Running SRUs at low throughput, effects and considerations

Khaleelullah Nizamuddin Syed

Abu Dhabi Gas Industries Ltd. (GASCO)

Abdul Kader Al Kasem

UniverSUL Consulting

Doug Cicerone

Cicerone & Associates, LLC

ABSTRACT

Abu Dhabi Gas Industries Ltd. (GASCO) – Habshan site has fourteen Sulphur Recovery Units (SRU) with different configurations and efficiencies. The first two trains (built in 1986) are 3 stage Claus units (recovery efficiency 97%), whereas the latest units commissioned in 2013 are Claus process with TGTU (recovery efficiency 99.9%+) and others are CBA and SuperClaus processes with efficiency of 99%. Based on the feed availability and prioritizing feed to higher efficiency units, acid gas is distributed amongst these trains. As a result of this, some of these units are running at low throughput. Running at low throughputs may lead to challenges in plant operations and maintenance and decreasing energy efficiencies. This paper outlines challenges that are encountered when Habshan SRUs run at low throughput, such as controlling flow to the reaction furnace, sulphur blockage in condensers (leading to higher pressure drop), and higher energy consumption in incinerator. Other considerations such as mothballing low efficiency trains, co-firing requirements and changing feed quality to SRUs will be discussed. INTRODUCTION

GASCO Habshan Plant currently operates 14 sulphur recovery units. A number of different sulphur recovery technologies are employed, including the modified Claus process, Cold Bed Adsorption (CBA), SUPERCLAUS®, and amine-based Tail Gas Treating (TGT) using proprietary FLEXSORB® solvent. A summary of the 14 SRUs is provided in TABLE 1.

Since Habshan 5 has the highest sulphur recovery, all efforts are aiming to maximize throughput there. Although Habshan 5 has dedicated acid gas header, maximizing feed will be achieved by routing sour gas feed to Habshan 5 plant as much as possible based on daily gas allocation plan. All other units (10 units) are taking acid gas feed from a common header. The distribution between these units depends on their sulphur recoveries and the availability of other SRUs. Normally units 152/153 will be maximized first followed by Habshan 1&2 units and finally with Habshan 0 units.

TABLE 1 – GASCO Sulphur Recovery Facilities [3]

Site Unit No. SRE (%) Feed

Sulphur Recovery Technology

Habshan-0 50

97.0 Acid gas 3-stage Claus 51

Habshan-1

52

98.8 Acid gas

1-stage Claus with 2 parallel CBA beds

53

54

57 99.0

Habshan-2 58

99.0 Acid gas 1-stage Claus with 2 parallel CBA beds 59

Habshan-HGCE 152

99.0 Enriched acid gas with natural gas co-firing

3-stage Claus with 1-stage SUPERCLAUS® 153

Habshan-5

550

99.9 Acid gas with natural gas co-firing

2-stage Claus with amine-based TGTU (FLEXSORB®)

551

552

553

EFFECTS OF RUNNING SRUs AT LOW THROUGHPUTS

In this section, possible effects (both advantages and disadvantages) on running SRUs at low throughputs will be discussed. The main effects will be on reaction furnace performance and control. Other equipments will be affected as well.

The following discussion is mainly for turndown condition. Since this paper will be focusing on running GASCO SRUs at low throughputs (which is higher than turndown in most of the cases), some of these effects will not be seen but they are still potential effects while running in low throughputs.

Combustion Air Blowers

Advantages

None

Disadvantages

Centrifugal blowers need a minimum air flow to prevent surge. The excess air is blown-off at low throughput which leads to wasted energy (electricity or steam). Inlet Guide Vanes can help reduce this inefficiency but won’t necessarily eliminate it.

Reaction Furnace

Advantages

Residence time increases as throughput decreases. This helps increase sulfur conversion and helps with the destruction of BTEX.

Disadvantages

Butterfly valves typically do not control well below 30% of design flow, so precise control of the air and acid gas flows to the Reaction Furnace below this limit can be difficult. Controllers can be placed in manual to help stabilize flow, however this can lead to poor control of the H2S:SO2 tail gas ratio. Split range control can be used to get to 10% or below.

Some Reaction Furnace burners won’t turn down below 30%. Operating a burner below its turndown limit can cause damage as the flame backs up against the burner face.

Reaction Furnace operating temperatures decrease at low throughput (particularly in smaller furnaces) as heat losses to ambient become significant. This can reduce BTEX destruction.

Skin temperatures decrease as the operating temperature decreases. This can lead to low temperature acidic corrosion.

Co-firing with fuel gas can help with most of these issues by increasing air flow and Reaction Furnace temperatures.

Condensers & Rundown Lines

Advantages

None

Disadvantages

Sulphur fogging can be an issue when operating condensers below their turndown limit. Fogging occurs at low gas velocities when the sulphur does not condense against the condenser tube walls. Instead submicron droplets are formed in gas phase and carried through to the condenser outlet.

Rundown lines are more prone to plugging at low sulfur flow rates. Catalyst and/or refractory dust can accumulate since the liquid velocities are low.

Rundown lines are also more sensitive to small disturbances in the steam jacket heat transfer at turndown (inadequate insulation, rain storms, cold weather, etc.).

Catalytic Converters (Claus, CBA and SuperClaus®)

Advantages

Lower space velocities can increase sulphur conversion in the reactors.

CBA cycle times can be increased which increases the overall recovery efficiency by reducing the number of times the CBA reactors switch from lead to lag (the most inefficient part of the CBA cycle).

Disadvantages

Channeling of gas flow through the catalyst can reduce sulphur conversion at low turndown.

Heat losses to ambient become more significant which can lead to corrosion issues (particularly at nozzles and manways). Insulation is important to minimize the corrosion and steam tracing/coils can be added if the unit is to be run at turndown for long periods.

Reducing Gas Generator

Advantages

None

Disadvantages

Burner controls can be problematic at turndown (same as Reaction Furnace). In extreme cases this can lead to oxygen breakthrough from the RGG causing catalyst fire in the Hydrogenation Reactor.

Damage to the burner can occur if operated below its minimum turndown (same as Reaction Furnace).

Hydrogenation Reactor

Advantages

Lower space velocities can increase sulphur and SO2 conversion to H2S in the reactor.

Disadvantages

Channeling of gas flow through the catalyst can reduce sulphur and SO2 conversion at low turndown.

Low temperature acid corrosion is typically less of a concern than in SRU catalytic converters due to the much lower SO2 partial pressure in the Hydrogenation Reactor.

Quench and Amine

Advantages

Increased gas/liquid contact time increases the H2S pickup in the amine system.

Disadvantages

Trays will typically turn down to 40% to 50% liquid flow. This leads to excess liquid flow when the gas flow falls below this limit, which causes excess energy usage in the reboiler and pumps. Packing can have a similar limit.

Incinerator

Advantages

Residence time increases as throughput decreases. This increases the destruction of all reduced sulfur species.

Disadvantages

Burner controls can be problematic at turndown (same as Reaction Furnace). Natural draft incinerators can be particularly difficult to control stack oxygen content to the typical 2% minimum, leading to excess fuel gas usage.

Damage to the burner can occur if operated below its minimum turndown (same as Reaction Furnace). Incinerator burners typically will not turn down as low as the SRU tail gas, leading to excess fuel gas usage.

Stack temperatures can get cold at low rates particularly if there is a Waste Heat Boiler. This can lead to stack corrosion and inadequate dispersal of the flue gases.

EFFECTS ON RUNNING SRUs AT HABSHAN PLANT

In this section, effects on running SRUs at Habshan Plant will be discussed. Most of the SRUs in Habshan were designed for 30% turndown and data used in this paper is DCS data collected from January 2015 to April 2016.

Units which are running on low throughputs are CBA (Cold Bed Adsorption) units most of the time, therefore analysis will be focused on them. Figure 1 is showing the configuration of Habshan CBA units.

The Habshan Gas Plant is equipped with six CBA SRU trains. The first three trains were started up in 1996 and the others followed in 2001. The basic configuration of the sulphur recovery units is fundamentally identical. That is each of the units includes the following sections [1]:

One Thermal Reaction Section

One Catalytic Claus Conversion Section

Two Stage CBA Section

One Tail Gas Incineration Section

One Sulphur Storage and Degassing Section

Figure.1 Typical CBA Process at Habshan Plant [1]

Data used in this paper is for CBA unit which was commissioned in 2001, this unit has six design cases (different in flow and H2S concentration of acid gas feed) with sulphur recovery capacities of 396 TPD to 733 TPD. The H2S concentration is varied in these cases from 35.9 to 62.9 mole% with total feed flow rates from 26 Knm3/h to 47 Knm3/h

DCS data for the time period mentioned above were filtered to consider fixed H2S concentration of 55 mole %. This will eliminate the effect of H2S concentration while feed is changing. The variation of H2S concentration in Habshan SRUs was most of the time in the range of 50 to 57 mole %.

Feed Control:

Controlling the acid gas flow is a challenge at lower throughputs. In Figure 2, the acid gas feed flow is shown; in lower feed flow the control valve was unstable and a lot of fluctuation is there.

Therefore in this particular day, acid gas was cut totally to replace valve positioner, solenoid and relay after running for few days at low throughput. The unit was started with higher feed flow to avoid such fluctuation again.

Operations also reported that reaction furnace flame scanner alarm was frequently coming at low feed flows which indicates flame instability. Flame scanner has been adjusted to look to the area of flame which will reduce alarms on flame scanner indications to avoid tripping the unit.

Figure.2 Acid Gas feed flow

For air control valves, it was unstable as well (trim air) therefore, it was kept in manual to avoid this fluctuation as in Figure 3.

Figure.3 Combustion Air valve opening

Reaction Furnace:

Reaction furnace temperature is coming down with less feed flow as shown in Figure 4. The most interesting point in this graph that there is difference in Reaction Furnace temperature for the

same feed flow. The reason behind this is mainly the calibration of the temperature measurements before and after shutdown. Before shutdown temperatures were less than after for same feed flow which was forcing operators to introduce fuel gas co-firing to maintain temperature within the range required for BTEX destruction.

The temperature increase at lower feed flow is mainly due to fuel gas co-firing.

Figure.4 Reaction Furnace Temperature

In Figure 5, when feed flow become less than 15 Knm3/h, combustion air will not follow the same trend of higher acid gas feeds. This will lead to higher than expected energy consumption in the incinerator because of the additional tail gas flow from fuel gas firing in the Reaction Furnace as we are going to see this in the Incinerator Performance effects.

In this figure also, fuel gas has been used always whenever feed is less than 15 Knm3/h but not continuously with higher feed, for example, at feeds higher than 20 Knm3/h fuel gas firing was rarely used.

980

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ºC

Feed Flow Knm3/h

RF Temp

Figure.5 Combustion air and fuel gas co-firing flow

Reaction Furnace Skin Temperature:

Skin temperatures in reaction furnace are monitored in order to provide a first indication of possible corrosion: low skin temperatures could lead to low temperature acid attack.

The skin temperature of Reaction furnace was affected when feed came down less than 22 Knm3/h. From Figure 6, the majority of data are above 150 ºC when feed flow is above 22 Knm3/h whereas they are below this temperature for less acid gas feeds.

There are many skin temperature indications in this reaction furnace and most of them are not affected much. The only one affected is which presented in Figure 6. This indication is in the bottom from burner side.

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55

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FG t

o R

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m3

/h)

Air

Flo

w K

nm

3/h

H2

S co

nce

ntr

atio

n (

mo

le %

)

Feed Flow Knm3/h

Air Flow H2S Conc FG to RF

Figure.6 Reaction Furnace skin temperature

Reaction Furnace in this unit is equipped with thermal shroud to keep the metal temperature between the desired temperature ranges. The louvers in this shroud are intended to be closed slightly to increase temperatures and open them slightly to decrease temperatures. This need operators to keep monitoring skin temperature and keep them closed whenever feed flow is less and especially when ambient temperature is low (below 25ºC).

The skin temperature was not a concern in the past (before Habshan 5 commissioning on 2013) where most of the units where running on maximum throughput but they become a concern when these units run in low acid gas feeds.

Condensers

While running at low throughputs, there will be a danger of plugging in condensers and rundowns working at low temperatures. Catalyst and/or refractory dust can accumulate also as result of low liquid velocity.

Many units at Habshan are running at low throughput without having pluggage, so it’s more related to precautions taken while running at low feed flow.

There was a case at Habshan where reaction furnace pressure increased significantly as in figure 7.

A pressure survey was carried out and most probably this was happening due to a partial plug in CBA Condenser First Pass. The plant pressure drop was increasing during the heat-up steps because condenser sees the peak sulfur flow during this time. The pluggage in the condenser causes it to partially flood with sulfur which increases the backpressure on the SRU. The

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170

180

190

10 15 20 25 30 35 40

RF

Skin

Te

mp

ºC

Feed Flow Knm3/h

operators are reducing throughput during the heat-up steps because of the increased pressure drop. The increased pressure drop is not caused by running the unit at low throughput directly but as result pluggage in the unit; the operators are turning down the unit because of the pressure drop.

Figure.7 Capacity Ratio Trend

While the increased pressure drop is not a result of running the plant at turndown, it is a common reason for operators to turn down the plant if they have pluggage.

Incinerator Performance

When feed flow becomes less than 25 Knm3/h, the amount of energy per unit of feed increased and maintaining O2 less than 4 mole % in stack will become a challenge as in Figure 8.

0

1

2

3

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7

8

9

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Cap

acit

y R

atio

= A

ctu

al(P

ress

/Flo

w^2

) /

De

sign

(Pre

ss/F

low

^2)

Figure.8 Incinerator fuel gas and Oxygen in stack

UPSTREAM CONSIDERATIONS

Feed stream preheat: All Habshan SRU’s are equipped with air/acid gas preheaters because they have lean acid gas design case. The key design feature is to maintain thermal reaction temperature (to avoid the instability of burner flame and destroy hydrocarbon based acid gas components).

The use of preheaters in addition to high intensity burners will improve acid gas feed processing especially while running in low throughputs.

Most of preheaters in Habshan plant are indirect type by employing high pressure steam as heating medium. Maximum temperature can be achieved through these preheaters is 225 to 235 ºC.

While running SRUs in low throughputs, it’s important to maximize outlet temperatures of air/acid gas preheaters to reduce chances of SRU trip due to flame instability and to increase reaction furnace temperature for better destruction of hydrocarbon based acid gas components (BTEX) and this may also reduce co-firing requirements.

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le %

O2

in s

tack

Nm

3/h

FG

to

Inci

ni /

Kn

m3

/h A

G F

eed

Feed Flow Knm3/h

Incin FG O2 in Stack

Co-firing with fuel gas: In this method, a plant fuel gas stream is added to the acid gas feed stream in the hope of substantially increasing the heating value of the furnace feed gas. In principle this method can be very effective, however, there are several significant disadvantages which may limit the usefulness of this approach [2].

In GASCO Habshan SRUs Operation, whenever acid gas feed flow is lower than ~40% of design capacity, fuel gas co-firing is used.

Co-firing fuel gas introduces a problem: CS2 production in the furnace is roughly proportional to the amount of fuel gas added. Recovery would suffer unless nearly all CS2 is hydrolyzed in the first converter.

Mothballing SRUs: In order to maximize feed flow to running SRUs and reduce operational costs of running SRUs at Habshan plant, the lowest sulphur recovery units 50/51 were mothballed. So, a complete long term mothballing was prepared according to ADNOC and GASCO procedures.

The entire unit was divided to eight loops and long term shutdown procedure was followed for making system sulphur free. Based on the loops identified, about 40 blinds were installed. Reaction furnace was opened and about 35 Kgs of Silica gel were kept inside before boxing up.

These two units were running with minimum feed of 24 – 30 Knm3/h. After mothballing, this amount of acid gas was distributed to other SRU trains. By doing this, emission reduced because of lower sulphur recovery in these two units, feed increased to other units and it was contributed to energy conservation.

Acid Gas Enrichment: Acid gas enrichment (AGE) is a method used to upgrade low-quality off-gas from treating units to higher-quality Claus plant feed. The process objective is to maximise CO2 slip and minimise the H2S leak into vent gas from the system, thereby producing a gas enriched in H2S to the greatest extent possible.

The most common method to enrich is by using a separate absorber for treating the low-grade acid gas stream coming from the regenerator. The AGE system is designed to increase the H2S concentration in the acid gas to a typical target value of 50%. This increases reaction furnace temperatures and flame stability.

For the case of low feed flow, acid gas enrichment will help in getting higher temperature for BTEX destruction and will make reaction furnace flame more stable.

In GASCO Habshan, there is only one AGE unit with MDEA as solvent and the H2S design concentration in the enriched gas is 41 mole % (H2S in feed gas is ~12 mole %). This enriched acid gas will be injected to the common header and will be processed mainly in the two SuperClaus units. Therefore it will not be helpful directly for the CBA units which are running at low feed flow.

There were some cases where AGE bypassed and acid gas become so lean (~36 mole %) and co-firing was there even at higher unit feed.

Oxygen Enrichment: Oxygen enrichment is well known for SRU capacity enhancement based on the principle of approximately 65 to 70% of the hydrogen sulfide entering the sulphur plant is removed as liquid sulphur in the first sulphur condenser. The remaining gas flowing through the Claus sulphur plant is primarily nitrogen, which enters with the combustion air. The equipment must be sized to accommodate this nitrogen. Adding oxygen is the equivalent of removing this nitrogen.

Using oxygen enrichment can help when running SRU at low throughput by improving reaction furnace performance but this will lead to reduced mass flow which may make the situation worse if burner has low turndown ratio and it will increase the probability of channeling in converters and lower gas velocity in condenser tubes.

However, none of Habshan SRUs are designed with Oxygen enrichment, therefore this option can’t be investigated and need more study.

CONCLUSIONS

There are many effects on SRUs while running at low throughputs. Some of these effects are

resulting in reduced operating and maintenance challenges and increased energy consumption.

Sometime these effects are misleading operators who are running the plant. For example, at acid

gas concentration of 55 mole %, preheating feed streams to 235 º C will be enough to get reaction

furnace temperature above 1050 º C (for BTEX destruction), whereas temperature indications are

showing less temperature either because they are faulty and not calibrated for long time or

because they are reading refractory temperature. In such conditions, where flame scanners will

be unstable, fuel gas co-firing will likely be admitted to reaction furnace to increase temperature

and stabilize the flame.

This results in extra fuel gas consumption and will increase the possibility of soot formation on

the catalyst.

Understanding the effects of running SRUs at low throughputs will help Operators to identify the

main causes of unstable conditions and will help in energy conservation especially in reaction

furnace where flame scanners may give instable flame indication.

Managing acid gas allocation among many SRUs (Habshan case) is an important factor while

running at low throughputs. It’s important also to make sure they are running above feed flow

value that can effect SRU a lot in terms of smooth operation, energy efficiency and environmental

impact.

NOMENCLATURE

°C degrees Celsius H2S hydrogen sulphide ADNOC Abu Dhabi National Oil Company Kg Kilogram AGE acid gas enrichment mol% mole percent AGRU acid gas removal unit SRU sulphur recovery unit Barg bar gauge SRE sulphur recovery efficiency BFW boiler feed water SO2 sulphur dioxide BTEX benzene, toluene, ethylbenzene, xylene TGTU tail gas treating unit CBA Cold Bed Adsorption DCS distributed control system GASCO Abu Dhabi Gas Industries Ltd.

REFERENCES

1. A. Alkasem, J. Al Marzouqi, Mohamed Salem Al Matroushi “Lessons Learned over 20 Years of Sub-Dewpoint SRU Operation in UAE” Presented at the 2nd Annual Brimstone Middle Eastern Sulfur Recovery Symposium, May 2016, Abu Dhabi, UAE.

2. Hydrocarbon Destruction In the Claus SRU Reaction Furnace, Bruce Klint, P.Eng, Sulphur Experts Inc., 50th Annual Laurance Reid Gas Conditioning Conference, University of Oklahoma, February 27, 2000.