Home > SOLUTION > Air separation device DCS system solutions

Air separation device DCS system solutions

I.Process description

This process device is mainly used for providingnitrogen and oxygen for other devices. By taking whole physical processincluding purification, compression, cooling and fractionation, usingatmospheric air as raw materials for production and utilizing the differentboiling points of oxygen and nitrogen under the same pressure drop, the airseparation device can get high-purity oxygen and nitrogen.

The heat exchanger, which plays animportant role in the whole control, is mainly used to remove impurities suchas water, carbon dioxide and inert gas in compressed air, directly affects the yieldand quality of products and can cool the air in the system for convenience ofnitrogen and oxygen separation after the air enters lower tower. The impuritieswhich are absorbed in heat exchanger are removed from system by airflowexchange. The heat exchanger is mainly provided with sequence control and shallbe programmed according to sequence chart of sequence control.

The whole process is implemented inhigh-pressure and low-temperature conditions; the related interlock protectionshall be used to ensure system safety since the air compressor and expansionmachine have high power. For details, please refer to control plan part.

II.Control plan

By analyzing the air separation device,it is found that the device has a lot of control loops but its plan is relativeeasy; this is not due to the easy air separation process but most of them canbe realized by simple loop and that the quantity of parameters controlled istoo large; there is strong relevance among these parameters and most of loopsare controlled by manual controller since the ordinary loops fail to controlthem. From the view of overall process, the air separation device belongs tocomplex process object, which can be obviously represented from the airfractionating part. Most of the air separation devices at home and abroad areprovided with manual control and other plans are not realized even a lot ofresearches and on-site tests in this field are made by many personnel.

As a typical system with multivariableinput and output, the air separation tower of air separation device has strongrelevance between variables and it fails to establish accurate mathematicalmodel; it is difficult for air separation device to realize fully automaticcontrol or optimal control through ordinary control laws since the controllingthrough modern control theories are basically focused on certain mathematicalmodels. In reality, the system safety is guaranteed by adding analysis functionto process system, providing proper prompts to operators and optimizing processby operations of operators instead of sending optimized results to on-siteactuators.

2.1 Sequence control during switch ofplate heat exchanger

The plate heat exchanger is veryimportant in the whole system and it’s also the key and difficult point inoperation of DCS system. The actions of 14 solenoid valves such as startup and shutdownare controlled by switch system according to sequence to ensure the normalworking of two heat exchangers. The switch purposes include: 1. Heat exchange:after pretreatment such as impurity removal and cooling, the outside air willabsorb cooling capacity again, reduce temperature and reach the critical stateof condensation for convenience of nitrogen and oxygen separation after the airenters air separation tower; 2. Impurity removal: when the solenoid valve isswitched and the waste nitrogen is discharged from system, the impurities whichcome from the air of heat exchanger and are absorbed on the tube wall and willbe removed from system. The discharging of impurities of heat exchanger isrealized by the constant switches and these impurities have big influence onthe rectification of whole air separation tower; the switch control of plateheat exchanger is very important in the whole control since the process systemmay be stopped if there is no switch, the failed switch or the switch cannot berestored in short period. The switch programs of plate heat exchanger shallhave the following functions: in manual mode, it can directly operate on-sitesolenoid valves and set solenoid valves after it completes setting the operatedtime in current period; in automatic mode, it can complete switching actionaccording to the set sequence; the system will send out alarm information,reminder that this switch has errors and request for intervention by operatorsif the system cannot receive right feedback of on-site solenoid valve after ithas actions to solenoid valve for several seconds.

2.2 Control of interlocking logic

The air separation device belongs tohigh-pressure and low-temperature treatment process according to its processcharacteristics. The process device includes multiple large power equipment andif they have abnormities in startup, shutdown or during operation, the controlsystem shall have corresponding responses to ensure safety of processequipment. The interlock includes:

Emergency shutdown available if oilpressure of 1# expansion machine <0.1Mpa, shaft temperature >65℃;

Emergency shutdown available if oilpressure of 2# expansion machine <0.1Mpa, shaft temperature >65℃;

Electrical linkage and emergency loadingavailable if revolving speed of 1# brake fan >4200r/m

Electrical linkage and emergency loadingavailable if revolving speed of 2# brake fan >4200r/m;

Auxiliary oil pump starts if oilpressure of air compressor <0.15Mpa; air compressor stops if oil pressure<0.1Mpa;

Air compressor stops if axialdisplacement of air compressor is too large;

Allowable conditions for startup ofcompressor: shutdown signalsunavailable;

Water pump stops if outlet pressure offractionating tower <0.35Mpa;

1# heating furnace stops if outlettemperature of dryer >80℃;

1# heating furnace pauses heating if itsoutlet temperature >200℃;

1# heating furnace starts heating if itsoutlet temperature <170℃;

2# heating furnace pauses heating if itsoutlet temperature >100℃;

2# heating furnace starts heating if itsoutlet temperature <70℃;

2.3 Routine control

Limited by the characteristics ofprocess of routine loops in air separation device, most the control loops arecontrolled by manual controller and routine PID control is only available forparts of loops, including liquid level control for air cooling tower, watercooling tower and lower tower and anti-surge control of air compressor. Themanually controlled loops in air separation device include: remote control offraction liquid nitrogen entering upper tower, remote control of pure liquidnitrogen entering upper tower, remote control of air at inlet of expansionmachine 1, remote control of air at inlet of expansionmachine 2, remote control of fan outlet of expansion machine 1,remote control of fan outlet of expansion machine 2, adjustment of 1# air inletof heat exchanger, adjustment of 2# air inlet of heat exchanger, adjustment of1# oxygen inlet of heat exchanger, adjustment of 2# oxygen outlet of heatexchanger, adjustment of 1# nitrogen inlet of heat exchanger, adjustment of 1# purenitrogen outlet of heat exchanger, remote control of total air inlet offractionating tower, remote control of oxygen outside of tower and remotecontrol of nitrogen outside of tower.

2.4 Optimized operation part

This set of DCS can provide certainprogram space to make dynamic description of whole process operation in orderto help operators obtain comprehensive information about optimized process,after which the operators can properly adjust the on-site actuators bycombining their analysis in order to optimize the operation of on-site process.The main purpose of optimization is to improve oxygen purity and tap thepotentials of device; therefore, the system mainly judges the parametersrelated to oxygen, analyzes possible conditions and reminds the operators tocarry out operation; this part has its limitations since not all details can beconsidered during design. The outputted information of program is used for referenceonly since only common conditions can be speculated by the program. The programchart of optimized part is as follows:

The method for determining flooding willbe introduced as follows:

2.4.1 Judgment of towerflooding

The liquid level of liquid oxygen haslarge fluctuations (>=10%). The oxygen purity willincrease/decrease when liquid level of liquid oxygen decreases/increases and itwill be shown as vibration descending graphic in oxygen purity analyzer. Theresistance has large fluctuations (>=20%, the influence at the moment ofswitch must be eliminated). When resistance increases, the liquid level ofliquid oxygen will decrease and pressure of upper tower will increase. When themiddle resistance of upper tower has sudden decrease (>=20%), the liquidlevel of liquid oxygen will have sharp increase (>=10%), then the pressureof upper and lower tower starts to decrease, the air amount entering towerincreases and oxygen purity reduces and it can be judged as flooding of uppertower if the phenomenon above occurs again after a period of operation.

2.4.2 Judgment of lower tower flooding

The liquid air and its liquid level inlower tower have large fluctuations; when liquid level is provided withautomatic control, the adjustment-1 valve will have frequent and largemovements. The resistance increases obviously, fluctuation is large (>=20%)and the openness of adjustment – 1 valve reduces gradually (since the liquidlevel of liquid air is provided with automatic control). If the resistance hassharp decrease when it reaches certain level, the liquid level of liquid airwill have sharp increase, openness of adjustment – 1 valve expands, pressure oflower tower reduces, air amount entering tower increases and the pressure oflower power increases again and it can be judged as flooding of lower tower ifthe process above occurs periodically.

2.4.3 Flooding of both upper and lowertower

When resistance of lower tower increasesgradually (>=20%), the pressure of lower towerincreases, openness of adjustment -1 valve reduces obviously (since thedownstream liquid air amount reduces), the liquid level of liquid oxygen inupper tower reduces, the pressure and resistance of upper tower increase(fluctuation range of resistance may be >=20%)to certain level and causes large fluctuation of nitrogen flow meter (>=10%,liquid carrying phenomenon occurs), temperature of cold side TI-6-1, TI-6-2,TI-7-1 and TI-7-2 reduces, pressure of lower tower reduces, resistance reduces,liquid level of liquid air has sharp increase (>=10%),adjustment – 1 valve expands quickly, liquid level of liquid oxygen increases (>=10%),resistance of upper tower reduces quickly (>=20%),pressure of upper tower reduces and gradually becomes normal and the processabove will be repeated periodically after it is stabilized.