FR2838425A1 - Control of partial oxidation of gas flows containing hydrogen sulfide, uses a Claus furnace with structured injections of air and/or oxygen according to variations in the gas flow characteristics - Google Patents

Abstract

To control the partial oxidation of a gas flow (1, 2) containing hydrogen sulfide, a gas with an oxygen content (3, 3', 4) is fed into a Claus furnace with a maximum processing capacity for a given maximum hydrogen sulfide output. The Claus furnace is formed by at least six concentric tubes (T1-T6), forming six spaces for the injection of the gas, air and oxygen. To control the partial oxidation of a gas flow (1,2) containing hydrogen sulfide, a gas with an oxygen content (3, 3', 4) is fed into a Claus furnace with a maximum processing capacity for a given maximum hydrogen sulfide output. The gas flow containing hydrogen sulfide is measured. If the measured gas flow is below the maximum furnace output, air (3, 3') is injected into the furnace as a linear function of the gas flow. If the measured flow is above the maximum furnace output, oxygen (4) is injected into the furnace at a rate which is proportional to the gas flow minus the maximum output. Air injection is continued, reduced by the amount of air represented in the oxygen inflow. The Claus furnace is formed by at least six concentric tubes (T1-T6), forming six spaces for the injection of the gas, the air and the oxygen.

Description

The present invention relates to a process for controlling oxidation.

  part of a gas comprising hydrogen sulfide using air and oxygen in a Claus furnace by managing the introduction and the flow rates of the various gas flows in the

Claus burner.

  Gas streams rich in hydrogen sulfide are wastes produced by many industries, including the petroleum refining and natural gas production industry. For reasons linked in particular to the environment, these gases rich in hydrogen sulphide cannot be released as such into the atmosphere. It is therefore necessary to treat them in order to significantly reduce their hydrogen sulfide content. A well known process for treating these gases rich in hydrogen sulfide is the modified Claus process commonly known as Claus. This process includes a thermal part and a catalytic part. In the thermal part, two main reactions are carried out. The first reaction consists in reacting part of the hydrogen sulfide with oxygen to produce water and sulfur dioxide in the following way: H2S + 3/2 O2 H2O + SO2 (i) By this first reaction, about 113 of the hydrogen sulfide to be treated is oxidized. The remaining 2/3 are reacted with the sulfur dioxide formed during the first step above, according to the following reaction, known as Claus resection: 2 H2S SO2 3/2 S2 + 2 H2O (ii) The products of combustion are then cooled in a heat recovery boiler, then it is supplied in the form of which the elemental sulfur is recovered in liquid form. The gases are then heated to a temperature allowing their treatment on one or more catalytic beds (each of these beds being followed by a condenser). On the catalytic beds, the Claus reaction continues until a conversion rate into hydrogen sulphide is obtained which is compatible with the standards for rejection into sulfur dioxide coming from the final stage of the process which is the incineration of residual H2S. In the case where two or three catalytic beds do not allow to reach the sulfur dioxide discharge standards, a tail gas treatment unit can be added before sending

  waste gases at the final incinerator.

  The gas streams rich in hydrogen sulfide treated in a refinery can sometimes contain amrrroniac in addition to hydrogen sulfide. This is the case, for example, of the waste gases from the acid water strippers in which the co ndensates of the processes (for example, hydrocaking or catalytic cracking stage in particular for heavy hydrodesulfurization charges) are steam stripped to recover hydrogen sulfide and ammonia. These gases are typically composed of a third of hydrogen sulfide, a third of ammonia and a third of water vapor. We can therefore treat different types of gases rich in hydrogen culture: those which are poor in ammonia, called "acid" gases, and those which are rich in ammonia, called "ammonia" gases, The evolution of the qualities of fuels fuels towards a lower sulfur content imposes a load of sulfur gases often more and more important on Ciaus units. These often reach their operating limits due to hydraulic limitation. The pressure drop in the unit becomes too great with regard to the additional quantities of gas to be treated. An alternative to investing in additional treatment capacity is enriching the combustion air with oxygen. This makes it possible to reduce the quantity of inert gas contained in this combustion air and to reduce the hydraulic load of the unit. This oxygen enrichment can be carried out by premixing pure oxygen with the combustion air upstream from the furnace of the Claus unit. This solution is very inexpensive in terms of investment. It is, however, limited by security questions linked to the compatibility of the pipes and burners initially intended for the supply of air. It is therefore customary to limit the oxygen enrichment of the combustion air to a value of 28% by volume. Above this value, the solution using the direct introduction of pure oxygen into the combustion chamber must be used. There are burners allowing this type of injection of pure oxygen and ensuring the treatment of acid and ammonia gases in air and pure oxygen. Usually, when these burners operate with an air flow, the process is controlled as follows: - the flow of acid and ammonia gases is measured. The signal from this measurement is converted into an air request which controls a set point on a first air flow, called "main flow" (corresponding to approximately 90% of the air

total entering the oven).

  - there is also a control on the tail gas by measuring the proportion of the molecules of H2S and SO2 because the ratio H2SISO2 must be maintained at 2 in order to ensure the smooth running of the unit according to the st_chiometric conditions of the reaction (ii). The signal from the measurement of this ratio is converted into a correction of the air demand. This correction is made by controlling the flow of combustion air called "secondary" (corresponding to approximately 10% of the total incoming air

in the oven).

  The document US Pat. No. 5,266,274 describes an oxidation process control when the burner operates with air and oxygen flows: the oxygen injection is triggered as a function of the value of the pressure measured in the combustion chamber. . This type of query only allows feedback control: it does not allow an increase in gas load to be anticipated

  1Q comprising hydrogen sulfide.

  In the case of an oxygen injection and a burner with separate acid and ammonia gas flow supplies, the management of the air and oxygen demand must be adapted. In addition, when oxygen is injected, this injection should only take place when it is necessary, that is to say when the unit reaches a maximum production limit. It is therefore customary to trigger the oxygen injection only when a precalibrated threshold value has been exceeded. Gette threshold value

  can be, for example, the flow of sulfur gases sent to the Claus unit.

  However, the sulfur gases treated by the Glaus units often come from refinery units located upstream. Consequently, the quantities of sulfur gases entering Claus units depend directly on the operating conditions of the upstream units: it is very common to have a flow of sulfur gases fluctuating around an average value. In the case where this average value corresponds to the oxygen triggering threshold, this results in an undesired fluctuation of the oxygen control system: the oxygen opening valve opening and closing unexpectedly. This can lead to difficulties in terms of operation (alarms on untimely low oxygen flow and automatic shutdown of the oxygen valves, necessary resetting of the oxygen regulation equipment, etc.) An object of the present invention is therefore to propose a method for controlling the partial oxidation of a gas comprising hydrogen sulfide using a gas

  Q including oxygen in a Claus oven.

  Another aim is to propose a process for controlling the partial oxidation of a gas comprising hydrogen sulphide using a gas comprising oxygen in a Claus oven whose burner allows injection of pure oxygen and air

  and ensures the treatment of acid and ammonia gases.

  Another object is to propose a process for controlling the partial oxidation of a gas comprising hydrogen sulfide using a gas comprising

  oxygen in a Claus oven operating at capacity limit.

  To this end, the invention relates to a process for the partial oxidation of at least one gas stream comprising hydrogen sulfide using at least one gas comprising oxygen in a Claus furnace having a capacity maximum treatment of a flow rate of hydrogen sulfide QMAX in which: - the flow rate of the gas flow comprising hydrogen sulfide QAG is measured, - S; QAG is lower than the maximum furnace treatment flow rate (QMAX), then air is injected into the oven via the Glaus burner, the flow rate of this air flow (QAIR) being a linear function of QAG, - If QAG is greater than the maximum oven treatment rate (QMAX), then: oxygen is injected into the oven via the Claus burner, the flow rate of this flow of oxypene (QO2) being proportional to (QAG - QMAX), and 15. the air is continued to be injected into the furnace, the air flow rate (QAIR) being reduced by the equivalent air flow introduced by oxygen (QO2 eqAR) Other characteristics and advantages of the invention will appear on reading

  of the following description Of the forms and embodiments of

  the invention are given by way of nonlimiting examples, illustrated by the accompanying drawings in which: - Figure 1 is a schematic and partial section of the end of a burner, - Figure 2 is a graph representing the flow of oxygenated gas inection

  as a function of the gas flow rate comprising hydrogen sulfide.

  The invention therefore relates to a process for partial oxidation of at least one gas stream comprising hydrogen sulfide using at least one gas comprising oxygen in a Claus furnace having a maximum treatment capacity a flow rate of hydrogen sulfide QMAX in which: - the flow rate of the gas flow comprising hydrogen sulfide QAG, - S, is measured; QAG is less than the maximum furnace treatment flow rate (QMAX), then air is injected into the oven via the Claus burner, the flow rate of this air flow (QA) being a linear function of QAG, - S; QAG is greater than the maximum furnace treatment flow rate (Q4x), then: oxygen is injected into the oven via the Claus burner, the flow rate of this flow of oxypene (QO2) being proportional to (QAG - QMAX), and the air is continued to be injected into the oven, the air flow rate (QA [R) being reduced by

  equivalent air flow introduced by oxygen (QO2 eqA, R).

  Generally, the maximum furnace treatment flow (QMAX) is calculated when sizing the Claus unit. The flow rates of the various flows comprising hydrogen sulfide are usually measured using installed flowmeters

  upstream of the flow control valves. The proportionality coefficient is.

  similarly to the maximum furnace treatment flow (QMAX), calculated during the dimensioning phases of the Claus unit from the average compositions of the gases which must be treated there. The ratio of the flow rate of the air flow to that of the gases is also generally calculated during the initial design phase of the Claus unit. The ratio of the flow rate of the oxygen flow to the flow rate of the gas comprising hydrogen sulfide and ammonia can be calculated during the phase of

  dimensioning of the renovation of the Claus unit.

  The equivalent air flow introduced by oxygen (QO2eqA, R) is defined by the

  following relationship: QO2-eqAIR = QO2 / 0.23.

  According to a preferred mode, the start and stop of the oxygen injection do not take place at the same set values. According to the invention, it is therefore preferable to implement a hysteresis cycle for the start and stop of the oxygen injection. Thus, preferably: - when QAG is an increasing function of time and: if QMAX + A 2 QAG 2 QMAX, then we inject oxygen into the furnace with a fixed flow of QO21 if QAG> QMAX + A, then we inject the oxygen in the furnace with a flow proportional to (QAG - QMAX), - when QAG is a decreasing function of time and: if QAG QMAX + A, then oxygen is injected into the furnace with a flow proportional to (QAG - QMAX), if QMAX + A 2 QAG 2 QMAX - B. then oxygen is injected into the oven with a fixed flow rate of QO21

  30. if QAG <QMAX- B. then the oxygen injection is stopped.

  According to a particular mode, the ratio (A + B) IQMAX can be 10%. As for QO211, it is generally fixed as a function of the technical minimum of the oxygen injection valve, which generally represents 10 to 30% of the nominal flow rate of the valve. According to a variant of the invention, an additional flow of oxygen of fixed flow rate QO2-O can be permanently injected into the furnace. This variant may correspond in particular to the case where the gas comprising hydrogen sulphide also comprises ammonia. Generally, the hydrogen sulfide concentration in this ammonia gas is between 10 and 90 mol% and the ammonia concentration is generally greater than QU equal to 5 mol% and preferably between 10 and 60 mol%. This gas can also

  include 10 to 60 mol% of water vapor.

  The process according to the invention is particularly suitable when using a gas burner for a Claus oven composed of at least six concentric tubes (T1 T6), forming six concentric spaces for the introduction of gas, the first tube (T1) being the tube of smaller diameter and the fifth tube (T6) being that of larger diameter, and in which: A gas comprising hydrogen sulphide is injected into the space formed by the first and second tubes (T1, T2) and in the space formed by the fifth and sixth tubes (T5, T6), À we inject oxygen into the space formed by the second and third tubes

(T2, T3),

  À we inject air into the space formed by the third and fourth tubes (T3,

  T4) and in the space formed by fourth and fifth tubes (T4, T5).

  For this type of burner, the diameters of the five tubes and the diameters of the orifices of the crowns (injectors) are generally defined as a function of the speeds in the tubes and the gear ratios at the nose of the burner that it is desired to give to each of these gases injected into the burner. The speeds in the tubes depend on the average flow rates of the flows entering the burner. The flow rates of oxygen-rich gases are a direct result of their oxygen concentration, the flow rates of acid gas flows and the average NH3 ammonia gas content. The average flow rates are imposed by the refinery in which the process of the invention is implemented, itself limited by the processing possibilities of the Claus unit (dimensions of the Claus furnace and characteristics of the heat exchanger located at the outlet of the Claus oven). Depending on these parameters, the skilled person is perfectly capable of determining the diameters of the tubes and the diameters of the orifices so that the speeds and ratios of

desired speeds are obtained.

  Generally, the diameters of the orifices of the crowns must be such that: - the ratio of the speed of the ammoniated g on the speed of the oxygen-rich gas is between 0.1 and 0? 8 or between 1.2 and 5, the speeds being taken at the end of the burner on the furnace side, and - the ratio of the speed of the acid gas to the speed of the gas less rich in oxygen is between 0.1 and 0.8 or between 1.2 and 5, the speeds being taken at

  the burner end on the furnace side.

  During the implementation of the process according to the invention, it is possible to treat two gases comprising hydrogen sulphide having different compositions. Thus the process according to the invention can implement: 15. a gas comprising hydrogen sulfide and ammonisc, said gas being injected into the space formed by the first and second tubes (T1, T2) of the burner previously defined, and a gas comprising hydrogen sulphide and no ammonia, said gas being injected into the space formed by the fifth and sixth tubes (T5, T6)

  2Q of the previously defined burner.

  The ammonia content of the gas comprising hydrogen culture and no ammonia, called acid g, is generally less than 5 mol% (<5%). The concentration of hydrogen sulfide in this gas flow can be at least mol%, and is more generally between 60 and 95 mol%. The flow of acid gas consists essentially of hydrogen sulfide and at least one of the following components: water vapor, carbon dioxide, hydrocarbons and other sulfur compounds. The ammonia and hydrogen sulfide content of the gas taking hydrogen sulfide and ammo niac correspo nd to the valu rs

indicated above.

  The control of the oxidation process during the iniection of these two gases can then be carried out as follows: When the sum of the flow rates of the gases comprising hydrogen sulfide (QAG3 is less than QMAX, then: the flow rate of the air flow injected into the space formed by the fourth and fifth tubes (T4, T5) (QAIR-) is proportional to the flow rate of the gas comprising hydrogen sulphide and no ammonia, and the flow rate d air injected into the space formed by third and fourth tubes (T3, T4) (QAIR-2) is a linear function of the flow rate of the gas comprising hydrogen sulfide and ammonia, and

Ie flow of oxygen (QO2) is zero.

  When the flow rate of the gas comprising hydrogen sulfide and no ammonia (QAG-) is greater than the maximum acceptable flow rate in the tube T6 of the gas comprising hydrogen sulfide and no ammonia (QAG - MAX) . then: part of said gas is mixed with gas comprising hydrogen sulfide and ammoniaG and is injected into the space formed by the first and second tubes (T1, T2) with a flow rate of (QAG-1 - QAG --MAX), the flow rate of the air flow injected into the space formed by third and fourth tubes (T3, T4) (QAIR-2) is the sum of two air flow rates, one being proportional to the flow rate of the gas comprising hydrogen sulfide and ammonia (QAG-2) and the other being proportional to (QAG-1- QAG - MAX) A When the sum of the flow rates of the gases comprising hydrogen sulfide (QAG = QAG-1 + QAG-2) is greater than QMAX, then: 2Q. part of the gas comprising hydrogen sulfide and no ammonia (QAG-) is mixed with the gas comprising hydrogen sulfide and ammonia is injected into the space formed by the first and second

  tubes (T1, T2) with a flow rate of (QAG-1 - QAG - MAX).

  The flow of oxygen is proportional to (QAG - MAXJ, 25. The flow of the air flow injected into the space formed by the fourth and fifth tubes (T4, T5) (QA 1) is proportional to QAG - MAX , The flow rate of the air flow injected into the space formed by third and fourth tubes (T3, T4) (QAIR-2) is the sum of two air flow rates, one being proportional to the flow it gas comprising hydrogen and lammammniaC suffu re (QAG2) and the other being proportional to (QAG-1 - QAG MAX), said sum being reduced by the equivalent air flow introduced by oxygen ( QO2 eqA). The value of the maximum acceptable flow rate in the tube T6 of the gas comprising hydrogen sulphide and no ammonia (QAG - MAX) is generally set at half.

  from QMAX. maximum capacity for processing the hydrogen sulfide flow from the unit.

  FIG. 1 illustrates a burner allowing the injection of pure oxygen and the place of introduction of the various gas flows into this burner. The burner consists of

  6 concentric tubes (T1, T2, T3, T4, T5, T6) forming 6 concentric spaces.

  In the first tube (T1) is generally placed a pilot burner for lighting the flame. A combustion gas can also be introduced into this space

  refinery to light and maintain the flame.

  The gas comprising hydrogen sulfide and ammonia (2) is introduced into the burner in the space formed by the first and second tubes (T1, T2), - the oxygen-rich gas (4) in the space formed by the second and third tubes (T2, T3), - the air (3, 3 ') in the space formed by the third and the fourth tubes (T3, T4) and in the space formed by the fourth and the fifth tubes (T4, T5), - the acid gas includes hydrogen culture and no ammonia () in

  The space formed by the fourth and fifth tubes (T5, T6).

  The graph in FIG. 2 represents the rate of infection of oxygenated gases in

  function of the gas flow rate comprising hydrogen suHide.

Claims (8)

  1. Method for partial oxidation of at least one gas stream comprising hydrogen sulfide (1, 2) using at least one gas comprising oxygen (3, 3 ', 4) - in a Claus furnace having a maximum capacity for treating a flow rate of hydrogen suHide QMAX in which: - the flow rate of the gas flow comprising hydrogen sulfide QAG is measured, - if QAG is less than the maximum treatment flow rate from the oven (QMAX), then air (3, 3 ') is injected into the oven, the flow rate of this air flow (QAIR) being a linear function of QAG] - if QAG is greater than the maximum flow rate of treatment of the furnace (QM, V (), then: oxygen is injected into the furnace (4), the flow rate of this oxygen flow (QO2) being proportional to (QAG - QMAX), and we continue to inject in the air furnace, the flow rate of the air flow (QAIR) being reduced by the equivalent flow rate of air introduced by the oxygen (QQ2-eqA [R) 2. Method according to claim 1, characterized in that: - when QAG is u increasing function of time and: if QMAX + A> QAG> QMAX, then inject oxygen into the furnace with a fixed flow of Qa2, if QAG> QMAX + A, then inject oxygen into the furnace flow proportional to (QAG - QMAX), - when QAG is a decreasing function of time and: if QAG> QMAX + A, then oxygen is injected into the furnace with a flow proportional to (QAG - QMAX), À if QMAX + A> QAG> QMAX - B. then oxygen is injected into the oven with a fixed flow rate of QO21!   30. S; QAG QMAX- B. then we stop the oxygen injection.   3. Method according to claim 1 or 2, characterized in that an additional flow   of fixed flow oxyrene QO2 o is continuously injected into the oven.   4. Method according to one of the preceding claims, characterized in that   uses a Claus oven gas burner composed of at least six concentric tubes (T1-T6), forming six concentric spaces for the introduction of gas, the first tube (T1) being the smaller diameter tube and the fifth tube (T6) being that of larger diameter, and in which: A gas comprising hydrogen sulphide (1, 2) is injected into the space formed by the first and second tubes (T1, T2) and into the space formed by the fifth and sixth tubes (T5, T6), À we inject oxygen (4) into the space formed by the second and third tubes (T2, T3),   At the air (3, 3 ') is injected into the space formed by the third and fourth tubes   (T3, T4) and in the space formed by fourth and fifth tubes (T4, T5).   5. Method one of the preceding claims, characterized in that it sets   works two gases comprising hydrogen sulfide having dlfférentes compositions. 6. Method according to claim 5, characterized in that it uses: a gas comprising hydrogen sulphide and ammonia and in that this gas is injected into the space formed by the first and second tubes (T1, T2) and a gas co mp renant of hydrogen sulfu re and not of moniac and in that this gas   is injected into the space formed by the fifth and sixth tubes (T5, T6).   7. Method according to claim 6, characterized in that when the sum of the gas flow rates comprising hydrogen sulfide (QAG) is less than QMAX, then: Ie flow rate of the air flow injected into the space formed by the fourth and fifth tubes (T4, T5) (QAIR-) is proportional to the flow rate of the gas comprising hydrogen sulphide and no ammonia, and 30. The flow rate of the air stream injected into the space formed by third and fourth tubes (T3, T4) (QA [R-2) is a linear function of the flow rate of the gas comprising hydrogen sulfide and ammonia, and Ie flow of oxygen (QO2) is zero.   8. Method according to claim 6, characterized in that when the flow rate of the gas comprising hydrogen sulfide and no ammonia (QAG-) is greater than the maximum acceptable flow rate in the tube T6 of the gas comprising hydrogen sulfide and no ammonia (QAG-fM, V;), then: 5. a part of said gas is mixed with the gas comprising hydrogen sulfide and ammonia and is injected into the space formed by the first and second tubes (T1, T2) with a flow rate of (QAG-1 - QAG - MAX), the flow rate of the air flow injected into the space formed by third and fourth tubes (T3, T4) (QAIR-2) is the sum of two air flow rates, one being proportional to the rate of the gas comprising hydrogen sulfide and ammonia (QAG-2) and the other being proportional to (QAG-1 - QAG-f-MAX) 9. Method according to claim 6, characterized in that when the sum of the gas flow rates comprising hydrogen sulfide (QAG = QAG-1 + QAG-2) is greater than QMAX, al ors: part of the gas comprising hydrogen sulphide and no ammonia (QAG-) is mixed with the gas comprising hydrogen sulphide and ammonia is injected into the space formed by the first and second tubes ( T1, T2) with a flow of (QAG-1 - QAG - MAX),   20. the oxygen flow rate is proportional to (QAG - QMAX), the flow rate of the air flow injected into the space formed by the fourth and fifth tubes (T4, T5) (RA-) is proportional to QAG-- MAX, the flow rate of the air flow injected into the space formed by third and fourth tubes (T3, T4) (QAIR-2) is the sum of two air flow rates, one being proportional to the flow rate of the gas comprising hydrogen sulfide and ammonia (QAG2) and the other being proportional to (QAG-1 - QAG - MAX). said sum being reduced by the equivalent air flow introduced by oxygen (QQ2-eqAIR)