Low-cost retrofit cuts feedstock costs, emissions at Chinese complex

April 6, 2015
In August 2010, Liaoning Huajin Tongda Chemicals Co. Ltd. (Huajin), a subsidiary of China North Industries Group Corp. (Norinco), completed a process retrofit at its Panjin integrated petrochemical complex in Liaoning Province, China.

Zhifang Tang Wusheng Huang Tao Zhang Dajun Xiao Keng H. Chung
Liaoning Huajin Tongda Chemicals Co. Ltd.

In August 2010, Liaoning Huajin Tongda Chemicals Co. Ltd. (Huajin), a subsidiary of China North Industries Group Corp. (Norinco), completed a process retrofit at its Panjin integrated petrochemical complex in Liaoning Province, China.

The retrofitting project consisted of adding a cooler, demister, and separator to the absorber of the styrene tail-gas recovery system to reduce water content of the gas. The retrofit resulted in the recovery of hydrogen from styrene tail gas, allowing the tail gas to meet specifications for use as a refinery feedstock.

The low-cost improvements Huajin implemented to enhance its hydrogen recovery from the tail gas have enabled the 8-million tonne/year (tpy) refinery to reduce both its feedstock costs and CO2 emissions.

Background

Huajin's petrochemical plant operates an ethylbenzene dehydrogenation process under a vacuum pressure of 44 kilopascals (kPa) to produce 150,000 tpy of styrene. Tail gas from Huajin's styrene production unit, however, contains a substantial amount of hydrogen, the unit producing about 0.9 tonnes/hr (tph) of gas containing 92% hydrogen.

Before the commercial retrofitting of Huajin's complex, tail gas produced from the unit was used as fuel to power other refining processes at the plant. Due to increasingly stringent environmental regulations, however, Chinese refineries began to examine projects that would increase their hydrogenation capacities as a way to produce cleaner transportation fuels and maximize recovery of refinery byproducts that could be further processed to help reduce feedstock costs.

In 2010, Huajin began to evaluate ways it could lower hydrogen feedstock costs by using hydrogen-containing byproducts already available from its own operations.

Fig. 1 shows the hydrogen balance of Huajin's petrochemical complex in 2010.

Most of the hydrogen used in Huajin refinery was for gas oil hydrocracking and diesel hydrotreating, while a small amount was used in tail-gas recovery at the complex's sulfur plant. High-purity hydrogen (99.9 mole %) production came primarily from a 50,000-cu m/hr steam methane reforming (SMR) hydrogen plant and a 30,000-cu m/hr pressure swing adsorption (PSA) unit. The PSA unit was fed with low-purity hydrogen (87-93 mole %) from various sources of process off-gases, including a 50,000-tpy naphtha reformer and a 1.8 million-tpy gas oil hydrocracking unit from the refinery, as well as 700,000-tpy ethylene and 150,000-tpy styrene production units from Huajin's integrated petrochemical plant.

A total of 0.6 tph of styrene tail gas was needed to meet Huajin's hydrogen requirement (Fig. 1).

After a review of the refinery's PSA unit process requirements, Huajin determined that the styrene tail gas would need to be dewatered to meet PSA feedstock specifications.

Huajin's styrene production

Huajin used styrene production technology consisting of a 160,000-tpy ethylbenzene alkylation unit and a 150,000-tpy ethylbenzene dehydrogenation unit. (See accompanying box.)

From the principal and side reactions of ethylbenzene dehydrogenation in styrene production, the reaction byproducts are hydrogen (H2), benzene (C6H6), toluene (C6H5CH3), methane (CH4), ethylene (C2H4), CO2, and carbon monoxide (CO). Of these, hydrogen comprises 92% of the total reaction byproducts.

Since steam is used in the catalytic dehydrogenation reactions, Huajin determined that water in the styrene product stream would be inevitable.

Retrofitting

The initial design of Huajin's styrene tail-gas recovery unit used an absorber-desorber system to dewater the styrene tail gas and make it suitable as a fuel (Fig. 2). To recover hydrogen from styrene tail gas, however, its water content would have to be reduced to meet PSA feed specifications of 1,500 ppm of water. Measures also would be needed to prevent pipe freeze-ups during winter operations.

After evaluating various options, a low-cost ($100,000) retrofitting project on the styrene tail-gas recovery system was executed during scheduled plant maintenance in August 2010.

The retrofitting project consisted of the following modifications to the styrene tail-gas recovery system:

• A cooler was added to the absorber inlet. Cryogenic brine at 2-3° C. was used to cool the styrene tail gas to 10° C. from 30° C. Hydrogen and hydrocarbon vapors were routed to the absorber, while condensed water was routed to a knockout tank.

• A mesh demister was added to the top section of the absorber to reduce entrainment of water and hydrocarbons to the absorber outlet.

• A gas-liquid separator was added to the absorber outlet to improve gas-liquid separation.

• For safety purposes, a flare line was added to the outlet of a four-stage hydrogen compressor located at the outlet of the gas-liquid separator.

• An 800-m heat tracing (77-95° C.) coupled water pipe was added to the piping system from the outlet of the styrene tail-gas system's hydrogen compressor at Huajin's petrochemical plant to the PSA inlet at the integrated refinery.

Fig. 2 shows (in blue) the units modified as part of the retrofit.

Process performance

Since the retrofitting of the styrene tail-gas recovery system, which took place in August 2010, 0.6 tph of styrene hydrogen steadily has been transferred to the PSA unit to produce 0.24 tph of high-purity hydrogen.

Table 1 shows the properties of Huajin's styrene tail gas both before and after retrofitting, as well as feed specifications for the complex's PSA unit.

Post-retrofit testing showed the water content of styrene hydrogen reduced to 750 ppm from 4,400 ppm, or lower than PSA feed specification requirements.

Contingency for process upset

Process upsets, such as an unstable supply of ethylene to the ethylbenzene alkylation unit in the petrochemical plant, can cause the CO concentration of styrene tail gas to fluctuate and exceed PSA feed specifications (0.02 mole %).

Such a process upset occurred Jan. 3-13, 2011, at Huajin's complex following the 2010 retrofit. In some cases, the maximum CO concentration of styrene tail gas at Huajin's complex was 0.1 mole %, while maximum CO2 concentration was 4 mole %.

Table 2 shows the properties of styrene tail gas and PSA product during the January 2011 process upset.

Results showed the CO concentration in the PSA product (0.017 mole %) as similar to that of the styrene tail gas (0.020 mole %). The CO2 concentration of PSA product (0.013 mole %) was much lower than that of styrene tail gas (3.50 mole %). Huajin project engineers, however, expected this because the PSA unit is equipped with CO2 absorbent.

The hydrogen concentration of styrene tail gas remained relatively constant (92 mole %) during the process upset. The hydrogen concentration at the PSA outlet, however, was slightly lower than the 99.9 mole % purity required for gas oil hydrocracking applications. This illustrated that high CO concentrations had a detrimental effect on PSA hydrogen purity.

To maintain high-purity hydrogen for gas oil hydrocracking applications, the total concentration of CO and CO2 at the PSA outlet was kept to less than 30 ppm by introducing hydrogen from the SMR hydrogen plant. Ethylene production also remained relatively constant, which allowed CO concentrations of styrene tail gas to remain at less than 100 ppm.

Economic benefits

Before completing the retrofit, styrene tail gas produced at Huajin's complex was used as fuel to power processing activities at the integrated refinery at an equivalent value of about $500/tonne. Following the 2010 retrofitting and process improvements, 0.6 tph of styrene tail gas was used as PSA feed at a value equivalent to about $1,650/tonne, resulting in additional net profit of about $5.52 million/year during 2011-12.

In late 2012, the styrene tail-gas recovery system was further optimized to use the spare capacity of the plant's newly installed cooler and gas-liquid separator.

The water concentration of styrene tail gas from the unit currently remains at less than 200 ppm, which allows all of the styrene tail gas (0.9 tph) to be routed to the PSA unit, yielding a net profit of about $8.28 million/year.

These improvements have reduced the amount of hydrogen produced from the SMR hydrogen plant by 10%, which in turn has reduced both the amount of natural gas used in hydrogen production at the plant and its CO2 emissions.

To date, the Panjin complex has lowered its use of natural gas by 10 million cu m/year, while CO2 emissions at the complex have been cut by 8,500 tpy.

Innovation

Even though the use of a PSA unit to purify hydrogen in styrene tail gas is not new, the complexity of the process and economic return varies widely with styrene production technology and plant integration.

At Maoming Zhonghe Chemical and Plastic Ltd.'s stand-alone petrochemical plant in southern China, the hydrogen content of styrene tail gas was 8% lower than that of Huajin's Panjin integrated complex. In addition to installing a grassroots PSA unit, Zhonghe also had to pretreat PSA feed using a carbon-based de-oxidation agent as well as low-temperature absorption. The total cost for a styrene tail-gas hydrogen recovery system at Zhonghe, consequently, amounted to $2 million, or 20 times more than the cost of Huajin's retrofitting project.1

Huajin attributes its ability to execute a lower-cost retrofit at its Panjin complex to its adoption of an advanced styrene technology that produces high concentrations of hydrogen in styrene tail gas; and its integration of petrochemical and refinery operations, which allows the complex to process styrene tail gas from the petrochemical plant in an existing PSA unit in the refinery.

The success of Huajin's styrene production and tail-gas hydrogen recovery project has served as an impetus for Jiangsu Leasty Chemical Co. to adopt a similar system for the second stage of its capacity expansion project at its plant in Jiangyin City, Jiangsu Province, China.2

Potential country-wide benefits

Styrene production in China had increased to 3 million tpy in 2010 from 2.06 million tpy in 2005. Typically, the ratio of tail gas to styrene is around 4.5%, with tail gas containing at least 50% hydrogen.

If hydrogen were to be recovered from all styrene production units in China, it would be equivalent to 67,500 tpy of hydrogen production. This, in turn, would lead to an equivalent savings of 260,000 tpy of natural gas used in steam reforming for hydrogen production and a 600,000-tpy reduction in CO2 emissions.3

References

1. Qin, Zhonghua, "The recycling and application of residual gas of styrene plant," Journal of Low Temperature and Specialty Gases, 2004, Vol. 22, No. 5, pp. 21-22.

2. "Leasty Chemical Company planned expansion of styrene system," China Chemical Industry News, Mar. 25, 2014.

3. Personnel Dept. of Sinopec and Personnel Service Center of CNPC, "Hydrogen production equipment and operation," China Petrochemical Press, Sept. 1, 2007, pp. 5-7 and 22-23.

The authors
Zhifang Tang ([email protected]) serves as chief engineer for Liaoning Huajin Tongda Chemicals Co. Ltd. and also is on the company's board of directors. With more than 30 years of experience in refining and petrochemicals, he previously held senior technical and executive positions with PetroChina Fushun Petrochemical Co. Ltd. He holds a petrochemical engineering degree from Fushun Petroleum Institute, Fushun, China.
Tao Zhang ([email protected]) serves as process contact engineer for Liaoning Huajin Tongda Chemicals Co. Ltd. He holds a degree in chemical engineering from Shenyang Chemical Institute, China, and has 20 years of styrene production experience.
Wusheng Huang ([email protected] ) is the refinery manager of Liaoning Huajin Tongda Chemicals Co. Ltd. He holds a degree in chemical technology from Shenyang Institute of Chemical Technology, Shenyang, China, and has more than 20 years of refining experience.
Dajun Xiao ([email protected]) serves as deputy director of petrochemical plant technical services for Liaoning Huajin Tongda Chemicals Co. Ltd. He holds a degree in chemical engineering from Shenyang Institute of Chemical Technology, Shenyang, China. Xiao has more than 10 years of experience in ethylene production.
Keng H. Chung ([email protected]) serves as refinery adviser for Liaoning Huajin Tongda Chemicals Co. Ltd. He also acts as a process troubleshooting expert for petroleum operations and actively is involved in developing sustainable heavy oil technologies. Chung holds a BS and MS in chemical engineering from Queen's University, Kingston, Ont., and a PhD in chemical and petroleum engineering from the University of Calgary. He is a registered professional engineer in Alberta.