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Jun-2017

Exchanger cleaning contest: chemicals vs jetting

Chemical cleaning proved to be more effective than water jetting to increase the efficiency of a high heat flux exchanger

HARADI ANAND ASHOK and RITWIK KUMAR HATIAL
Mangalore Refinery Petrochemicals
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Article Summary
High flux thermal coatings in gasoline stabiliser reboilers have the effect of increasing thermal conductivity by two to three times, thereby decreasing the overall heat transfer coefficient. In Mangalore Refinery, both the hydrocrackers units are equipped with similar types of reboilers. The coatings on the non-process side are supposed to be removed by special chemical treatment; we carried out a chemical treatment in one of the units and a normal water wash in the other unit and found that, in the case involving chemical treatment, the specific reboiler duty (that is, the exchanger duty per unit of light naphtha production) reduced markedly and that the efficiency of the exchanger has improved. The gain was calculated and the payback period was estimated.

In detail, inside a hydrocracker unit, light naphtha is stabilised in the gasoline stabiliser column after removal of LPG from the feed stream. The feed, unstabilised light naphtha, is fed to the column and medium pressure steam reboiler. The feed stream is reboiled to separate LPG from the feed stream and the stabilised naphtha. The medium pressure steam reboiler is a thermo-siphon exchanger for the gasoline stabiliser column; it has a high thermal flux coating on the outer surface of the tubes. This high flux coating increases the heat transfer coefficient to two or three times that of a conventional type. To saturate the medium pressure steam, boiler feed water is injected into the medium pressure steam inlet line for effective condensation. Normally, the medium pressure steam header pressure is 14kg/cm2 gauge. At this pressure, the medium pressure steam dew point is 205°C and this is maintained at the inlet of the exchanger by applying boiler feed water at the inlet for effective medium pressure steam condensation in the reboiler itself.

During a shutdown of hydrocracker unit-I (HCU-I) in September 2016, a chemically enhanced cleaning procedure was adopted for the exchanger EA2109. As one of the reactors was showing a thermal crack near the nozzle area which needed significant time for repair, it was decided to start HCU-1 with two reactors at 70% capacity; after the reactor-3 repair work, the plant was restarted at 100% capacity during January 2017. The performance of exchanger EA2109 was checked before the chemical wash at 100% capacity utilisation, after the chemical wash at 70% capacity utilisation, and finally at 100% capacity utilisation taking all the reactors online. During September 2015, there had been a shutdown in hydrocracker unit-II (HCU-2) for work on a similar kind of exchanger, EA42109, when a conventional water wash was carried out. This article presents a comparison of the results with a chemical wash and a normal water wash. In HCU-2, boiler feed water was lined up all the time whereas in HCU-1 boiler feed was brought on-line after the start-up with 70% capacity utilisation.

Chemical cleaning
A 250 litre fibreglass reinforced plastic tank was used as the source of cleaning fluid and circulation was established in the shell side of the exchanger with the help of a pump; from tank depletion data, the net volume of the circuit was found to be 1000 litres. Hot water (85°C) circulation was started at 11:30h on 27 September 2016. At 12:00h the chemical cleaning began as follows:
• Inhibited hydrochloric acid (HCl plus 50 ml Rodine chemical R-213 per 40 litres of HCl)
• Soda ash solution for neutralisation; 50 kg with 250 litres of water
• Passivation with sodium nitrate (NaNO3) solution; 2 kg with 250 litres of water.

Post chemical cleaning
After chemical cleaning, the exchanger was handed over to mechanical operations for jet cleaning. The exchanger was water jetted at 2500 psi. The shell was water cleaned and air dried with plant air; the shell side of the exchanger was mildly pressurised with nitrogen after bundle insertion. The deposit on the tubes was sulphidic in nature and this was confirmed by emissions of hydrogen sulphide during the chemical wash.

Observations
Before and after the chemical wash, the performance of reboiler EA2109 was checked and its duty calculated; a similar type of calculation was carried out before and after the water wash of EA42109 (see Table 1).

After chemical cleaning, the net condensate flow had fallen by 35% without boiler feed water. With boiler feed water to saturate the medium pressure steam and the gasoline stabiliser bottom temperature put in cascade control to deliver effective condensation, the net 
condensate flow had come down further, by 43%, and more effective heat recovery was observed. The performance of the exchanger was checked with full load in three-reactor operational mode (see 
Table 2).

During the September 2015 shutdown, normal water wash was carried out on the HCU-2 gasoline stabiliser reboiler, EA42109. A comparison study was made for a normal water wash of EA42109 compared with a chemical wash for EA2109 and the observations are shown in Figures 1 and 2 and in Table 2.

Assumption made
The HCU-1 boiler feed water was not lined up before chemical cleaning whereas in HCU-2 it was lined up before water wash. For purposes of normalisation, the same specific reboiler duty was considered for both HCU-1 and HCU-2 before the exchanger cleaning. This has been accounted for as a normalised value in the comparison study and pay-back calculation.

Conclusions
Chemical cleaning of a high heat flux exchanger (EA2109) protected the high flux tube coating and 
further increased the efficiency of the exchanger.

Unlike HCU-2, HCU-1 is not fitted with a pressure control valve in the boiler feed injection line and there is no pressure indicator downstream. Further downstream, pressure has to be maintained at at least 14.7 kg/cm2 gauge to get boiler feed water into the medium pressure steam header. Random opening leads to a safety valve popping downstream of the flow control valve. To line up the boiler feed water, one temporary pressure gauge is fixed at the bleeder downstream of the flow control valve and, by adjusting the down stream pressure to 16 kg/cm2 gauge, the boiler feed water is lined up to avoid the safety valve popping at 22 kg/cm2 gauge.

The benefits of chemical cleaning are observed to be higher than for normal water jet cleaning. Chemical cleaning resulted in energy savings of 132 270 Kcal, equivalent to $81.3 per day with a payback period 16 days.
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