Operation of an online gas chromatograph with reduced use of calibration gas

Abstract

Online gas chromatograph, comprehensive: a trial admission; at least one chromatography column, which is coupled to the sample inlet and ready for operation; at least one valve inserted between the sample inlet and the at least one chromatography column; a detector that is fluidically coupled to at least one chromatography column; and a controller coupled to the detector and to the at least one valve, wherein the controller is configured to control the flow from the sample inlet through the chromatograph using the at least one valve, wherein the controller is configured to generate a plurality of sequential calibration cycles, each calibration cycle having a calibration gas purge, and wherein a first calibration gas purge lasts longer than a second calibration gas purge.

Description

STATE OF THE ART

[0001] Gas chromatography is the separation of a mixture of chemical compounds based on their migration rates through a chromatography column. This separates the compounds based on differences in boiling point, polarity, or molecular size. The separated compounds then flow through a suitable detector, such as a thermal conductivity detector (TCD), which determines the concentration of each compound represented in the overall sample. Knowing the concentration of the individual compounds allows for the calculation of certain physical properties, such as BTU or specific gravity, using equations that are standard in the field. For example, US Patent 2013 / 0304393 A1 discloses a method for operating gas chromatographs based on statistical considerations.

[0002] A gas chromatograph is an analyzer that passes a small volume of gas through chromatography columns to separate and individually measure the gas components of the sample mixture. The analysis cycle can be divided into two general phases: the sample injection phase and the separation and measurement phase. Each analysis cycle requires a specific amount of time to complete, typically measured in minutes, depending on the application.

[0003] An online gas chromatograph performs sequential analysis runs of a single or multiple sample streams. After the sample injection phase of the analysis cycle, the next stream gas to be analyzed is then purged through the internal sample flow paths and the sample loop to the end of the analysis cycle to ensure that a fresh, uncontaminated sample is present in the sample loop for injection into the columns at the beginning of the next analysis cycle.

[0004] A calibration sequence consists of three or more consecutive analysis cycles of calibration gas with a known composition, used to calculate the individual calibration factors (known as "reaction factors") for each component measured by the gas chromatograph. In the natural gas transportation market segment, where gas chromatographs are used as part of the calibrated measurement system, the gas chromatograph is typically set to calibrate itself automatically once daily to ensure measurement accuracy and to validate the analyzer's proper operation. The calibration gas is typically a custom-blended gas with a certified composition. The cost of the calibration gas can be a significant component of the gas chromatograph's operating costs.Conserving the amount of calibration gas consumed by the operation of the gas chromatograph would reduce the overall cost of such online gas chromatography. A solution to this problem is provided by a gas chromatograph according to claim 1 and a method according to claim 9. SUMMARY

[0005] An online gas chromatograph is provided. The online gas chromatograph includes a sample inlet and at least one chromatography column, which is coupled to the sample inlet and ready for operation. At least one valve is inserted between the sample inlet and the at least one chromatography column. A detector is fluidically coupled to the at least one chromatography column. A controller is coupled to the detector and to the at least one valve, and the controller is configured to control the flow from the sample inlet through the chromatograph using the at least one valve. The controller is configured to generate a plurality of sequential calibration cycles, each calibration cycle including a calibration gas purge. The first calibration gas purge lasts longer than the second calibration gas purge. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic system view of a gas chromatograph, with which embodiments of the present invention are particularly useful. Fig. Figure 2 is a flowchart of a method for operating an online gas chromatograph in accordance with an embodiment of the present invention. Fig. Figure 3 is a schematic view of sequential calibration gas purge cycles in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF ILLUSTRATIVE FORMATIONS

[0006] Fig. Figure 1 is a schematic system view of a gas chromatograph with which embodiments of the present invention are particularly useful. The gas chromatograph 100 includes a carrier gas inlet 102, a sample inlet 104, a sample vent outlet 106, and a measurement vent outlet 108. Carrier gas is supplied to the flow panel 110, where it passes through a regulator 112 and a dryer 114 before entering the analyzer oven 116 and passing through the carrier gas preheater coil 118. During measurement, sample gas enters the chromatograph 100 via the sample inlet 104 and passes through the sample gas preheater coil 120 in the analyzer oven 116. During calibration, calibration gas enters the sample inlet 104 and passes through the sample gas preheater coil 120.Both the sample gas (during measurement) or calibration gas (during calibration) and the carrier gas ultimately enter a multitude of pneumatically controlled multi-port multi-way valves 60 to selectively allow different volumes of a sample and / or carrier gas to flow through different chromatography columns 112 in accordance with known gas chromatography techniques. Each of the pneumatically controlled multi-port multi-way valves 60 is fluidically coupled to a respective solenoid 112, which receives its control signal from the controller 126. As in . Fig. As shown in Figure 1, each pneumatically controlled multi-port multi-way valve 60 has a pair of states. In the first state, the fluid connections of each valve 60 are shown as solid lines. The fluid connections of each valve 60 in the second state are shown as dashed lines. The controller 126 is also coupled to the detector 128, which in one embodiment is a thermal conductivity detector located in the analyzer oven 116. Thus, the controller 126 can completely control the flow through the gas chromatograph 110 by controlling the solenoids 124. Furthermore, the controller 126 can determine the response of the detector 128 to the gas flow through it.In this way, the controller 126 can selectively introduce the sample into a chromatography column for a selected duration, reverse the gas flow through the column, and direct the reverse flow through the detector to observe and / or record the detector response over time. This provides a chromatographic analysis of the sample.

[0007] The controller 126 preferably includes a microprocessor or other suitable device capable of executing a sequence of instructions, calculating analytical parameters, and storing information. The controller 126 may include or be coupled to both volatile and non-volatile memory.

[0008] Fig. Figure 2 is a flowchart of a method for calibrating an online gas chromatograph in accordance with an embodiment of the present invention. The method 200 begins at block 202, where a sample analysis cycle is completed immediately before the first calibration analysis cycle. Upon completion of the sample analysis cycle, the control transfers to block 204, where calibration gas is purged through the system. During this first calibration gas purge, the system waits at block 206 until the time t1 has elapsed. In one embodiment, the time t1 is an equal length of time currently used in prior art calibration purge cycles. However, the time t1 can be any suitable value that effectively purges calibration gas through the system. Once the time t1 has elapsed, the method 200 continues at block 208, where a second calibration cycle is performed.During the second calibration cycle, additional calibration gas is purged through the system. However, during the purge of the second calibration gas cycle at block 210, the system waits until time t2 has elapsed. In accordance with embodiments of the present invention, time t2 is shorter than time t1. In this way, less calibration gas is used for the second calibration gas cycle than was used for the first calibration gas cycle. The configured time t2 is selected to significantly reduce the amount of calibration gas purged through the sample injection system, while also providing sufficient purge time to completely fill the sample loop with calibration gas before the injection routine. Next, a third calibration cycle is started at block 212.During the third cycle, the calibration gas is purged a third time, and the system waits for a time t3, as specified at Block 214, while the calibration gas is purged. The time t3 can be less than, equal to, or greater than the time t2. However, the time t3 is always less than the time t1. Furthermore, as specified above, the configured time t3 is selected to significantly reduce the amount of calibration gas purged through the sample system while also providing sufficient purge time to completely fill the sample loop with the calibration gas before the injection routine. Finally, Procedure 200 at Block 216 is complete, and the gas chromatograph is ready to analyze the next sample.

[0009] Fig. 3 is a calibration gas purge timeline representing the calibration gas purge in accordance with an embodiment of the present invention. As in Fig.Figure 3 shows that a sample stream analysis 300 is followed by three subsequent calibration gas cycles 302, 304, and 306. Each cycle has two identical initial steps. First, atmospheric referencing takes place, as shown at reference 308. Atmospheric referencing is then followed by sample injection, as shown at reference 310. Separation and analysis then take place, as shown at reference 312. At the last sample stream analysis before a calibration gas cycle, the system is purged with calibration gas. This is shown at blocks 314 and 316. Furthermore, section 318 is shown above section 320 to distinguish prior art techniques from embodiments of the present invention.Section 318 shows calibration gas purges in accordance with prior art techniques, while section 320 shows calibration gas purges in accordance with embodiments of the present invention. As shown, the initial calibration gas purge prior to calibration gas cycle 302 is essentially the same for embodiments of the present invention compared to prior art techniques. However, embodiments of the present invention differ from prior art techniques in the subsequent calibration gas purges. As shown, the second calibration gas purge 322, which takes place prior to the second calibration gas cycle 304, is essentially of the same duration as the calibration gas purge 314 for prior art techniques.However, in striking contrast, the subsequent calibration gas purge 323 for embodiments of the present invention takes place only over a time t2, which is considerably shorter than the time t1 for the initial calibration gas purge 316. Likewise, the third calibration gas purge 324 for prior art techniques is essentially the same as purges 314 and 322. However, embodiments of the present invention provide a third calibration gas purge 325 with a duration t3, which is also considerably shorter than time t1. After the third calibration gas cycle 306 is completed, the sample stream can be purged into the system to measure the next sample, as shown in blocks 326 and 328. Additional calibration cycles beyond the three described can also be configured, with these additional calibration cycles also using a shorter purge time than t1.

[0010] It is assumed that embodiments of the present invention significantly reduce the amount of calibration gas consumed by a gas chromatograph during a typical calibration sequence. A calibration sequence consists of three or more analysis cycles in which acquired calibration gas is purged with high accuracy through the system and sample loop of the analyzer, with a fixed volume of the gas being injected into the chromatography columns at the beginning of the analysis cycle. Embodiments of the present invention reduce the amount of calibration gas purged through the system without compromising sample integrity or measurement accuracy.

[0011] Therefore, embodiments of the present invention generally require less purge time and calibration gas volume to ensure that a consistent sample is injected into the columns. Furthermore, the reduction in calibration gas consumption is particularly advantageous for online or process gas chromatographs, which are typically not located in a laboratory but may be situated at an outdoor process site, such as a refining facility or on a long-distance transmission pipeline system. In such cases, the calibration gas is supplied in the form of a cylinder or container with a fixed volume. Thus, when the calibration gas is depleted, an operator or technician must physically travel to the gas chromatograph location and replace the calibration gas cylinder.This procedure can take up valuable time from the technician and may require the gas chromatograph to be taken offline during the change of the calibration gas.

Claims

[1] Online gas chromatograph, comprising: a sample inlet; at least one chromatography column operatively coupled to the sample inlet; at least one valve inserted between the sample inlet and the at least one chromatography column; a detector fluidly coupled to the at least one chromatography column; and a controller coupled to the detector and to the at least one valve, the controller configured to control flow from the sample inlet through the chromatograph using the at least one valve, the controller configured to generate a plurality of sequential calibration cycles, each calibration cycle including a calibration gas purge, and a first calibration gas purge lasting longer than a second calibration gas purge. [2] The online gas chromatograph of claim 1, wherein the plurality of sequential calibration cycles includes a third calibration cycle having a third calibration gas purge, and wherein the first calibration gas purge lasts longer than the third calibration gas purge. [3] The online gas chromatograph of claim 2, wherein the second calibration gas purge takes approximately the same amount of time as the third calibration gas purge. [4] The online gas chromatograph of claim 1, wherein the gas purged during the first and second calibration purge gas operations is identical. [5] Online gas chromatograph according to claim 4, wherein the calibration gas has a known composition. [6] An online gas chromatograph according to claim 5, wherein the calibration gas is an individually mixed gas having a certified composition. [7] The online gas chromatograph of claim 1, wherein the gas chromatograph is located outdoors. [8] The online gas chromatograph of claim 1, wherein the controller is configured to periodically generate the plurality of sequential calibration cycles. [9] A method for calibrating a gas chromatograph, the method comprising: Purging calibration gas through the gas chromatograph for an initial duration; Analyzing the calibration gas during a first calibration gas analysis cycle; after analyzing the calibration gas during the first calibration gas analysis cycle, purging calibration gas through the gas chromatograph for a second duration, the second duration being less than the first duration; and Analyzing the calibration gas during a second calibration cycle. [10] The method of claim 9, and further comprising: after analyzing the calibration gas during the second calibration gas analysis cycle, purging calibration gas through the gas chromatograph for a third duration, where the third duration is less than the first duration; and Analyzing the calibration gas during a third calibration cycle. [11] The method of claim 10, wherein the third duration is the same as the second duration. [12] The method of claim 10, and further comprising: After analyzing the calibration gas during the third calibration cycle, purging a sample to be analyzed through the gas chromatograph. [13] The method of claim 9, wherein the calibration gas has a known composition. [14] The method of claim 13, wherein the calibration gas is an individually mixed gas having a certified composition. [15] The method of claim 9, wherein the method is performed periodically. [16] The method of claim 15, wherein the period is daily.

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