HBS9,ВОДОРОДНАЯ ЭНЕРГЕТИКА И ТРАНСПОРТ
Хранение водорода HYDROGEN ENERGY AND TRANSPORT
ч Hydrogen storage
LOW-TEMPERATURE REGENERATION f
OF CRYOSORPTION DEVICES IN HEAT-INSULATION CAVITIES 1 OF HYDROGENOUS CRYOGENIC TANKS i
и
ь
A.L.Gusev ы
с a
Scientific Technical Centre "TATA" ^
P.B.O. 787, Sarov, Nizhny Novgorod region, 607183, Russia §
Tel.: +7 (83130) 97472; Tel./Fax: +7 (83130) 63107; E-mail: gusev@hydrogen.ru ®
As a result of hydrogen leaks into heat-insulating cavities through microcracks as well as at the expense of gas release from the walls, a situation may arise related to untimely supersaturation of cryosorption pumps (CSP) with hydrogen [1, 2]. As it is known, the complete regeneration of ceolyte CaEH-4B requires the temperature of 473 K. It is impossible to reach this temperature without terminating the technological storage process for built-in CSPs. To reduce a negative effect related with this situation the possibility of low-temperature built-in cryosorption device regeneration was investigated on the base of the CaEH-4B adsorbent in a heat-insulating cavity of a hydrogenous cryogenic tank with substituting feeding. Using the Henry equation for the interval 20.2-32 K the desorption dynamics were obtained for gases sorbed by the adsorbent at the temperature of 20.2 K. The efficiency of using low-temperature adsorbent regeneration is shown for various temperature levels of cryogenic liquid [3]. Based on experimental research a chemical patron design is proposed. This makes possible the removal of hydrogen from the vacuum heat-insulating cryogenic tank cavity and monitoring the hydrogen absorption process according to the thermal chemical reaction effect.
Introduction
CSP belong to periodic operation devices — as an adsorbent becomes saturated, their speed of response reduces. As a result, the pressure in the vacuum cavity of the cryogenic tank is an increasing function of time.
For the complete reproduction of the ceolyte CSP absorptibility, a low-temperature adsorbent regeneration is used. The present work describes the possibility of using low-temperature regeneration for these purposes. It is assumed that for CSP operating at the temperature level of 20.7 K, the low-temperature regeneration will be a useful addition to the existing high-temperature regeneration method. For some particular cases, the low-temperature regeneration will just be the only possible tool that will allow avoidance of a number of emergency situations. The built-in CSP operating at 20.7 K are made as built-in models according to the design scheme [4, 5]. A similar pump layout, along with big advantages as compared with the side-mounted pump scheme (flange CSP), has one significant disadvantage. It is related to the necessity of discharging all cryogenic liquid to an empty vessel at the adsorbent regeneration. Additionally, the inner cryogenic tank cavity should be heated. For ceolyte CSP, the regen-
eration temperature should not be less than 200 °C. The factors enumerated above define large energy losses during the regeneration of big cryogenic tanks.
In the course of operation of big cryogenic tanks, a situation often arises when it is necessary to regenerate an adsorbent, but the component can not be merged into another vessel. Moreover, this could occur in two cases. The first case is as follows: the maintenance period has been completed, but it is to be extended at least for a short time. However it is impossible to prolong this period for the built-in CSP by means of conventional methods. The pressure in the tank HIC may arise up to the limiting level, at which the tank operation is forbidden. It may cause a high i evaporation capacity of the cryogenic liquid and even t lead to an accident. The second case corresponds to an | increased leak in the HIC. In this case, both hydrogen dissolved in the metal thickness and being released 3
T
through the defect pores and hydrogen being released | in the HIC through the defects from the cryogenic ¿; tank may take part in the gas release process. This | results in the super-saturation of the adsorbent. *
c
In order to overcome the enumerated situations, ° a method was proposed providing the low-tempera- 0 ture regeneration of the built-in CSP adsorbent [6]. This method allows regeneration CSP without merg-
Статья поступила в редакцию 06.11.04 г. Article has entered in publishing office 06.11.04.
Notation
B — the experimental constant, m3/kg;
CCSP — the thermal capacity of the CSP, J;
CINS — the thermal capacity of the SVHI, J;
CSP — the cryosorption pump;
D(x) — the diffusion constant;
e — the efficiency criterion regarding the cryogenic liquid storage method;
- ML
tym — m — the mass degree of the cryogenic tank filling with liquid;
Gins — the mass of the shield-vacuum thermal insulation, kg;
GT — the cryogenic tank mass, kg;
CT — the thermal capacity of the cryogenic tank, J;
CL — the specific heat of the cryogenic liquid, J/kg;
GCSP — the mass of the in-built cryoadsorp-tion pump with the adsorbent, kg;
HIC — the heat-insulating cavity;
L, L0, LC — the CSP absorptivity, with index "0" corresponding to the initial value and "C" — to the current value;
MH — the mass of liquid hydrogen together with the gaseous pad, kg;
M' — the passport liquid mass for the given tank, kg;
Mv — the mass of vapour, kg;
AMN t — the cryogenic liquid losses caused by the pre-term pressure pickling under the drainage-free storage, kg;
AMp — the evaporating liquid losses due to the molecular component of the heat inflows from the environment defined by the residual gas pressure in HIC, kg;
AMn — the cryogenic liquid losses due to the liquid trampling into an empty vessel while conducting the built-in CSP regeneration, kg;
AMQ — the evaporating liquid losses caused by the molecular component of the heat inflows from the environment defined by the pressure of the hydrogen residual gas component in HIC, kg;
Mi' — the mass flow-rate of the evaporating liquid per second, kg/s;
n — the degree of meeting the passport requirements regarding the CSP normal operation time;
N
G.C.
— the number of gas cushion replacements in the cryogenic tank;
PHIC — the pressure inside the heat-insulating cavity of the tank, Pa;
Pv P2 — the pressure values in the cryogenic tank, MI2 a;
PGC — the gas cushion pressure, Pa;
q — the differential heat of adsorption, J/(moleK);
QH2 — the intercrystalline hydrogen flux from the cryogenic tank casing, Pam3/s;
QH2 — the hydrogen flux produced by the gas release of intercrystalline hydrogen from the cryogenic tank metal as well as by fluxes through wall defects of the cryogenic tank and from the cryogenic tank due to diffusion through the wall, Pam3/s;
QH2 — the desorption hydrogen flux from the CSP adsorbent;
QHC — the cryosorption pump capacity as to hydrogen, Pam3/s;
QH2 — the chemical patron capacity as to hydrogen, Pam3/s;
dQe — the heat delivered to the cryogenic tank from the environment, J;
QL — the total heat inflow from the environment to the liquid, W;
Qj — the heat inflow per second caused by the molecular component of the heat inflows from the environment defined by the residual gas pressure in HIC, J/s;
QQ2 — the heat inflow per second caused by the molecular component of the heat inflows from the environment defined by the residual hydrogen pressure in HIC, J/s;
r — the evaporation heat, J/kg; Rg — the gas constant, J/(moleK); R(T) — the ratio of the specific adsorbent capacity at variable temperature to the specific adsorbent capacity at the temperature of 20.2 K, percent;
rV — the density of the cryogenic liquid vapour, kg/m3;
SVHI — the shield-vacuum heat insulation; tmp — the passport maintenance period of the CSP adsorbent, s;
tL — the time, within which the cryogenic liquid temperature in the cryogenic tank rises from
TL1 to T^ s;
Tj, T2 — the temperatures of the cryogenic liquid, accordingly, at the following pressures in the cryogenic tank: (Pp P2), K;
0 — the degree of the adsorbent saturation with gas indicating what part of the CSP absorptivity is spent: 0 = m/L, where m is the amount of gas absorbed by the adsorbent, Pam3;
tmtp — the actual time of the CSP turnaround period, s;
t — the current time lapsed from the moment of start of the low-temperature regeneration technique, s;
dU — the change of the inner energy of the system comprising the cryogenic tank with the cryogenic liquid, J;
v — the specific capacity of the adsorbent under the operating pressure, Pam3/kg;
v1, v2 — the specific adsorbent capacities, accordingly, at the following temperatures: (Tp T2), Pam3/kg;
VGC — the gas cushion volume in the cryogenic tank, m3.
ing the cryogenic liquid. The analysis of the possibility of using the low-temperature regeneration for CSP of cryogenic tanks of various temperature levels (96 K, 77 K, 20.7 K) has showed that it is applicable only the for hydrogenous built-in CSP.
Generation of a residual cryogenic tank atmosphere. Gas emission type influence upon CSP absorptibility
In Fig. 1, a cryogenic tank with the built-in CSP is shown.
In case of long periods of operation of big cryogenic tanks, the hydrogen gas emission of from the casing metal QH is of great importance.
Investigation of the opportunity to perform the low-temperature regeneration
Let's investigate the opportunity to perform the low-temperature regeneration of built-in cry-oadsorption devices on the basis of the widely spread specialised vacuum ceolyte CaEH-4B. Let us first discuss a configuration when the cryoadsorption devices are located inside a heat-insulating cavity within a big hydrogenous cryogenic tank with the cryogenic liquid supercharge. Let us consider a case of early adsorbent super-saturation with hydrogen. Based on the Henry equation one can express the desorption dynamics of gases absorbed by the adsorbent at the temperature of 20.2 K for the interval 20.2-32 K (Fig. 2).
100
R (T
Fig. 1. Cry
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