several aggregate banks spread about that area waiting to be used. Keeping these aggregate banks dry is a problem in itself, but whether the aggregate arrives at the outset of the manufacturing process from such a bank, or as individually deposited layers delivered to site when actually needed, the first stage in the manufacturing process is to attempt to dry the aggregate before (usually) mixing it with the heated bitumen binder. The Problem to be Solved The volumes of aggregate and binder needed to be heated and dried; the temperatures to which the aggregate, in particular, must be raised in order to do this satisfactorily; and the energy input needed to make all this happen, are intense. The costs of the manufacturing process are accordingly high, and whilst in theory these costs may be able to be absorbed in (for example) the laying of a major length of highway, a good deal of asphalt manufacture and usage is on an altogether smaller scale - repairs, minor byways, and so on. In the latter instances particularly, but in all instances generally, any way of reducing those costs of asphalt manufacture is an advance in the known state of this particular developed art. If, for example, the amount of moisture reduction needed to be generated in the aggregate could be reduced by just 1% before it is dry enough for final mixing with the binder and subsequent delivery, then at current rates in even a relatively small asphalt plant a saving of approximately £500,000 per annum in energy and other operational costs could be achieved. The essence of the invention stems from the realisation that savings are possible if, instead of concentrating on (for example) cutting the drying time, as such, and / or varying the heat applied to dry the aggregate, attention is focused on positively using the heat present in the now-dried aggregate as it leaves the drying chamber. Accordingly, the invention is embodied in the concept of capturing that heat - which at that stage of the process will be at its maximum, because the inevitable part-cooling effect of the heating jets is no longer acting on the aggregate at and post-discharge - and conducting it for use in its own right; possibly other than as part of that continuing asphalt manufacturing process; possibly again as part of a separate drying operation elsewhere on the manufacturing site. Ways of achieving this will now be described and illustrated. Brief Description of the Drawings In the accompanying drawings: Figure 1 shows in simplified diagrammatic outline the basic asphalt manufacturing process; and Figure 2 shows in more detail a process of the kind exemplified in Figure 1 but modified to incorporate the teachings of the invention. Description of the Preferred Embodiment In Figure 1, Aggregate (A) enters a drying Drum via a hopper as shown. The Drum (D) rotates anticlockwise as indicated by arrow (R) and simultaneously hot gas jets (H) are blasted continuously along and around the axis of the Drum (D) so that the combined rotary and hot-jet-blasting movement gradually dries the Aggregate mixture. This drying process continues until enough moisture has been removed from the aggregate mixture, at which point a Chute (C) is opened through which the dried Aggregate mixture is discharged onto an Elevator (E) to either join a pre-heated bitumen binder (not shown) or in some cases the binder - or part of it - may already have been premixed with the dried Aggregate in an associated process (known in itself) and then the whole asphalt mixture is elevated for storage or delivery to site. The temperatures involved in this Aggregate-cooling and drying process are intense. The time needed before the Aggregate is sufficiently de-moisturised is also not minimal. Whilst the rotating Drum (D) can be cooled, and the Aggregate inside it will inevitably be cooled albeit to a very minor extent in relative terms by the gasses (H) as well as the rotation of the drum, the finally-ready mixture discharged into Chute (C) has neither of these benefits; and the Chute itself is not only fully enclosed (for obvious safety reasons) but is typically and conventionally a steel chute whose external temperature can be as much as 250"C. Little if any attention has been paid to this stage of the process until now. The discharge into Chute (C) has traditionally been regarded, not unreasonably, as the ''at last, fully dried and now ready for delivery" stage and attention has been focused on how best to get it, without undue and unacceptable cooling, into either a storage silo or one of a succession of dump trucks for delivery to site usage. The invention however is embodied in the realisation that if this hitherto wasted heat in the Chute body - discharging only to atmosphere because apart from anything else the Chute tends to be of short length for obvious “give it (i.e. the Aggregate) the minimum time to cool post-drying’' caveats - is instead captured; then it can be conducted elsewhere and used to aid the overall operation and economics of the asphalt manufacturing site; and the continuing discharge and storage or transport of the asphalt can proceed as previously either without interference from the now separated heat stream or, optionally, with that latter heat stream being used to aid the continuing asphalt producing process from which it had been extracted. Once specific application of this realisation is shown in Figure 2. Heated and dried Aggregate leaves Drum (D) and is discharged into Chute (C) which takes the form of a cylindrical closed-circumference steel tunnel from the exit end of which the Aggregate mixture begins its journey to storage or site delivery. The outer circumferential surface of Chute (C) is surrounded by a Jacket (J) filled with a fluid - in this case, water. The hot Aggregate as it passes through and out of the end of Chute (C) rapidly heats the internal surfaces of the steel tunnel forming the Chute, and that acquired heat rapidly heats water in Jacket (J) which is in direct contact with the tunnel outer walls; and whilst there will inevitably be some temperature loss, the water in the Jacket will quickly get hot. The hot water from the Jacket is then taken by a Feed Pipe (F) to the first Pl of two Pumps pumping into and around an heat-insulated Water Tank (W) and so provide the Water Jacket (J) surrounding Chute (C) with a continuously recycled cooling water flow. A second Pump P2 takes the hot water from Tank (W) and feeds it to and from a Storage Bay (S). This feed also is a continuous closed loop feed and return system, and the thus-warmed Bay (S) can be put to a number of usages. For example, it was mentioned earlier that asphalt manufacturing plants of any sizeable scale will have banks of Aggregate spread about them in the vicinity of the plant processing unit as such. The bases of those banks could be kept continuously warmed by a storage Bay (S) in the form of a grid of just-below-ground small diameter pipes, fed by the system to warm a sand-bed on which the Aggregate bank sits. Other uses will become apparent to the intended skilled addressee of this specification now that the inventive concept has been revealed. The applications are many, and again it is stressed that the conventional continuing manufacture of the bitumen and Aggregate mix into a deliverable asphalt product is unaffected. The savings at scale, by contrast, of using this hitherto wasted heat can readily be imagined; and whilst a Chute-surrounding-Jacket based heat transfer system has been described and illustrated, other means (e.g. one or more fans mounted externally of and adjacent to the Chute outer surface and blowing that outer surface heat directly into a takeaway path - or, additionally or alternatively, an outlet in the Chute wall transmitting the hot air surrounding the discharged Aggregate itself directly to a takeaway path) are all covered within the scope of the invention. 5 The wording of each and all of the numbered claims which now follow is incorporated by reference as part of this disclosure; as is the wording of the abstract which follows those same claims.

Claims

1. An asphalt concrete manufacturing process wherein heat is recovered indirectly or 5 directly from dried aggregate when it is within the discharge apparatus of a drying apparatus and conducted elsewhere for use other than as part of that continuing asphalt concrete manufacturing process.

2. The invention of Claim 1 and in which the heat thus recovered is used as part of a 10 separate drying operation elsewhere on the asphalt concrete manufacturing site.CXI153. Apparatus adapted to modify an otherwise conventional asphalt concrete manufacturing apparatus so as to make that otherwise conventional apparatus capable of putting into practice the invention of Claim 1 or Claim 2.

Images