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Surface Modification of Metallic Reaction Vessels to Resist Scaling by Calcium Salts


Project Description

The CDT in Advanced Metallics is a partnership between the Universities of Sheffield and Manchester and the I-Form Advanced Manufacturing Centre, Dublin. CDT students undertake the CDT training programme at all three locations throughout the 4-year programme.

Inorganic chemical scales form rapidly in certain chemical processes, reducing process performance. One example is in bioethanol sugar juice evaporators, which require frequent manual cleaning in confined spaces. This project aims to develop modifications to steel surfaces via chemical surface treatments which resist the formation of these scales, thereby reducing safety risks and improving process efficiency.

Sugar cane ethanol plants use the addition of lime to the raw extracted juice in order to neutralise and clarify the juice. This leads to the juice becoming saturated with a range of calcium salts, notably sulphate, phosphate, silicate, and oxalate. This leads to the deposition of these salts as a hard scale on the insides of the tubes in the evaporators, where the sugar juice is concentrated by boiling off the water. The hard scale reduces the heat transfer rate in the evaporators, and would eventually lead to blockage of the tubes. Normal practice in Brazil is for the evaporators to be cleaned manually, as often as weekly. This involves personnel being lowered into the upper chamber of the evaporators and removing the scaling by “rodding”, i.e. physical abrasion of the inner tube surfaces. Not only does this involve undesirable working in confined space, but it inevitably also leads to significant wear on the tubes leading to their frequent replacement. In countries where labour is most expensive, chemical cleaning has been attempted with rather mixed results. One difficulty is that such a diverse range of calcium salts is involved, a range of different (and mutually incompatible) chemical cleaners, including strong alkali, are needed to remove the deposits, so several cycles of cleaning are required each time, and some of the chemical cleaning agents also introduce additional hazards. Furthermore, each of the evaporator stages has a different combination of scalants, and requires different treatment.

This project forms part of BP Group Research’s response to this issue, and seeks to find means of treating the insides of evaporator tubes (and potentially other vulnerable surfaces) such that the calcium salts do not nucleate on those surfaces. It is understood that the evaporation of water must inevitably take the juice beyond the solubility limit of the salts concerned, so the aim must be to ensure that the salts either crystallise in the bulk rather than on the surface, or form solids which readily detach from the surface rather than form hard scales. In a practical implementation, this would require installation of filtration or settling equipment downstream of the evaporators to collect these calcareous precipitates.

We envisage that a range of different surface treatments will be investigated. One promising line will consider a number of different coating options (alkyl phosphonates, silanes, and thiol terminated molecules designed to provide a positively charged surface) that have been used in biomedical applications to preserve the chemical composition of metal implants and prosthetic heart valves, in order to prevent accretion of foreign materials on them. A key feature of the experimental work will be to ensure that evaporation takes place on the modified surface, which will involve designing and building a piece of experimental equipment which allows heat to be directed through the test sample so that this evaporation takes place at the right location. We envisage that work under this programme will be conducted with synthetic solutions – initially with individual problematic calcium salts, but subsequently with mixtures possibly including sugar. We also have the option of subsequently commissioning a test programme at larger scale with authentic sugar solutions at a Brazilian governmental research facility, should a sufficiently promising solution be found.

Funding Notes

Current UKRI PhD stipend (£15,009 in 2019/20) plus a top-up of £2,500p.a. in Year 1 and £3,500p.a. in years 2, 3 and 4, for UK and eligible EU students

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