IMPROVED USE OF THE RESIDUAL GAS FROM A PRESSURE SWING ADSORPTION PLANT

Abstract

The invention relates to a process for providing a fuel gas (4) which is generated at regeneration pressure as residual gas (3) during the regeneration of a pressure swing adsorption plant (D) used for fractionation of synthesis gas (1) and after an intermediate storage in a buffering vessel (P) is passed through a control valve (Z1) in order to be passed to a burner (B) with a controlled mass flow. The characterizing feature here is that by specification of a manipulated variable (8) determined by the load on the pressure swing adsorption plant (D) the control valve (Z1) is positioned at an operating point wherein the pressure in the buffering vessel (P) is in a defined range.

Claims

1. A process for providing a fuel gas (4) which during regeneration of a pressure swing adsorption plant (D) used for fractionation of synthesis gas (1) is obtained as residual gas (3) at regeneration pressure and after intermediate storage in a buffering vessel (P) is passed through a control valve (Z1) to be supplied to a burner (B) at a controlled mass flow, characterized in that the control valve (Z1) is positioned at an operating point by input of a manipulated variable (8) determined from the load on the pressure swing adsorption plant (D), wherein the pressure in the buffering vessel (P) is in a defined range.

2. The process as claimed in claim 1, characterized in that the manipulated variable (8) is input to the control valve (Z1) such that over the entire load range of the pressure swing adsorption plant (D) a pressure is established in the buffering vessel (P) whose temporal average is less than 300 mbar(g).

3. The process as claimed in claim 1, characterized in that the lower limit of the defined pressure range is between 50 and 150 mbar(g) and the upper limit is between 200 and 300 mbar(g).

4. The process as claimed in claim 1, characterized in that the control valve (Z1) is positioned at its operating point via a flow controller (FC) coupled to a position analysis controller (ZC), for which purpose by comparison of the actual position value for the control valve (Z1) with the load-dependent manipulated variable (8) the position analysis controller (ZC) determines a value which it inputs to the flow controller (FC) as a target value.

5. The process as claimed in claim 1, characterized in that the control valve (Z1) is positioned at its operating point via a flow controller (FC) coupled to a pressure controller (PC1), for which purpose by comparison of the pressure in the buffering vessel (P) with the load-dependent manipulated variable (8) the pressure controller (PC1) determines a target value which it inputs to the flow controller (FC).

6. The process as claimed in claim 1, characterized in that the control valve (Z1) is positioned at its operating point via a flow controller (FC) coupled to a pressure controller (PC1), for which purpose the pressure controller (PC1) obtains a target value calculated as a function of load.

7. The process as claimed in claim 1, characterized in that the burner (B) is used for firing a steam reformer (S).

8. The process as claimed in claim 1, characterized in that the pressure swing adsorption plant (D) is used for removal of hydrogen (2) from a synthesis gas obtained in a steam reformer (S).

9. The process as claimed in claim 8, characterized in that synthesis gas or a gas mixture (1) obtained by fractionation of synthesis gas is diverted upstream of the pressure swing adsorption plant (D) and passed directly into the buffering vessel (P) in bypass to said plant as soon as the pressure in the buffering vessel (P) falls below the lower limit of the defined pressure range.

Description

[0010] The recited object is achieved according to the invention when the control valve is positioned at an operating point by input of a manipulated variable determined from the load on the pressure swing adsorption plant, wherein the pressure in the buffering vessel is in a defined range.

[0011] An operating point is to be understood as meaning a position of the control valve in which the fuel gas flows from the buffering vessel to the burner at a mass flow corresponding to the load on the PSA and the pressure drop over the control valve which for control purposes is varied around the operating point is in a range which allows trouble-free execution of the control task.

[0012] To determine the manipulated variable for the control valve the load on the PSA is measured at time intervals normally in the seconds range and averaged over a plurality of consecutive measured values. Between two consecutive load determinations the manipulated variable remains unchanged independently of the actual load on the PSA. In order to be able to compensate short-duration pressure variations of the residual gas in the range of seconds, the control valve which is preferably in the form of a control flap and provided with remote operation and position feedback is advantageously controlled via a flow rate controller set with correspondingly fast control parameters.

[0013] To determine the load on the PSA the current residual gas amount may be determined and for example compared to the residual gas amount at nominal load. Since direct measurement of the residual gas amount is normally possible only with considerable errors, the current residual gas amount is advantageously not measured directly but rather calculated from the amount of synthesis gas arriving at the PSA and the known yield of the PSA. However, it is preferable to determine the PSA load by determining the amount of the synthesis gas arriving at the PSA and comparing it to the synthesis gas amount at nominal load.

[0014] The manipulated variable is preferably input to the control valve such that over the entire load range of the PSA a pressure is established in the buffering vessel whose temporal average is less than in the prior art, thus resulting in a reduction of the regeneration pressure of the PSA compared to the prior art. The temporal average value of the pressure is preferably between 100 and 250 mbar(g).

[0015] The correlation between the load on the PSA and the manipulated variable for the control valve is characteristic for the production plant of which the PSA forms part. Said correlation must be determined experimentally or by simulation and is preferably recorded as a curve or table, electronically or otherwise.

[0016] The size and position of the defined range in which the pressure in the buffering vessel may vary likewise depend on the characteristics of the production plant and the operating conditions thereof and are specific to the system. They are chosen such that stable plant operation is ensured as long as the pressure in the buffering vessel is in the defined range. Especially when the synthesis gas to be fractionated is generated in a burner-fired steam reformer which uses the residual gas for heating, the lower limit of the defined pressure range is between 50 and 150 mbar(g) and the upper limit is between 200 and 300 mbar(g).

[0017] The process according to the invention makes it possible to achieve hydraulic balance of the controlled system between the outlet of the buffering vessel and the opening into the burner over the entire load range on the PSA. The hydraulic balancing is preferably performed such that the maximum pressure drop over the control valve is less than 70%, and particularly preferably less than 50%, of the total pressure drop over the controlled system. Short pressure variations in the buffering vessel in the range of seconds, such as occur for instance when switching between the adsorbers of the PSA, may therefore be efficiently compensated even in the lower PSA load range for example via a flow controller acting on the control valve and operated with markedly faster control parameters than in the prior art. This has not hitherto been possible in a concept according to the prior art since the high pressure drop over the control valve causes severe disruption to the system even for small position changes especially when operated at low load.

[0018] At its particular operating point the control valve advantageously has sufficient distance to its end positions over the entire load range on the PSA. In order especially to ensure sufficient scope for interventions of a flow controller used for compensation of short-duration pressure variations in the buffering vessel, the control valve is preferably 70% to 90% open at its operating point at full load operation, wherein the pressure in the buffering vessel is about 30 to 50 mbar from the upper end of the defined range. During operation at minimal load the pressure in the buffering vessel is 30 to 50 mbar from the lower end of the defined range and the control valve is 20% to 40% open.

[0019] Provided it does not deviate from the defined pressure range, the pressure in the buffering vessel is not a responding variable. At least for an unchanged load on the PSA the control valve remains at its operating point under these conditions. Only when the pressure reaches the limits of the defined range do additional high pressure and low pressure controllers become active.

[0020] The proposed process may be realized in different ways. Preferably, the position of the control valve is altered via a flow controller coupled to a position analysis controller. The position analysis controller which is input with the operating point dependent on the load on the PSA and derived from the recorded curve or table as the manipulated variable compares said manipulated variable with the actual position value for the control valve and from the deviation of the two values determines a target value for the flow controller. If the operating point for the control valve is smaller than the actual position value, i.e. the control valve is opened further than required, the currently applicable target value for the full controller is reduced so that the control valve moves in the closing direction. If, by contrast, the position analysis reveals that the control valve is currently in an excessively closed position, the flow controller is input with a higher target value, thus causing the control valve to be opened further. The flow controller is also used to compensate short-duration pressure variations in the buffering vessel, for which purpose it is set with markedly faster control parameters than the position analysis controller.

[0021] Another option is that of dispensing with the position analysis controller and instead controlling the flow controller via a pressure controller which monitors the pressure in the buffering vessel and which is input with its target value from the recorded curve or table according to the current load on the PSA as a manipulated variable. The target value for the pressure controller may also be determined via a load-dependent calculation which uses for example the desired pressure drop over the control valve as an input.

[0022] In order that the pressure in the buffering vessel can be kept in a limited range under any operating condition of the plant, especially in exceptional operational cases and in cases of disruption, the use of a high pressure controller and a low pressure controller is proposed.

[0023] If the pressure in the buffering vessel exceeds the upper limit of the defined pressure range, the high pressure controller opens a conduit through which residual gas may be discharged from the buffering vessel. The high pressure controller keeps the conduit open until the pressure in the buffering vessel has once again fallen below the upper limit of the defined pressure range. It is preferable when the conduit is a connection conduit to a flare in which the residual gas discharged from the buffering vessel is disposed of by incineration.

[0024] Especially when the PSA is under partial load the buffering vessel is operated at a pressure only slightly above atmospheric pressure and a correspondingly reduced storage efficiency. To ensure that the buffering vessel may be advantageously utilized as a storage means under any operating condition it is therefore provided that a low pressure controller opens a conduit by means of which a flammable gas is introduced into the buffering vessel as soon as the pressure of the residual gas falls below the lower limit of the defined pressure range. The low pressure controller keeps the conduit open until the pressure in the buffering vessel once again exceeds the lower limit of the defined pressure range. This conduit is preferably a bypass conduit by means of which synthesis gas or a gas mixture obtained by fractionation of synthesis gas, for example crude hydrogen, is diverted upstream of the PSA and introduced into the buffering vessel in bypass to said PSA. The direct supply of synthesis gas/crude hydrogen into the buffering vessel makes it possible to utilize the entirety of the residual gas present in the buffering vessel in case of disruption in the PSA and consequent interruption of the residual gas supply. This has the result that compared to the prior art a significantly longer time is available for provision of a substitute gas from an external fuel gas source.

[0025] The invention shall be more particularly elucidated hereinbelow with reference to an exemplary embodiment illustrated schematically in FIG. 2.

[0026] FIG. 2 shows a production plant for hydrogen having a burner-fired steam reformer for generating synthesis gas and a pressure swing adsorption plant whose residual gas is used for heating the steam reformer according to a preferred variant of the invention. Identical plant parts and material streams as in FIG. 1 are provided with identical reference numerals.

[0027] From the steam generator A fitted with a burner-fired steam reformer S, the crude hydrogen 1 separated from a synthesis gas is passed to a pressure swing adsorption plant D to obtain pure hydrogen 2 and a residual gas 3 which is intermediately stored in buffering vessel P and subsequently supplied to the burners B of the steam reformer S as fuel gas 4.

[0028] To control the fuel gas stream 4 the position of the control valve Z1 is in normal operation of the plant altered via the flow controller FC which is coupled to a position analysis controller ZC. To achieve a higher precision the actual value 7 for the fuel gas flow may be corrected with the current fuel gas density 10 which is determined using the density analyzer Q1. The position analysis controller ZC which is input with the operating point for the control valve Z1 which is dependent on the load on the pressure swing adsorption plant D and derived from a recorded curve or table as the manipulated variable 8 compares said manipulated variable with the actual position value for the control valve Z1 and from the deviation of the two values determines a target value 9 for the flow controller FC. If the operating point for the control valve Z1 is smaller than the actual position value, i.e. the control valve Z1 is opened further than required, the currently applicable target value for the flow controller FC is reduced so that the control valve Z1 moves in the closing direction. If, by contrast, the position analysis reveals that the control valve Z1 is currently in an excessively closed position, the flow controller FC is input with a higher target value, thus causing the control valve Z1 to be opened further. The flow controller FC is set with fast control parameters so that it is capable of compensating flow variations of the fuel gas 4 caused by short-duration pressure variations in the buffering vessel P. In normal operation the pressure in the buffering vessel P is not a responding variable and may vary freely in a defined range which preferably extends between 100 and 250 mbar(g).

[0029] In order to keep the pressure in the buffering vessel P in the defined range in any operating condition, especially in exceptional cases and in case of disruption, the plant comprises a high pressure controller PC2 and a low pressure controller PC3.

[0030] If the pressure in the buffering vessel P exceeds the upper limit of the defined pressure range, the high pressure controller PC2 opens the shutoff element Z2 so that residual gas can flow out of the buffering vessel P via the flare conduit 5 to a flare (not shown) where it is disposed of by incineration. The high pressure controller PC2 keeps the flare conduit 5 open until the pressure in the buffering vessel P has once again fallen below the upper limit of the defined pressure range.

[0031] If the pressure in the buffering vessel P falls below the lower limit of the defined pressure range, the low pressure controller PC3 opens the shutoff element Z3 so that crude hydrogen 1 is introduced directly into the buffering vessel P via the conduit 6 in bypass to the pressure swing adsorption plant D. The low pressure controller PC3 keeps the conduit 6 open until the pressure in the buffering vessel P once again exceeds the lower limit of the defined range or a substitute gas for the residual gas 3 is provided from an external source.