Cold recycling

Cold recycling is a pavement recycling method in which no heating is required at any stage of work. The advantages and disadvantages of cold recycling are outlined in Table 18.4. 

The binder used is exclusively bitumen emulsion and cold recycling may be carried out in situ or in a stationary plant (central plant). The in situ cold recycling is distinguished into full-depth recycling, also known as full-depth reclamation (FDR), and partial-depth cold recycling, also known as cold in situ recycling (CR). 

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The per pass ethylene conversion in the primary reactors is maintained at 20—30% in order to ensure catalyst selectivities of 70—80%. Vapor-phase oxidation inhibitors such as ethylene dichloride or vinyl chloride or other halogenated compounds are added to the inlet of the reactors in ppm concentrations to retard carbon dioxide formation (107,120,121). The process stream exiting the reactor may contain 1—3 mol % ethylene oxide. This hot effluent gas is then cooled ia a shell-and-tube heat exchanger to around 35—40°C by usiag the cold recycle reactor feed stream gas from the primary absorber. The cooled cmde product gas is then compressed ia a centrifugal blower before entering the primary absorber. 

Major process conditions for experiment HGR-13 are plotted vs. stream time in Figure 6. The total recycle ratio was held relatively constant at 10 1 which resulted in a constant temperature rise of about 100°C across the catalyst bed (300°C at the inlet, 400°C maximum). Near the end of the experiment, the cold recycle ratio was varied from 8 1 to 1 1. The experiment was ended at 1368 hrs. 

Effects of Cold Gas Recycle and Approach to Equilibrium. Product gases resulting from various CGR ratios were analyzed (Table XI). For the experiments tabulated, a decrease in the cold recycle ratio resulted consistently in increases in the product gas concentrations of water vapor, hydrogen, and carbon dioxide and a decrease in methane concentration. These trends may be noted in experiment HGR-12 as the CGR ratio decreased from 8.7 1 to 1.2 1, in experiment HGR-13 as it increased from 1.0 1 to 9.1 1, and in experiment HGR-14 as it decreased from 3.0 1 to 1.0 1. These trends indicate that the water-gas shift reaction (CO + H20 —> C02 + H2) was sustained to some degree. Except for the 462-hr period in experiment HGR-14, the apparent mass action constants for the water-gas shift reaction (based on the product gas compositions in Table XI) remained fairly constant at 0.57-1.6. These values are much lower than the value of 11.7 for equilibrium conversion at 400°C. In... 

Lesueur, D., Herrero, L., Uguet, N., Hurtado, J., Pena, J.L., Potti, J.J., Walter, J. and Lancaster, I. (2008) Bitumen nano-emulsions and their interest for cold recycling of bituminous mix. Carreteras, 4 (158), 48-53. 

The cold recycle gas from the separator is fed to the pre-cooling heat exchanger and is recompressed to the pressure in the distribution line. The work from the expander may be utilized in a compressor for the recompression. Dependent on the conditions, additional recompression may be needed. 

The downflow fixed-bed reactor has been used widely for hydrodesulfurization processes and is so called because of the feedstock entry at the top of the reactor while the product stream is discharged from the base of the reactor (Figure 5-6). The catalyst is contained in the reactor as stationary beds with the feedstock and hydrogen passing through the bed in a downward direction. The exothermic nature of the reaction and the subsequent marked temperature rise from the inlet to the outlet of each catalyst bed require that the reaction mix be quenched by cold recycle gas at various points in the reactor. Hence the incorporation of separate catalyst beds as part of the reactor design. 

Considerable engineering judgment and effort are needed to ensure that the heat recovery is efficient, yet has low pressure drop. Since a large portion of the heat contained in the reactor effluent has to be transferred back to the cold recycle gas, this exchanger arrangement received our special attention. 

Option 3 (Figure 1) which contains all basic and additional processing units can be described as follows. Acrylic acid is produced by partial oxidation of propylene in a fluidized-bed catalytic reactor. To prevent any side reaction, a cold recycle quench is used immediately after reactor. Deionized water in the off-gas absorber absorbs off-gas from the quench tower, containing acetic acid, acrylic acid, unreacted propylene, and byproducts. In the next step, an acid extractor is used for liquid-liquid extraction to... 

Additionally, the effect of asphalt recycling is positive, particularly when cold recycling is used on carbon emissions. It has been found that carbon emissions were only 624 kg eq C/1000 m during the reconstruction of a 7 cm thick asphalt layer using cold recycling, in contrast to 2381 kg eq C/1000 m by way of the conventional technique with hot bituminous mixture (Mauduit et al. 2011). 

Pavement recycling methods differ from the flexible to the rigid type of pavement. In flexible pavements, two basic methods are distinguished hot recycling (HR) and cold recycling (CR). Each one is further distinguished into in situ (or in-place) and in-plant (central plant) recycling. 

In situ full-depth cold recycling, also known as FDR or structural road recycling, is a cold recycling method where all asphalt layers and, in some cases, a predetermined portion of the underlying base material are uniformly pulverised and usually mixed with stabilising additives to produce a stabilised base course material. The pulverised reclaimed material may also be used as it is untreated for unbound base course material. 

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