Boiloff rate

Diking is a common industry practice for spill retention around flammable liquid storage vessels. Regulations and codes require that exclusion zones be placed between storage vessels to maintain the structural integrity of nearby vessels or process equipment in the event of a fire within the dike (Welker, 1987). Studies of flammable vapor boiloff rates as a function of dike design have been used in the natural gas industry to evaluate optimal materials of construction (Arthur D. Little, Inc., 1974). In recent... 

Since the purpose of diking is to contain a spilled fluid effectively while reducing the vapor generation rate, a secondary containment system that will be impervious to the fluid must be provided. It should also have favorable thermal properties to retard the boiloff rate. 

The thermal properties of a dike can have a profound effect on the vapor boiloff rate, especially for materials with boiling points below ambient... 

We may note immediately that although determining the vessel inlet and outlet flows in equations (12.2) should be a straightforward matter, calculating the boiloff rate is not so easy. 

We now have in (12.1) and (12.6) equations in our chosen state variables liquid mass and temperature. But the fact that we do not know the boiloff rate prevents us from solving these equations without the additional differential equation (12.2) for the mass of vapour and the algebraic equations for volume and pressure, (12.9) and (12.13). We may now use the Method of Referred Derivatives, outlined in Chapter 2, Section 2.11 (and in Chapter 18, Section 18.7 in further detail), to put these algebraic constraints into the appropriate form to be used in the differential equations defining the state variables. 

By is the boiloff rate of component j from the liquid in plate i (kmol/s),... 

The several flows into and out of the distillation plate make the form of the differential equations rather busy , but it is possible to simplify the algebra without any loss of information by grouping together the expressions that are independent of the boiloff rates. Hence we may rewrite equation (12.58) as ... 

Substituting into equation (12.73) and rearranging gives the required set of equations in the boiloff rates, B,y ... 

For binary distillation, it is a simple matter to write an explicit solution for the boiloff rates for the two components on plate i ... 

Once the component boiloff rates have been found, it becomes possible to calculate the derivatives of the liquid masses and temperature using equations (12.77) and (12.74) ... 

The boiloff rate was measured to be approx. 1,3 %/d (about 0.7 m /d) for the filled open tank, while it was decreasing to 0.6 %/d at filling levels below 40 %. The decrease of the boiloff rate is attributed to the stronger heat uptake of the steel at higher temperatures which occur when in contact with the gas phase. In the self-pressurization tests, mass transfer from liquid to gas was found to have a dominant influence. The tests with the model tank on the BAM test site near Berlin have been terminated in the meantime, the tank is now being offered to third parties for further experimentation. 

For a large-scale liquid hydrogen storage, tanks of spherical shape are used to minimize boiloff losses. Two spherical dewars with a capacity of 190 of LH2 plus 10 % ullage each were components at the Nuclear Rocket Development Station, Nevada, during the ROVER project starting in 1955 (see section 9.5.1.). The insulation consisted of a vacuum jacket and a 0.9 m perlite powder layer allowing an estimated boiloff rate of... 

The largest LH2 tank constructed so far is the NASA 3407 vacuum perlite-insulated spherical storage tank at the Kennedy Space Center, Florida, used in the US space shuttle program. The outer sphere is made from carbon steel with an inside diameter of 21.34 m and the inner sphere is made from austenitic stainless steel with an inside diameter of 18.75 m the ullage is about 10 %. The tank has a boiloff rate of 0.03 % or approx. 800 1 per day [51]. For comparison purposes the LH2 storage tank within the External Tank (47 m height, 8.4 m diameter) of the US Space Shuttle has a volume of almost 1600 m. ... 

Tank specifications for a large-scale LH2 storage tank as projected within the WE-NET project, are a storage capacity of 50,000 m with a target boiloff rate of 0.1 % per day. Various insulation concepts are currently being under investigation [32]. 

Transportation of LH2 in rail cars has started in the 1960s by the Linde company using a 107 m tank witii a multiple layer insulation. The measured boiloff rate was 0.2 %/d. The US company Praxair operates a fleet with 16 hydrogen railcars. The hydrogen is saturated at a working overpressure of the tank of 55 kPa with a pressure control system to open the relief valve at an overpressure of 117 kPa. The amounts of LH2 transported in railcars in recent years were approx. 70 tons or about 10 railcar loads per month [8]. 

The benefits achieved by this liquid-shielding arrangement are obvious. Equation (7.1), the modified Stefan-Boltzmann relation for radiant heat transfer through a vacuum, shows that the reduction in the boiloff rate with... 

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