36


EQUIPMENT


Typically for VRU systems the lowest


emissions that can be achieved in a single stage system are between 150 mg/Nm3 100 mg/Nm3


. A second stage is therefore


required to achieve an end emissions requirement of 35 or 50 mg/Nm3


. This second


stage has tended to be either a Catalytic Thermal Oxidiser (CTO) or Regenerative Thermal Oxidiser (RTO). The question might be asked for these


to the vent of the VRU. An inlet vapour blower will draw the vapours from the loading system, pushing the vapours through the VRU. The blowers are typically required to operate in a Zone 0 rated environment and consequently require all of the associated safeguards. A vent vapour blower draws the vapour through the VRU, effectively providing a zero back pressure from the VRU on the vapour manifolding system. A vent blower can usually be rated Zone 1 or 2 and is a much lower cost item compared with the inlet blower, although it may be less flexible.


EXTREME STANDARDS Where authorities have introduced extremely tight emissions standards – as in Germany, the Netherlands and Oman – these normally apply only to new VCSs. However, they can also apply in cases where operational changes at the terminal might require amended permitting requirements. There are, however, cases where retrofits have been applied to meet these tighter standards.





OLDER VCSS MAY BE RETAINED TO PROVIDE BACK-UP OR ADDITIONAL CAPACITY WHEN NEEDED


low emissions requirements, why not simply oxidise the vapours? It is often the case that the legislators require some form of recovery. Combining a VRU to meet the recovery requirement with a CTO/RTO unit generally provides recovery efficiencies greater than 97 per cent, while the oxidiser package ensures the facility meets the emissions requirement. Products containing hydrogen sulphide – such as crude oil – raise another issue, particularly in terms of corrosion in the VCS. It can also lead to the deposition of elemental sulphur in the carbon bed. A switch to such a cargo may require the addition of an H2


S guard bed located in the


inlet to the VRU. This contains a sacrificial H2


reduce the H2


S-selective activated carbon that can S concentration in the vapour


“OPERATING AND ENVIRONMENTAL REQUIREMENTS HAVE CHANGED, REQUIRING MORE FROM


VAPOUR CONTROL EQUIPMENT”


phase to ≤1 ppm. When fully saturated, the guard bed carbon needs to be replaced. In most instances the guard bed system is designed on the basis of a one- to two-year lifespan before replacement is required. This type of bed should not be confused with the VRU regenerative activated carbon bed, where operational life spans often exceed 10 years. In his presentation, Shipley concentrated on


activated carbon adsorption since this approach is viewed as the best available technology as opposed to cold liquid absorption, membrane and pressurised absorption. It is comparatively simple and has good overall operating efficiency; it can cope with a wide range of products and applications; and it is familiar to operators. Zeeco is also a specialist in activated carbon adsorption as well as other technologies, and offers a team of field service engineers fully trained in the supervision of new VCS installation and commissioning, retrofitting of existing equipment and routine maintenance and inspection of units. HCB www.zeeco.com


and


HCB MONTHLY | SEPTEMBER 2016


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