Zhejiang LNG terminal dump line simulation of precooling BOG

Time: September 15, 2013 Source: million net VIP HowNet hits: Secondary

Abstract: As the LNG low temperature properties, before its first entry into the receiving station technology system, you need to use LNG cryogenic LNG vapor dump line (BOG) pre-cooled to -120 ℃, and then reintroduced into the LNG pipeline will discharge was cooled to - 150 ℃. Pre-cooling the discharge line is to ensure that the focus of the work of LNG terminal successfully put into trial operation. To this end, Zhejiang LNG terminal as an example, the use of self-programming model for long distance LNG discharge pipe diameter l000mm pipeline BOG pre-cooling process, the establishment of a one-dimensional flow and heat transfer model, with the BOG MATLAB simulation tool pre-cooling the entire process of LNG unloading lines, the results show: the discharge pipe wall temperature does not exceed the maximum rate dropped 10 ℃ / h, calculation time step taken l0 s, calculated 737m unloading of LNG pipeline was cooled to - the required time is about l20 ℃ 30.25h. Also analyzed the influence of different factors on the discharge line of the pre-cooling process, the results showed: ① cooling over time increases with the BOG flow at the end of the cooling phase, BOG flow rate of 40.95kg / s, the cumulative consumption BOG as l4330kg; ② pipeline within the BOG flow rate increases with increasing cooling time; ③ pipeline BOG pressure within the cooling time and with the increase of pipe length decreases. Suggesting practical operation, the pipeline was cooled to -100 ℃ to enter the LNG cooling stage, the cooling time and save the entire amount of BOG pipeline.

Keywords: Zhejiang LNG terminal dump line BOG MATLAB modeling precooling A simulation study of boil-off gas (BOG) pre-cooling process in unloading pipelines in all

LNG terminal

in Zhejiang Abstract:

Before LNG enters into a terminal from cargo tanks, the pre-cooled LNG boil-off gas (BOG) steam will be utilized first to cool the unloading lines down to -120 ℃, and LNG will then be introduced to cool the lines down to -150 ℃. This will be essential for a LNG terminal to be smoothly put into operation. In view of this, a case study was performed in a LNG terminal in Zhejiang province. First, a self-programming system was adopted to build a one dimensional flow heat transfer model for the BOG pre cooling process in a long distance LNG unloading line with the diameter of l000mm. The results were achieved in this simulation case study assisted by MATLAB. The maximum temperature drop rate on the

pipe wail surface was not more than l0 ℃ / h; when the time step was l0s, the cooling time would be 30.25h for a 737m LNG unloading line cooled down to -120 ℃. Additionally, the impact of different factors was analyzed on the BOG pre-cooling process in LNG unloading lines. The BOG flow rate increases gradually over time and reaches up to 40.95kg / s by the end of cooling stage with the cumulative consumed BOG quantity of l4330kg; its flow speed in the lines increases with the increasing cooling time; and its pressure decreases with the increase of cooling time and pipe length. Thus, it is suggested that in practical operation, the LNG cooling should be got involved as soon as the unloading line is cooled down to -100 ℃, which will help save the cooling time and the BOG consumption for the whole line.

Key words: Zhejiang, LNG terminal, BOG pre-cooling, MATLAB simulation Zhejiang Zhejiang LNG terminal is the introduction of liquefied natural gas project

application core project. The main function is to receive LNG terminal, LNG storage and gasification, and gas supply [1] through the pipe network to downstream users. Part LNG receiving station also set up LNG tanker loading station, providing LNG directly to users. Since LNG low temperature properties, before its first entry into the LNG receiving station technology system, the need for the use of LNG cryogenic LNG vapor dump line (BOG) to precool -l20 ℃, and then reintroduced into the LNG pipeline will discharge cooled to -l50 ℃. Priorities is to ensure that pre-cooling LNG terminal successfully put into trial operation. By pre-cooling at room temperature so that the LNG pipeline and tank reaches the low temperature working conditions, to prevent the sudden entry of LNG caused a sharp contraction of pipes and storage tanks, resulting in damage to the pipes and tanks; In addition, you can also pre-cooling cryogenic equipment and pipeline inspection facilities pipes cold shrinking, pipe bracket support change, cryogenic valves sealing, tank low temperature properties [2]. Zhejiang LNG terminal as an example, the use of self-programming model, using MATLAB tool to simulate the unloading of LNG BOG cooling line the whole process, for the LNG terminal actual intake air cooling operation and provide a theoretical basis for optimization tips.

Establish a model 1.1 Zhejiang LNG Receiving Terminal Process Description

Zhejiang LNG terminal process shown in Figure 1. The LNG from the LNG terminal LNG carriers (-162 ℃) through the discharge line on the dock transported to the LNG storage tank. Pressure pump located inside the tank and then transporting the LNG storage tank to LNG within the tank and then the condenser, the high pressure pump and then through a further condenser pressurized LNG delivered to the gasifier, the final LNG gasification in the gasifier after delivery to the gas pipeline network in Zhejiang Province, the natural gas

pipeline natural gas delivered to the end user [3-4]. Zhejiang LNG terminal discharge pipeline consists of 40in (1in = 25.4mm) main discharge pipe, 36in feed manifold and the end of the pipe 4in sprinkler piping components.

Establish Model 1.2

LNG belong cryogenic liquid in the discharge pipeline before officially put into use, it needs to be pre-cooled. Ting Zhejiang LNG terminal dump line pre-cooling and the use of LNG BOG staged a cooling mode, which is to use BOG from the cabin of the discharge arm discharge pipeline to the LNG storage tank cooling, the process pipe wall temperature maximum the cooling rate control in less than 10 ℃ / h, after cooling gas into the tank through the end of the pipe 36in 1.5in bypass valve and spray line 4in discharged into LNG storage tanks. When the end of the pipe surface temperature reaches 120 ℃ End BOG cooling phase. Finally, the introduction of the LNG, the depth of the second pre-cooling stage, until the pipeline is full of LNG. The simulation model is mainly BOG is precooled discharge pipeline process.

Model dump line is divided into a number of the total 737m infinitesimal segment within the outer tube and pipe natural convection boundary conditions for convective heat transfer under BOG conditioning, heat from the outside into the interior. Wall insulation layer and the initial temperature is room temperature, in the pre-cooling process, the temperature of the wall and the insulation material is gradually decreased. 1.2.1 tube fluid temperature, pressure, speed and other parameters

According to the energy conservation equation, momentum equation, mass conservation equation and the equation of state is obtained is calculated BOG outlet temperature, pressure, velocity and density parameters [5-14].

among them:

1.2.2 wall insulation and unsteady heat conduction equation

Considering only the outer wall of natural convection, heat radiation is not considered the case, the model reduces to a two-dimensional unsteady heat conduction equation solving pipe wall temperature.

At each time step, the initial calculation conditions (inlet temperature, pressure, velocity

and density) infinitesimal segment 1 and the boundary conditions on behalf of the segment infinitesimal population temperature of the gas into the formula (1) to (4), calculated 2 , pressure, velocity and density, and finally the use of the formula (9) to calculate the pipe wall temperature. Calculation stopped until the temperature reaches the wall -l20 ℃. 2 calculation results

BOG precooler inlet condition input program (BOG inlet gas temperature of -120 deg.] C, an inlet pressure of 24 kPa, the maximum pipe wall temperature decrease rate of l0 ℃ / h), 12h after the calculation, the following calculations. 2.1 discharge end of the pipe is cooled to the target temperature -l20 ℃ time required

Dump line in Figure 2 the wall temperature changes. Expected to discharge cooling tube end to a desired target temperature -l20 ℃ time 30.25h.

As can be seen from Figure 2, with the extension of the passage of time and asked the

length of the pipeline, the discharge line temperature is gradually reduced to the target temperature. Specific feature point discharge pipeline wall temperature calculation results are shown in Table l.

2.2 cooling over time increases with the BOG flow

In order to maintain l0 ℃ / h cooling rate, cooling BOG flow rate increases over time, at the end of the cooling phase, BOG gas flow to 40.95kg / s, the cumulative consumption of BOG l4330kg. Population BOG dump line mass flow changes shown in Figure 3, the discharge line BOG inlet mass flow results in Table 2.

Within 2.3 Pipeline BOG flow rate increases with increasing cooling time

BOG flow rate increases due to the mass, and therefore, the flow rate of the BOG conduit increases. The BOG flow rate changes in the discharge line in Figure 4.

2.4 pipeline BOG pressure increases cooling time and length of the pipe decreases

The dump line BOG pressure variation shown in Figure 5.

3 Conclusion

1) After calculation, the end of the pipeline is expected to discharge cooled to a desired temperature of -120 ℃ when the target time is 30.25h.

2) In order to maintain l0 ℃ / h cooling rate, cooling BOG flow rate increases over time, at the end of the cooling phase, BOG flow rate of 40.95kg / s, the cumulative consumption of BOG l4330kg.

3) at 26h after cooling, the BOG mass flow rate significantly increased, from 10 to 40 times the initial. Cooling around 28h of mass flow rate of BOG 5.65kg / s, it was cooled to line can -96 ℃. Late into the cooling needs not only greatly increase the amount of the cooling gas and the time is extended accordingly. Suggesting practical operation, the pipeline was cooled to -100 ℃ to enter the LNG cooling stage, the cooling time and save the entire amount of BOG pipeline.

4) with a flow rate of the cooling pipe BOG time increases.

5) pressure pipe BOG asked to increase the length of the pipeline decreases upon cooling. Symbol Description

Paper A is the area, m 2; c is the specific heat, J / (kg · K) ; C p is the specific heat, J / (kg · K) ; D is the pipe diameter; f of evaporation coefficient; G r is the mass flow rate, kg / (s · m 2 ); h is the heat transfer coefficient, W / (m 2 · K ); h conv heat exchange coefficient, W / (m 2 · K ); i is infinitesimal segment label; J is the time step numbers; L is the length, m; m is mass flow rate, kg / s; sound as pressure, Pa; q m is the mass flow rate, kg / s; R g is the gas constant, J / ( kg · K); T is the tube fluid temperature, K; T bao the insulation temperature, K; TT is BOG

temperature, K; T w is the wall temperature, K; u is velocity, m / s; V is the volume, m 3; r is the density, kg / m 3; ti is time, s; l is the thermal conductivity, W / (m · K) ; n is the specific volume, m 3 / kg; and the subscript a is a tube in fluid contact with the wall surface. references [1] First Yanqun, Chen Wenyu, Niu Jun Feng. LNG terminal Application Technology (T) [J]. Gas industry, 2007,27 (1): 120-123.

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Author: Jiashi Dong Lu Jun Deng Qing Author: CNOOC Zhejiang Ningbo LNG Co.

(Author: Gas Magazine Editor: Gas HowNet)

Article hot words: Zhejiang receiving station line pre-cooled simulation study

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