Monday, December 18, 2017

Modification in Blow Heat Recovery SystemPrint


- By ITC Ltd. Paper Boards & Specialty Division Bhadrachalam

To encourage “Energy conservation & Environment Protection” in the Industrial sector, PCRA has been continuously upgrading their knowledge data base thru energy audits, experiences in those industries and has been developing case studies on experience shared by various Industrial groups and organization. The case study detailed below is one of the modifications carried out by M/s ITC Ltd’s Paper Boards & Specialty Division Bhadrachalam in association with TERI Banglore which is being produced to disseminate the information for the benefit of Paper Industries.

 

Introduction

M/s ITC Ltd’s Paper Boards & Specialty Division at Bhadrachalam was established during 1975 and is one of the most modern unit.

The installed capacity of the plant is 2.1 Lac tons of paper per annum with four paper machines. They have taken up installation of 5th paper machine & modernization of pulp mill. M/s ITC Ltd. Bhadrachalam carried out modification in their Blow Heat Recovery system thus resulted in a reduction of steam consumption by 155 tons/day plus other benefits. Net saving amounting to Rs. 151 Lac/yr.

 

Process

For manufacturing pulp the wood chips are cooked in digesters under pressurized conditions (7 Kg/cm2g). Batches of 18 to 19 MT of chips are loaded into each digester. The wood chips after cooking with white liquor. (NaOH, Na2S and Na2 CO3) are converted into pulp. Heating and pressuring is done with the help of MP and LP steam. The pressure in the digester vessel itself is utilized for emptying the pulp from the digesters after completing the cooking cycle (4.5 Hrs.) for further processing in the pulp mill.

This operation is called as blowing, which takes about 15 to 20 minutes per digester per batch. The blown pulp is stored in vessels called as blow tank. As soon as the pressurized pulp is let into the open blow tank, huge amount of flash vapours (contaminated with white liquor) are generated.

 

Present Blow Heat Recovery System

Along with original equipment supply, a blow heat recovery system was supplied to condensate these flash vapours. Blow heat recovery system is used to generate hot water which is further used for the process in the downstream of pulp processing.

The major equipments involved in the blow heat recovery system are.

a. Blow tank

b. Primary and secondary condensers

c. Accumulator and foul condensate tank

d. Plate heat exchangers

e. Hot and warm water tanks

f. Primary and secondary pumps

Hot pulp from the digesters after cooking is blown into the blow tanks for further processing. The flash vapours generated from the pulp are condensed in primary and secondary condensers. The foul condensate thus generated from flash vapours is collected in foul condensate tank. This hot foul condensate exchanges its heat with warm water in plate heat exchanger, to generate hot water.

About 200 M3/Hr. at 700C of hot water is required for the downstream process. The present blow heat recovery system is not facilitating required quantity of water at required temperature. Thus LP steam was used to supplement this shortage which was about 327 TPD, involving Rs. 8.62 Lac/day. Requirement of LP steam for hot water also affected power generation since being a co-generation plant. This also resulted in drawl of power from State grid.

Thus only about 33% of flash vapour heat was being recovered.

 

Problem Areas Identified

- Reduced condensing capacity.
- Less heat transfer area in the plate heat exchanger.
- Frequent fouling of fiber filter, leading to reduced foul condensate flow rate to the plate heat exchanger.

 

Modifications

M/s ITC Ltd. Bhadrachalam carried out the following modifications in consultation with TERI Bangalore.

1 Replaced the foul condensate pumps with suitable pumps such that the cooler foul condensate from outlet of the plate heat exchanger is directly pumped to the primary and secondary condensers.
Two pumps of different capacities were installed. The higher capacity pump (630 M3/Hr. & 40 M Head) for operation during the blow period and the smaller capacity pump 275 M3/Hr. for operation during non-blow period.

2 Replaced the plate heat exchanger with increased heating surface area from 45 m2 to 83 m2

3 Provided online back flushing of the fiber filter.

The diagram below indicates the modified system

 

Table--Comparison between past and present system conditions

 

Parameters Past Condition Present Condition Remarks
Temperature of foul condensate at inlet of PC and SC 700C 570C Decreased temperature has enhanced the performance of the condenser
Hot water temperature at outlet of plate heat exhanger 600C 680C Achieved the desired hot water temperature
Heat transfer area of plate heat exchanger 45m2 83 m2 Increase in heat transfer area enabled to attain hot water temperature of 680C
Additional LP steam usage in the hot water tank 327 TPD 172 TPD Reduction in steam consumption at hot water tank as well as overall steam consumption
Power consumption by PC, SC and foul condensate pumps 44 KW 103 KW Increase in power consumption, which is compensated by reduction in steam consumption

Benefits

The above modification resulted in reduction of LP steam form 327 TPD to 172 TPD amounting to saving of Rs. 169.7 Lac.

Improved power generation from co-generation plant due to reduction in extracted steam quantity thus reduction in drawl of power from State grid.

Improved environment thus operating & working condition and reduction in water consumption.

 

Conclusion

The net saving considering the reduction in steam consumption and increase in overall power consumption was to the tune of Rs. 151 Lac/yr. The project implementation cost was Rs. 40 Lac with payback period of 4 months.

 

The energy saving achieved due to energy conservation amounts to Rs. 1.51 Cr./yr.