To engineer a bitumen fit for purpose, made to measure we recommend a Technix Multistage Bitumen Reactor.
The Technix multistage bitumen reactors can be used for modifying bitumens, manufacturing multigrade bitumens, producing bitumen from refinery distillation residues (vacuum bottoms) or for producing bitumen from refinery solvent precipitated ‘asphalts’.
With this process it is possible to produce for instance a 90/95 penetration bitumen within the 70/100 specification. This illustrates how precise the process is.
AIR BLOWING PROCESS
Bitumen manufactured in oil refineries is commonly produced and categorised in terms of its major physical properties, viscosity and penetration, using oxidation with air. Blowing processes using air for the source of oxygen are well known and they may be batch or continuous. A variety of chemical reactions takes place in bitumen blowing and the key products should enable the viscosity and penetration of the bitumen to be suitable for a variety of end uses. For the blowing reactions to take place effectively the oxygen must be transferred from the gas phase to the reaction sites and then the necessary chemical reactions must occur at an appropriate temperature.
It is possible that the traditional blowing process is interface mass transfer restricted as relatively low conversions occur, and high oxygen concentrations occur in the exit gas. At the relatively high temperatures involved, the reaction kinetics are likely to be rapid and hence the reactions should not be limiting. Unfortunately the traditional blowing process involves relatively benign fluid dynamics with respect to the bitumen phase and possibly laminar flow. This does not assist the mass transfer process which must occur before the reactions can take place. The Technix Reactors production process as described in the patents and as designed and built, centers around a recycle (loop) reactor with a high recycle ratio. The reactor tube is of a relatively low diameter and the high flow rates generated by having a large recycle stream produce highly turbulent flow with Reynolds number approaching 160,000 or more. In addition the tubular reactor includes specially designed static mixers which break up the flow and generate an extremely high degree of mixing which significantly reduces the mass transfer resistance of the blowing operation.
The reactor is a stable, high performing continuous process and modifying or producing bitumen within the required specifications. The overall plant is relatively easy to operate, and can be operated safely. The hydrocarbon generation is relatively very small and the exit gases are low in oxygen concentration, demonstrating a high oxygen consumption. Air entering the reactor with an oxygen content of 21% typically exits the reactor with oxygen concentrations (depending on the feedstock and product being produced) of between 3 % and 6 %.
Air supply from the compressors is in the order of a relatively modest 5 m³/minute to each of the reactors – significantly less than traditional blowing columns which typically operate with large blowers and air volumes at up to 10 times this rate.
To further increase the rate of reaction the reactors work with column pressures of up to 6 bar as against ambient pressures for traditional blowing columns.
However, the degassers operate at atmosphere pressure.
HEAT EXCHANGE PROCESS
The reactors require a controlled feed stock from a supply tank at or above 170 °C and lower than 230 °C. An oil or gas fired heater is used for heating thermal oil which is then used in heat tracing pipes and jackets for preheating the reactor heat exchangers, pumps, related pipework, and the feedstock pipeline and the bitumen delivery pipeline. The thermal oil is also used in the heat exchanger, to heat the product coming from the heat recovery heat exchanger to c. 230 °C for supply to the reactors. The modification reaction is exothermic and bitumen temperatures increase in the reactors by 5 °C to 10 °C.
Heat energy is recovered very efficiently through cleanable plate heat exchangers where the bitumen coming from the reactor at around 230 °C – 240 °C is used to heat the c. 170 °C feedstock. The bitumen is delivered to the receiving tank at around 180 °C.
UNDERSTANDING BITUMENS CHEMICAL COMPOSITION
Bitumen is a construction material comprising highly complex compounds of over 100,000 different hydrocarbons with a small proportion of sulphur, nitrogen, oxygen and traces of metals.
Bitumen is commonly regarded as a colloidal system of various substances and it is therefore fundamentally insufficient to define the quality of bitumen by its physical properties only.
Bitumen can be subdivided in four different substance groups which are described by the acronym SARA.
SARA stands for the three (n-heptane) soluble groups of bitumen, being the saturates, aromatics and resins (also known as maltenes) based on molecular mass and polarity, and the asphaltenes, higher molecular weight micelles, defined as the (n-heptane) insoluble group.
The Gaestel Index is a ratio of asphaltenes and saturates percentage to aromatics and resin percentage ie it is an indication of the chemical make-up of bitumen. It can be used to determine the colloidal stability of bitumen.
Bitumen that is within the Gaestel Index ranges is more resistant to rutting, heaving, shoving, has lower temperature susceptibility, and less brittleness at low temperatures. You can expect the bitumen to have a much better and longer life.
The chemical composition of bitumen can be indicated through fractionation based on polarity. This can be achieved by firstly separating bitumen into asphaltenes and maltenes through precipitation of the asphaltenes using the solvent n-heptane. The maltenes are chemically fractionated further using an iatroscan device. The iatroscan works by a chromatographic technique that involves separation of bitumen molecules according to the strength of their attraction to an activated substrate. Usually the substrate is made up of a thin layer of silica powder which is fused onto a quartz rod (chromarod). The separated fractions are detected in the iatroscan by passing the chromarod through a hydrogen flame. The conductivity of the flame, which is measured by a Flame Ionisation Detector (FID), changes with the quantity of material burnt off the rod (number of ions present). By this means the mass of each fraction can be determined.
The selection of crude is an important aspect of bitumen manufacture, as not all crudes can be used to make bitumen. Few of the, on a global basis, nearly 1,500 available crude oils are suitable to manufacture a good quality bitumen. In an oil refinery the crude oil initially undergoes an atmospheric distillation process, followed by a vacuum distillation process. The vacuum distillation further yields the so called vacuum distillates and leaves a non-volatile, fairly high viscosity residue, called short residue or vacuum residue or vacuum bottoms.
In a few cases, depending on the crude origin, this vacuum residue may be directly used as bitumen without further processing, providing it fulfils a bitumen specification. For most vacuum residues though, additional processing such as air blowing and subsequent physical blending is required to meet the various bitumen specifications. Sometimes bitumen is produced by blending vacuum residue with other side streams of a refinery, for instance solvent (e.g. propane) precipitated asphalt (bitumen), which is derived from the production of lubricating oils. Sometimes bitumen is produced by blending vacuum residue and, or, solvent asphalt with lighter flux oils. Both latter processes are targeting the physical properties of bitumen only and can produce bitumen to fit in a bitumen specification, but disregard the importance of the chemical composition of bitumen for its performance in the field and it tends to produce rather weak and inferior construction materials.
The Technix Reactor can address these issues.
All the above mentioned compositions have in common that they make a good feedstock for Technix Reactors. Using the Technix Reactors on the basis of a prior SARA analysis, and on occasions reagents and, or, fluxes, allows the production of a whole range of specification conforming bitumens from a single feedstock and to also engineer the chemical composition for better bitumens at the same time. Through molecular re-arrangements by the air rectification process with Technix Reactors asphaltenes are created and for the system precious resins are preserved, thereby improving the overall bitumen composition.
EMISSIONS
The Technix Reactors plant, with its multiple static mixes in the reactor columns, provides very vigorous and intimate mixing of the oxygen contained in the air, with the bitumen and reagents, and any other products, as they move through the reactor columns. During this process there are extraordinarily high oxygen, and bitumen reaction sites interchanges. As a consequence, relatively low volumes of air are required for the production process – considerably less than standard bitumen blowing plants.
The Technix Reactors plant process is highly efficient. As a consequence, very high conversion efficiencies are obtained and minimal feedstocks are expected to be lost in the process – less than 0.1 % for bitumen modification compared to typically 2 % to 5 % in traditional blowing columns and less than 1 % for distillation column residues (vacuum bottoms) or less than 2 % for solvent precipitated ‘asphalts’. Accordingly very low levels of emissions are produced by the plant.
A minor amount of entrained gases in the reactors are transported with the bitumen to the bitumen receiving tank. If the client proposes to install, or has a vapour recovery system, Technix suggests that system could handle the emissions from the bitumen receiving tank. Alternatively emissions can be removed from the bitumen receiving tank by an extraction fan and fed into the Technix Reactors incinerator system. A suitable liquids knockout vessel and flash back preventer will be required for this option.
At the Technix Pacific Limited plant in Fiji the liquids retained in the knockout unit are principally water and a minor amount of volatiles – the amount recovered is approximately 0.5 litres per tonne of bitumen production with approximately 90 % water and 10 % volatiles.
The liquids retained in the knockout vessel are pumped into the incinerator, and together with the remaining emissions are incinerated in the LPG, or natural gas, fired 45 kw incinerator.
The emissions gases are of a low oxygen content (typically 3 % – 6 %) and low pressure within the system. There are no odours other than those that one experiences in a bitumen refinery. The exhaust gases from the emissions afterburner contain no unburnt hydrocarbons, are appropriately vented and are not toxic.
Noise emissions emanating from the bitumen plant are minimal – such sounds relate to the compressors and rotating equipment.
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