System for control of a nitric acid plant

Claim 2

In a system for operating a chemical plant utilizing compressed process gas wherein the energy for compressing said gas is furnished at least in part by the augmented enthalpy of a waste gas derived from such chemical plant, said system including: gas compressor means driven by gas expansion turbine means, first conduit means for conducting compressed air to a chemical processing unit, second conduit means for conveying the discharged waste gas from said chemical processing unit to said gas expansion turbine means, and means in the path of said second conduit means in advance of said turbine to raise the temperature and augment the enthalpy of said tail gas; the improved arrangement for monitoring and controlling the flow rate of the compressed process gas supplied to said chemical processing unit, said arrangement comprising: a. a heater provided with an inlet for waste gas discharged from said chemical processing unit and a discharge conduit for heated effluent gas; b. a fuel gas supply line for introducing combustible fuel into said heater; c. an air supply line for introducing air for supporting combustion in said heater; d. flow control means in (b), e. flow control means in (c), f. a temperature sensing means in the discharge conduit of said heater; said sensing means being operatively connected to temperature control means responsive thereto; said temperature controls means being effective in manipulating the flow control means (d) to maintain a substantially constant temperature at (f); g. ratio control means operatively associated with said temperature control means of (f) and said flow control means (e), to maintain a preset constant ratio of air to fuel supplied to said heater; h. a steam supply line communicating with said heater for injection of steam therein; i. flow control means on said steam supply line; j. flow rate measuring means in the said first conduit conducting compressed air to said chemical processing unit; and k. means for signal communication between said flow measuring means (j) and said flow control means (i), whereby said control means (i) is responsive to variations in air flow rate measured at (j) to supply controlled quantities of steam to the heater to augment the enthalpy of the gas powering said turbine means, to the extent required to compensate deviation and to maintain the air flow rate to the chemical processing unit substantially constant.

Claim 3

A nitric acid plant involving oxidation of ammonia vapor by oxygen-containing gas at superatmospheric pressure; said plant having an ammonia oxidation convertor and an absorber for absorption of produced oxides of nitrogen in liquid aqueous media, said absorber being provided with conduit means for discharge of unabsorbed tail gas; said plant being further provided with compressor means for compressing said oxygen-containing gas for introduction into said converter, gas expansion turbines operatively connected to said compressor means for driving the latter and means for heating the tail gas discharged from said absorber; an inlet conduit for introduction of tail gas into said heater, a conduit for introduction of fuel gas into said heater, a conduit for introduction of combustion supporting gas into said heater and a steam line for injection of steam into said heater; a discharge line for conveying heated gas from said heater to a catalytic combustion unit downstream of said heater; said catalytic combustion unit being provided with a conduit for introduction of reducing gas thereinto for reduction of residual oxides of nitrogen to elemental gas, means for conducting hot gas from said catalytic combustion unit to said expansion turbines, and a control system comprising: a flow rate indicator measuring the rate of flow of oxygen-containing gas into said converter, a flow controller on said steam line responsive to deviations in gas flow rate from the value preset at said indicator; a temperature sensor in the line from said heater to said catalytic combustion unit, a temperature control device operatively connected to said temperature sensor; flow control means in said fuel introduction conduit responsive to said temperature control device adapted to maintain a substantially constant temperature in the gas stream monitored by said temperature sensing device; flow control means in said conduit supplying combustion supporting gas to said heater, said flow control means being responsive to a ratio control means to maintain a preset ratio of combustion supporting gas to fuel introduced into said heater; a flow measuring device on the inlet conduit conveying tail gas to said heater, and a flow control device in said conduit for introduction of reducing gas into said catalytic combustion unit, said last named flow control device being responsive to the measured tail gas flow rate entering said heater to maintain a preset ratio of reducing gas admitted to the catalytic combusion unit to the flow rate of tail gas entering said heater.

Claim 4

A nitric acid plant involving oxidation of ammonia vapor by oxygen-containing gas at superatmospheric pressure; said plant having an ammonia oxidation convertor and an absorber for absorption of produced oxides of nitrogen in liquid aqueous media, said absorber being provided with conduit means for discharge of unabsorbed tail gas; said plant being further provided with compressor means for compressing said oxygen-containing gas for introduction into said converter, gas expansion turbines operatively connected to said compressor means for driving the latter and means for heating the tail gas discharged from said absorber; an inlet conduit for introduction of tail gas into said heater, a conduit for introduction of fuel gas into said heater, a conduit for introduction of combustion supporting gas into said heater and a steam line for injection of steam into said heater; a discharge line for conveying heated gas from said heater to a catalytic combustion unit downstream of said heater; said catalytic combustion unit being provided with a conduit for introduction of reducing gas thereinto for reduction of residual oxides of nitrogen to elemental gas, means for conducting hot gas from said catalytic combustion unit to said expansion turbines, and a control system comprising: a flow rate indicator measuring the rate of flow of oxygen-containing gas into said converter, a flow controller on said steam line responsive to deviations in gas flow rate from the value preset at said indicator; a temperature sensor in the line from said heater to said catalytic combustion unit, a temperature control device operatively connected to said temperature sensor; flow control means in said fuel introduction conduit responsive to said temperature control device adapted to maintain a substantially constant temperature in the gas stream monitored by said temperature sensing device; flow control means in said conduit supplying combustion supporting gas to said heater, said flow control means being responsive to a ratio control means to maintain a preset ratio of combustion supporting gas to fuel introduced into said heater; a flow measuring device on the inlet conduit conveying tail gas to said heater, and a flow control device in said conduit for introduction of reducing gas into said catalytic combustion unit, said last named flow control device being responsive to the measured tail gas flow rate entering said heater to maintain a preset ratio of reducing gas admitted to the catalytic combusion unit to the flow rate of tail gas entering said heater.