Acid Gas

The usual name for SRU feed gas because it contains H₂S and C0₂, as the main components.

Air Demand/Excess H2S

These two terms are essentially the same, the percentage change in the current process air flow rate required to obtain the correct ratio of reactants in the tail gas (IE, 2 parts H₂S to 1 part SO₂).

BTX

Benzene/Toluene/Xylene, three similar types of aromatic (benzene like) organic compounds that can be absorbed by the amine, present in the acid gas and difficult to combust in the reaction furnace.

Catalytic Reaction

A reaction that is has a promoted rate of reaction without the catalyst being consumed or undergoing chemical change. For the Claus SRU the catalyst is activated alumina, made under several trade names and proprietary forms.

Catalytic Stage

A linear group of process vessels comprised of a reheater, catalytic converter and sulfur condenser.

Coking

Formation of carbon in the tubing or glass tube of a total sulphur hydrogenation furnace. It is the result of the incomplete reaction of a hydrocarbon with hydrogen.

CSA

Canadian Standards Association.

Demister

A device designed to remove entrained sulfur mist from sample gas. Demister is registered trademark of the Koch-Otto York company.

Dry Gas Analysis

A method of stack gas analysis where the sample gas is dried before analysis and measured on a dry basis. The opposite of this is hot/wet analysis where the sample is measured as is, IE with the water component included.

Efficiency (conversion)

Based on the conversion of the reactants (H₂S and SO₂) to sulfur vapor in the catalytic converter.

Efficiency (recovery)

Based on the recovery of the converted sulfur vapor in the sulfur condenser. The significant difference between these two measurements is the loss of sulfur vapor and sulfur liquid from the condenser outlet.

Equilibrium

A condition where a reaction and its opposite or reverse reaction occurs at the same rate resulting in a constant concentration of both the reactants and the products.

Exothermic

A reaction where energy in the form of heat is produced.

F Factor

f factors are used for calculating flue gas emissions (eg pounds per million BTUs) based on a concentration measurement of the pollutant and several other inputs (BTU value of the fuel, fuel flow rate etc). This method was developed for power and combustion sources but it is not practical to utilize it for SRUs and other chemical type processes where mass flow rate of the stack gas must be measured.

F M

Factory Mutual.

Feed Forward Control

Control of a process based on the measurement of process variables before the reaction. In the case of the Claus SRU, this is the measurement of flow, temperature, pressure and possibly composition of the acid gas/gases.

Feedback Control

Control of a process where a measurement is made of the variable to be controlled and this is compared with a reference (set) point. In the case of the Claus SRU this is measurement of the tail gas composition. It is an important objective of feedback control to match the time response of the control system to the process.

FID

Flame Ionisation Detector.

FPD

Flame Photometric Detector.

Hydrogenation

Reaction where hydrogen displaces another molecule in a compound.

Hydrolysis

The reaction of a compound with H₂0 to produce one or more new compounds. In the case of the Claus SRU this is the desired reaction of converting COS and CS₂ back to H₂S in the presence of the water vapor that is formed from the Claus reaction itself.

Kinetic Limitation

Chemical reactions that do not reach thermodynamic equilibrium due to various inhibiting factors (residence, time, mixing etc).

Lean Feed Acid Gas

Acid gas that is less than 50% H₂S, the balance normally being C0₂. This type of acid gas is found mostly in gas plant SRU where the C0₂ content of the raw gas is high and selective treating is not used.

Mercaptans

Other reduced sulfur compounds such as methyl mercapton ethyl mercapton and di-methyl di-sulfide.

NEMA

National Electric Manufacturers Association.

NEMA 12

Enclosures intended for indoors to provide a degree of protection from dust, falling dirt and non-corrosive dripping liquids.

NEMA 4

Enclosures intended for indoors or outdoors to provide a degree of protection from wind blown dust and rain, and hose directed water.

NEMA 4X

Enclosures intended for indoors or outdoors to provide a degree of protection from corrosion, wind blown dust and rain, and hose directed water

NEMA 7

Enclosures designed for explosion proof areas.

Odorant

Normally a mixture of methylethyl sulphide (MES) and tertiary butyl mercaptan (t-BuSH) added to natural gas to warn users of its presence.

Paramagnetic

O₂ detection: An analytical method where the polar properties of the O₂ molecule are exploited in order to measure the O₂ concentration. This method is used where there are other combustibles in the sample gas and a bulk O₂ method such as zirconia would not be accurate.

Permeable Membranes

When used in the context of sulfur measurement, these devices will pass a constant rate of a pure sulphur compound based on concentration on either side of a membrane. These devices are useful as dilutors for measuring high concentration of H₂S.

PFPD

Pulsed flame photometric detector.

Pluming (stack)

A condition where the stack gas is visible (fluffy and white) due to the off ratio conditions, incomplete combustion in the incinerator resulting in the presence of SO₂ in the stack gas.

Rich Feed Acid Gas

Acid gas that is more than 50% H₂S, the balance normally being CO₂, water vapor and some hydrocarbon (1-2%). Refinery acid gas from an amine source are normally rich feed, in the order of 95% H₂S.

Selective Absorption

In an amine absorber, acid gas is selectively enriched in the presence amine, which absorbs the H₂S and rejects most of the CO₂.

Sour Water Stripper (SWS) Gas

A SRU feed gas found in refineries that is composed of approximately 1/3 each H₂S /ammonia/water vapor plus a trace of heavy hydrocarbons. It is a special type of lean acid gas that must be handled in a special way in order to ensure complete combustion of the ammonia and hydrocarbons.

Stoichiometry

A condition where there is the perfect amount of reactants to form the desired reaction. In the case of stack gas combustion this would be zero excess O₂ (not always desirable) and in the case Claus tail gas 2 parts H₂S to 1 part SO₂ (normally the desired condition).

Sulfur Dew Point

The highest temperature at which the process stream is saturated with sulfur vapor.

Thermodynamics

The energy changes associated with chemical reactions. In practical application a reaction that reaches thermodynamic equilibrium has not been inhibited by kinetic limitations and has achieved maximum yield.

Total Sulfur

The sum of all sulphur compounds in a stream expressed normally in ppm of elemental sulphur by weight.

Trim Valve

A control method for SRUs where the feedback signal from the tail gas analyzer is used to control a separate (smaller) trim valve. Some SRU designers (most notably TPA) do use this method but utilize proprietary methods using the main valve only.

TRS

Total Reduced Sulfur. Common term used in the pulp and paper industry for the sulphur emitted from a Kraft process. TRS is the sum of H₂S, methyl mercaptan, ethyl mercaptan and dimethyl disulfide.

UL

Underwriters Laboratories Inc.

UL

Underwriters Laboratories Inc.

UV Light Bench

A term used to describe the complete UV photometer, source, cell, detector, electronics.