Chlorine Liquefaction -Performance Monitoring & Benchmark Chlor-Alkali

Abstract

This study reports importance of the chlorine liquefaction in chlor-Alkali process. It is in the third place when looking to the energy usage in any Chlor-Alkali (CA) plant. The conventional plants still operating with very crude level of monitoring and focus. In India, most of CA plants are operating with medium pressure compression and liquefaction. It is single stage recovery process and chlorine recovery ranges from 90% to 93%.

The refrigeration system removes heat from the chlorine gas stream. This paper provides the necessary information for the system performance and associated practices to maintain the benchmark performance. Now the benchmark figures are available with Indian chlor-alkali against the typic figures shown below in the table.

Chlorine Liquefaction:

Chlorine liquefaction is a key to efficient operation of Chlor-Alkali site. As chlorine is a driving factor for any caustic producing plant. Any disturbance and underrated capacity will impact the energy use and productivity of the plant. Normally, chlorine liquefaction capacity installed based upon the site configuration and many of time it is installed based upon the total chlorine load.

Chlorine liquefaction aspen model is prepared and it was simulated at different scenarios to identify parameters and its optimization. The modeling experience for chlorine liquefaction will be share next.

The following major challenges faced by the chlorine condensing process. These are:

•  Poor Evaporator and condenser performance
• Refrigerant leaks
• Upstream process upset
• Gas composition
• Machine Loading 

Chlorine Liquefaction & operating condition in Chlor-Alkali

Chlorine has many uses such as making of chlorine derivatives like chlorinated paraffin's, stable bleaching powder, hydrochloric acid, water treatment chemicals, cleaning solvents etc.

Refrigeration is needed to condensed and recover chlorine from a gas mixture. Higher the recovery, lower the evaporative temperatures required. Figure-1 is showing the typical chlorine condensing curve for a typical chlorine mixture 95% chlorine and 5% non-condensable at the inlet pressure of 2.35 bar.

It is not difficult to find vapor compression cycle that are in good mechanical condition but not working efficiently due to wear, off design operating condition, refrigerant leak, and external issue.

In these cases, plant is facing challenges in terms of energy use, continuous freon charging, poor recovery of chlorine, rated load operation, breakdowns, poor performance of the system. With through system knowledge, experiences, and improved monitoring system, engineers can detect inefficiencies and maintain best practices to run the system at acceptable performance. Some common operational issue and process parameters are identified to help energy performance of chlorine liquefaction. 

The higher chlorine recovery rate is not economical when it is designed over 95%. From the figure-1, the 95% chlorine recovery possible at -12DgC.

The chlorine recovery never be 100% at any point of time.

Advance Measurement and monitoring systems:

In recent years, the suppliers of refrigeration system and specialized companies have developed sophisticated system for continuous monitoring of the entire system.

At present, the related analysis are obtain in real time and can be overlapped to do more complex studies when required.

Newer monitoring system can survey more online parameters that allow the engineers the ability to access the compressor status, condenser status, and evaporator status and identify the critical readings and its impact, easy to take mitigation step. The primary justification to install the monitoring system with critical field primary measurement, responsible for breakdown. Without such system, it would be very difficult to detect a catastrophic or high-cost failure. 

Conversely modern monitoring system have a great potential in planning predictive and preventive maintenance of the system, as well as to detect energy inefficiencies, leakage, internal circulation, or inefficiencies can be determined by analyzing the operating parameters as presented in figure-2.

Design and operating condition:

In refrigeration system, the compressor operating away from the design condition can cause serious inefficiencies. The compressor efficiency of screw compressor depends upon many factor including valves opening, compression ratio, gas composition, internal clearance. These factors depend upon process condition. The factors can be divided in to groups that depends upon the process conditions and those related to inappropriate design.

In chlorine liquefaction refrigeration system, the refrigerant compressor is used twin lobe screw type, due to it is robustness and partial loading compression when compared to centrifugal compressors. The machine availability with screw compressors are in range of 10 TR to 1200 TR. The screw compressor design efficiency is offered in between 60-65%. Normally they are frequent used in configuration from 100 TR to 300 TR.

The compressor is provided with different loading pattern and its power consumption. Refer figure-2 for the 259 TR machine. The specific power consumption and evaporator loading represented in the same figure.

For capacity control, sliding valve control is used to bypass some gas to the suction side and hence reduce the volumetric efficiency of the compressor. The variation in power consumption and cooling capacity to a screw compressor is shown in figure-3. Due to internal losses, capacity control by sliding valve below 60% capacity is not an efficient option. 

Screw compressor has ability to handle liquid slugging and less sensitive compared to other compressors. 

----- Constant line represents with constant condensing temperature.

------- Dashed line represents with drop in condenser pressure. 

Reciprocating compressor part load operation.

Many of time, people struggling for the key parameters suction and discharge side. It is recommended to use the suction temperature, pressure and flow parameters into the digital panel to observe the pattern of operating.

Control Temperature and Impact of measurement :

The suction gas from evaporator is normally equivalent length of 30-40 meter. The refrigeration system always designs with suction superheat and piping pressure drop.

The chlorine liquefaction system is presented in figure 4, the condenser is water cooled and evaporator is flooded with refrigerant to ensure proper recovery of chlorine. Receiver is used to store the liquid refrigerant. 

The suction temperature and pressure indicate about the refrigeration performance at evaporator side. This temperature is directly linked with evaporator. In chlorine system, it is observed that higher the temperature, higher the system capacity, and higher the power input and lower specific power consumption (kW/TR). It clearly indicates that cooling effect increases in greater proportion than the power consumption.

The approximate thumb rule is that every 1°C higher temperature in the evaporator, the specific power consumption will decrease by 2-3%.

Hence, the evaporator temperature will be input for the optimum capacity operation against heat load.

Keeping lower evaporator temperature is not good and its waste of energy via subcooling of condensing chlorine. This increases the compressor power consumption by lowering the suction pressure and operating at high compression ratio. Normally chlorine liquefaction machines are design in the range of 4.6 to 4.8 compression ratio. As most of all plant are operating at medium chlorine pressure.

Most vapor compression machine use superheat sensing expansion valve, which does not give accurate temperature control especially when compressor is operating at part load, resulting in significant temperature fluctuation.

When the chlorine liquefaction load is lower and machine capacity significantly more than expansion valve control based on superheat sensing often leads to super cooling, resulting in an energy penalty due to unnecessarily lower temperature and lower COP at lower temperatures.

This can be avoided by the electronic expansion valve, which are modulating valves that operate based on electronic sensing of the end use temperature. 

Tracking the sub-cooling of refrigerant in Chlorine liquefaction system: 

Now in latest system, the machines are provided with economizer and even extra sub-cooler. The performance of economizer is not evaluated in any Chlor-Alkali site.

Normally, the economizer & sub-cooler are often estimated at 10°F below condensing temperature.

Sub-cooling is another way of reducing the vapor flow to the refrigeration machine. In a simple cycle. The refrigerant to the throttle valve is at the bubble point. In sub-cooling the liquid is cooled below its bubble point before pressure let down. The subcooled liquid produces less vapors on its pressure reduction resulting in lower compressor power consumption. The liquid refrigerant can be cooled by any process stream, with a small stream of liquid refrigerant evaporating or with vapors from evaporators.

In chlor-Alkali sites, it is observed that the condenser cleaning frequency are too long, and cleaning frequency is not maintained as desired. This condenser inefficiency is loading the economizer in excess and even partial condensing of refrigerant would eliminate the purpose optimization.

For Chlorine liquefaction system, It is must to provide proper measurement at condenser and economizer side. Condenser can be provided with pressure and temperature for both sides and economizer can be provided with refrigerant flow measurement and in/out temperatures.

Effect of poor condenser & evaporator its impact on power consumption is given below.

Matching Capacity with system Load: 

In Chlor-Alkali site, it is observed that every plant has varying load due to configuration and market condition. Hence, the consideration at part load operation is must when designing chlorine liquefaction system.

The chlorine liquefier would be attached with minimum two or three machines. For example, if the rated capacity of the plant is 300 TPD having provided with 300 TPD liquefier to cater the rated load demand. The wet chlorine use can be provided as a backup capacity in case of non-availability. The production chlorine gas close to 90%, if the plant is using 100 TPD wet chlorine, it means during normal operation the chlorine liquefaction load is 170 TPD for 300 TPD capacity liquefier. The table provided below is example of the mapping the system load and avoid part load operation.

The capital cost and spare cost will high but opex will be all time lower. This can evaluate based upon the life cycle analysis. The table-4 is providing all time 90% loading and it’s a perfect match for provided configuration.

In many plants, it was seen same capacity two machines are provided and part load operating day always higher side and providing poor energy efficiency. The table is showing selection of machine for 300TPD plant based upon the wet chlorine load.

Maintain dedicated cooling tower for chlorine liquefaction system

In chlorine liquefaction system, the condenser is hook up with centralize cooling tower. The condenser water flow is very critical to the overall performance as shown in table-3.

In any CA plant, the refrigeration condenser is major consumer after caustic concentration units. In survey of chlor-alkali plants, it was observed that most of the time condenser performance is poor due to low water, high cooling water temperature, poor cooling water quality, not adequate pressure at condenser inlet. These all cause effecting the performance of entire refrigeration system.

Hence, cooling tower supply temperature, flow and pressure should be inline and need to comply on priority.

The best practices for maintain condensers.

1.     Provision of backwash

2.     Maintain cleaning frequency based upon the water quality.

3.     In case of poor water quality, the standby condenser could be plan to frequent cleaning.

4.     Installed flow meter for condenser water flow.

Evaporator or Chlorine Liquefier:

The evaporator condition is dependent on downstream of the process and mainly gas composition, drying and liquid ring compressor. The pressure of chlorine gas is one of driving factor, which guide the evaporator temperature. Higher the chlorine gas pressure, lower the evaporator temperature.

Hence, always seek the best possible pressure from chlorine compressor house.

The second important factor is dryness of chlorine gas. The inefficiency in drying system led to high moisture content in the gas. The liquid chlorine has capability dissolve moisture elevated temperature.

The -10 °C chlorine liquid easily dissolve 10 ppm moisture. In level of -5 to -4 °C, it goes up to 30-40 ppm. This is impurity and effecting the operation at end user side.

The second issue of entrainment of acid from liquid ring compressor. The sulfuric acid is used in chlorine compressor, it provides two benefits.

1.     Removal of heat from compressor

2.     Worked as desiccant to absorb traces of moisture carry over.

Hence, the change over of sulfuric acid in chlorine compressor is one of important factor to avoid and problem in chlorine liquefier. High moisture content with chlorine gas can cause freezing. 

The entrainment of sulfuric acid with chlorine gas is major source of sludge formation. The sludge formation keeps building and reduces the heat transfer area of the liquefier.

The compressor knock-out drum/separator are need check for their internal condition in regular basis. The chlorine gas sample analysis also carried out in 15 days period to check the quality of chlorine passing to the liquefier.

The higher sludge formation will lead to poor system performance and poor recovery of chlorine. The back pressure method also works well, when chlorine compressor is kept increasing, this is possible due to line choking and sludge deposition. The liquefier cleaning needs to carry out once in a two-year period. 

Conclusion 

There is an important improvement margin in terms of energy efficiency and chlorine liquefaction system health. Some modifications and best practices can implement easily and with very little investment. Some improvement can be obtained by adjusting the loading compressor to the real needs of the process, thus avoiding unwanted gas circulation. The daily work of checking process conditions and machine capacity is critical to improving compressor efficiency and performance. The service and maintenance team identify the measurement part and take advantages digitization.

The chlorine liquefaction is modeled on aspen plus to create the different economical options and it utilization to meet the best value figures. In our part-2, we will sahre aspen plus model on chlorine liquefaction and their output.