i. r TT-098 CONEXPO ARPEL’96

RECENT ADVANCE IN CLAUS PLANT TAIL GAS CLEAN-UP FOR AIR QUALITY IMPROVEMENT

. Daniel Benayoun SUMMARY: . Armando Koskas

Overall sulphur recovery yields of 99,8% can be attained using Clauspol II technology, which is a tail gas treatment process. H2S and S02 in Claus effluents are removed in a simple, low capital operating cost process that uses no hydrogen and low utilities. The continuous withdrawal of product sulphur explains the high efficiency of the process. The flexibility of Clauspol II technology allows to reach 99,9% and even higher overall conversion, by using IFP Booster 99.9 option.

ABSTRACT:

This paper outlines the sulphur emissions situation and presents the advantages of IFP Clauspol technology and the highlights of the new IFP process, Clauspol II. That includes the chemical principles, the process description, process controls' among with typical performance and investments. It finally concludes with description of the highly performant Booster 99.9 process which can reach 99.9% sulphur recovery and even more.

1. BACKGROUND

Petroleum, gas, petrochemical and coal processing produce acid gas streams that contain sulfur, usually as HgS, a noxious and extremely toxic material. Although the incineration or flaring of acid gases has been tolerated in some locations, the resulting sulfur dioxide emissions are unacceptable; it is better to recover the sulfur in its elemental form using a Claus Unit

Unfortunately, a Claus unit by itself is not enough today. A two-stage Claus unit removes only 96% of the sulfur content in a refinery tad gas whereas most emissions standards today require 99.5%.

In addition, sulfur contents in crude oils arc on the rise, not good news for refineries unable to cope with sour crudes, and those that can may have to process much more. Therefore, merely maintaining the environmental status quo implies increasing the loads to existing Claus plants, revamping them or constructing new units.

Another influence on global sulfur recovery capacity is the trend towards conversion of refinery heavy ends to middle distillates and gasoline. Where emission standards are more lenient on heavy fuels, they are relatively tight concerning gasoline and diesel fuel. The burden will fall on the refiner to increase HDS capacity to meet the existing standards. It follows that the additional H;S formed will be have to be treated also.

Institut Frangais du Petrole DISCLAIMER

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One can see there will be a trend for years aheat to add sulfur recovery facilities to gas treatment plants, refineries, coal processing and petrochemical plants. We expect that the sulfur recovery capacity in refineries will need to double in the next few years. So far, the Claus process has no major competitor for basic sulfur recovery, but its tail gas must be treated in a supplemental process. So what we are really talking about is Claus plus one: a Claus unit plus Tail Gas Treatment (TOT).

Now several processes are available; the choice depends on the degree of emissions permitted, the investment costs and operating costs. IFP's process, Clauspol D, would seem a good candidate because of its cost-effictiveness.

IFF was one of the first to develop and offer a TGT, Clauspol 1500 y and has licensed over 40 units since 1971. These units have capacities ranging from 30 to 800 metric tons per day and. typically, boost the sulfur recovery of a Claus unit to over 98.5%. Drawing from IFF R&D and worlwide Clauspol operating experience, IFF can now claim a substantial increase in sulfur recovery for practically the same low investment, with its new Clauspol U process.

2. PRINCIPLES

2.1. Chemistry

Sulfur recovery in Claus units is limited to 94-97 per cent. There are three main sulfur- containing components in the tail gas coming from Claus units:

Unreacted hydrogen sulphide and sulfur dioxide are due to the thermodynamic equilibrium limitation of the Claus reaction:

2H2S + SO2 | Sn + 2H20

This conversion towards sulfur is favoured by low temperatures but is in fact limited by the exothermicity of the reaction and the physical properties of sulfur; a continuous process must operate at a sufficiently high temperature to keep the sulfur in the vapour phase and to prevent it from depositing on the Claus catalyst, thus reducing the conversion.

Elemental sulfur vapour is present at a concentration in the range 500 to 2,000 ppm (expressed in Si). Some liquid sulfur is also carried along m the tail gas.

Organic sulfur compounds (COS, CS%) are formed in the Claus thermal stage. Their concentration may be greatly reduced in the first Claus catalytic stage under special precautions (catalyst, operating temperature). TT-098

Table 1 presents the usual range for sulfur component concentrations in Claus tail gas.

Table I

Most usual Sulfur Components Range in Claus Tail Gas

Component Vol.-%

H2S/S02 0.5 - 1.8 Sulfur (as Si) 0.05 - 0.3 COS + CS2 (as Si) 0.02-0.16

The IFF Clauspol U process has been developed to reduce the sulfur content of the Claus tail gas. It belongs to the family of tail gas processes based on the Claus reaction. The process is operated as close as possible to the sulfur freezing point: this favours the equilibrium towards sulfur formation and also reduces the sulfur vapour content in the treated gas to a minimum.

Basically the Clauspol Q process consists of treating the Claus tail gas at the Claus battery limits conditions, i.e. pressure and temperature, with a special solvent containing dissolved catalyst. The operating pressure is near atmospheric and the operating temperature of the solvent is 120-122°C. just above the sulfur freezing point. The solvent is a polyethylene glycol (PEG) of molecular weight 400, and the catalyst is a sodium salt an organic acid.

There are two successive discrete steps in the overall reaction:

- mass transfer of H%S and SO2 from the gas phase into the liquid phase; - Claus reaction between H2S and SO2 in the liquid phase.

In operation, the PEG is saturated with sulfur (sulfur solubility in PEG is low), so the produced sulfur, being heavier than the solvent, constantly separates from the PEG. Water is soluble in the solvent, but owing to the equilibrium between gas and liquid phases, only a small quantity of water remains in the solvent and the produced water evaporates in the gas phase. So, as the reaction products constantly separate from the solvent, the Claus reaction proceeds to the right.

The catalyst is needed to obtain a fast enough reaction in the liquid phase. The active principle of the catalyst is sodium salt of a sulfur oxyacid complexed by the organic acid.

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All the sodium introduced in the process is transformed into sodium sulfate, which is insoluble in the solvent. This is due to a side reaction of the Claus reaction in the liquid phase, although the selectivity towards sulfur is very high - more than 99.9%. The vapour pressure of the organic acid is the cause for. its consumption in the treated gas stream. (It occurs to a lesser extent than would be expected on the basis of calculations from vapour pressure data: this is becausethe organic acid is complexed in the liquid phase). So a continuous catalyst make-up is necessary to maintain the catalyst activity in the process.

Concerning organic sulfur compounds (COS, CS2). they arc partially hydrolysed in the Clauspol process into H2S, owing to the water content of the tail gas, according to following reactions:

COS + H20 -> CO2 + H2S CS2 + 2H20 -» CO2 + 2H2S

The conversion into H2S is approximately 40% for COS and 15% for CS2. The H2S produced then behaves as the H2S present in the feed gas.

Entrained liquid sulfur is recovered by the process, the reactor acting as a mist eliminator.

Sulfur vapour is partially recovered, as the process operating temperature is always lower than the tail gas inlet temperature. The losses in the treated gas correspond to the sulfur vapour pressure at the operating temperature (some 300 ppm expressed in S% in the treated gas).

Water vapour is one of the species involved in the Claus reaction, and the water content of the tail gas in one of the parameters that must be taken into account at the design stage. The total amount of water vapour at the outlet of the unit is increased by the quantity produced by the reaction.

Carbon monoxide and dioxide, hydrogen, nitrogen and hydrocarbons, if any, have no effect on the process.

Oxygen is theoretically not present in Claus tail gas, as the Claus process operates under reducing conditions. But in practice some oxygen may remain as a result of incomplete mixing of H2S and air in the Claus section or imcomplete H2S combustion. EFP industrial experience shows that usually the oxygen content of Claus tail gas is very low, less than 100 ppm.

It should be noted that it is now existing catalysts to be placed in Claus catalytic stages for oxygen traces elimination in the tail gas and protection of catalyst again sulfatation. TT-098 >V

Ammonia is nowadays often present at higher concentrations in the Claus tail gas owing to the increasing capacities of hydrotreating units of middle and vacuum distillates and sometimes even residues. Theoretically, ammonia is destroyed in Claus furnaces by using appropriate design and operating conditions. In fact, it is difficult to continuously destroy ammonia because of the fast changes in water stripper overhead gas flow or composition which sometimes occur.

As the combustion of ammonia and H%S need different amounts of air, the complete destruction of ammonia is not continuously ensured. In refineries ammonia contents of 300 ppm arc not unusual in Claus tail gas (in one case, an extreme peak value of 5,000 ppm has been measured).

The difficulties associated with the presence of the ammonia in Claus units are well known: deposition of solid ammonium salts, sulfation of alumina catalysts, for example. Lucidly, although the Clauspol Q process uses the same reaction as the main Claus plant, it operates without any trouble in presence of the ammonia in the Claus tail gas without any modification of the unit. The simplicity of the Clauspol unit and the use of a liquid phase provide a perfect protection against ammonium salt plugging (ammonium sulfides, sulfites, etc., are soluble in the PEG). Moreover, ammonia, being a catalyst for the Claus reaction, has a positive action on the process.

2.2. Process description

As shown before, the Clauspol process is requiring a single reactor, a solvent circulating pump, and a tempered water cooler.

The limiting factor of the overall reaction is the mass transfer of H%S from the gas phase into the liquid phase. So the reactor is designed to provide a big enough interfacial area between the gas and liquid phases to treat a specific feed to the required degree of conversion. No booster on the tail gas is needed but. because of the small pressure drop available, a packed tower has been selected for the reactor. The packing type is ceramic Intalox saddles. The interfacial area is provided by wetting the saddles with the solvent flow. TT-098

Gee to Incinerator

Solvent /S temperature (f\j control

Liquid Sulfur

The tail gas is contacted to countercurrent with a circulating stream of PEG containing dissolved catalyst. The treated gas leaves at the top of the reactor and goes to the Claus incinerator while the product sulfursettles at the bottom of the reactor and drains through a seal leg to the sulfur pit A constant liquid flow is maintained in the tower by means of a centrifugal pump.

A heat exchanger is needed to cool down the circulating solvent. It ensures the correct packing temperature, and is used also for solvent warm-up and cool down during plant start-ups and shutdowns.

2.3. Process controls

The process needs only a few controls.

Temperature

The Claus reaction is exothermic, and the incoming Claus unit tail gas is usually at temperature between 125 and 144°C. sometimes even higher. As the circulating stream must be maintained in the 120-122°C range, heat must be rejected. This is accomplished by a cooler using tempered water to avoid cold spots on the solvent side. The temperature is controlled from the treated gas line. A cooling water exchanger ensures the cooling of the tempered water loop. TT-098

Although the recommended temperature range is close to the sulfur freezing point, operation of the unit is quite reliable and untroubled by sulfur plugging owing to its high thermal inertia: the total operating mass of the reactor (metal, packing, solvent) is high in comparison to the tail gas mass passing through it.

HgS/SOg ratio

The Clauspol D process needs rather accurate H2S/SO2 ratio control, as it is based on the Claus reaction between remaining H%S and SO2 in the Claus tail gas.

In many Claus units, the H2S/SO2 ratio is controlled by means of an air-to-HaS feed flow ratio controller at the inlet of the Claus unit. This is known to be far too inefficient to obtain a high sulfur recovery in the Claus unit. Better ratio control is obtained by using a continuous on-line analyser installed on the Claus unit tail gas line. The anaiyser compares its reading with a present value and opens or closes a secondary air valve at the Claus unit inlet in proportion, while the main air admission is still kept proportional; to the H2S feed gas flow to the Claus unit.

The continuous analyser used is a photometric one (ultraviolet). This type of analyser provides an instant analysis and is able to control the H2S/SO2 ratio within the recommended range even when vacations of the feed to the Claus unit are important. This is the case in refineries: very often, several H2S gas streams are fed to one Claus unit, and the flow, the composition and battery limit conditions of each one can vary.

3. RECENT IMPROVEMENTS

3.1. Indirect Water Cooling and On-Stream Analysers

The major developments to the Clauspol 1500 process that enable the high sulfur recovery are • the indirect water cooling in the solvent loop and the reliable, accurate on-stream analysers that help control the Claus unit itself more efficiently and smoothly.

The tempered water cooling loop improves the stability of the catalyst complex by avoiding the introduction of free water. This in turn allows a more uniform catalyst make-up rate and improves the catalyst activity. The exchanger's special design copes with the solvent characteristics while the tempered water is cooled by means of a normal heat exchanger.

The new analysers and sample handling techniques available today (UV) provide for more accurate and reliable Claus operation. An improved H2S/SO2 = 2 ratio control at the Clauspol inlet is accomplished by an on-line analyser mat resets the H2S/au ratio at the Claus inlet. The benefits of a well-controUcd feed quality arc exploited to the maximum by the Clauspol unit. Minor fluctuations in feed quality (H2S/SO2J raoo are then easily smoothed out by the Clauspol solution's mass and its reactor residence time. TT-098

3.2. Boo«ter99.9

Booster 99.9 is an upgrade path for Clauspol units that, when combined with Claus Units, enables recovery of 99.9% of the potential sulphur in refinery, gas and petrochemical plant gas streams. This is usually enough to satisfy the tightest of govermentai controls on sulphur emissions.

Function of Booster 99,9: Existing Clauspol units circulate a saturated solution of solvent entraining a tiny amount of entrained liquid sulphur. The liquid sulphur has a vapor pressure, although small, that contributes to the sulphur losses in the Clauspol effluent. Booster 99.9 eliminates the liquid sulphur phase and thereby the extra 300 ppm sulphur that it adds to the Clauspol effluent. Booster 99.9 now allows to circulate as an unsaturated solution at lower temperatures - both of which promote a better yield and reaction equilibrium.

Booster 99.9 Features: Besides its ability to increase the overall recovery of Claus/Clauspol units to 99.9% from 99.8%, Booster 99.9’s outstanding features are its simplicity, its ease of installation and operation, its low capital cost, that it adds little in the way of utility costs, and its relatively small space requirement. Furthermore, like the Clauspol unit, there is no need for hydrogen. Compared to other TGT plants, Clauspol and Booster 99.9 continue operating even if the hydrogen source stops running, and there is no H2S recycle stream to the Claus unit to consume Claus capacity.

Booster 99.9 can be easily installed along side of an existing Clauspol: perfect for an upgrade. The original Clauspol unit is pratiosUy unchanged.

It is a continuous process but it can be maintained without shutdown: either Booster 99.9 or the Clauspol unit. Clauspol Booster 99.9

Gas to Incinerator

Low Level Heat Cteus Tall Gas

CW

Low Level Heat

Liquid Sulfur TT-098

4. TYPICAL PERFORMANCE AND INVESTMENTS

Claus Unit Number of catalytic stages 2 Sulfur in feed, tons per day 100 Sulfur recovery, % 95.5 Claus tail gas, Nmtyi 9450 Claus tail gas, kmol/h 422 HjS + SO* vol. % 1.22 Sulfur (as Si), vol. % 0.16 COS + C$2 (as Si), vol. % 0.03

Clauspol Unit Overall sulfur recovery. wt % 99.7 Investment, battery limits. 1993, excl. engineering and license fees, million USD 3.4 Solvent and catalyst Initial charge, thousand USD 125 Consumption, thousand USD 24 Utilities Power, kWh/h 170 Cooling water, m3 /h 17

CONCLUSION

Sour crudes and tight emission's standards will encourage construction of new sulfur recovery units soon. This will quite likely involve more Claus plus TOT. units. GFP, whose licensing activities embrace all phases of petroleum processes, has two new Clauspol II industrial .scale references whose sulfur recovery exceeds 99 8%. Moreover another significant improvement has been performed with the Booster 99.9 technology, which enables recovery of 99 u% of the potential sulphur in refinery, gas and petrochemical plant gas streams. All this technology now allows the most stringent sulphur regulations to be met.